Ask the Physicist!

QUESTION:
My eight year old has a question. Get ready. If two particles are in entangled, and one is on the earth and the other has passed the event horizon of a black hole. Would the entanglement be broken or even if it's unmeasurable still be entangled. Does the medium "the ether" transcend the event horizon? Silas Massicotte (8) and Malachi Massicotte (6).

ANSWER:
Seriously? Your 8-year old understands quantum mechanics and cosmology?! That would be pretty amazing. I usually don't do, as stated on the site, astronomy/astrophysics/cosmology, but I did a little research and think I can give you an answer. When one of the two entangled particles is totally destroyed, the other one will not know at all because there is no communication between the two. Since the wave function of the destroyed one simply disappeared, the wave function of the other will remain in the mixed state it was in originally, but it will no longer be entangled with anything. If a measurement is made on the survivor, there will be no change in whatever remains of the other particle. There is a pretty good explanation at this link. There is no such thing as the ether.

QUESTION:
In the known universe, is there anything else in existence besides matter, waves, forces, movement and change? In other words what are the very basic essential physical things and the effects, excluding any measures that we seem to give a reality to quite often? Maybe waves could exclude matter or maybe waves are just matter in motion? Space itself seems not to be truly part of existence? Time to me is a measure of forced motion and change and the same with energy. What are the bare essentials of the known universe that I can detect through my senses, not just concepts alone? Would you include fields? What are the indisputables?

ANSWER:
This is a strange a very strange question. There is only one way I know to encompus everything which exists. Mass is energy, light is energy, electric and magnetic fields are energy, motion of a mass is energy, etc.. The answer is that the universe is ENERGY.

REVISED ANSWER:
It occurs to me that time also exists. Time is not energy, so the revised answer is ENERGY and TIME.

FOLLOWUP QUESTION:
What is energy and where has it come from? Does the ability to do work come from energy or the forces or both? It seems to me that of all the forms, shapes, colours, sizes, amounts of energy that can be stored or transferred but not created or destroyed.. energy is a quantitative measure of the movement that forces create on an object or particle. Could energy be dispersing waves of just motion through fields since the big bang. Anyway, if not a measure, what is it? Please not the ability to do work.

ANSWER:
You seem to have a talent for submitting questions which, contrary to site ground rules, are not "…single, concise, well-focused questions…" You are expecting me to give you a complete tutorial on energy, something which takes a couple of years of classes to fully understand. Essentially we invent a quantity which, for any isolated system, is conserved, i.e. remains constant no matter what happens inside it. By being isolated we mean that there are no external forces doing work on the system. [Sorry, there is no way to talk about energy without mentioning work.] Conceptually, isolated means any system to which energy is neither being added to nor taken from the system. Assuming that the entire universe is all that there is (or not interacting with other systems outside the universe), the total energy of the universe never changes.

QUESTION:
So I just read an article about virtual particles and Hawkins radiation, and how a black hole can take virtual particles, separate them, and take one of them while the other Hawkins radiation is ejected out and that's how a black hole can evaporate, which I still find hard to believe. If that is the case, and a virtual particle pops existence and one is taken away and the other is ejected what happens with that energy? So, if there's energy between the virtual particles and that energy can't be destroyed only transferred could that energy end up as dark energy? I'll explain it again in the hopes of clarifying, near a black hole, virtual particle, can pop into existence, be separated, with one particle, being devoured by the black hole, and the other one being ejected out into space, if there is energy between the two particles at the moment of them being separated could that energy simply turn into dark energy or would it stay with the Hawkins radiation being pushed into outer space?

ANSWER:
If a particle/antiparticle pair is created near the Schwarzschild radius, and one of them is inside and one outside, the one inside cannot escape but the one outside can. If this had happened either inside or outside the black hole, they would quickly have recombined to conserve energy consistent with the uncertainty principle. But, now, because the particle outside has energy, energy would appear to be not conserved. But that does happen, so that energy excess has to be provided by the black hole which means that it must lose a bit of mass. The details doesn't matter of how this happen, it has to happen to conserve total energy.

QUESTION:
I am writing you to seek your expertise in the field of physics. As you know, Newton's law of universal gravitation demonstrates that the gravitational force between two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. The use of r² instead of r arouses my curiosity. Any further explanations would be highly appreciated.

ANSWER:
At the Newton was proposing his three laws there was a lot known about the solar system. Most importantly, Tycho Brahe had made thousands of precise measurements of planetary orbits, their shapes and periods. His assistant Johannes Kepler continued after Brahe's death but he also used those data to formulate the properties, the three Kepler's laws. Keep in mind that Kepler's laws were purely empirical, nothing we would call a theory. It was now known that the orbit of a planet was an elipse one focus being at the sun, that the quantity T²/a³ was the same for all the known planets (T is the period and a is the semimajor axis of the elipse); also, the line from the center of the sun to the orbit swept out equal areas in equal times. Newton's laws introduced the notion of force and its relation to motion. He realized that the planets moving around the sun implies that some force was acting between the sun and the planets. And surely he realized that the force would not be constant nor would it get bigger as the objects got farther apart. Maybe he did try a force which went like 1/r, but if he did it would not explain the data—Kepler's three laws were the data. But it would not work. Nor would 1/√r, 1/r³, etc., but a 1/r² force did work.

There is another way you can look at forces. A force usually may be thought of as a field where we think of lines of force directions in the direction a mass would be attracted; the closer the lines are together, the stronger the field; and total number of lines is proportional to the mass. The red lines in the figure show the earth's gravitational field but only in one plane. The blue lines show spheres again only in two dimensions. You should imagine the lines coming in from all directions and many imaginary spheres. The strength of the field would be the number N of lines per unit area A. But every sphere is punctured by exactly the same number of lines as all the others. The area of a sphere is 4πr² so the number of lines per unit area, strength of field, is N/(4πr²). Eureka! There's your 1/r².

QUESTION:
Is it theoretically possible for a black hole to contain other smaller black holes?

ANSWER:
If you mean by contain inside the Schwarzschild radius, the answer is yes, although it wouldn't last for very long. If you mean inside the black hole itself, what does it mean for a point to "contain" anything? Keep in mind that I, as stated on the site, do not normally do astronomy/astrophysics/cosmology.

QUESTION:
I wonder, how step by step, is parallax method used to find distance of a star? When you find the parallax angle, then it's easy to find distance using tangent formula or other trigonometric formule. But how is that parallax angle exactly determined? In some online articles, like in Wikipedia or Sky and Telescope they show such pictures. But in this picture, only distance between A to B is known. Distance between A to Eros or B to Eros is unknown. Angle of A and B are unknown. So we can't know angle of p with so little information. What we see only is a star on the sky do change its position (like you see right side of the picture) slightly (some millimeters) every 6 months. How do astronomers determine from that motion the parallax angle of the star?

ANSWER:
The angles A and B are not unknown. These are measured by the direction in which the telescopes at those locations are pointed. Once you know those angles you know the angle p; and if you know p and the distance AB you can also calculate the distances from A and B to Eros.

QUESTION:
How did the eclipse of the sun move from east to west across the earth.

ANSWER:
The shadow the moon casts on the earth (which is the eclipse) moves west to east, not east to west. A clear explanation was provided by NASA: "Because the Moon moves to the east in its orbit at about 3,400 km/hour. Earth rotates to the east at 1,670 km/hr at the equator, so the lunar shadow moves to the east at 3,400-1,670 = 1,730 km/hr near the equator. You cannot keep up with the shadow of the eclipse unless you traveled at Mach 1.5."

QUESTION:
With the recent discovery of "seeing" gravitational waves emitted by two merging neutron stars, how does one "see" gravitational waves.

ANSWER:
There is a good description of the detectors here. What is particularly exciting is that light from the merging neutron stars could be seen; earlier observations, of merging black holes could not. Also, the abundance of heavy elements like gold is much larger than could be understood using standard models of stellar evolution; it had been hypothesized some years ago that perhaps the excess could be produced in neutron star collisions, and the current observation seems, at first glance of the spectroscopy of the light, to support this hypothesis.

QUESTION:
How did the eclipse of the sun move from east to west across the earth.

QUESTION:
As I'm sure you know, LIGO has detected 3 black hole mergers in the last few months. This means that there have been and likely are a huge number of black holes in the universe. In parallel, the math says that 95% of the mass of the universe is not visible to us. Could it be that there are millions or billions of black holes floating around a few light-years from the nearest thing and that they would be virtually invisible - we could only detect them if they happened to line up with further distant light sources such that we might detect gravitational lensing. Could they account for the 95%? I know you don't do astrophysics but thought that the answer might be so simple that you might be happy to address it.

ANSWER:
Keep in mind that I have no real expertise in cosmology. Two things, I think, will shoot down your idea. The 95% number you quote is like 20% dark matter and 75% dark energy. The dark energy component cannot be thought of as "mass-like" because it makes itself known in the accelerating expansion of the universe, so it would appear to be some kind of repulsive force. The dark matter which everyone is trying to observe cannot be in black holes because to cause the things it is thought to cause (like the anomalous rotation of spiral galaxies) it needs to be spread out over huge volumes of space which, of course, black holes are not. My take on dark matter (absolutely not mainstream) is that perhaps dark matter is the luminiferous æther of the 21st century, i.e. it is not some invisible "stuff" but rather its purported effects are indicative that we do not really understand gravity as well as we think we do. Certainly general relativity is an incomplete theory since no one has successfully quantized it. You may know, but dark energy, at least the evidence for it, has an interesting history. When Einstein formulated general relativity, the expansion of the universe was not known and the universe was thought to be static; but with just gravity this is not possible—it will expand forever or expand until it collapses, depending on the initial conditions. Einstein therefore empirically inserted something he called the cosmolgical constant. Later when Hubble discovered that the universe is expanding, Einstein removed the cosmological constant and dubbed it his "biggest blunder"; now it is coming back into play, but still just empirically. There are doubtless millions or billions of black holes as you speculate, because all stars greater than a few solar masses end up that way. It is known that nearly all galaxies have a supermassive black hole at their centers. I would guess these came from early-universe stars coalescing. And early-universe stars were huge, thousands of times more massive than the sun because only hydrogen was available to make early stars. Still, I have learned that the size of the universe is just about impossible to comprehend and at its relatively young age still, I would guess these remnants of earlier stars are still a pretty small fraction of the mass of the entire universe.

QUESTION:
Good Day ! Thank you for being there and dedicating the time to answer our questions. I have many... Can I start with the "Inverse Square Law" as it pertains to light. E = I/r2 , Using 100 as "S", S/4pi2 = I ( intensity at surface of sphere ) as the source strength, the farthest we would be able to "see" would not even get us to PC. it is 4.2 light years away, 5,878,499,817 x 4.2 = 24,689,699,231.40 miles... Assuming 100% at "S". Reaching the surface of the earth, using a nominal, 8,000 mile diameter, that works out to be about 1 (one) photon for every 1,205. square miles.... And, that is purported to be our closest star. How can this be ?

ANSWER:
I generally do not try to find errors in questioners' calculations, I just do the calculation myself. The sun is a pretty average star and its photon rate is about Φ≈10⁴⁵ photons per second. Using R=4.2 ly=4x10¹⁶ m, I=Φ/(4πR²)=10⁴⁵/(64πx10³²)≈5x10¹⁰ photons/s/m². That would be the photon intensity 4.2 ly from a sun-like star.

QUESTION:
Every day my wife reads the latest fake news about planet 9 and sits weeping in fear the whole time . As a scientist ,surely you can tell me one thing that just shuts this whole fiasco down . Please. Tell me that undeniable fact that will convince her that we are in fact safe from a collision or near miss from this nonexistent space oddity and it 's cohorts . It s affecting her health . And I love her . So it' i m feeling her pain as well .

ANSWER:
First of all, "Planet 9" is a serious astronomical topic. Minor irregularities in the orbits of some of the distant planets suggest the presence of another planet farther out. Serious efforts are underway to try to observe it. But its anticipated period is more than 10,000 years, so if it is far away now, it is not likely to be a problem for us for far longer than the lives of any of us. All reputable astronomers have declared that (if it actually exists) it is absolutely no danger to earth. Google "planet 9" to get lots of good information. But, even if there were a planet much closer, like the "fake news" "planet x", there is an amazingly low likelihood of its colliding with earth. Most lay folk like your wife have no comprehension of the vastness of space; the probability of two particular objects in the solar system colliding is, for all practical purposes, zero. It is often posited that a "rogue planet" passing through the asteroid belt would "shake loose" a "storm" of asteroids toward the earth. I recently answered a question addressing this possibility which you might find useful.

QUESTION:
The movie "Interstellar" did a nice job of explaining how time dilation works in a massive gravity field. My question relates to how we on Earth measure the age of the universe to be 14.3B years. If I could make that measurement on the planet orbiting the "Interstellar" singularity [and since, theoretically, if I could view life on the singularity planet from Earth, it would all look to be in super slow motion], what would my measurement of the universe's age be from the my perspective on that planet? If time moves more slowly on the singularity planet, wouldn't my estimate of the universe's age be much less?

ANSWER:
As I say on the site, I do not usually answer questions on astronomy/astrophysics/cosmology. Maybe I can make a stab at this. The microwave background radiation which pervades the universe is generally considered the best source for information about the big bang and measurements are probably the best determinations of the age of the universe. In your high-gravity position, you would see the same microwave radiation I do. Likely, you would have to make corrections to your observations due to the intense gravity, but you would still conclude the same age as I did.

QUESTION:
The following question came up during lunch with the ROMEOS (retired old men eating out) yesterday: Why is a star's spectrum mainly a black-body spectrum?

ANSWER:
Innumerable photons are produced by innumerable processes in a star. These diffuse around in the plasma which comprises the star. Those in the photosphere, the outermost layer of the star, come to thermal equilibrium. A relatively small number "leak out"; this then is analagous to the classic cavity with a small hole containing a photon gas. Thermal equilibrium is maintained by photons diffusing in from the more interior regions of the star.

QUESTION:
I am a writer. A plot idea came to me, and I am perplexed as to how to explain it using physics. I refuse to write something if there is no way it could plausibly happen, so I am hoping you can help me with some explanation as to how this could happen. The premise is, some celestial event occurs that causes Earth to be repeatedly, and fairly regularly bombarded with devastating asteroid strikes. These would be sufficient enough to destroy our infrastructure, plunging the world into darkness and challenging the heros to survive what would seem to them, a shooting gallery. I was wondering, if there is some way to explain how this could happen, perhaps an exoplanet disrupts one of the asteroid belts on a regular basis, perhaps by its orbit passing through it. I know this may seem unusual, but as a writer of hard sci-fi, I refuse to do a project, no matter how intriguing it is, if I can not offer at least some plausible explanation, even if it is a bit stretching the limits, to justify my work. I trust you might take sympathy on me and help me with this, because I would hate to think this idea is a dead end.

ANSWER:
The only possibility I can envision is the one you suggest—a planet-sized object somehow gets inserted into an orbit which periodically passes through the asteroid belt. The period of a typical asteroid in a roughly circular orbit is about 3 years, so I imagine an elliptical orbit for our rogue planet which has a semimajor axis about equal to the radius of the asteroid belt as shown in the picture. Kepler's first law then says that this orbit would also have a period of about 3 years and therefore pass through the asteroid belt about once every 1½ years; it would pass through the asteroid belt twice each orbit spending only maybe a couple of months in the belt. So, we are off to a good start for your scenario! To get some numbers I found the a Physics Stackexchange discussion useful. An estimate of the distance between asteroids larger than 1 km is shown to be about 2 million miles (about 3 million km). This alone is sort of eye-opening—we tend to think of the asteroid belt, ala sci-fi movies like Star Wars, a dangerous densely-packed hazard whereas your rogue planet would be extremely unlikely to encounter even a single one as it passed through the belt. With a passage taking only a couple of months, the likelyhood of even seeing a single large asteroid is small, let alone being close enough to significantly alter its orbit. But, suppose that a large asteroid is shaken loose. Now you have to think of the probability that it will collide with the earth; this will be even tinier than the probability of its having been kicked out of the belt in the first place. I have not done any quantitative probability calculations because I think the main point can be made with very rough qualitative estimates as I have done here. The bottom line is that space is unimaginably vast and even in places where the density of objects is relatively large, it is still extremely small in absolute terms. If this rogue planet were to appear, I doubt that, on average, even a single asteroid collision with earth would occur in a lifetime.

QUESTION:
I have a question about black holes,: If they are infinitely dense, how can one black hole be "bigger" than another i.e "stellar" black holes and "super massive" black holes... Obviously if two black holes merge or obtain mass through eating stars 1 + 1 = 2 but if the mass already equals infinity how can it get any larger. This would assume that it is equal to infinity + 1.

ANSWER:
You might first read an earlier answer. Now, infinite density does not mean infinite mass; rather the mass is some finite number and the volume is zero. So, clearly, the rest of your question does not apply to black holes since their mass is never infinite. Super massive black holes have masses much larger than the mass of a star.

QUESTION:
If planets are the part of stars around which they revolve then the stars and the planets must have the same stuff then why the stars do not cool down as the planets do? Why they continue to burn and expand while planets cool down?

ANSWER:
The solar system formed from a huge interstellar cloud of material. The material was overwhelmingly composed of hydrogen. A very large fraction of the material collapsed gravitationally to form the sun and, as this mass collapsed, it got hotter and hotter until eventually nuclear fusion of the hydrogen ignited and it became a star. A smaller fraction if the material from the cloud, because of angular momentum, did not become part of the sun and, through a long period of condensation and collisions among clumps, became the planets and their satellites. But the hydrogen did not ignite because the gravity of the planets was too small. Eventually some of the hydrogen combined chemically with heavier elements (for example, water) but most of it escaped back into space because the gravity was too weak to hold lighter elements in the atmosphere.

QUESTION:
Can gravity be focused like light thought a lens? Say following the plane of a qalaxy. If so, could stars be traveling around galaxies at the same speed because gravity effects time? The reason for the question above, IF gravity focuses out along the plane of a galaxy, could it be slowing down the stars closer to the center of the galaxy because gravity effects time?

ANSWER:
Gravitational lensing is light being focused by gravity, not gravity being focused. So I am afraid that your idea to explain the anomalous orbiting properties of stars in a galaxy will not work. An example of gravitational lensing is shown to the right; shown is "Einstein's cross" which shows four images of the same quasar.

QUESTION:

Where happens all the matter that enters a black hole? Is all the matter in a black hole crushed into a "singularity" What are the current theories you suggest I study?
If a black hole can consume matter without limits, could a super massive black hole also consume the mass our universe?
So, could the "Big Bang" be from this "singularity", a singularity that holds all the mass of our universe? (This makes sense to my way of thinking and boggles my mind.)
What are White Holes? Have they been proven to exist yet? Or like "worm holes" are they only theoretical today? And does current thinking say White Hole evolve from Black Holes?

ANSWER:
Well, I certainly appreciate your donation—almost nobody does, even for my best, most creative answers. I do, however, recommend that people wait for an answer before donating unless you like my site so much that you just want to support it and I greatly appreciate that! (Also, I have no way to "refund" contributions.) In your case, you did not read enough on the site before submitting your question(s). Two quotes from the site: "If your question is clearly astronomy or astrophysics, particularly detailed questions about black holes, stellar evolution, dark matter or dark energy, the big bang, etc., areas in which I am not expert, I may not answer" and "Please submit single, concise, well-focused questions". Because of your contribution, I will cut you a little slack this time.

The simplest idea of a black hole is an object with mass, charge, and angular momentum which has infinite density and where time has stopped. I believe the latest thinking is that it is not really of infinite density. Predictions of singularities are from classical general relativity; if you view the situation quantum mechanically, then uncertertainty ideas do not want zero size or stopped time because spacetime itself should have a granularity. A discussion which you might find accessible is at Physics Forums.
This depends on how things happen to be moving around. You could imagine a much older universe composed of nothing but black holes and the radiation they emit when they undergo Hawking radiation but moving in such a way that they will never encounter each other.
There is a very interesting book by Lee Smolin, Time Reborn, where he reprises an idea originally by Wheeler and deWitt that black holes are the seeds of new universes. There is a nice interview with Smolin on space.com and a lecture by him that you can watch.
White holes are hypothetical and have never been observed. I know nothing about them.

I hope I have earned your donation!

QUESTION:
How is the light red shift, observed in the universe, a factor of distance AND speed and the Doppler affect (in which it is often compared) is only a factor of distance? Galaxies farther away have a greater red shift so we are told they are farther away and moving away faster than nearby galaxies. With Doppler the farther the object is away the greater the shift, but the speed of travel could be constant, say using the historic train example. I could buy into the universe is expanding, I'm not convinced we can predict how fast relative to time.

ANSWER:
The Doppler effect does not depend on distance, only speed. Many years ago astronomers discovered that distant stars and galaxies exhibited red-shifted spectra which they could use to determine the speed at which those objects were traveling. Later methods were developed which allowed an independent measure of the distance of those objects. When the two separate measurements were compared, it was found that the velocity of the objects was a linear function of their distances. The graph shows an example where distance was determined using the brightness of type Ia supernovae. I do not understand your question beginning with "…With Doppler the farther…"

QUESTION:
People say that Universe is expanding, right? We all know that Gravitational force is an attractive force and it is predominant at the planetary level among all other forces. So shouldn't the universe come closer together as time passes? Not at a very fast rate, but at least very slowly. I don't understand why still people do say it expands, Can you please help me reasoning it out?

ANSWER:
Right after you throw a ball straight up, it is moving upward ("expanding" relative to you); the harder you throw it, the farther it goes before you and it "come closer together". The universe is still in the early stages where it is still "going up". But, if you throw it hard enough (called the escape velocity), it will never come back. And, if you throw it with a velocity greater than the escape velocity, it will never stop going. So, the ultimate fate of the universe is that it will either expand forever or everything will turn around and fall back together (the big crunch). All that is out of date, though, because it was discovered almost two decades ago that gravity is not purely attractive or else there is some other force which is operative as well (called dark energy); this results in the fact that the distant objects in the universe are not just moving away from us, they are actually speeding up. This repulsive force is not yet fully understood and the ultimate fate of the universe is not known.

QUESTION:
Is dark energy real? If all matter in the universe expanded from a single point the size of an atom (the big bang) wouldn't things be moving faster from each other because of geometry? If you start with a sphere the size of an atom and it expanded outwards over billions years even a Planck length size degree difference would be immense. It would cause things to move farther apart at faster rates as time went on. Are things moving faster away from each other because of geometry and angles, and not from hypothesized dark energy?

ANSWER:
I do not usually answer questions in astronomy/astrophysics/cosmology but think I can answer this one. If a collection of objects interact only via an attractive force (gravity in this case), any one of them can only speed up when moving toward their center of mass. The details of the motion would be determined by the initial conditions. If all the objects were moving away from some common point at some time, the only possible motions would be

for all to move forever away from each other, but forever slowing down;
for all to slow down and eventually turn around and speed up back together; or
for some to come together and some to keep going.

The simple reason is that the potential energy of such a system increases as the objects get farther apart so the kinetic energy must decrease to conserve energy.

QUESTION:
Does the existence of gravitational waves imply the existence of gravitons?

ANSWER:
No, gravitational waves have nothing to do with gravitons. Gravitational waves are predicted by general relativity, the best current theory of gravity. Gravitons would be the quanta of the gravitational field is a successful theory of quantum gravity is ever devised. You can look in the faq page for earlier answers about gravitons and general relativity.

QUESTION:
So if black holes suck in everything in including light that must mean everything is getting pulled in as fast or greater then the speed of light. So if light is weightless and it is sucked in. What happens to any mass as it is sucked in. Would the mass of the object then cease to have mass? Because im pretty sure anything traveling at the speed of light has to be mass-less correct? And how does gravity effect something with no mass? I dunno if it is a good question or not but i couldnt find a whole lot on the subject.

ANSWER:
Nothing ever goes faster than the speed of light and only light can go at the speed of light. When an object merges with the black hole, its energy, E=mc², is not lost and the black hole becomes more massive by the amount of the mass energy. When light is swallowed by the black hole, the mass increases even though the light does not have any mass because light does have energy and the energy shows up as increased mass of the black hole, m=E/c².

QUESTION:
How fast are we moving? Besides Earth rotation and revolving, there's our path in the galaxy, and even greater in relationship to all other galaxies, and please include the expansion of space-time.

ANSWER:
First you should understand that it only makes sense to ask what our velocity is relative to some inertial frame of reference. And, also, it depends on things like what time of day it is, what time of year it is, what year it is, etc. because all these things affect your velocity; e.g., relative to the earth's axis, you are moving in opposite directions at noon and midnight. There is a nice qualitative discussion of this question at Curious about Astronomy; adding up all the various velocities, she finds a maximum speed relative to the center of our supercluster of about 900 km/s. Given the roughness of this calculation, trying to fine-tune it by adding in the expansion of space-time is pointless.

QUESTION:
Two questions for you concerning the recent proof that gravity flows in waves. 1). How fast do these gravity waves move? Do they move at a constant speed or is their speed predicated by the mass of the object that generated the waves? 2). Since the earth is trapped in orbit around the sun by the sun's gravity, how can a wave moving out from an object in turn pull a second object back towards the object that generated the wave? In other words, how can gravity waves moving out from the sun at the same time pull the earth towards the sun?

ANSWER:
The answer to your first question is in a recent answer. In your second answer, you are confusing gravity with gravitational waves. Our picture of gravity is that mass (like the sun) deforms the space around as in the animation above. Something like the earth orbiting the sun is not really feeling a force, it is following the contour of the space. (Do not take this simplistic model too seriously, it is really the four-dimensional space-time which is "deformed".) But, if something accelerates, like the earth going around in its orbit, it will send out ripples and those are the gravitational waves. On the scale of this animation, the waves the orbiting earth is sending out are far too tiny to be seen. But, if the objects orbiting are much more massive, like the two orbiting black holes which were observed in the recently reported observation of the waves, the waves are much bigger.

QUESTION:
How fast do gravitational waves move? Is that rate constant?

ANSWER:
The speed of gravitational waves has never been measured. The speed at which a gravitational field propogates should be the same but it has never been measured either; this would determine, for example, if the sun suddenly disappeared how long it would be until the earth stopped orbiting. The theory of general relativity, which predicts gravitational waves, say that the speed of gravity should be the same as the speed of light. The recent observation of gravitational waves determined the distance to the source to be about 1.33 billion light years, but the uncertainty was very large, about 40%, so it really provides no measurement of the speed. Whatever the speed, there is no reason to think that it would not be constant everywhere in empty space.

QUESTION(submitted by my daughter!):
Is this exciting for you?? I dont really understand it but I'm trying to...it seems cool!

ANSWER:
Observing gravitational waves has been a holy grail of physics since before I was an undergraduate (like 55 years ago, gasp!). So yes, it is pretty exciting news. This is actually just the first direct evidence for gravitational waves. Indirect evidence was found when observing a pair of stars orbiting each other and spiraling in toward a collision. The loss of energy turned out to be exactly equal to the amount of energy they would lose if radiating gravitational waves. A nobel prize in physics was awarded in 1993 for this observation. The animation below shows the predicted gravitational waves from two neutron stars orbiting each other.

QUESTION:
I work in a high school where this question was posed by one of the pupils in a class I support. The question is this.... If you could attach a rope to a rocket, which would also be attached to earth, and sent it into space (out of our atmosphere) until the rope went taught and then cut the string. Would it stay in space or would it fall back to earth?

ANSWER:
This is one of those problems which I had fun with and I hope it will not be too exhaustive an answer. I will make the following assumptions:

the rocket always goes straight up;
the rocket stops moving vertically when the rope is taught;
the rope is cut the instant that the rocket stops;
fuel and the weight of the rope are not issues; and
the launch is from the equator. This makes things much simpler and I will briefly talk about a similar launch from some latitude at the end.

My view of this problem, therefore, is the same as if the rocket were on top of a very long stick vertically straight up and the stick is suddenly removed.

The thing to appreciate is that even though the rocket goes straight up, it will have the same angular velocity ω as the earth so its speed will be ω(L+R) where L is the length of the rope and R is the radius of the earth. The angular velocity is ω=[(2π radians)/(24 hours)]x[(1 hour)/(3600 seconds)]=7.27x10^-5 s^-1. If L is just right, the rocket will assume an orbit like the geosynchronous communication satellites; this turns out to be if L=5.6R.

So, if the rope happens to be 5.6 times larger than the radius of the earth, the rocket will remain (apparently) stationary above its launch point; it is actually going in a circular orbit with a period of 24 hours. (The animation above illustrates this, although not to the correct scale.) For any other L the orbit will be an elipse with the center of the earth being at the focus. Visualize.
If L>5.6R, the starting point of the rocket will be the perigee (closest point to the earth) of its orbit. Visualize.
If L<5.6R, the starting point of the rocket will be the apogee (farthest point from the earth) of its orbit. Visualize.
For L<5.6R, though, there will be some critical distance L=L_c when the perigee of that orbit is exactly equal to R; in that case the orbit will just skim the surface of the earth. After some laborious algebra I found that L_c≈3.7R, about 2.2 earth radii inside the geosynchronous orbit. Visualize.
For L<L_c, the rocket will crash back into the earth but not where it was launched from because it is a projectile which has a horizontal speed greater than that of the earth's surface. Visualize.
Finally, if the horizontal speed of the rocket ω(L+R) is greater than or equal the escape v_e=√[2MG/(R+L)], where M is the mass of the earth, the rocket will escape the earth and never come back. I calculated this to be when L=7.6R, two earth radii beyond the geosynchronous orbit. Visualize.

For any other latitude θ the speed of the satellite will be ω(L+R)sinθ. The resulting orbits, while all elliptical, are much more difficult to visualize and maybe we should save that for another day! However, the simplest launch of all would be from the north or south pole (θ=0⁰ or 180⁰) because it acquires no horizontal velocity (ω(L+R)sinθ=0) where the rocket would fall straight back down regardless of how high it went.

QUESTION:
Would it be possible physically to encapsulate a black hole with solar panel type devices and use its energy to power a civilization? Like when we reach trans-galactic civilization status and run across a black hole and we want to utilize it.

ANSWER:
Well, that is a pretty crazy idea because a black hole is an energy sink, not an energy source!

QUESTION:
When in the early universe did life become possible? I'm assuming it would have had to have been at some point when the temperature had cooled down enough so that the protons and neutrons of basic elements of life like carbon and oxygen could bond and come together. Also, what do you believe is the purpose for intelligent life in the Universe?

ANSWER:
I usually do not answer questions about astronomy/astrophysics/cosmology as stated on the site. I can give you a little information here, though. The universe just after the big bang was almost exclusively hydrogen. No elements which could be used to form planets where life might develop or the material for life itself. The first stars started forming about 100 million years after the big bang, basically pure hydrogen stars. These had to be much larger than the sun (perhaps 300-1,000 times heavier) because of the lack of heavier elements and consequently their lifetimes were too short (a few million years) for life to evolve (it has taken about 5 billion years to reach our stage of evolution, about half the sun's lifetime). Inside these stars nuclear fusion caused heavier and heavier elements up to about iron to form; then when those stars used up all the fuel, they exploded (super novae) scattering the heavier elements to eventually be part of clouds of dust and hydrogen which subsequently formed new stars with the possibility of planets. (As you can see, it is not "cooling down" which is responsible for heavier elements which are created in stars which are super hot.) So, we are now at least several billion years into the age of the universe. More detail on early stars can be read in a Scientific American article. The "purpose" for anything is not within the purview of physics.

QUESTION:
In the event horizon of the black hole, a pair production of a particle and an anti particle and the falling of one into the black hole and the releasing of the other from it gives us the Hawkins radiation, but my question is , isn't it possible for both the particle and the anti particle to fall into the black hole. If that happens defiantly the vacuum energy of the space decreases how can the universe compensate this decrease in energy to validate the law of conservation of energy.

ANSWER:
There are two ways you can imagine the creation of the particle-antiparticle pair (PAP):

Vacuum fluxuations where a particle-antiparticle pair is spontaneously created just outside the event horizon, violating energy conservation. However, the uncertainty principle (UP) allows this violation but only for a very short time. If one escapes, it adds energy to the universe so the captured particle must have negative total energy thereby decreasing the mass of the black hole and maintaining energy conservation. If both particles are captured, they are required by the UP to recombine thereby keeping the mass of the black hole constant.
When a virtual pair is created just outside the event horizon, the intense gravitational field of the black hole can provide the energy for the PAP to become real rather than virtual. In this case, the energy acquired by the pair will be lost by the black hole reducing its mass. If only one of these is captured, the energy of the escaped particle will be added to the universe but the energy of the captured particle will be added to the (already reduced) energy of the black hole resulting in a net loss of energy of the black hole equal to the energy of the escaped particle.

Energy of the whole system is conserved.

QUESTION:
How can it be said that the universe is only 13.7 billion years old? This seems ridiculous to me that anyone can make this assumption. Taking in the fact that there are black holes almost as old as the universe itself how is this possible? Stars live billions of years so the first group of black holes would be billions of years after the "big bang". Also due to the fact that the universe is expanding way faster and not slowing down like predicted it would seem to me the current model is wildly inaccurate. Can you please explain this to me? My understanding is that stars form and die out all the time so to be able to accurately determine the age of the universe another method would need to be devised. It is my understanding that galaxies form with the help of super massive black holes.(Which aren't even holes at all but super compacted spheres of matter so dense light cannot escape created by countless other bodies of mass.) Which attracts matter into tight clusters of stars we call galaxies. These galaxies would have to take billions of years to form after the creation of the first stars ever created and after the first black holes were created from some of those stars. Does the current model explain this?

ANSWER:
As clearly stated on the site, I do not do astronomy/astrophysics/cosmology. I can at least say a little about your question, though. The age of the universe is not an "assumption", it is based on careful analyses of many measurements. The stars in the early universe were much more massive than the sun; this resulted from the fact that the early universe was essentially all hydrogen and a bit of helium. Stars began forming only 100-200 million years after the big bang. Very large stars burn much hotter than smaller stars and therefore have much shorter lifetimes, a few million years rather than a few billion years for less massive stars (e.g. the sun with an expected lifetime of about 10 billion years) which would explain why many black holes are very old. The rate of expansion of the universe is not inconsistent with the model of a big bang about 14 billion years ago (in fact, the rate is one of the measurements used to estimate the age of the universe) even though we do not understand all the details (e.g. dark energy and accelerating expansion).

QUESTION:
What is the true shape of the Sun?

ANSWER:
I was surprised to learn that the sun is almost a perfect sphere.

QUESTION:
Why do the planets revolve only around the sun? There are stars which are bigger than the sun, and apply more gravitational force, but other planets do not attracts towards the stars. Please tell why?

ANSWER:
I guess you have not been following news articles over the past dozen years! In fact hundreds of exoplanets (planets revolving around stars other than the sun) have been observed.

QUESTION:
I am really curious and I would really appreciate if you take some time to answer some questions for me! Where does all this rotation come from? Stars can rotate at incredible speeds, planets rotate, satellites rotate, etc. Why do certain bodies rotate at significantly higher speeds than others? Pulsars can have rotational periods in the milliseconds.

ANSWER:
I state clearly on the site that I do not do astronomy/astrophysics/cosmology. However, I have earlier answered a question about rotations in the solar system, so you can have a look at that. For any individual body, the rotation rate is normally determined by how much smaller or larger it has become over time. For example, some star may have a certain rotation rate in the "prime of its lifetime", slow down when it grows to a red giant when dying, and speed up when it collapses into a very tiny neutron star. Also any interaction with other bodies which causes a torque can change the rate of rotation, the moon being the best known. The moon originally had a much larger rotation rate than its current rate of one rotation every 28 days. The torque exerted on it by the earth resulted in the change of rotation.

QUESTION:
How do they figure out phases of the moon in the past? in other words, how can they take a date hundreds of years ago and determine what phase the moon was in and specifically when it rose in a particular spot? Short of going month by month back tracking. There are sites that calculate it online but what are they actually doing?

ANSWER:

My usual disclaimer�I am not an astronmer and often do not answer astronomy questions. I think I can give at least a semi-quanitative answer to this one, though. The details would be more complicated than I am presenting, but what we know with great accuracy is the moon's orbit around the earth and the earth's orbit around the sun. Both are, of course, elipses and their periods are known: the length of a year is 365.2425 days and the time of one lunar orbit is 27.321661 days. The phase of the moon is determined by the relative positions of the earth, moon, and sun; so all you need to know is the phase of the moon on one particular day and to know exactly where both the earth and the moon are at that time (which I will call the reference point). When you choose a day in the future or past, it is fairly simple to compute how many days that is from the reference point; you have to be careful to correctly apply the rules for leap years, every 4 years except at the century. You can then calculate the whole number of orbits of both the earth and the moon in that time and the fractional time elapsed of the last uncompleted orbit. It will be a little tricky to compute just where some fractional time of an orbit will take you relative to the reference point because, the orbits not being quite circular, the speed varies depending on where you are in the orbit (you go fastest when you are closest to the body you are orbiting); doing this, however, is straightforward, just the Kepler problem. So now you know precisely where the earth and moon are relative to the sun and, from that information, you can calculate the moon phase. There are other details you might need to worry about, for example when the Gregorian calendar was instituted and precession of the lunar orbit (which, not being an astronomer, I do not understand in detail). Also, you cannot go millions of years using this method because the moon's orbit is getting larger with time on this kind of time scale. My final disclaimer�this may all be wrong!

QUESTION:
I understand that because of time dilation that we could never see something actually entering a black hole. That is to say, from our earth view, time would appear to stop at or near the event horizon and observed objects would appear to move slower and slower until they seemed to be standing still (to be clear, I certainly understand that the object does not actually halt its motion, and will continue to accelerate and fall into the hole). I understand (layman's level) the basic concept of time dilation, and that it is how the speed of light remains constant for all observers, but the idea of something appearing to halt its motion is pretty mind-blowing...I picture (when it becomes technically possible to actually see one) an event horizon cluttered with incoming matter but that matter never descending (according to our observation) into the black hole. That seems far fetched to me, so I am sure I'm missing something.

ANSWER:

First of all, once an object passes the event horizon, you cannot see it at all. I would say that we really do not know how a clock would behave inside the event horizon because, as shown below, the field is two intense for the usual gravitational time dilation expression to be valid. You might want to look at an earlier answer on gravitational time dilation. The rate at which the clock runs slower in a gravitational field (compared to your clock in a very weak gravitational field) is √[1-(2MG/(Rc²))] where R is the distance from the center of a spherically symmetric mass M. Note that if R=2MG/c² this is zero and the clock stops. That magic R is called the Schwartzchild radius and so if you are watching this clock, you see it stop at this R. From the point of view of someone traveling with the clock, though, time clicks on as usual and you enter inside R. Your fate would be that as you got closer to the black hole (imagine you are falling head first), the gravitational force would be much larger on your head than your feet; this kind of gradient of force is called a tidal force and it would rip you apart. A nice little animation may be see here.

QUESTION:
Would it be acceptable/correct to say that the Sun orbits the Earth, with respect to the Earth? For example, when describing the rising/setting of the sun and the alteration of night/day on Earth (respect to Earth), in that context, could one say that the Sun is orbiting the Earth?

ANSWER:

No, because to an excellent approximation the sun is in an inertial frame (at rest or moving with constant velocity) and the earth is an accelerating frame because of its orbit. There are also innumerable other examples which could not be understood (like the motions of the planets in the sky) by an earth-centric model.

QUESTION:
why Mercury have less gravity as compared to earth,while its mass is almost same as earth.is it because of lower spin speed

ANSWER:

Sorry, but the mass of Mercury is almost 20 times smaller than earth and its radius is almost 3 times smaller. You can calculate the acceleration due to gravity at the surface, g=MG/R² where G=6.67x10^-11 N∙m²/kg². The data for Mercury (earth) are: mass M=3.3x10²³ kg (6x10²⁴ kg); radius R=2.4x10⁶ m (6.4x10⁶ m); gravitational acceleration at surface g=3.7 m/s² (9.8 m/s²).

QUESTION:
I'm wondering about whether it would be possible for human activity, given current technology, to move a moon such as Europa or, better yet, Ganymede out of Jupiter's orbit and into a solar orbit (down the road, ideally it would be nice to have it impact with another planet with water resources for, you guessed it, terraforming). The parameters of this situation are as follows. Assume cooperative human action which would allow the use of nuclear weapons on the far side of the moon to push it away from Jupiter, but at regular enough intervals to keep it from exploding. Assume also any "tractor" technologies, such as a large, solar-powered spacecraft which uses (some of) the electricity created by the solar panels to create lasers which push it away from this moon. Am I right in thinking that either or both of these techniques this could possibly move an "earth-like" moon into the habitable zone, eventually?

ANSWER:

I am not going to address any of your hypothetical scenarios for delivering the necessary energy because, as I shall show, this is not going to work for any "current technology" you wish to put into play. The energy necessary to remove Ganymede (M_G≈1.5x10²³ kg) from its orbit (R≈10⁹ m) around Jupiter (M_J≈2x10²⁷ kg) is E=GM_JM_G/R≈2x10³¹ J, where G=6.67x10^-11 J∙m/kg² is the universal gravitational constant. Suppose that you wanted to achieve this over a period of T=100 years≈10¹⁰ s. The required power would be P=E/T≈2x10²¹ W=2,000,000,000 TW. The current average power consumption of the entire earth is about 15 TW. (By the way, Ganymede is not really "earth-like" at all since the acceleration due to gravity at its surface is only about 1/5 of earth's.)

QUESTION:
What causes the birth of a black hole?

ANSWER:
When a star has exhausted most of its hydrogen fuel, it begins to collapse under its own gravity. The next thing that happens is that it explodes, a supernova. Then, providing that the star was massive enough, it will continue collapsing until it has essentially collapsed into zero volume, a black hole.

QUESTION:
I was at a training and learned that the Andromeda Galaxy revolves around a black hole. Is that true of all galaxies?

ANSWER:
Astrophysicists believe that nearly all large galaxies have a supermassive black hole at their centers.

QUESTION:
If one stood on a typical neutron star and turned on a flashlight pointed horizontally, would the light bend down to the surface much like water coming out of a garden hose here on earth?

ANSWER:
The density of a typical neutron star is about 5x10¹⁷ kg/m³ and the radius is about 12 km, so the mass is about M=5x10¹⁷x4xπ(12x10³)³/3=3.6x10³⁰ kg. To find the acceleration due to gravity, g=GM/R²=6.67x10^-11x3.6x10³⁰/(12x10³)²=1.7x10¹² m/s². So your weight on the surface would be about a trillion times what it is on earth. You would be crushed totally flat. We can estimate how far light would "fall" in such a gravitational field. The time it would take to go 100 m would be about t≈100/(3x10⁸)=(1/3)x10^-6 s. The distance it would fall would be y=�gt²≈10 m. On earth the speed of something which dropped 10 m in 100 m would be about 71 m/s≈160 mph, quite a bit faster than a garden hose. But qualitatively, yes the light would fall quite a lot.

QUESTION:
Can you please tell me the difference between dark matter and dark energy.

ANSWER:
There are many instances where astronomical objects behave as if there were more mass present than we can see. The best-known example is the dynamics of galaxies where the speeds of stars far from the centers of the galaxies are much bigger than expected on the basis of the mass that can be seen; see the figure to the left. The conclusion usually drawn is that there is much more mass present than we can see and this hypothesized stuff is called dark matter. Nobody knows what it is nor has it yet been detected directly. A second anomalous feature of the universe is that it has recently been discovered that the expansion of the universe is actually accelerating; the conclusion usually drawn is that there is a repulsive force not heretofore observed which is "winning" a tug-of-war with the attractive force of gravity. This is referred to as dark energy.

QUESTION:
I understand you don't answer questions about stars, but im wondering about the nuclear dynamics involved in nuclear fission. In a star with 250 solar masses or more there is something called photofission, where high energy gamma rays cause elements as light as tin to go through nuclear fission. I was wondering, how much energy would be required to cause fission to occur in the element tin?

ANSWER:
Right, this is certainly more nuclear physics than astrophysics, so I can do some rough estimates for you. I can tell you approximately how much energy is released in the symmetric fission Sn¹¹² �>2Mn⁵⁶. The binding energies of the tin and manganese are 953.5 MeV and 489.3 MeV respectively. So the energy released in the fission would be 2x489.3-953.5=25.1 MeV. What I cannot tell you is what energy photon would be required to cause the fission. This depends on the structure of the Sn¹¹² and how it interacts with the photon. Fission might be induced by a much lower energy photon than 25 MeV. After all, uranium can fission with no energy whatever added to it (spontaneous fission).

QUESTION:
What is the Cosmological Constant and why it couldn't be described perfectly by Einstein? Was only because of it that Einstein thought the universe was static?

ANSWER:
When Einstein proposed the theory of general relativity, around 1918, it was generally believed that the universe was static. General relativity is the theory of gravity, and if gravity is the main interaction among stars and galaxies, this is not possible; with only gravity, the universe would have to be either expanding and slowing down or compressing and speeding up. Therefore Einstein had to introduce something to balance the universal gravitational force; this something was called the cosmological constant. It was later discovered that the universe is expanding and he later denounced the cosmological constant as "�my greatest blunder�" Interestingly, several years ago it was discovered that the expansion of the universe is actually speeding up implying some kind of repulsive force, akin to the cosmological constant, often referred to as dark energy; this has reignited interest in Einstein' "greatest (or maybe not) blunder". I do not know what you mean by "�couldn't be described perfectly�"

QUESTION:
If I shine a light it emits a light particle outwards. Assuming it is travelling in vaccum all the way out and hits no light absorbing medium. The farther it travels it starts losing energy and shifts to the red. Now how long in Earth years will it take for that photon particle to lose "ALL" its energy. How far will it travel in distance (light years) and what happens to that particle at the end of it all.. i.e it just cannot "disappear" or become void or null. Or perhaps it will all be converted to some other form of energy but then what would it lose its energy to given it was hypothetically travelling in a vaccum/void all the way out?

ANSWER:
Let's first consider the particle to be a ball. If you throw it up it will begin immediately losing energy and eventually stop. You can now think of the earth as having acquired the energy the ball had, now in the form of potential energy. Being a ball, it then falls back and extracts all its energy back. However, if you throw the ball hard enough it will keep going forever and the farther away it gets, the less energy it loses, essentially moving with constant speed when it has gotten pretty far away. The same is true of a photon, as you note, except for two important differences: it cannot stop and it has no mass, just pure energy. The red shift you refer to is the gravitational red shift and as the photon becomes redder it is, as you stated, losing energy (also to the mass it is leaving) but not speed. For the earth (and almost every object in the universe) the effect of gravity is insufficient to ever cause the photon to lose all its energy. Only black holes can rob a photon of all its energy; if a photon is inside the event horizon of a black hole, as it tries to escape it gets "redder and redder" until it has finally lost it all and it disappears. The energy it had will now be found in a slightly increased mass of the black hole.

QUESTION:
In looking at the photgraphs from the Rosetta comet mission, I notice what seems to be "loose boulders" on much of the surface. How much gravitational attraction does that comet have? Is it enough to keep the boulders on the surface as it would on Earth, or are the loose parts of the comet simply in a coinciding orbit?

ANSWER:
If they remain in the same place relative to the main body of the comet they are resting on the surface. If they were moving alongside, there would be an attractive force which would bring them together; otherwise, to remain separated, the boulder would have to be orbiting the main body. I once answered a question which surprised me�two dice, in empty space and separated by 10 cm, take only about 8 hours to come together.

QUESTION:
If I am at a point in space where very, very, very little gravitational pull exists, what happens to the time clock on my spaceship as opposed to clocks on earth?

ANSWER:
A clock in a gravitational field runs slower.

FOLLOWUP QUESTION:
What I am asking, if you were at a point in space, where the gravitational forces were almost 0, would your clock, in your space craft be running extremely fast as observed from earth? I was thinking about a spot in the universe were the gravitational effect would be almost opposite of a black hole. Like the trampoline example you used on the web site. I set 5 or 6 bowling balls on the trampoline so each makes a dent. I then use a marble and find a spot to set it where it is not affected by the bowling balls. Now I know my ship is moving always towards some mass but there has to be a spot where gravitational effects all almost 0. Maybe that spot in space would be inaccessible to my ship, kind of an anti event horizon where I would not have enough power to get too. a spot where Light can not even enter. I Know that my question is flawed, but I can't help to think about a black hole as a gravity pit, and that makes me think of a spot in space where there is a gravitational mountain (Anti-Black Hole, with an anti-Event horizon). In this spot time runs so fast as not to exist....

ANSWER:
First of all, all your thrashing around with black holes etc. trying to find a spot with small gravitational field is totally unnecessary; all that is required is that you need to be far away from any mass and nearly the whole universe satisfies that criterion. We generically call such places intergalactic space. So, choose some place, maybe halfway between here and the Andromeda galaxy, our nearest galactic neighbor. For all intents and purposes the field there is zero and a clock would run at some rate. Now, take that clock and put it on the surface of a sphere of radius R and mass M. The gravitational time dilation formula you need to compute how much slower the clock would tick at its new location is √[1-(2MG/(Rc²))] where G=6.67x10^-11 N∙m²/kg² is the universal constant of gravitation and c=3x10⁸ m/s is the speed of light. Putting in the mass and radius for the earth, I find that the space clock runs about 7x10^-8% faster than the earth clock, certainly not "extremely fast"! The bottom line here is that, except very close to a black hole, gravitational time dilation is a very small effect. An interesting fact, though, is that corrections for this effect must made in GPS devices because they require extremely accurate time measurements to get accurate distance measurements.

QUESTION:
How fast would a more massive planet than Earth, say twice as massive, with the same diameter have to spin to produce an Earth-like sense of gravity, due to centripetal force, at its equator? Might there exist a massive Earth-like planet, environmentally speaking, that we could still live on because its spin negated its gravitational pull enough to produce an Earth-like sense of gravity that wouldn't crush us?

ANSWER:
Let's just do it in general, the pseudo-earth being N times more massive than our earth. The apparent weight at the equator would be Nmg-F_c=mg where F_c=mv²/R=mRω²is the centrifugal force. Solving, ω=√[(N-1)g/R] where ω is the angular velocity in radians per second. For example, if N=2, ω=√[g/R]=√[9.8/6.4x10⁶]=1.24x10^-3 s^-1=2π/T where T is the period. So, T=5.08x10³ s=1.41 hours.

QUESTION:
Suggest that we have constructed a model of the solar system by decreasing the sizes of all orbits by n = 0.005. We have kept the average density of the planets and that of the Sun the same as in the original solar system. How will the periods of rotations of planets change in our model?

ANSWER:
Is this homework? Are the sizes of the planets also scaled by n?

QUESTION:
No this is not homework-it was in a past paper and I was curious as to how to answer it! And the question does not say, although I assume so!

ANSWER:
The period T of an elliptical orbit with semimajor axis a around a body of mass M which is much bigger than the mass of the orbiting body is given by T=2π√[a³/(GM)] where G is the universal gravitational constant 6.67x10^-11 N∙m²/kg². So the answer to the question depends on how the masses are scaled. If all the masses stay the same, the new periods T' are simply scaled by √0.005³=3.54x10^-4. If the densities remain the same and the sizes of the sun and planets themselves are also scaled down by 0.005, then the masses will scale by the factor M'=0.005³M because the mass is the density (unchanged) times the volume (proportional to the cube of the radii). Therefore the periods would remain unchanged, T'=2π√[(0.005∙a)³/(G∙0.005³∙M)]=T.

QUESTION:
I have a question that been thats been unanswered from quite some time now. I even tried asking that to a person in NASA houston and he couldn't even understand. Hope you can help. We know that space dust and debris keep on falling on earth. And I have read that its many tons a day. Considering mass of earth is increasing every day for millions of years now, how does it affect its revolution, speed and axis. Or does it even affect that.

ANSWER:
One estimate is that the earth gains 40,000 metric tons (4x10⁷ kg) per year. So, in a million years that would be a gain of 4x10¹³ kg. The mass of the earth is 6x10²⁴ kg. The moment of inertia will be proportional to the mass times the square of the radius, so, assuming the change in radius is negligible, the fractional change in moment of inertia over a million years would be approximately 4x10¹³/6x10²⁴≈10^-9=10^-7 %. Since angular momentum (moment of inertia times angular velocity) is conserved, this would mean that the angular velocity would decrease by about 10^-7 %, an increase in the length of a day of about (24 hr)(3600 s/hr)x10^-9≈10^-3 seconds in a million years!

QUESTION:
This might seem dumb, but i cannot understand the laws of thermodynamics applied to gravity. If a moon orbits a planet in a stable orbit, it still pulls on the planet, and the planet pulls on the moon. This seems to generate heat (the heatsource on europa) and motion (tidalwaves on earth). How can this not violate the laws of thermodynamics?

ANSWER:
The first law of thermodynamics, which seems to be the crux of your question, is nothing more than conservation of energy. Energy conservation is true only for isolated systems, so let us think about a single planet and a single moon isolated from everything else. In addition to the simple central force of gravity, the two interact with each other via tidal forces which result from the sizes and relative masses of the two. Just what the effects of the tidal forces are depends a lot on the relative sizes and distances of the two, but, as you note, often appear to violate energy conservation. So, let's look at that more closely. Soon after the moon was created, probably by a cataclysmic collision with an asteroid, it was much closer to the earth than today and it rotated on its axis at a much faster rate than it does today (about 28 days, which is why it always shows the same side to us). The tidal force caused what is called "tidal locking", the same side always facing the earth. It would seem that this would represent a loss of energy since it is not spinning as fast as it used to be, but as this happened, the moon moved farther and farther away from the earth which represents an increase in energy. So, the energy of the whole system stayed the same. Today, the tidal effect is mainly due to the ocean tides on the earth and now it is the earth which slows down its rotation�the earth is losing rotational kinetic energy and tending toward being tidally locked with the moon. This is an extremely small effect�a day gets about 2 milliseconds shorter during one century. But, just as was the case when the moon was slowing down its rotation, the way energy is being conserved is by the moon moving farther away, resulting in an increase in the earth-moon energy keeping the total energy constant. If the tidal forces are great enough to cause significant frictional heating because of the tidal force distortion of the whole moon (your Europa example), energy is lost because some of the heat is radiated into space. So the first law would be violated, but you would expect that since some energy is escaping from the planet-moon system. You can see that your error was to assume a "stable orbit".

QUESTION:
Isn't the fact that Andromeda galaxy (being the galaxy supposedly going through a collision with the Milky way in millions of years) defying the dynamics of dark energy? Shouldn't it be moving AWAY?

ANSWER:
The expansion of the universe is the behavior on average, not some kind of absolute rule. Imagine that you drop a stone�shouldn't it move away from the earth? Local gravitational fields and initial conditions often trump the overall expansion properties. The Andromeda and Milky Way galaxies are attracted to each other gravitationally and since they never have had a large velocity moving away from each other, they are doomed to fall into each other.

QUESTION:
ok, two stones. Both spherical and same mass and density evenly spread in each stone. Set each about 1/2 the distance to the moon. One leading the earth's orbit and one following the earth's orbit. Both not moving relative to the earth, yet the same speed as the earth as it moves around the sun. No tangent or orbital speed, the stones are starting in freefall. Which gets to earth's surface first, neglecting air drag.

ANSWER:
Since you stipulate that the stones are not orbiting, the stones are at rest with respect to the sun and the earth is not. Therefore, the stone on the leading side of the orbiting earth will win the race because the earth is moving toward it and away from the other when the stones start dropping.

BETTER ANSWER:
I see that I misread this question. I guess you meant there is no orbital speed around the earth. To make this manageable at all I will neglect the influence of the moon and assume a spherically-symmetric mass distribution of the earth. In the figure above, I show your two stones and the forces (blue arrows) on them. The down-pointing forces are the from the sun (keeping them in orbit) and the horizontal forces are the weights making them want to fall toward the earth. The distances and forces are not drawn to scale; when the stones are about 30 earth radii away from the earth center (the moon is about 60 earth radii away), the weight forces are about 2 times larger than the sun forces. The big blue arrow shows the direction everyone is orbiting the sun. So, the leading stone slows down its orbital speed and so will slightly fall toward the sun as it falls toward the earth; the trailing stone increases its orbital speed and so will slightly fall away from the sun as it falls toward earth. These deflections are shown (probably quite exaggerated) by the red arrows. Given the symmetry of the situation, I would expect the two to be at the same distance from the center of the earth at any given time�which is the crux of your question, I think. To actually do this more quantitatively, though, would be very hard because as the stones were deflected the weight force would change direction now having a vertical component in the figure.

QUESTION:
What effect (if any) would there be on the solar system if the sun were to lose the mass of Jupiter? This question is based on various schemes to power a "warp drive" by converting mass about the size of the planet Jupiter into pure energy. Since vaporizing Jupiter is probably not a good idea, it might make more sense to get the necessary mass by skimming off a chunk of the sun's surface. Assuming mankind might have the technology to do this someday, would the sun be appreciably different if it lost the mass of Jupiter? Would the orbits of the planets change to a significant degree?

ANSWER:
I usually do not answer astronomy questions, but will take a stab at this one since I have earlier talked about effects which Jupiter has on other objects in the solar system. If Jupiter disappeared it would have a negligible effect on other planets. It would, though, have a large impact on many asteroids whose orbits are controlled by Jupiter's gravity (see that earlier answer). This would release a swarm of asteroids into earth-crossing orbits considerably increasing the likelihood of catastrophic collisions with earth. The mass of Jupiter is only 0.1% that of the sun, so taking that much mass from the sun would probably cause fairly minor effects on solar system orbits. "Skimming" that much mass from the sun could have pretty serious short-term effects on the sun's radiation. Oh, by the way, there is no such thing as warp drive!

QUESTION:
Something ridiculous I thought of, if the Moon suddenly stopped moving and began to fall toward the Earth, how long would it take to impact? I'm stumped as to how to calculate this, as the force on the Moon gradually increases as it falls, and the Moon also pulls the Earth toward it, and the radius of each object would have to be included.

ANSWER:
I guess I am going to have to put questions like this one on the FAQ page. You should read the details of these earlier questions since I do not want to go over all the details again. It is tedious and uninstructive to try to do this kind of problem precisely. I, being a great advocate of "back of the envelope" estimates, use Kepler's laws to solve this kind of problem; I have found that a very excellent approximation to fall time can be found this way. I note that the mass of the moon is only about 1% of the mass of the earth, the period of the moon is about 28 days, and the moon's orbit is very nearly circular. The trick here is to use Kepler's third law and recognize that a vertical fall is equivalent to the very special orbit of a straight line which is an ellipse of semimajor axis half the length of the line. Kepler's third law tells us that (T₂/T₁)²=(R₂/R₁)³ where T_i is the period of orbit i and R_i is the semimajor axis of orbit i. Now, T₁=28 and R₂=R₁/2 and so T₂=T₁/√8=9.9 days. But this is the time for this very eccentric orbit to complete a complete orbit, go back out to where it was dropped from; so, the time we want is half that time, 4.9 days.

But this is not what you really wanted since I have treated the earth and the moon as point masses. What you really want is when the two point masses are separated by a distance of the sum of the earth and moon radii, 6.4x10⁶+1.7x10⁴≈6.4x10⁶m. To see how much error this causes, I can use the equation for the velocity v at the position r=6.4x10⁶ m if dropped from r=R_moon-orbit=3.85x10⁸ m which I derived in one of the earlier answers: v=√[2GM(1/6.4x10⁶-1/3.85x10⁸)]=1.1x10⁴ m/s. It would continue speeding up if the collision did not happen, but even if it went with constant speed the time required would be about t=R/v=6.4x10⁶/1.1x10⁴=580 s=9.6 min. This is extremely small compared to the 4.9 day total time, so, to at least two significant figures, 4.9 days is the answer to your question.

An important part of doing physics, or any science, is knowing when to eliminate things which are of negligible importance!

QUESTION:
If our solar system was formed from a cloud of dust, created by numerous super nova explosions, where we get various elements, and the sun is comprised of the greater part of this cloud, how is it that the sun is comprised of hydrogen burning to make helium? How did the sun separate all the other elements?

ANSWER:
Again, I am not an astronomer/astrophysicist. However, I was just reading in the April 2014 Scientific American an article about stars early in the universe. When the universe was young, it was composed almost entirely of hydrogen with only a tiny amount of helium and lithium. When stars form, they contract gravitationally and heat up as they do so. But, heating up, the pressure increases and keeps the star from collapsing further until it can shed some of the heat so that it can continue contracting. Eventually, a core dense and hot enough forms where the fusion can ignite. It turns out that hydrogen is not very good at getting rid of the heat and so more and more hydrogen accumulates; eventually ignition occurs but the typical early star, because of the inabilty of hydrogen to cool, is hugely bigger than the sun, anywhere from 100 to 1,000,000 solar masses. These giant stars now eventually die after burning much hydrogen and creating lots of heavier elements up to iron; they explode in supernovae and fall back into black holes, scattering the heavy elements (including heavier than iron made in the supernovae) into space. Now, to answer your question, the sun has lots of the heavier elements in it, just a smaller fraction than the planets. It turns out that these heavier elements are much more effective in cooling the protostar as it is forming which allows much smaller stars like the sun to form.

QUESTION:
What is the moon's orbit around the earth? I was wondering if you could send me pictures and diagrams or whatever you could about the orbit.

ANSWER:
I am not sure what you want. The moon has a nearly circular orbit of radius about 385,000 km. It takes about 28 days to go around earth once (which is how it came to be that a month was a standard time measurement). The same side of the moon always faces earth which means it also takes about 28 days to rotate once on its axis. The picture above is drawn to scale. For more detailed information, see the Wikepedia entry.

QUESTION:
The number of known Mars Meteorites on Earth at last count I know of was greater than the number of Lunar meteorites. This is the opposite of what you would expect: The moon is 140 times closer than Mars with a weaker gravitational field to recapture the meteorites and no atmosphere to slow material blasted off the planet. The number of Lunar meteorites on Earth should be hundreds of times greater than Mars Meteorites.

ANSWER:
As always, a disclaimer that I am not an astronomer. The origins of either lunar or Martian meteorites are major asteroid or comet impacts; after impact, some of the debris has enough velocity to escape the gravitational field. First of all, the cross sectional area of the moon is about 4 times smaller than Mars, so all things being equal, the probability of a major impact is 4 times greater for Mars. Second, Mars is closer to the asteroid belt and would therefore, I presume, be more likely to suffer an asteroid impact. Most major impacts occurred early in the history of the solar system when embryonic planets were "sweeping up" material in their orbits and there was a much greater potential of major impacts. These early meteorites from the moon which landed on earth would have been eroded away or taken under the crust by tectonic action; those from Mars would have gone into orbits around the sun and land on the earth over a much longer period, probably still landing today.

QUESTION:
During a discussion with my 6th grade class about the Law of Inertia and space travel, a student asked: "If a spacecraft leaves a solar system, for example Voyager 1, will its velocity increase due to the lack of gravity from the sun." Fairly certain the answer is no; however, it did spark a rather interesting debate. Can you explain?

ANSWER:
Perhaps the key is to note that "leaves the solar system" does not mean that there is no longer any gravity from the sun. Rather there is a boundary where the solar wind, particles like protons and electrons, stops; this is called the heliopause and has been definitely observed by the Voyager 1 spacecraft as shown in the graph to the right. This shows the amount of solar wind the Voyager detected as it moved during the months of 2011-2. In September 2012 it dropped to near zero. This is where we define the edge of the solar system to be and it is about 50 Astronomical Units (AU) from the sun; the earth is one AU from the sun. But the gravity from the whole solar system is still present and the craft will continue slowing down, but ever so slightly since as you get farther away from a mass the gravity gets weaker. This craft has enough speed that, if it never encountered any other mass it would keep going forever. I did a rough calculation and found that the acceleration of Voyager is about a=-0.01 mi/hr² which means that it loses about 1/100 mph per hour; but the speed is about 40,000 mph, so I think we could agree that it is moving with an almost constant speed. As it gets farther away, the acceleration will get even smaller (physicists call slowing down negative acceleration, not deceleration). Until it gets close to something else, like some other star, it will keep going with an almost constant speed. When it does approach another star it will begin speeding up. Your students should appreciate that the only thing which can change the speed of something is a force, a push or a pull.

QUESTION:
Isnt the gravitational pull of an object determined SOLELY on the mass of that object and NOT its size or density? obviously distance from said object plays a role just as size and density affect mass.... however, if what i believe is true, there can be no such thing as black holes, which even today their existence is only theoretical, not proven... anyway my point is this... i believe that if our sun was the size of a basketball but still had the same mass it has today, (giving it almost infinite density)..(what many scientists today consider a 'black hole') but the Earth was still the same 93,000,000 miles from the actual surface as it is today, the orbit of the Earth and the planets would remain, and the sun would not be considered a 'black hole', nor would it behave like one, (sucking in all its orbiting masses, and even light itself).. it would be much smaller, but given the exact same mass and distances surface to surface, little would change as far as orbits go... could this be correct theoretically?

ANSWER:
If the object is spherically symmetric and you are totally outside the mass distribution, then you are right�only the total mass matters. But, this does not mean that black holes do not exist. It is dramatic to say that nothing can escape a black hole, not even light, if you are inside a critical distance called the Schwartzchild radius, but that does not mean that objects could not orbit the black hole, either inside or outside that radius. If the sun were a black hole, its Schwartzchild radius would be R=2GM/c²=2x6.7x10^-11x2x10³⁰/(3x10⁸)²=3,000 m; so all the planets would be outside and would orbit just fine; they could even be dragged away if you wanted. Only objects inside 3000 m would be "trapped" but even they could still orbit the black hole. The figure to the left shows some orbits of stars around the supermassive black hole at the center of the galaxy which have been observed by astronomers.

QUESTION:
I'm not a physicist or mathematician, but could there be a better explanation to the expansion of the universe without assuming so much dark matter? The galaxies accelerating away from one another seems to be analogous to the same way a sudden release of gas molecules would disperse across a large room, or the way a drop of liquid would disperse across a vast body of water.

ANSWER:
First, let's get some things clear. Dark matter is a proposed type of matter which exerts an attractive force (gravity) on all other matter and therefore works against the expansion of the universe because it is trying to pull the rest of the universe toward it. The acceleration of the universe is the result of some proposed repulsive force trying to push matter away, and this is called dark energy. Even without dark energy or dark matter, we can talk about the expansion of the universe. Imagine there is an object, say a planet, in the middle of empty space which suddenly explodes in to numerous pieces. There are essentially two possible ultimate outcomes. If a relatively small amount of energy was added by the explosion, the pieces will all move away from each other slowly but, because they are all attracting each other gravitationally, they will all eventually turn around and come back to recoalesce into the planet. However, if enough energy is added by the explosion, the pieces will have such a large velocity that they will never come back, they will just keep expanding forever. (This is analogous to giving a rocket a high enough velocity, the escape velocity, so that it will never fall back to earth.) One of the most important goals of astrophysics over the last century has been to determine which is the case for our universe, the big bang being the explosion which started it all. Now, the problem which prompted astrophysicists to hypothesize dark matter was not the expansion of the whole universe, but the way that galaxies move; the way the stars in a galaxy revolve around the center cannot be understood as resulting only from gravitational interactions among all the matter in that galaxy. The more recent discovery of the observed acceleration of very distant galaxies is what requires something like dark energy, the source of something pushing them apart, "beating out" the gravity which is trying to slow them down. Neither of your two examples have any mechanism which would accelerate the expansions. My own feeling, totally not mainstream, is that perhaps these problems are simply indicative of our not understanding gravity as well as we think we do.

QUESTION:
How far from Earth would you have to travel before the constellations changed shape?

ANSWER:
Of course, this is impossible to answer in a quantitative was because different constellations are made up of different stars which are different distances away from us. Also, you need to have some criterion for "changed shape". In some sense, you already can observe a tiny change in shape due to moving around the sun as shown in the picture to the left. When the earth is at position 1, a nearby star (yellow) is observed at one location relative to very distant stars (orange and blue) and 6 months later at position 2 it will be observed to have apparently moved, thereby changing the shape of this 3-star constellation. But by how much? The nearest star to us is about d=4.22 ly (light years) from us and the radius of the earth's orbit (called an astronomical unit, AU) is about B=1 AU=1.6x10^-5 ly, so the angle p (called the parallax angle) is about 1.6x10^-5/4.22=3.8x10^-6 radians=0.00022⁰≈1" (one arcsecond, 1/3600 of one degree). You would probably not consider that to be much of a change! So, suppose we now asked how big the earth's orbit would have to be to have a parallax angle of 10⁰=0.175 radians. In that case we would have B/4.22=0.175 or B=0.74 ly. So finally we can conclude that if you want the position of a star in the sky to change by about 10⁰, you will need to move laterally about 20% of the distance to that star. Be sure to note that most stars are enormously more distant than a few light years from us.

QUESTION:
Is there something wrong with the theory that says, if there is enough dead matter from dying stars that comes together, gravity will take over and eventually produce a new star from this dead matter? I am aware that stars form from hydrogen, but what if 999 trillion tons of the heaviest known elements got together all in one place? Most say a star would not form because there is no hydrogen, but�what then? It would appear that gravity, being indiscriminate, will continue crunching this big iron ball until critical mass is surpassed, regardless of what the result might end up being. What would that result be?

ANSWER:
As I state on the site, I usually do not do astronomy/astrophysics/cosmology questions, so take my answer here with a grain of salt. But, sometimes the question is sufficiently interesting to me that I attempt an answer. I would say that a "normal star" might be defined as an object which makes energy by nuclear fusion, fusing light nuclei to heavier nuclei. However, when you reach iron on the periodic table you no longer gain energy by fusing nuclei and therefore the energy production ceases. Before the whole star becomes iron, though, other instabilities cause the star to, in one way or another, end its life, the details depending on the mass of the particular star. The most dramatic end is a supernova (of which there are several types, again depending on details), a very dramatic explosion of the whole star; the energy from such an event is what is generally thought to be responsible for creation of heavier elements beyond iron. One possible end following a supernova explosion is called a neutron star. The remnants of the star, mostly heavy elements like you refer to, will undergo gravitational collapse and, in essence, electrons will be pushed into protons such that the star is made mostly of neutrons and is extraordinarily dense (it is essentially a giant nucleus). With sufficient mass, that is what I believe would happen to your collection of heavy elements. However, your number, about 10¹⁸ kg, is way too small for the requisite mass. Neutron stars have about 1.4-3.2 solar masses and the mass of the sun is about 2x10³⁰ kg.

QUESTION:
If we can talk about "escape vel. for light ,(in case of black holes)" that mean light is affected by gravitational fields and thus like other masses, must be "decelerated", while moving away from a heavy planet. Thus, can we slow down light to near zero speed? I know the ans is 'no', but WHY? if light is affected by gravity then we should be able to slow it down? Also if light is affected by gravity , then what's it's mass??

ANSWER:
As light travels upward in a gravitational field, it loses energy. If light were a classical object like a baseball, you would say that, since its energy is �mv² and the mass is constant, the velocity must decrease. However, the energy of light, since it has no mass, is not �mv² but rather each photon has an energy hf where h is Planck's constant, and f is the frequency of the light. So, as it goes higher its frequency gets smaller�the wavelength gets longer; that is why it is called gravitational red shift. In the case of a black hole, eventually the energy of the light is zero and the black hole has gained all the energy which the light had. Similarly, if light is aimed down in a gravitational field it does not speed up but rather increases frequency as it goes down, gravitational blue shift. Light does not have mass but it is affected by gravity. Newton's universal law of gravitation is not the last word; the modern theory of gravity is general relativity and if you go to my FAQ page you can learn about how gravity affects massless objects.

QUESTION:
If energy can neither be created nor destroyed, how can people who believe in the bing bang believe what they believe since we have energy now so they are saying that it was created by something in the natural world which we know is not possible.

ANSWER:
The law of conservation of energy applies only to our universe after its creation. Nobody knows where the original energy came from for the "bing bang". I should also point out that energy conservation can be violated in nature; the Heisenberg uncertainty principle allows energy to appear out of nothing but only for short times.

QUESTION:
i am a university senior student. my department is elementary science education. I know that every atom that is in our planet was once created in stars. I know that first H used to create He and all the way to iron. However, iron is not the last element in periodic table how other elements are formed? Thank you for your answer.

ANSWER:
As you note, iron is the heaviest element created in stars; the reason is that iron is the most tightly-bound nucleus and therefore any heavier elements will require that energy be added rather than energy being released by fusion (which is what stars do). Heavier elements are created in very energetic events like novae and supernovae. Recently it has been found that collisions between neutron stars also play an important role, particularly for the element gold.

QUESTION:
From what I understand, the vast majority of the large bodies in the solar system rotate and orbit in a counterclockwise fashion relative to their magnetic north pole. Is this true for most/all solar systems in this galaxy/the observable universe, or is it different every time? Why is this?

ANSWER:
First, you have a misunderstanding. Most rotations in our solar system are counterclockwise as viewed from the North Star (Polaris). The magnetic pole of any particular body has nothing to do with this rule. In fact, the earth's NS poles have swapped positions many times in the distant past. Also, many of the bodies in the solar system (like our moon) have no or negligible magnetic field. So, to state the rule you need to specify whether you are viewing the solar system from "above" (which I will call looking down on our north pole) or "below" (which I will call looking down on the south pole); in the latter case, the rule would say all rotations are clockwise. The reason is that in the current best theory for formation of the solar system says that a huge cloud of gas and dust collapsed to form it. That cloud will have an angular momentum (net rotation) which remains and speeds up as the cloud collapses. To see more detail, see an earlier answer. Any other planetary system in the galaxy will have a similar rule with most objects spinning the same direction, but not likely in the same direction as ours. Other galaxies do not have the same rotation as our Milky Way galaxy because when we see them, some are seen edge on and others are seen face on. (Both of those pictured are spiral galaxies which spin like a big plate.)

QUESTION:
i am currently working on a book that deals a lot with space. while it is technically fiction, it's not heavily science fiction (no aliens or new planets or anything of the like) and i want it to be as accurate as possible. my question is, is it possible for a satellite/spacecraft to be able to have and elliptical orbit around the earth and moon, with the two bodies being the elliptical centers. the main problem i see with this idea is that the moon, well, moves, which could wreak havoc on the orbit, but i'm still wondering if it is still theoretically possible.

ANSWER:
If, by "elliptical centers" you mean foci, the answer is absolutely no. The three-body problem has no general analytical solution. In your special case, one approximate solution would be that if the satellite/spacecraft were at all times very far away compared to the earth-moon distance, the orbit would be approximately an ellipse with the center of mass of the earth moon system at one focus of the ellipse. Another interesting case is if the moon-earth system is approximated to have circular orbits (about their center of mass) and the satellite/spacecraft moves in the same plane. In that case the satellite will have a closed orbit (called a horseshoe orbit) or even be stationary points (called LaGrange points) as seen from the (rotating) earth-moon system. The picture to the right shows some possible orbits around the sun-earth system, but would be similar for the earth- (replacing the sun) -moon (replacing the earth) system; also shown are the LaGrange points (L1-L5). The picture to the left shows the asteroid belt (white dots) along with other asteroids (green dots) which orbit with Jupiter near the L5 and L4 LaGrange points and still other asteroids (orange points) which are referred to as Hildas; for a more detailed discussion of the Hildas which have elliptical orbits, see the article by John Baez. An animation of the Trojans, Greeks, and Hildas is shown below; keep in mind that this is in the frame of Jupiter and the sun which is approximately stationary but the picture from outside would have Jupiter going around the sun counterclockwise. I think these are the most interesting solutions for your situation, but there are numerous other special analytical situations of the three-body problem which you can research starting with the Wikepedia article.

QUESTION:
Im sitting here watching the science channel, which I love! And they are talking about the core of Jupiter, which is by popular discussion they believe it is mostly metallic hydrogen. Now my question is, "if you had a 1 inch cube of metallic hydrogen, and took away the extreme pressure, would it stay metallic or dissolve into back into a gas?

ANSWER:
Metalic hydrogen is atomic hydrogen (not H₂, molecular hydrogen) which behaves like a conductor and which occurs at high pressures. The phase diagram to the left shows that metallic hydrogen cannot occur at pressures less than about 100 GPa≈1000 atmospheres or temperatures above about 2000 K. Lower temperatures, say around 300 K (near room temperature), require pressures more like 350 GPa≈3500 atmospheres. To the right is a phase diagram for H₂ at much lower temperatures and pressures and you can see that hydrogen will be a gas of H₂ for normal conditions. Note that atmospheric pressure is about 1 on the pressure scale and the temperatures are far below room temperature So, the answer to your question is that putting 1 in³ of metallic hydrogen in an environment of normal pressure and temperature would cause it to change to a gas of H₂ molecules (hugely expanded in volume).

NOTE ADDED:
I found this "Jupiter cake". Cake depicting the layers of Jupiter! The center (mudcake) is the theoretical rock/ice core, then comes the (almond butter) layer of liquid metallic hydrogen, and finally the (colored vanilla) liquid molecular hydrogen.

QUESTION:
What causes matter to rotate/spin/orbit? All I can find is the statement that in space particles of dust/gas/matter contract into a spinning disk due to gravity (to form stars, solar systems, galaxies etc.), with no explanation as to why the spin. I see a lot about the conservation of angular momentum, but these discussions all presume that the 'spin' already exists. What caused the spin in the first place? Shouldn�t gravity simply attract particles of dust, gas or matter together along a straight path till they collide, as a magnet does to a paper clip? The magnet does not make the paper clip revolve around it, and if I fall off of a building, I don�t spin around the earth. I fall in a straight path till I collide with the earth. What am I missing?

ANSWER:
A star and its planets form from a cloud of dust and gas which is hugely bigger than the size of the final star-planet system. Imagine that this cloud is a million times bigger than the final system. Each dust particle and molecule is moving with a random velocity; labeling each member of the cloud by the number i, the angular momentum ℓ_iof each particle of mass m_iand speed v_i is m_iv_iθr_i=ℓ_iwhere r_i is the distance to the center of mass of the cloud and v_iθis the component of the velocity which is perpendicular to the vector r_i. Notice that if at large distances out into the cloud all the v_iθdo not add up to zero exactly, even if the sum is a very tiny number (the cloud is almost not rotating) the angular momentum can be appreciable because r_i is very large. Probably the whole cloud has an imperceptibly small spin but it still can have a large angular momentum. Now the cloud starts collapsing because of its gravity. There is a very important law in physics called conservation of angular momentum: the total angular momentum of any isolated system must remain constant. Here, isolated means that there is nothing which exerts torques on the system and since this cloud is far from other large objects, it is essentially isolated. As the cloud collapses, r_i for each particle gets smaller and so v_iθfor each particle must get bigger by the same factor to conserve momentum. So, if the cloud shrinks by a factor of a million, it will end up with a million times greater spin. Also, see an earlier answer.

QUESTION:
I am a teacher in New Zealand, and am much more familiar with Chemistry than with Physics. I have been trying recently to understand magnetism better, in general, and as applied to Earth and Space Sciences. When a permanent magnet is created, what is actually lining up to create the magnetic field? I have heard it described as atoms, or pockets of charge, or micromagnets, but what does this chemically look like? Are they polar molecules within the solid? My understanding of bonding of metals doesn't include any polarity, they are positive nuclei in a 'sea of electrons', so how could a metal be aligned by charge? Is it an alteration of electron orbitals? Am I over-thinking this?

ANSWER:
An electron is, itself, like a tiny bar magnet, called an electric dipole moment. In most materials, the electrons point in random directions resulting in no bulk magnetism. In a ferromagnetic material like iron, there is a quantum mechanical effect which causes the electrons responsible for bonding to neighbors in the crystal to align their dipole moments. An ordinary piece of iron is normally not a magnet because the aligning of electron magnets happens only below a certain temperature called the Curie temperature and when the iron cools down the alignment occurs in local small volumes, each of which is pointing in a random direction; these are called domains and are hugely bigger than atoms but still microscopic. To magnetize the iron, you put it in a strong magnetic field and the domains already aligned with the field will grow at the expense of those not aligned with the field. You can also cool iron in a strong magnetic field and then most domains will form being aligned with the external field.

CONTINUED QUESTION:
Also, how does the movement of charged particles in the Earth's liquid outer core, or in the Sun's convective zone, actually produce a magnetic field? I have read about this effect many times, but I just can't picture chemically what this would look like. Is it that the general movemnet of ions within a fluid tends to align in the same direction over time, thus influencing other ions to do the same, and this spreading the force?

ANSWER:
(In future, please follow site groundrules and submit single questions!) Any electric current causes a magnetic field. So a simple model of the earth's field would be to imagine a huge ring of current inside the earth. The magnetic field of a current ring is shown to the left. Compare this to the field of the earth shown to the right. Clearly a model where the core is thought of as a collection of current loops gives rise to a reasonable description of the earth's field. You are barking up the wrong tree by trying to understand it chemically. I should also note that the details of the earth's field mechanism are not fully understood.

QUESTION:
So if black holes have so much mass that light can't even escape and gravity has almost an infiniate range of influance, when our universe is at its end and black wholes have swallowed up everything will the black holes start to move toward each other and then come together? If so would this singularity be so heavy that it would rip space time and then "big bang" again?

ANSWER:
Again, I emphasize that I usually do not answer astronomy/astrophysics/cosmology questions. I believe, though, that the current best model of the universe has, because of dark energy, the most likely fate of the universe not recollapsing (big crunch) but ever expanding and black holes eventually all evaporating via Hawking radiation.

QUESTION:
Is there a factor in Newtonian Gravity similar to the factor for precession as in GR(ie., the factor in the Schwarzschild equation for planet precession)?

ANSWER:
If you are wondering whether an orbit precession is possible in Newtonian gravity, the answer is yes. For example, if the sun were not perfectly spherical (which it is not), orbit precession is possible.

QUESTION:
I�m not sure if you would consider this an �off the wall� question (it seems like a �deceptively simple� type to me), but here it goes: what would be the relative density (to water�s 1 (1000 kg/m^3) say) of something with a theoretical �complete density,� i.e. something with no empty space between mass components (atoms, quarks, etc.)�a celestial goo, if you will. Would this be a singularity material or would this be less than singularity? Some sources list a singularity as being infinite density, so, if that is the case, I�d guess relative comparisons might be pointless. Part of what I am trying to figure out is how deceptive the term "density" actually is; I am thinking it will end up being something like the differential between the Fahrenheit scale and Kelvin. I would also like to know the how much �more� surface area a 1 meter by 1 meter square of this material would have than the traditional one square meter (of water I guess) of any traditional material. In that case, I figure ewe have always assumed complete density with surface area calculations, but, for some calculations I need to do, I need to know what �true� surface area is.

ANSWER:
Well, you are pretty close to "off the wall" here! First, a singularity has infinite density and zero surface area. A black hole is thought to be such a thing. I could quit there since this is certainly the limit. However, maybe you want something somewhere between the density of normal matter and infinite density, something which does not depend on what the material is. If we think about all matter as being composed of electrons, neutrons, and protons, we can imagine somehow compressing them all together to make some "primal stuff". Since electrons have very small mass compared to neutrons and protons, the number you seek is the density of nuclear matter, that is, the mass of a typical nucleus divided by its volume; that number is about 2x10¹⁷ kg/m³; so, that is 2x10¹⁴ times more dense than water. There are actually, in nature, objects with this density. Some stars, late in their lifetimes, collapse under gravitational attraction to this density and the enormous pressure causes electrons to be "forced" into the protons to make neutrons; they are called neutron stars.

Regarding your question about area, it really has no meaning because you cannot have a square meter of something, it always has a volume as well. If you had a sphere of nuclear matter of radius 1 m, it would have a surface area of 12.6 m² and a mass of 8.4x10¹⁷ kg. I guess you could imagine a "sheet of neutrons" with this density and ask what its area was. Taking the radius of a neutron to be about 10^-15 m, the volume of a sheet of area 1 m² would be 2x10^-15 m³ so its mass would be 400 kg. Taking the radius of a water molecule to be about 10^-10 m, a sheet of water of mass 400 kg would have an area of about 4x10⁹ m².

QUESTION:
With all the space junk circling our planet, I wonder how much mass do we have to lose before it affects our orbit around the sun?

ANSWER:
First of all, we have not lost that mass, it is still attached to the earth. Second, the orbit of an orbiting body is independent of the mass; a 1 gram earth, given the same orbit and speed, would take one year to go around. For example, there are hundreds of communications satellites in orbits high above the equator and all have a period of 24 hours, although they have many different masses.

QUESTION:
If all the galaxies are accelerating away then is nothing an intertial reference frame? If so, why do Newton's laws work the way they do?

ANSWER:
I am not, as I emphasize on the home page, a cosmologist, so take what I say with a grain of salt. I do believe, though, that there is no such thing as an inertial frame of reference and it really has nothing to do with acceleration. General relativity posits the equivalence of accelerating frames and frames in a gravitational field, so any frame where there is any gravitational field is not an inertial frame. Because there is no place in the universe where there is truly zero field, there are no inertial frames. Therefore, Newton's laws are false! However, there are many frames which are very close to inertial and for which they are almost not false. On earth, a patently noninertial frame, Newton's laws work remarkably well in many situations. For much of celestial mechanics (calculating orbits of satellites and planets, for example), Newton's laws work almost perfectly. Special relativity originally overthrew Newtonian mechanics which does not work, even in an inertial frame, at very high speeds without serious modification of definitions of things like linear momentum and energy.

QUESTION:
which takes more fuel- a voyage from earth to moon or from moon to earth?

ANSWER:
You can get a pretty good idea by just looking at the escape velocities from the earth and from the moon. From the earth, v_e=11.2 km/s and from the moon, v_e=2.4 km/s. Since kinetic energy is proportional to v_e², it would take about 23 times as much fuel to escape from the earth as to escape from the moon.

QUESTION:
We've all been reading about dark matter and dark energy for some years now and I believe you've said that you (among many others) are not yet persuaded that dark energy and dark matter exist. If matter and energy, as traditionally and conventionally understood, comprise only a very small part of the substance of the Universe, does it follow that classical mechanics, thermodynamics, relativity and quantum theory, etc., correspondingly apply only to that (seemingly) tiny aspect of the world? Is there any reason to think that the laws and theories of physics that humanity has discerned to date would apply also to dark matter and dark energy?

ANSWER:
First, a disclaimer: as I state on the site, I am not an expert in astrophysics, astronomy, or cosmology, so you can take my opinion with at least a grain of salt or ignore it altogether! I would not say "many others"! Most astrophysicists, astronomers, and cosmologists talk about dark matter as if it is surely there but just not directly observed yet. My own point of view is that I need to see some direct evidence before I accept that such a thing really is there; there is lots of indirect evidence of dark matter―the dynamics of galaxies, the time when galaxies first began to form, to name a couple―but it is altogether possible that we do not understand gravity as well as we assume that we do. The best theory of gravity, general relativity, makes many assumptions which are not necessarily true over really large distances. If this were the case, maybe dark matter is the 21^st century equivalent of the lumeniferous �ther (see the following answer) and is something we are looking for in vain because there is no such thing. There are lots of good ideas about what dark matter might be (including WIMPS, for which some evidence has recently been observed in the observed excess of high-energy positrons) and I will be happy to accept experimental evidence when it happens. Dark energy is a different matter in that it has not caused a search for some "stuff". There is already a place for dark energy in general relativity, known as the "cosmological constant"; in other words, many cosmologists do have the point of view that dark energy does result from an incomplete theory of gravity.

QUESTION:
why can nothing escape event horizon when all it needs to escape is just to exceed the gravitational force and not to reach the escape velocity-speed of light in this case- the way rockets on earth do as far as I know they do not cross the escape velocity

ANSWER:
The escape velocity V_escape is defined to be the minimum speed something must have to escape an object of mass M to a distance infinitely far away and from a distance R away from M. This is easily shown in classical physics to be given as V_escape=√(2GM/R) where G is the universal gravitational constant. Suppose that an astronomical object exists such that M is large enough and R can be small enough that V_escape=c, c being the speed of light. Then R=2GM/c²≡R_s, this being the Schwartzchild radius. Since no object with mass can travel as fast as the speed of light, anything inside this radius cannot escape. Also, light itself inside this radius (for which V_escape>c) could not escape either since the speed of light is always equal to the speed of light. Technically, the calculation of V_escape is not correctly calculated using classical physics for either photons or masses going at relativistic speeds, but this is one of those cases where a correct relativistic calculation gives the same results as the classical calculation.

QUESTION:
Even though mars weighs one ninth of earth why does is have 38 percent of earth's gravity?

ANSWER:
For a spherically symmetric mass, the gravity at the surface is proportional to the mass of the planet and inversely proportional to the square of its radius. For Mars the radius is R_M=0.533R_Eand the mass is M_M=0.107M_E, so M_M/R_M²=(0.107/0.533²)M_E/R_E²=0.38M_E/R_E². There is your 38%.

QUESTION:
Are the eliptic orbital planes of all our solar systems planets on the same 2d plane, or are there angles between the planes of the planets orbital elipses' ?

ANSWER:
Orbits of the planets are nearly coplanar. Inclination, measured relative to the plane of the earth's orbit, are all less than 3.5⁰ except for Mercury which is about 7⁰. Pluto, no longer considered a planet, has an inclination of 17⁰.

QUESTION:
Could you explain the origin of the inverse square law that occurrs in nature? Why can't Newton's law of gravitation vary as R^3 or R^5? How did Newton deduce this so long ago? Could it be because of the geometry of space when something radiates from a central point source...as a sphere gets larger & larger radii?

ANSWER:
Let us start with the historical question of how Newton could deduce the inverse square law for gravity and that will take care of your questions of why it cannot possibly be some other power law. There were data available to Newton which described the motion of the planets; these were Kepler's three laws. These can only be described by an inverse square law. So once again we see that, at its heart, physics is an experimental science and data dictate theory. Is there a deeper reason? As I see it, that is part of your question too. The electrostatic force is also an inverse square law (Coulomb's law) and in this case the theory of quantum electrodynamics correctly predicts the 1/r² dependence of the force. One reason is that the quanta which transmit the field, photons, are massless; forces like the nuclear strong interaction are mediated by particles with mass and the force is a much shorter-ranged force. For gravity, it is, as you suggest, related to the geometry of space (the area surrounding a point increases like r² as the distance r increases) as best we understand. General relativity, our best theory of gravity, correctly predicts the force to be inverse square law. Although there is no theory of quantum gravity, the mass of a graviton, if it exists, is expected to be zero because the force law is what it is.

QUESTION:
Why the orbit of Mercury around the Sun cannot be all accouted from the Newton's Law of Gravity, because there is a disprency between observation and Newton's Law.

ANSWER:
The orbit of Mercury is an elipse and the semimajor axis of this elipse precesses in time and this should not happen in Newtonian physics. However, this is predicted by the theory of general relativity and is one of the conerstone tests of that theory.

QUESTION:
I'm doing a report for my physics class on gravity and I was wondering if the force of gravity from the sun and the size of the planets determines the distance that the planets are from the sun.

ANSWER:
The size of the planets has nothing to do with it. The orbital period is independent of the mass. For example, any object, regardless of its mass, which orbits the earth near the earth's surface (like the shuttle) has a period of about an hour and a half. The distances of the planets from the sun is purely accidental. The distance does determine the period of the orbit (the length of a "year" on that planet). If the orbit is circular (which is approximately true for all planets), then the square of the period is proportional to the cube of the distance to the sun; this is Kepler's third law. For example, if a planet is a distance twice as far from the sun as the earth, then its period T may be found from (1/2)³=(1/T)² , so T=√8=2.83 years. To learn about Kepler's laws, see the Wikepedia entry.

QUESTION:
Hey dude, With the endless amount of stuff floating around in space, how come none of it hits earth? And to be more specific, how come nothing so massive and unstoppable by earths defensive capabilities ever comes towards earth and destroys it?

ANSWER:
Hey Dude back at ya! It happens all the time�what do you think shooting stars are? And, tons of space dust hit the earth every year. In the early solar system, there were a lot more pieces of stuff "floating around" and there were many more collisions with bigger objects than there are today. One collision with an asteroid-sized object is thought to have caused a major extinction of life millions of years ago. But most remaining objects are in reasonbly stable orbits now. And, you just have no idea how mostly empty space is�just statistically it is immensely improbable that collisions will occur. Imagine two guys with rifles 5 miles apart firing their rifles; what is the likelihood that two of their bullets will collide? There was one major collision in 1994 when Comet Shoemaker-Levy hit Jupiter. Many asteroids have highly variable orbits which could possibly result in a collision with earth, but it is a very low probability event.

QUESTION:
Galaxies recede from us at a velocity proportional to their distance. Why then is Andromeda on a collision course with our beloved Milky Way?

ANSWER:
The overall average behavior is that the universe is expanding. However, at distances much smaller than the size of the universe there is no fundmental reason why everything has to be moving apart. For example, the sun is not moving away from us. And, there are attractive gravitational forces which can be important between nearby galaxies. Astronomers have observed many examples of galaxies colliding. The Milky Way and Andromeda galaxies will collide in about 5 billion years. You can see a simulation of that collision at this link.

QUESTION:
If a black hole pulls material in doesn't it gain mass and therefore slow down?

ANSWER:
Yes, it gains mass. But, it gains the mass by interacting with something else. So, to find out what the black hole is doing after the collision, you have to conserve momentum of the event when it gained that mass. It is pretty much like the collsion of two putty balls�you start with two and end with one. For example, suppose the black hole is at rest and the to-be-captured object is far away and has a velocity v directly toward the black hole and a mass m. So the momentum of the system is mv. After the collision, the mass must still be mv (that is what momentum conservation means). So, if the mass of the black hole were 999m, the speed after the collision would be V=mv/(1000m)=v/1000. You could go through the whole thing for other initial conditions, for example if the black hole were moving with a speed v and the object were at rest, the speed after the collision would be V=999mv/(1001m)=0.998v, slower, as you suggested, but not really for the reason you suggested. (I have done this nonrelativistically, but it conveys the idea.)

QUESTION:
If the Big Bang is the result of everything in the universe is rushing away from each other then why is the Andromeda galaxy hurtling toward the Milky Way galaxy? Should they move in opposite directions? Did something more massive than the Andromeda galaxy fling it our way?

ANSWER:
The fact that the universe is expanding does not mean that everything is moving away from earth. For example, sometimes Venus moves in a direction toward the earth, sometimes away, depending on the positions we are in our orbits. Almost everything very far away moves away from us, but things closer, like other stars in our galaxy or nearby galaxies, may move toward us. This can be understood in terms of initial conditions and forces. For example, if two galaxies form reasonably close together initially at rest with respect to each other, their mutual gravitational attraction will pull them together.

QUESTION:
I read that the Andromeda Galaxy will collide with the Milky Way Galaxy in about 3 billion years. How could this occur unless the Big Bang allowed for a change of direction, OR there was more than one Big Bang?

ANSWER:
I do not normally answer astrophysics questions, but I can handle this one. Just because the universe is expanding does not necessarily mean that everything is moving away from everything else. Imagine two galaxies which are originally moving away from some point in space but fairly close to each other. Then, if they keep moving like this, each will see the other moving away. But, they are gravitationally attracted to each other. Therefore, as they move away from the point, they will begin moving toward each other; so looking at one from the other, we will see it approaching. We know that the universe is made up of local clusters of galaxies which are gravitationally bound to each other and therefore orbit each other, collide with each other, etc. But if you look at that cluster, you will find that the average of all those motions is that the cluster moves away from us.

QUESTION:
I read the other day that Tycho Brahe didn't think the earth could be rotating partly because if it did, then a cannonball fired in the direction of the earth's rotation should go farther than if it were fired in the opposite direction. I'm pretty sure I understand the idea of relative motion to understand why it doesn't do so, but what I don't understand is why he thought it would travel further if it were shot in the direction of the earth's rotation. It seems if I were going to be confused about that topic, I would think the cannonball would travel further if the ball were fired in the opposite direction of the earth's rotation. So, I imagine I'm not understanding his confusion. Can you help me understand why he was confused

ANSWER:
In the simplest terms, the cannonball should go the same distance either way because it is fired relative to the earth which is moving. If you are in a moving train and throw a ball forward or backward, it will go the same distance in the train. However, if someone watches from outside the train, the ball goes faster when you thow it forward than backward. That is why most satellites are launched in an easterly direction�to get the extra boost of the earth's rotational speed. Brahe was right in thinking about this issue, though, because things behave strangely in rotating coordinate systems and do not behave the same as in stationary systems. Most notable is the motion of projectiles due to the Coriolis force where cannonballs curve left or right. But the effect was too small to be observed for cannons of his time and, since he was born 20 years before Galileo, these effects were unknown.

QUESTION:
We know the Earth's rotational speed is 24 hours approximately. Using T= sqr (4 x pi^2 x R^3/GM), where R=radius of earth= 6400km, M=Mass of earth= 6x10^24 kg, I always get 1.41 hours and not 24 hours. Whats wrong with my concept?

ANSWER:
This is a perfect example of using a perfectly good equation for a situation where it is not valid. The equation you state is Kepler's third law for the period of a circular orbit of a mass mm going around a mass M if m<<M. For example, if you set R equal to the radius of the earth's orbit and M equal to the mass of the sun, T=1 yr. Or, if you set R equal to the radius of the moon's orbit and M equal to the mass of the earth, T≈1 month. What you are doing is calculating the orbit of a satellite which orbits near the earth's surface (like the International Space Station) which does, indeed, orbit the earth about every hour and a half. If R=6.6 R_E, R_E being the radius of the earth, T=24 hr; this is called a geosynchronous orbit where a satellite over the equator appears to remain fixed in space and is where communication satellites are placed.

QUESTION:
This lady lost her legal battle to have the Large Hadron Collider shut down. She believed that the atom smasher could create a black hole and suck up the earth. The courts sided with the scientists. The scientists said it's not possible, and even if it did create a black hole, it would be a micro black hole and collapse in on itself. That seems to go against the laws of physics and quantum physics. We now know that matter sucked up by a black hole is permenantly imprinted on the black holes surface and the size of the black hole increases. Did I miss something?

ANSWER:
I do not normally answer astrophysics questions, but I can deal with this one, I think. First, some cosmic rays (radiation which strikes earth from space) have energies much greater than the proton energies in the LHC and if a black hole could be created and have sucked in the whole earth, that would have happened long before we evolved. Second, you did miss something�Hawking radiation whereby a black hole can radiate energy. For something to be "sucked into" the black hole, it must come within the Schwartzchild radius which is 2Gm/c²=6.67x10^-11m/(3x10⁸)²≈1.5x10^-27m where m is the mass of the black hole. The maximum kinetic energy of each proton in the collider is 7 TeV which is about equal to the mass energy of 7000 protons, so the heaviest black hole they could make would have a mass of about 14,000 protons, about 2.3x10^-23 kg; the corresponding Schwartzchild radius is 3.5x10^-50 m! It seems to me that even if this black hole never evaporated, it could go a really long time before it got close enough to anything to suck it in.

QUESTION:
Why is everything spinning? Moons, planets and stars all rotate on their axes, moons orbit planets, planets orbit stars, spiral galaxies rotate around super-massive black holes, etc. What force is causing all this spinning?

ANSWER:
First, the fundamental fact: if something is spinning it takes a force (technically a torque) to change that spinning, nothing is required to keep it spinning. Imagine that you are in the middle of empty space and a mass moving with a constant velocity goes by you; no force is necessary to keep it moving and it will move like that forever as long as no forces act on it. (This is called conservation of linear momentum.) Now, imagine that you are in the middle of empty space and there is some object in front of you spinning; then, if you do nothing to it, it will keep spinning like that forever. (This is called conservation of angular momentum.) Take the solar system, for example. The sun and all the planets all formed around five billion years ago when a huge cloud of gas and dust in space collapsed because of gravitation. If that cloud had any rotation at all before collapse, then the collapsed cloud (aka solar system) has to retain that angular momentum. Although the collapsed cloud rotates much faster than the original cloud did, it still has the same amount of angular momentum, just like a spinning figure skater pulling in her arms spins faster.

QUESTION:
Is there a mechanism that create Hydrogen? It seems to me that over billions of yrs all of the H would be burned up in stars. Where does it get converted back to H after its been used up???

ANSWER:
This is the nature of our universe. When all the hydrogen is burnt up (that is, converted to heavier elements), that will be the end. After more than 14 billion years, there is still quite a bit left.

QUESTION:
Hydrogen is the most abundant element in the universe. A star, such as the sun in our solar system can produce all of the naturally occurring elements on the periodic table. Are all of the naturally occurring elements on the periodic table created by adding hydrogen atoms together under extreme temperatures and extreme pressures? (Ex: 1 hydrogen atom equals hydrogen, 2 hydrogen atoms added together equals helium, 3 hydrogen atoms added together equals lithium, and so on.)

ANSWER:
Starting with hydrogen, stars fuse nuclei together to get heavier elements, but not all heavier elements. The net effect is to make helium from two hydrogen, but the process is a little circuitous. Heavier elements are made by fusing lighter elements, but not three hydrogen for a lithium, rather something like a helium and a hydrogen; for example, see this link to see how carbon, oxygen, and nitrogen are made.. After iron, energy is lost when fusion to heavier nuclei occurs so it will not happen. The elements heavier than iron are made when the star explodes (supernova) and the energy from the explosion can be used to fuse heavier elements.

QUESTION:
Does everything in the universe rotate in the same direction?

ANSWER:
No. However, most things in the solar system do; if viewed from the north star, most objects in the solar system rotate counterclockwise.

QUESTION:
As a shotgun fires, the bb's make a pattern that gets wider and less dense with more distance. So how can light leave a small source and fill all the void in between for what seems could be an indefinate distance? i.e. the sun

ANSWER:
Because a shotgun is more like a flashlight than the sun. The light from a flashlight is pretty well collimated and spreads a little. The light from a source like the sun goes out in all directions. However, the intensity of the light from this kind of source gets smaller as you get farther away quite rapidly--if you go 10 times as far away, the intensity of the light gets 100 times smaller. That is why, of course, star appear much dimmer than the sun even though the sun is a pretty average star.

QUESTION:
OK. I have been thinking about this for a very long time, however since i do not possess the intimate knowledge that a well-versed person in the ream of physics might have. My question is this: From my understanding, there are four main forces which keep the universe together. One of them is gravity. Gravity affects the heavens and all that it encompasses even light. Light is bended by the gravity exerted from a massive object. From what ive learned, this is one way black holes are observed. As is commonly known, nothing can escape the gravitational pull of a black hole, not even light. So what we have are two extremes: in one extreme, a beam of light would travel (theoreticlly) in a straight line if no force of gravity was imposed on it and in the other extreme, light is bended so far that it is incapapble of escaping when it enters a supermassive graviational field. The question i have is is there a medium between the extremes? Would it be possible for a beam of light to orbit an object of a specific mass, trapping it in orbit?

ANSWER: (answer provided by L. A. Magnani)
Yes, light can orbit around a black hole of any given mass. The key question is, how far from the singularity can the light maintain a circular orbit? It turns out that, according to General Relativity, light can maintain a circular orbit at 1.5 times the Schwarzschild radius from the singularity (the Schwarzschild radius is 2GM/c² where M is the mass of the black hole, G is the gravitational constant and c is the speed of light. This region is sometimes called the photon sphere. Inside 1.5 times the Schwarzschild radius, no circular orbits exist; all orbits are unstable and all objects moving inside of this distance fall into the black hole.

QUESTION:
I was told to ask this question to a phyicist, so here goes. Where did air come from?

ANSWER:
Well, how far back do we want to go? All heavy elements (essentially heavier that hydrogen) were produced in stars and then, when the star was "all burnt up", it exploded and sent all the heavy elements flying into space and then they eventually come together again to form planets, etc. (Scientists like to say that we are all made of "star dust". Then, depending on the chemistry of the planet, its temperature, and other factors, some of the planet will become an atmosphere, i.e. gases will escape from the surface somehow In some cases (like the moon) the gravity is not strong enough to hold the atmosphere and it eventually "leaks" off into space. In the case of the earth, there is virtually no hydrogen or helium in the air because it has all leaked off. The detailed composition of the atmosphere depends on chemistry and biology. For example, it is thought that originally the earth had much more carbon dioxide in its air but that evolution of green plants resulted in there being much more oxygen now.

QUESTION:
Do gravitons exist?Or are they only a theory?

ANSWER:
There is no working theory of quantum gravity and so a graviton is a purely hypothetical particle. A graviton has never been detected experimentally. Trying to unify general relativity (the working theory of gravity) with quantum mechanics (the working theory of everything else) is one of the holy grails of theoretical physics.

QUESTION:
Given that the neutron has a magnetic moment - is this why neutron stars have very strong magnetic fields? Are all of the neutrons within the extreme density of the star aligned along their magnetic fields?

ANSWER:
The simple answer to your question is no! The reason is not simple but is related to the fact that a neutron star has lots of neutrons but it also has lots of other stuff. An astronomer here in the department is going to consult with friends of his who are neutron star experts and then we can get back to you with a more complete answer.

Have a look at http://www.astro.umd.edu/~miller/nstar.html#internal.

ANOTHER ANSWER: (Provided by M. C. Miller) (I referred the questioner to the above site. He emailed the author of that site, Professor Cole Miller of the University of Maryland who then provided the following answer.)

"The origin of neutron star magnetic fields is not, by any means, well understood. The existing field is supported by normal currents in normal fluids (superconductors exclude magnetic fields), but why it got so strong is an evolutionary question. Let me start by giving you the pat answer you'll find in many textbooks, then I'll explain why it's at best misleading and tell you a bit about current thinking.

It's often "explained" like this. The Sun has a magnetic field, with an average strength of about 100 Gauss. If you were to compress the Sun to the size of a neutron star and conserve magnetic flux, the strength of the field would be about a trillion Gauss, which is about the strength inferred for young pulsars. Voila, that's why you expect such a strong field.

The problem is that the Sun will not become a neutron star. Stars that do become neutron stars (massive O and B stars) have relatively little convection in their interiors, meaning that they don't generate a field via a dynamo. If you were to take their central cores and compress them to the size of a neutron star, you'd get a rather weak field.

So, people think that the field is generated during the collapse of the core that initiates the supernova. Somehow, with all the associated turbulence, you stir things up and produce a dynamo that generates the observed field. Again, the field itself isn't especially mysterious -- it's standard currents -- but how it got there is not well known. One problem is why it stops at a trillion Gauss for many or most neutron stars. In principle, the dynamo could generate something a good hundred thousand times stronger, so why doesn't it? Maybe there's something preventing such growth, or maybe it's just that such a star would spin down so fast we usually wouldn't see it. There is a class of objects thought to have magnetic fields up to 1000 times stronger, so perhaps such stars are out there."

QUESTION(1):
Today there is evidence that the big bang occurred. But do scientists have any theories as to WHY it occurred? Or is it, that they just don't know for sure and are working on it? Also, any theories are not testable right?

ANSWER(1): (Provided by L. A. Magnani)
Right now, there is no consensus on why the Big Bang happened. Modern physics breaks down in conditions typical of the Plank Time (10^-43 seconds after the Big Bang) so the safest thing to do is to discuss what happens after the Plank Time. However, people who work on Grand Unified Theories and string theory feel that when these theories are fully developed we will be able to discuss what happens before t=10^-43 seconds. People talk about vacuum energy fluctuations and phase transitions, but, at the moment this is just speculation backed up by theories which are in the developmental and not the testing stage. String theories and GUTs may be testable in the years to come, but they are not testable now. However, the next few decades could lead to breakthroughs in this area.

QUESTION(2):
Oops! I have another question. According to this article: http://www.space.com/scienceastronomy/generalscience/physicists_bigbang_000209_wg.html Does this mean that at the most we can have quarks come out of this "recreation" of big bang conditions? Or do scientists think they can make other matter, like trees, rocks, gold, etc. through this matter, or do they think that it only occurred through the big bang?

ANSWER(2): (Provided by L. A. Magnani)
I hadn't heard of this result and I would like to know more about it. But, I think the answer to the student's question is that the conditions in the early Universe which led to quark production subsequently led--as the Universe expanded and cooled--to the production of protons and neutrons. Hydrogen, Helium, Lithium, and Beryllium followed. But by then, the Universe had cooled to the point where nucleosynthesis could no longer proceed and the abundances of those elements and their isotopes were frozen in (some elemental and isotopic compositions changed with time because of processing of original material in stars). The rest of the elements, necessary for trees, rocks, gold, etc. were synthesized in stars.

QUESTION(3):
Hello, I have one last question!!! Do scientists believe that the big bang explosion came from a single atom, a glob of matter, or they just don't know and are working on it?

ANSWER(3):
See ANSWER(1).

YET ANOTHER FOLLOW-UP QUESTION:
Prof. Magnani had said that "elements, necessary for trees, rocks, gold, etc. were synthesized in stars." What does he mean by this? I mean, you never see gold, or a tree, or a rock or plants, or a coral reef coming out of a star or inside a star? How could these things come to earth through a star? Also, how are stars and the Sun formed? I couldn't find this on the Web!!! I mean, stars are balls of gas, and the sun is too right? So could we just put hydrogen and helium together to make a mini-sun?" How do scientists think the earth is formed? I have a feeling that the professor will tell me that stars were created through the matter by the big bang, but what about the earth? You'll never find the earth coming out of those balls of gases, right?

ANSWER:
As Dr. Magnani said, after the big bang the universe consisted entirely of the very lightest elements, mostly hydrogen--nothing you could make a tree out of. The reason is that what was created were the "elementary" particles (protons, neutrons, electrons) and to make a heavy element like gold you have to put together a bunch of protons which takes a lot of energy to push those positively charged protons together which want to keep apart. Therefore when things have settled down after the big bang you have what is essentially a universe full of hydrogen. And that would be the end of it if the universe had been homogenous (perfectly the same in all directions). But there were lots of places where the density of H gas was bigger than neighboring regions and the gas started to contract because of the gravitational attraction. Eventually the gravitational force has compressed this H gas so much that it gets real hot, that is the atoms are moving around with a very large velocity. Now the H starts "burning". It isn't really burning in the usual sense; rather it is colliding with other H atoms and causing nuclear fusion to occur. The main thing that happens in a normal star in the "prime of its life" is that H burns to Helium and the result is the release of huge amounts of energy. That is the energy that keeps us all alive. As the star gets older and begins to exhaust its H, things get complicated and things start happening relatively fast. Basically the He starts combining to form heavier elements, the star starts cooling down, so that it starts to contract under gravity again. In many cases the star at some point just explodes (a supernova) throwing all its matter out into space. Now we have in the instellar space material other than H. But there is still a lot of H around. So now a new star will form but when it does there is stuff other than H there. What apparently happens is that there is an "accretion disk" which forms around the new young star which has much of the heavier elements in it (sort of like Saturn's rings which are sort of an accretion disk) and the "stuff" in the disk condenses into lumps which become planets. So our sun must have formed long after the big bang or else there wouldn't have been any of the heavier elements around. A favorite poetic saying astronomers like to use is that "we are all made of star dust".

The answer to your question "So could we just put hydrogen and helium together to make a mini-sun?" is yes! That is what a hydrogen bomb is--a mini-sun. That is what scientists trying to harness fusion energy are trying to do is to make a controlled H bomb as an efficient power source. It is a very difficult job still far from finished.

You need to get yourself a good book on elementary astrophysics and one which addresses stellar evolution. There were things called books and libraries back before the internet and they actually still exist! Seriously, though, you clearly have a very inquiring mind and you should read a good book or take a course so that you can get the big picture. It will be better for you than surfing the web!