1. Is it possible for black holes to lead to other universes?

Using black holes to get from one place to another is an interesting story. Here, people sometimes talk about wormholes.  Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity. Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working).  In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless technojargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity.  Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely.  The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative.  This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes, and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity).  Therefore, if you have a wormhole you can time-travel.  This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox.  In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person.  I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother.  Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture!  Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

Cole Miller

2. What happens to all the stuff that get's sucked into the black hole? Would it pop out randomly at some other place or something? Or would things not survive a black hole?

It is thought that the matter that goes into a black hole gets crushed into a tiny point at the center called a "singularity".  That's the only place that matter is, so if you were to fall into a black hole you wouldn't hit a surface as you would with a normal star.  Once it's there, it's there.  As far as we know, nothing would survive going into a black hole.  People sometimes talk about "wormholes" as portals to other universes, but it is now thought to be very likely that these can't exist.

Cole Miller

3. If nothing escapes a black hole not even light, and  if gravity is perhaps a form of electromagnetic radiation (G.U.T.) how does gravity escape the black hole to pull things in?

The idea (in Einstein's conception) is that space and time are parts of one entity, spacetime. The presence of mass distorts and warps this spacetime; normally the warping is minor, but around a very compact object such as a black hole or a neutron star the warping is dramatic and all sorts of funky effects happen.  A good analogy to think of is a heavy ball on a rubber sheet. The ball distorts the sheet, so if something light rolls by its path is deflected.  Net result: the light thing curves, which is the normal expected effect of gravity.  Therefore, the key is that the effect of gravity does not emanate from inside the black hole; instead, it comes from the overall warping of spacetime *outside* the hole.

Assuming people eventually unify gravity with the other forces, how would this work with the idea that particles (gravitons) carry gravity?  You're right that these particles couldn't come from inside the black hole.  At the moment, people are working with ideas that just outside the event horizon of the black hole is a "brane" that contains particles that transmit gravity.  This would solve the paradox, in principle.  Sorry I can't be clearer about this part, but this is very much an unsolved problem at this time.  Mainly, you're right, gravity can't be transmitted from inside the hole.

Cole Miller

4. I was wondering, and I'm sure you've been asked this before, but I've heard that maybe in the center of the universe there is a black hole, and that's how the Big Bang came about.  Have you ever heard of this?  Anyway, if that is the case, then wouldn't the universe be pulled towards the black hole instead of expanding?

You have to be careful when you talk about the center of the universe, because in our current understanding there *isn't* any center.  All portions of the universe are equivalent.  There is, however, a *beginning* to the universe (the Big Bang), and at that time the universe was extremely hot and dense; indeed, so hot and dense that it qualified as a "singularity" (infinite density). This term is also used for the central point of a black hole, which may be where you heard the association.  When thinking about the expansion of the universe, an analogy may help.  Imagine painting dots on a balloon, and blowing the balloon up.  Every dot moves away from every other dot, and no dot is more the "center" of the expansion than any other.  The balloon does, of course, have a center. In our analogy, that center represents not a place but a *time*; the beginning of the universe.  So, all of space expands as time goes on.  A weird concept, but there you have it!

You are right that the gravity of the universe is working against the expansion of the universe.  However, although this gravity tries to slow down the expansion, there may be enough speed in the expansion to make it go forever; this isn't known yet.  An analogy is when you throw a ball upwards.  Gravity slows it down, and for a real ball will bring it back eventually.  But if you had a super strong arm, you could in principle throw the ball all the way off the Earth! Similarly, we don't know if the gravity of the universe will cause it to recollapse or whether it will expand forever.

Cole Miller

5. If we had some rope the length or ten light years how long would it take a black hole to completely suck the rope in?

It depends on how much mass the black hole had.  If the black hole had the mass of our Sun, it would take a long time, around a hundred million years.  For a really big black hole, with a hundred million times the mass of our Sun (as is thought to exist in the centers of some galaxies), it would take about ten thousand years.  One thing to remember is that at such great distances the gravity of a black hole acts like the gravity of anything else with that mass (e.g., if the Sun were magically turned into a black hole with the Sun's mass, the Earth would orbit as it always does. We'd just get cold without the Sun's radiation!).  The weird effects you read about for black holes happen only very close in.

Cole Miller

6. Black holes are supposedly started from a neutron star collapsing on itself so if the center of galaxies are powered by a super massive black hole would it still have been caused by a neutron star?  And if that is the case, how big of a neutron star would it have to be in order to cause a black hole?

There are actually several different ways to form black holes.  One of these is the possibility you mention, that enough matter could pile onto a neutron star or other object, causing it to exceed some critical mass and collapse to form a black hole.  Black holes can also form as stars die (in "supernova" explosions), or from 2 or more objects (stars or neutron stars or black holes) that are orbiting each other and eventually collide, or from other mechanisms early in the life of the universe.

Astronomers are actually not sure how supermassive black holes form.  It was not a giant neutron star that collapsed, though . . . if a neutron star is bigger than a certain size, it can't exist--it will collapse to form a black hole.  The size at which it will become a black hole is much smaller than the size of a supermassive black hole.  It is possible that the supermassive black holes started out small, however.  As time passed, other stars and gas joined with the black hole to make it larger, until it became the huge black hole that we observe now.

Dave Rupke

7. Has there been a case where a black hole has sucked it self in? And is there a possible way that could happen?

A black hole can't suck itself in, but it can suck in other black holes (as well as sucking in gas, dust and stars). When a black hole swallows another black hole (or anything else) it grows larger, and this is one of the ways to grow super-massive black holes that are millions of times more massive than our sun, and often found in the centers of galaxies. In addition, black hole mergers create "gravity waves" or ripples in space that spread out from the black hole and can be observed on earth (in principle, at least -- experiments sensitive enough to measure them are only just being built).

Andy Young

8. I was just wondering where the matter goes when it is sucked in a black hole?  Also where does a white hole get its matter?  Are these related in any way?

Any matter that disappears across the event horizon of a black hole makes the hole more massive and larger.  Inside the event horizon, the matter probably keeps on falling towards the very center of the black hole.  What happens when it gets to the center, we don't really know. Several years ago, it was thought that matter reaching the center of a black hole might reappear somewhere else in the Universe by being thrown out of a "white hole".  No good evidence for white holes has been found, and they might not exist.

Neal Turner.

9. Do white holes exist?

Thank you for your interesting question about black holes. As you mentioned in your question, the gravitational pull of a black hole is so strong that anything that gets too close to it cannot escape. The "event horizon" is the closest you can get to the black hole and still escape (but it gets much more difficult to escape as you get close to the event horizon). If you get to the horizon or cross it you can't escape -- even if you travel at the speed of light (and that's the highest speed possible). Since nothing can escape from the event horizon we can never see what is going on inside -- a form of "cosmic censorship". If you were unlucky enough to fall into a very big black hole you might drift past the event horizon without too much trouble (although it would be impossible to go back), and you'd find yourself falling towards the "singularity" at the center. As you get closer to the singularity the strong gravity would try to stretch you out like spaghetti, and when you reached the singularity you'd be squashed. Unfortunately the laws of physics as we know them don't work at the singularity (when everything gets squashed to a point). There is a lot of ongoing research to figure out a new theory of gravity that explains what happens at the singularity. Things may not be so bad, however, if the black hole is spinning. There is a theory that a spinning black hole may have an "Einstein-Rosen" bridge at the center (a bit like a tunnel) that leads from the center of a black hole to the center of a "white hole" in another universe. So, with a very strong spaceship you might be able to enter a black hole here and emerge from a white hole in another universe. A "white hole" is the opposite of a black hole, and has no "event horizon" so you would be able to see the singularity at the center -- and we'd be able to view something that our physics theories cannot explain. If they do exist, white holes should be easy to detect, but they have never been seen. Most physicists think what white holes do not exist, and that something falling into a black hole gets squashed at the singularity and the black hole grows a little larger.

Andy Young.

10. One question that I've always wondered about is, what would happen if two black holes or neutron stars become close together?

If two black holes orbit each other (perhaps because their progenitor stars orbited each other), then they can gradually spiral closer and closer until they merge.  The reason has to do with general relativity. In general relativity, we imagine spacetime as a rubber sheet that is flat in the absence of any masses.  If you add a mass, then spacetime is warped, in the same way that a rubber sheet will be dented if you add a bowling ball.  Now imagine that you have two bowling balls nearby, circling around each other.  They will produce ripples in the sheet that carry away energy, and the bowling balls will get closer together until they hit.  Similarly, two black holes (or any other two masses) that orbit each other create ripples in spacetime called gravitational waves. These ripples carry away energy, and cause the black holes to get closer until they hit.

What is the result?  A single, big black hole!  Black holes can merge in this way, but they can never split apart.  There are now several instruments that exist or are planned that will look for the signs of the ripples in spacetime.  These will be very tiny but might be detectable.

Two neutron stars orbiting each other will also spiral closer together, for the same reason.  However, since neutron stars have surfaces, when they hit they might produce more than gravitational radiation.  In fact, some people think that this will produce a burst of radiation that might be seen in the gamma rays.  It is likely that the merged remnant of the two neutron stars will be too heavy to remain as a neutron star, and will then collapse into a black hole.

Cole Miller

11. One day in class Mr. Edwards stated that Dr. Packer calculated when 2 singularities are close enough together, your able to pass through safely. Could that ever be a possibility?

Your question relates to wormholes: is it possible to have a spacetime in which you can not only go through the throat of what looks like a black hole, but do it safely?

Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity. Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working).  In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless technojargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity.  Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely.  The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative.  This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes, and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity).  Therefore, if you have a wormhole you can time-travel.  This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox.  In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person.  I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother.  Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture!  Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

Cole Miller

12. I've heard it said that whenever interstellar matter reaches or touches a black hole's event horizon, it is known to "belch" light and radiation.  What does this mean and why does it happen?  Also, if nothing can travel at the speed of light except light, how can a black hole pull light into itself?

It sounds like you may be confusing a few things about black holes: they don't actually "belch" radiation, because nothing can escape from a black hole (in a reasonable amount of time, anyway).  Infalling material typically forms an accretion disk around the event horizon, similar to water spiraling down the drain.  This material loses energy as it spirals around the disk closer to the hole, and radiates this lost energy away.  The amount of emission increases dramatically as one nears the event horizon, because This is the region where the gravitational potential well of the black hole is the steepest, so material falls in faster and loses more energy as a result.

You're right: nothing with mass can move at speeds at or greater than light, as a result of special relativistic effects.  However, black holes can still "pull in" light if it gets within the event horizon.  This is because by definition, the event horizon is the distance away from the central singularity that one would have to be for a black hole of a given mass to have an escape velocity equal to the speed of light.  So something would have to be traveling faster than c to escape the gravity of the hole at that distance!  This is not possible, since light moves at a maximum velocity of c, so even light will get pulled in when it's this close.  The escape velocity of a black hole is sqrt(2GM/c^2), where G is Newton's gravitational constant and M is the mass of the black hole.  So as the mass of the hole increases, so does the velocity an object would need to escape it.

-Laura Brenneman

13. What are the difference between Black holes and White holes?

We need to think about black holes before we talk about white holes. How do we know black holes exist? Well, we have two kinds of proof.   Firstly, we have real, hard evidence from telescopes. Telescopes have been used to observe light emitting by material in orbit around something dark (that we can't see with telescopes) and very heavy. These dark heavy objects seem to be very small relative to objects of similar or greater mass that we can see. These dark, massive objects are probably black holes.

Secondly, we have a really good theory of gravity, which has been well-tested, which predicts that we should see such objects in the universe.

So where do white holes come in?

Well, the theory of gravity that we have (Einstein's General Relativity), predicts black holes as a solution to a set of equations. Solutions to Einstein's equations can have a "+" (plus or 'positive')  or "-" (minus or 'negative') sign in front of them. We see this sort of thing all the time in mathematics. The solution to y*y=9 can be y=+3 or y=-3.

Depending on your choice of sign in Einstein's equations, you get a black hole which grows by sucking in matter from far away. Or you get the opposite, a "white hole". It's a black hole running backwards in time.

What would a white hole look like? Well, if you had a camera recording a black hole sucking in matter, and could then play the recording in reverse, you'd see a white hole. You'd see matter being blown out of the 'white' hole and the hole would gradually decrease in size as it spits out matter. But are white holes real?

Well, just because an equation that describes the world has a 'negative' solution as well as a 'positive' solution, doesn't mean that it's the correct description of things. If I tell you that I have the square of the number of baseball tickets that I have is 9, that means that either I have three tickets or minus three tickets. But minus three baseball tickets doesn't make sense. There's no such thing. There are plenty of examples of equations in dynamics where there are both positive and negative solutions in time, but only one describes the real world. This is because we can see just by looking around us that time moves in one direction. Time never seems to reverse itself and replay an hour, or a day.

So we think that the white hole solution to Einstein's equations is one of these solutions that don't apply in the real world. It's like having minus three baseball tickets...it just doesn't happen.

The final word goes to observations, since you can have all the pretty equations you want, but they have to agree with what we see otherwise they're useless. So far observations tell us that we're right. We see a lot of good evidence for black holes, but none so far, for white holes.

Barry McKernan

14. I know that the Black Hole has a great gravitational field. Everything will be disappearing if they go through the Black Hole. What would happen if the Sun went through the Black Hole? The Sun also has a great gravitational field. Would the Sun disappear?

If the Sun went through a black hole, it would disappear into the black hole just like anything else. When anything (light, a space ship, even the Sun) comes too close to a black hole, the gravity of the black hole is so strong that the object cannot escape, even if it traveled at the speed of light! The distance from a black hole inside of which nothing can escape is called the event horizon.

If you flew a space ship close to a black hole, you could remain safely in orbit around the black hole so long as you flew outside of the event horizon. However, if you went too close and passed inside the event horizon, there would be no escape! Similarly, if the Sun got so close to a black hole that it was inside the event horizon, it would be unable to escape and would disappear.

If you want to learn more about black holes or most anything else in physics, astronomy, chemistry, or math, check out this website: http://scienceworld.wolfram.com/

Matthew Knight

15. If we could control a singularity, it's placement, size, and/or destination, what do you suppose we would do with it?

First, it's useful to remember that most people think there is a form of "cosmic censorship" that means one cannot have a "naked" singularity. That means that a real singularity in the universe will have an event horizon around it, so that (1) you can't see the singularity directly, and (2) what you really have is a black hole.

If, therefore, one could control a black hole, then a constructive use would be to generate tremendous amounts of energy.  Gas spiraling into a black hole from far away will typically release about 10% of its mass-energy before finally falling into the event horizon.  The reason is that the gas is forced to move really fast, so that it experiences friction with itself and heats up, therefore radiating.  This is about 10-20 times as much energy per mass as is released by nuclear fusion, and applies to any matter at all.  That is, you could imagine throwing garbage at the black hole (maybe toxic waste!) and then using the energy generated in that way to power civilization.

However, I'm afraid that if the technology existed to control a black hole in the way you mentioned, it would be used for military purposes...

Cole Miller

16. On what evidence does it suggest that black holes can be warp holes?

I'm not aware of anything called a "warp hole" outside of Star Trek :).  Perhaps you were thinking of wormholes?  If so, let me describe what those are and what people think of them.

Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity. Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working).  In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless techno jargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity.  Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely.  The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative.  This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity).  Therefore, if you have a wormhole you can time-travel.  This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox.  In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person.  I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother.  Since he's dead, he can't produce my father, who can't produce me, which means that I could not have built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture!  Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

Cole Miller

17. When matter is sucked in, it must be destroyed because black holes are only a point but the Conservation of Mass theory says that matter can't be destroyed, it must be converted into another form of matter.  What does the matter in a black hole convert to? Black holes also have x-ray jets that are shot out from the middle of it. Is it possible that this is what becomes of the matter?  Thanks for your knowledge.

At the simplest level, one can say that what is really conserved is mass-energy, rather than just matter.  That is, for example, that when a nuclear reaction occurs the energy that is released means that the mass left behind is a little less to compensate. That's what Einstein's equation E=mc^2 means, where E is energy, m is mass, and c^2 is the speed of light squared.  Therefore, you can look at this as the matter being converted into gravitational energy.  Another way of putting this is that like the Cheshire Cat in Alice in Wonderland (which disappeared but left its smile behind), matter falling into a black hole disappears but leaves its gravity behind.

However, at a deeper level we really don't know what happens.  The problem is that according to our current theories, the matter would go to the center of the black hole, where at the singularity it would reach infinite density.  Now, infinities in one's theory usually mean that something has gone wrong.  Maybe it turns into some kind of super dense quantum nugget, but we'll need extensions to our current theories to have a better idea.

The jets of matter you mentioned are another interesting phenomenon. The matter in the jets never gets inside the black hole; in fact, in practice, nothing that goes inside a black hole ever comes out again.  However, the rapid motions near (but outside!) a black hole produce huge amounts of energy, so some of the matter can be flung away in a jet.  Along with two of my colleagues (Chris Reynolds and Anita Krishnamurthi), I have an article in the April Sky and Telescope magazine (page 42) talking about these jets, if you're interested.

Cole Miller

18. My question is what do scientists think is inside of a black hole or on the other side? Or is there anything that could have possible survived that extreme conditions of the gravitational pull throughout the black hole?

I'm a big science fiction fan, and I'd love it if it were possible to go through a black hole to travel across the universe.  But in reality it almost certainly isn't possible.  Sorry about that :).

Specifically, as we understand it now, if you fall into a black hole you are guaranteed to hit the center, which is called the singularity. At the singularity you would be crushed into a ball of almost infinite density, which would destroy anything, even atoms, protons, or quarks. Some people have considered the idea of a "wormhole", where you would never get to that density, but to make a long story short it appears that such things probably can't exist in reality.  Oh, well!

Cole

19. Is there a radius around a black hole where light can flow around?

If I understand your question, you're asking whether there exists a radius at which light can have a circular orbit about a black hole.  The answer is yes, but the circling photons are subject to a few constraints. Firstly, the photons must keep to their trajectories VERY precisely, since the photon sphere is really just a very precise, spherically symmetric radius at which objects moving at the speed of light (i.e. only photons) can orbit a black hole.  If a photon interacts with another photon and changes its trajectory it can easily get flung out of the system or plunge into the black hole.  So it is important to note that photons are not stable in this orbit for very long!

Secondly, the nature of the photon sphere depends on the spin of the black hole.  The easiest case is for a non-rotating hole: here the radius of the photon sphere is 1.5 times the radius of the event horizon (2GM/c^2). Things get a bit more complex if the black hole is spinning: in this case, there are actually two photon spheres-- one that rotates with the hole (closer in), and one that rotates in the opposite direction as the hole (further out).  The faster the hole spins, the more distance exists between the two photon spheres.  So photons with the right trajectory that orbit in the same direction as the black hole are temporarily stable in the inner photon sphere, and photons with the right trajectory that orbit in the opposite direction are temporarily stable in the outer photon sphere.  It also helps if the photons in question come in on equatorial trajectories with respect to the black hole... things get a bit more complex if they do not

I've included a few links that should help you visualize all this.  Check out the movies in particular!

http://starchild.gsfc.nasa.gov/docs/StarChild/universe_level2/black_holes.html

http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

Laura Brenneman

20. Since there are usually black holes in the center of galaxies, do we have one in the center of the Milky Way, and if so, will we eventually be sucked in? Exactly, how does that work?

Let me assure you that you have nothing to worry about from black holes!  The key point is that far from a black hole, its gravity acts just like the gravity from anything else of the same mass as the black hole.  For example, let's say that somehow the Sun were magically turned into a black hole, but the black hole had the same mass as the Sun.  The Earth's orbit wouldn't change a bit, and certainly we wouldn't get sucked into the black hole (we'd get cold, though!).  In the same way, the black hole at the center of our Milky Way is far enough from us that we won't ever feel its effects!

Cole Miller

21. Within my knowledge of black holes, I am aware that visible light cannot escape them. This leaves me wondering how x-rays can me emitted from black holes.

You are absolutely right that light cannot escape from black holes. That applies to X-rays or anything else as well as to visible light. The answer, therefore, is that X-rays are *not* emitted from inside black holes.  Instead, the gravity of a black hole forces gas around it (and outside it!) to move very rapidly.  This rapid motion causes friction within the gas, that friction generates heat, and that heat generates X-rays and other wavelengths.  Therefore, the X-rays you see come from outside black holes, which is why you can see them.

Cole Miller

22.  I have been thinking about our universe and how it all works, and I've developed a theory.  I'm sure something like this has already been explored if it is at all likely or possible, but I wanted to know your thoughts and explanations if possible.

I envision our galaxy (a spiral galaxy) complete with both a black and white hole.  As the black hole sucks in matter, the white hole shoots it back out at the same rate.  We are not aware of the white hole yet because we are not able to see or detect it.  If this process is occurring, I envision the galaxy's life cycle infinite--- as one arm disappears, another arms is forming and the galaxy is constantly replenishing itself.

Next, I theorize that if we could move far enough out into space where we could look at our universe in its entirety, I think we may find that all the galaxies it encompasses are also formed into arms, creating one spiral universe.  This spiral universe is also complete with a black hole and white hole.

Is this theory at all feasible?  Would this explain the acceleration of the universe, in that, the matter we are detecting as it speeds up, is just nearing a black hole?  Maybe the chaos that occurs inside a black hole does something to create new elements that are spit from the white hole making it possible for new star formation, planets, moons, etc.

Wow!  Your question encompasses a lot of scales of our Universe -- from galaxies to the structure of the entire Universe.  It's so cool that you're thinking on such grand scales.

Let me start by saying something about white holes.  As you probably know, white holes represent a solution to Einstein's equations, and are considered to be the opposite of black holes.  However, one very important point in physics is that just because a solution to an equation exists doesn't mean that the solution exists in the physical world.  White holes seem to be one of those things -- most astronomers and physicists agree that they can't exist in the real Universe.

Now on to spiral structure in galaxies.  Current theory says that the spiral structure is caused by a density wave.  A density wave is a wave of material that gets bunched up and then spread out -- you can make your own density wave with a slinky.  Stretch out the slinky between you and another person -- don't swing the slinky from side to side, but instead pinch several of the rings together and then let go.  The pinched rings will "travel" down the slinky and back again.  Here the slinky itself does move, but a few of the rings bunch up and then separate to make it appear as though the rings travel up and down the slinky.

The spiral arms in galaxies seem to do the same thing.  The stars in the galaxy don't orbit at the same speed as the spiral arms, which means that as a star orbits the center of our galaxy, it alternately becomes part of a spiral arm, and then leaves the spiral arm for a time only to join up with another spiral arm later.

The last part of your question was about the possibility of spiral structure to the Universe.  Astronomers have mapped a portion of the sky out to about billion light years, and found that the galaxies are arranged in large clusters, filaments and empty voids.  You can see the map at the following URL:

http://antwrp.gsfc.nasa.gov/apod/ap000614.html

Thanks for such a far-reaching question, and I hope you continue to think about the Universe.

Barbara Mattson

23. Black Holes are very interesting to me. I have a couple of questions about them. If two black holes with the same amount of gravitational pull got close to each other, what would happen? would one fall into the other, or would they combine to create a massive black hole with twice the gravity? Also what really happens to the light inside? where do things go? will they ever come back? Could a black hole's gravity be reversed so that it spit every thing back out into the universe?

Black holes seem to capture everyone's imaginations -- it's one of the reasons that I went into astronomy in the first place.

One thing to remember about black holes is that "far away" from the black hole, it behaves like anything else of the same mass -- it's only when you get close that things get weird.  So, it would be possible for two black hole to orbit one another, just the same as two stars might do. Having said that, it is entirely possible for two black holes to collide. If two black holes with about the same mass got close enough together, they would collide and merge to form a black hole with their combined masses.

You may already know this, but there are two parts of a black hole that astronomers often refer to -- the singularity and the event horizon.  The event horizon is the "point of no return" from a black hole -- anything inside the event horizon will stay inside the black hole.  Inside the event horizon, at the center of the black hole, is a singularity -- a near-zero volume where all the mass is mashed together.

So, when light crosses the event horizon of a black hole, it will get pulled into that singularity and will be trapped there.  All matter and light that are pulled into the black hole end up in that singularity, adding to the mass of the black hole.  And, as far as we know, they're stuck there forever.

Barbara Mattson

24. How is it that a black hole can capture light when light is the fastest speed known?

Even though the speed of light is faster than any thing else in the universe, it still cannot escape the gravitational pull of the black hole! Why is that?

Einstein tells us that really heavy objects, such as a black hole, warps the space around it, causing it to "sink down," in a manner of speaking. Think of it like this: space is a nice, flat blanket that you're holding up in the air on its four ends. When you put heavy stuff on the blanket, such as a brick, the blanket bends downwards, and that's what heavy stuff does to space. When light, which is made out of little packets of energy called photons, passes near these sinkholes, they try to go around it in a curve and sometimes get "sucked in" to where the mass is. Because these sinkholes made by blackholes are really, really steep, even light, moving really, really fast, cannot escape it!

Lisa Wei

25. I was hoping maybe you might be able to elaborate on the relation black holes have with  white holes, anti-matter, wormholes, and the path that (even hypothetically) the forms that matter after entering a black hole might become or the paths it might take. I realize that the information available to you is somewhat limited considering black holes ... eccentric existence and warping of laws of physics due to its properties, but any further information you could supply would be most appreciated.

You presume correctly! Based on our current understanding:

A. White holes and wormholes.

Wormholes, or "shortcuts" through spacetime, were first considered because they arise as solutions to some of the equations of Einstein's general theory of relativity.  White holes would be the "fountain" that would come out of the other side of the wormhole.  Unfortunately, the first such solutions were such that if you tried to travel through one of them, it would immediately collapse and crush you (that is, having you in it would prevent the wormhole solution from working). In the mid 1980s, interest in wormholes that could be traversed was rekindled because Carl Sagan wrote a novel ("Contact"; perhaps you've seen the movie?) in which he needed a wormhole that could be traveled safely. Given that he was a famous scientist, he didn't want to just babble meaningless technojargon, so he called up his friend Kip Thorne, a professor at Caltech who is an expert in general relativity.  Thorne and his colleagues have since worked out some solutions of the equations which *may* allow for wormholes that can be traveled through safely.  The only catch is that they have to be made of so-called "exotic" matter, which has a total energy which is negative.  This isn't quite as impossible as it sounds, but it still isn't all that likely!

By the way, one tricky thing about wormholes, and about faster-than-light travel in general, is that any time you can appear to go faster than light, you can travel backwards in time (this is one consequence of Einstein's theory of relativity).  Therefore, if you have a wormhole you can time-travel.  This can lead to lots of nasty paradoxes: one of the most common is called the grandfather paradox.  In this paradox, we suppose that I am a brilliant scientist but a real scumbag of a person.  I invent a time machine, and my use for it is to go back in time and kill my grandfather before he met my grandmother.  Since he's dead, he can't produce my father, who can't produce me, which means that I could not have  built the time machine and killed my grandfather, which means that he was able to meet my grandmother and produce my father, then me, so I was able to make the time machine and kill him, so... you get the picture!  Paradoxes like this convince some people that you can't travel in time, so you can't travel faster than the speed of light, but it isn't as clear-cut as it seems.

B. Antimatter.

If antimatter falls into a black hole, the same thing happens to it that happens to normal matter: it gets stretched and crushed, and ultimately just adds to the mass.  Black holes are equal-opportunity destroyers :).

C. The paths and forms of matter in black holes.

Once matter enters inside the "event horizon" it is compelled to fall all the way to the central "singularity", which is where current theory predicts that forces and densities will be infinite.  That would mean that this is the end of the line for all matter.

However, it must be said that the prediction of infinities suggests to most people that the theory is incomplete.  If so, maybe there are other things that would happen to the matter.  Mind you, it will still be utterly destroyed in its current form.  Some people think, though, that it will end up in an unimaginably dense (but still finite) type of quark nugget.  I even went to a talk yesterday in which the speaker, a professor specializing in quantum mechanics and strong gravity, said   he thought it was possible that the matter that goes into a black hole   could help create another universe!  Pretty wild, and it will be tough to figure out how to test such ideas, but that's the kind of fun you can have when thinking about black holes :).

Cole Miller

26. I was wondering what would happen if two black holes collided. Would they just become one? Would any x-rays/gamma rays be released?

If two black holes collided they would indeed become one black hole. No X-rays, gamma rays, or other electromagnetic radiation would be released, but there would be a lot of gravitational waves.  To understand these, consider the usual concept of Einstein's gravity: spacetime as a flat rubber sheet, which warps when a massive object is present (e.g., the dent produced by a bowling ball on a rubber sheet).  If a couple of massive objects are near each other and moving, the dents move and produce ripples.  These spacetime ripples are gravitational waves, and they might be detected in the next five to ten years with various experiments.

Cole Miller

27. If the escape velocity of a black hole exceeds the speed of light and at this time the speed of light is the fastest speed known, would the acceleration of an object due to a black holes gravity exceed the speed of light? Our thoughts were that friction or a terminal velocity factor would affect the final speed.

An interesting question. To rephrase, I think you are asking: if an object were to fall into a black hole, would it go faster than the speed of light when it got inside the black hole (that is, inside the event horizon of the black hole)?

The answer, like the answer to many questions involving relativity, can be obtained by being very precise about how you do the measurement. In this case, we need to explore exactly what we mean by a measurement of speed, so bear with me.

Suppose you do the following experiment, which you can actually perform if you like. Take a basketball and hold it outstretched, at about the height of your head. Now let it go. What happens? Well, obviously it falls! Therefore, it has acquired a decent speed when it hits the ground.

Now, get up on top of your desk, and again hold the basketball out. Jump off the table, and at the same time let the basketball go. What do you see? Until you hit the ground, the basketball doesn't appear to move at all relative to you, although it will to your students if they watch from their seats.

The point, of course, is that the speed you measure depends on how you are moving and not just how the basketball is moving.

So far we haven't introduced anything specific to black holes. We can, however, do the same experiment (although I don't recommend that you do this one!). Drop a basketball from rest towards a black hole; let's say you release the basketball from a long way away from the hole. How fast is it going at any point along its trajectory? Clearly, it depends on how you are moving. If you are stationary with respect to the black hole, the ball moves with one speed, but if you, too, are falling then the ball will move with a lower speed.

With this in mind, we can be more precise. Let's drop the ball from rest a long way from the black hole. Let us say that we measure the speed as the ball comes by us, some place outside the hole, and that we aren't moving with respect to the hole. What we can say is that the closer we are to the hole (always stationary!), the faster the ball appears to move. If we are just barely outside the horizon, the ball would appear to speed by at just barely under the speed of light. Extrapolation of this would seem to suggest that if we did the same measurement *inside* the hole, the ball would appear to move faster than the speed of light.

But it isn't so! To see this, consider how, exactly, you are managing to remain stationary. When you stand on Earth, you have gravity pulling on you, but you remain where you are because the floor pushes back in the same amount. The acceleration you feel is one Earth gravity. In the same way, remaining stationary with respect to a black hole requires some force to balance the hole's gravity.

Suppose that this is provided by a skyhook. The closer you get to a black hole, the more force has to be provided by the skyhook. Just outside the horizon the force is tremendous, and at the horizon it would theoretically be infinite. No skyhook, no rocket could keep you stationary at that point, and inside the horizon you *must* fall inwards.

We argued above that if you fell towards a black hole, you would measure a speed for the ball that is less than if you were stationary. It's the same thing inside. Since you are falling, you would measure a smaller speed, and it would always turn out to be less than the speed of light, if only by a little bit. In fact, it is the case that *no* observer could *ever* measure a speed greater than the speed of light for anything, inside a black hole or out. Mind you, I'd rather not do the experiment inside a black hole to find out if the theory is correct!

Cole Miller

28. As of late the question of, "Where is the edge?" has been a constant both in and out of class. It has been explained to me in a, Fabric of the Universe, idea in which our universe is like a sheet. Due to recent knowledge obtained from the studies of black holes it has become apparent to me that the fabric being warped around and into the hole has to go somewhere. While the question of "where" isn't my own, I am asking where does the fabric come from. Much like tectonic plates on earth, it would make sense if the universe "regurgitated" the fabric out somewhere else to form more material in the universe and keep the fabric going.

Will we run out of fabric eventually or is the fabric being brought out from somewhere else?

Thanks for your appreciation! We enjoy the chance to communicate with you.

I like your question, although I can't give a fully satisfactory answer. The question of where spacetime fabric originates is one, similar to "what happened before the Big Bang?", or "what is spacetime expanding into?" to which we do not have a full answer. It seems possible, for example, that time itself came into existence at the Big Bang, and also seems that spacetime simply gets bigger as the universe expands. Given that the universe has expanded in volume by an overwhelming factor, I suspect that we will not ever run out of fabric. Nonetheless, I like your analogy of tectonic plates, and wouldn't rule out the possibility that at some future point a theory could be developed along those lines. Maybe you'll be the one to do it!

Sincerely,

Cole Miller

29. While I was learning about black holes in my astronomy class, I came up with a couple questions. If a black hole keeps sucking in matter shouldn't the singularity become more massive? If a black hole is sucking in light shouldn't it be sucking in protons or electrons, thus becoming more massive?

Thanks for your questions. Black holes are a really interesting topic of research for astronomers, largely because we have more questions than answers at this point!

You're quite right: if a black hole is actively "sucking in" matter (we call this accretion), then that matter contributes its overall mass/energy to the black hole. One of Einstein's more famous equations links mass and energy as E=mc^2, where E is energy, m is mass, and c is the speed of light (in this case it's raised to the second power, or squared). This means that mass and energy are essentially two aspects of the same thing, so that as a black hole accretes matter it also accretes most of the energy associated with this matter. In reality some of that energy may escape as light, or perhaps as a relativistic outflow in the form of a jet, which you may also learn about in class. But for every bit of matter that actually makes it past the event horizon into the black hole, the hole gains that much mass/energy. Because of the equivalence between mass and energy, the same thing happens when a black hole is accreting radiation. We tend to think of matter as having mass, whereas radiation has energy, but both are the same to a black hole. So whether matter or radiation is being accreted, the black hole will experience a net gain in mass/energy, which can be thought of as the singularity itself gaining that much mass/energy.

Interestingly, the way this is outwardly manifested is by an increase in the effective size of the black hole, otherwise known as the radius of the event horizon. So basically if a black hole accretes, it generally gets bigger as well as more massive. I say "generally" because this is not necessarily the case if the hole were to accrete material moving in the opposite direction than the hole is rotating, but that can get rather complicated, so for now let's assume we're accreting material in the "prograde" sense rather than the "retrograde." So long story short, yes, the black hole does indeed gain mass/energy as it accretes material, and as a result it grows larger and effectively becomes a bigger gravitational sink in the fabric of spacetime.

Hope this helps,

Laura Brenneman

30. We have been talking about black holes for quite some time. I know that light cannot escape from a black hole, but how does gravity escape it? Isn't the speed of light the fasted speed we know of? How does a black hole suck in the light? Light doesn't have any mass, it is energy, or does it have mass?

You are right thinking that neither light nor other forms of matter and energy could escape a black hole. So how does it work for gravity and gravitational waves? First, lets understand what gravity is. The gravity is the effect that a massive body has on its surroundings, in such way that it attracts other objects. Gravity is communicated through the universe via gravitational waves. Similarly to the photons, gravitational waves are all energy and they travel at the speed of light. The reason why gravitational waves are not trapped by a black hole is because they are emitted on the *outside* and not on the inside of the black hole. If we could see gravitational waves, they would look something like the ripples on the surface of the water after it was hit by a stone. Similarly, in this case the ripples are caused by the stone, they do not emanate from the stone itself but they form in the water and propagate away.

Now, if gravity is the effect that one massive body has on another massive body, how can gravity of a black hole trap a photon that has no mass, but it's all energy? Escaping a gravitational pull of a black hole has all to do with the velocity and slightly less with the mass. An object gets irreversibly trapped by the black hole when its velocity is not high enough to escape. This is true even if the "object" in question has no mass, like a photon. This is because at some small distance from the black hole, called the "event horizon", not even the speed of light is enough velocity to escape, and photons too get trapped. Massive bodies are even easier to capture because we know that they MUST move slower than the photons. For example, if you would hang out in the vicinity of a black hole, and outside of its event horizon, the faster you are, the better your chances to escape. Also, the less massive you are, the faster you are allowed to move, which again increases your chances to escape. Just make sure you don't run out of gas!

Thanks for your interest in black holes and all the best!

Tamara Bogdanovic.

31. My question is concerning light being sucked into black holes. As it says in Chapter 3, the speed of light is the fastest speed possible. What I don't understand is if nothing goes at the speed of light except light itself, how can a black hole also pull light into itself? Do black holes have some kind of force beyond what we understand?

The Earth pulls objects towards it which is what makes them fall. The reason objects fall toward Earth is because there is gravitational attraction. But the earth also pulls on light because gravitational attraction acts not only on energy in the form of massive objects (i.e. E = mc^2) but on energy in general. A black hole in this respect isn't much different. It pulls on energy in the form of massive objects as well as energy in pure form such as light. The main difference is that while the earth can pull light only a bit so we don't notice it by eye, a black hole's gravitational pull is much greater so light gets pulled in much more. So you see that a black hole exerts the same forces that the earth exerts but only greater. Let me know if that helps.

David Garofalo

32. We have been learning about black holes. I understand that Black holes pull everything around them into it and we don't know what happens after that. Is it possible for all the matter in the black hole to form a solar system or planet? Black holes pull in everything, so wouldn't the black hole have a charge? because it pulls in electrons, protons, and neutron. if so what difference would that make in the pull of the black hole?

This is a very interesting version of the general question, "What happens inside a Black Hole?" Kip Thorne, one of the greatest physicists of our time, addresses it at length in Chapter 13 of his fantastic book "Black Holes and Time Warps" (which I highly recommend by the way!). As he explains, remarkably, you could actually fall into a black hole without even feeling it until you hit the singularity at the center. However, for a black hole the mass of our sun, the time it takes from crossing the horizon until you hit the center is only a matter of MICROSECONDS, so you wouldn't have much time to enjoy the bizarre scenery. This time is proportional to the size of the black hole, so for a giant quasar ten billion times the mass of the sun, you would actually get 20 hours of freefall before getting crushed by the singularity.

Now, to form a planet or solar system, we need even more time between the horizon and singularity. That means a bigger black hole. Our own solar system probably formed over a period of about one billion years, or 30,000,000,000,000,000 seconds. To free-fall for that long, the black hole would need to be 4,500,000,000,000,000,000,000 times the size of our sun, or roughly the size of the known universe!! So who knows, maybe we actually ARE inside a giant black hole right now, getting sucked inexorably towards our final destruction at the center.

Regarding your second question, YES, black holes can have a charge if they eat up too many protons and not enough electrons (or vice versa). But in practice this is very unusual, since these charges tend to be so evenly balanced in the universe. And then even if the black hole somehow picked up a charge, it would soon be neutralized by producing a strong electric field in the surrounding space and sucking up any nearby charges to compensate. These charged black holes are called "Reissner-Nordstrom black holes" or "Kerr-Newman black holes" if they also happen to be spinning.

best regards,

Jeremy Schnittman

33. Over the last few chapters we have been learning about black holes. I was curious if black holes grow larger in size when consuming matter, if so? i would also like to know if a black hole can be postively or negatively charged if it were to consume a large quantity of electrons or protons. For the final question, Does the matter that is being consumed by the black hole create a type of energy, because of the law of conservation of energy i i would think there is, or does the black hole consume everything even the energy that is being produced?

Black holes seem to capture everyone's imaginations -- in fact, it's one of the reasons that I wanted to become an astronomer in the first place.

First I'll answer your question about the "size" of a black hole. Your books probably talked about this already, but there are two parts of a black hole that astronomers often refer to -- the singularity and the event horizon. The event horizon is the "point of no return" from a black hole -- anything inside the event horizon will stay inside the black hole. Inside the event horizon, at the center of the black hole, is a singularity -- a near-zero volume where all the mass is mashed together.

Often when we talk about the "size" of a black hole, we're really referring to the event horizon. The quantity that astronomers use to talk about the size of the event horizon is the Schwarzschild radius (R_s), which is calculated with the following equation:

R_s = 2*M*G/c^2

where M is the mass of the black hole, G is the gravitational constant, and c is the speed of light.

When a black hole consumes matter, then, the mass will grow. When the mass grows, the Schwarzschild radius will also grow. So, a black hole will get larger when it consumes matter.

Your second question is about a "charged black hole". You're right that a black hole can consume charged particles and, indeed, would become charged. The problem with a charged black hole, though, is that it is unlikely to stay charged for very long. A black hole with any significant charge would attracts oppositely charged particles to it, and would quickly lose it's charge.

Finally, you ask about the energy associated with matter falling into a black hole. As the material falls into the black hole, it heats up to very high temperatures and about 10% of it's energy is radiated away. (The energy I mean is that given by Einstein's famous equation, E=mc^2, which gives the energy associated with a given mass, m). The rest of the material is just absorbed by the black hole, and as I mentioned above, adds to the mass of the black hole. Sometimes, the material doesn't have a chance to radiate away any energy, and essentially all of the mass goes right into the black hole.

Hope this helps with some of your question. Stay curious!

-Barbara Mattson