Can Black Holes Die? - Exploring Their Nature

[Kenneth Korsmo]

Hi i have a few questions about black holes.

1. Are black holes just stars that are so big, and have so much mass that no light can escape them?
2. Do black grow in size, or are they just an infinite small point of space?¨
3. Can black holes die?
4. are they actual holes, or are they spherical?
5. can you travel faster than the speed of light? I know that light bends to gravity, so in theory if you could get light to travel around a massive object, like a star or a planet. could you go straight through the object, like a tunnel and catch up with light.
6. does gravity lessen as the universe gets bigger?

 

[ShayanJ]

1. Black holes are singularities in space-time. They're a consequence of squeezing matter too much.
2. I think the size of black holes is defined by their event horizon and I think that can grow through combination of black holes.
3. Theoretically they can. Hawking proposed black holes radiate their energy through Hawking radiation and that can consequently cause the death of the black hole but that's a very very very slow process.
4. In the sense of a sphere in Euclidean space, no they're not. But their event horizon can be considered a sphere in 4 dimensional space-time with a singularity in the center.
5. Yes. locally you can't go faster than light but globally I think its not forbidden. You don't have to go through a planet or star. It can be done in curved space-time with no matter present in it.
6. I don't know!

 

[mfb]

"Size of a black hole" is a tricky concept. I'll use their Schwarzschild radius (the surface of no return) as "shape"/"size" and so on in this post. The singularity at the center does not have a size or shape - but the singularity is a point where our current theories don't work, so we don't know what exactly happens there.

1. No they are not stars. They can be created when stars "die" and collapse. They are much smaller than the stars that collapsed. If the sun would become a black hole (it won't), it would be smaller than the size of a small town.

2. Their Schwarzschild radius grows if they gain mass, their singularity at the center not.

3. See post#2.

4. If they are not rotating, there are spherical, otherwise they are ellipsoids.

5. As above, you cannot go faster than the speed of light, but you can take a shortcut. Just arriving at a destination faster than light is even easier, you can send it through a medium or various mirror setups.

6. What do you mean with "gravity lessens"?

 

[Kenneth Korsmo]

6. What do you mean with "gravity lessens"?[/QUOTE]

As I understand, gravity is not that everything is pulled towards planets and stars, but gravity pushes everything together. would not the pressure/gravity be less if the universe expanded.

 

[mfb]

Gravity slows the expansion of space (but something stronger, called dark energy, is accelerating expansion now), so a less dense universe will have a lower influence of gravity on its expansion.

 

[Chalnoth]

I'd just add that the singularity almost certainly does not exist. It's an artifact in the math that occurs due to General Relativity, an artifact that makes no sense whatsoever.

One way to understand black holes is that we can talk, with pretty high confidence, about how black holes behave right up to their horizons (e.g. spinning black holes cause all sorts of interesting effects on their surroundings). But discussing anything inside the horizon is speculative at the moment.

I think a good way to understand a black hole is this:
Matter can only exert so much pressure. Once the pressure climbs above a certain limit, nothing can stop the collapse. In fact, for very high pressures, even if you postulated some hypothetical force that could raise the allowable outward pressure, that pressure itself acts as a source of gravity, so that it just becomes impossible for anything to withstand the collapse, and it necessary collapses in on itself.

The result of such a collapse is a region of space-time that is so tightly-curved that not even light can escape. This region is surrounded by a boundary (the event horizon) that demarcates the "point of no return". The behavior of this boundary encodes all of the behavior of the black hole. What happens on the other side of the boundary is largely speculative (we can calculate things with General Relativity, but we can't be very confident that those calculations are accurate), but it isn't necessary to understand how the black hole acts on its surroundings.

In fact, the "radius" of a black hole is not actually a meaningful concept. People talk about it, but the actually-meaningful parameter that describes the black hole is its surface area. The radius is typically taken as the parameter which gives surface area following the familiar equation $A = 4\pi r^2$. The "r" isn't a radius in any real sense (remember: we can't actually describe the interior of the black hole, and the radius is by definition inside the black hole). But it's useful to try to wrap our heads around the size of a black hole, because we usually don't think of objects' sizes in terms of surface area.

 

[Lyle]

Hi i have a few questions about black holes.

1. Are black holes just stars that are so big, and have so much mass that no light can escape them?
2. Do black grow in size, or are they just an infinite small point of space?¨
3. Can black holes die?
4. are they actual holes, or are they spherical?
5. can you travel faster than the speed of light? I know that light bends to gravity, so in theory if you could get light to travel around a massive object, like a star or a planet. could you go straight through the object, like a tunnel and catch up with light.
6. does gravity lessen as the universe gets bigger?

Hi, I'll do my best to answer your questions.
1. Black holes were massive stars with a sufficient enough gravity that when they die (go supernova) the core crushes in on itself so that even the atoms are crushed. But no, a black hole is not a star. In fact, a when a star becomes a black hole, the matter ceases to exist. All that's left is a super dense point called a singularity. And of course the gravity is left behind too. That's what causes the event horizon, or the point where the gravitational influence is so great that not even light can escape.
2. No and Yes. The black hole can take on more matter/energy but the singularity itself stays infinitely small. However, the more matter/energy the black hole takes on, the larger it's gravitational influence will be. So the the event horizon will reach further from the center.
3. No, black holes can not die. In fact something cool about black holes is that time actually stops inside of them.
4. The truth is, we don't know. We call it a hole because everything that goes in never comes out. You could go in and be crushed or sent through a white hole into a whole other universe. But that's what's so fun about black holes.
5. Faster than light travel is theoretically possible. With a sufficient amount of the right types of energy you can contract space time in front of you and expand space time behind you. Inside the two waves the fabric of space time is flat. And since space time itself can expand at any speed, you could ride those waves many times the speed of light and still technically be standing still. You can look up "100 year starship project" on YouTube and it will get more in depth.
6. No gravity does not lesson because gravity comes from the mass of an object. The more mass, the more gravity. And as of yet, the universe hasn't expand enough to effect the state of matter. At least not since it came about.
I hope this was helpful. If you have any more questions. I would be happy to help!

 

[PeterDonis]

Lyle, you need to take your own response to the OP outside the quote block in your post. You should still be able to edit it for (IIRC) 4 hours after you posted.

Also, your post contains some errors and one statement that I don't understand:

Re your #3, time does not stop inside black holes.

Re your #4, when you talk about going through into another universe, you are talking about a wormhole, not a white hole. Wormholes are a different family of solutions to the equations of General Relativity than black holes/white holes. These solutions require exotic matter, which is not known to exist and which many physicists think cannot exist.

(In rotating black holes, if you extend the idealized spacetime geometry as far as possible, you do get the possibility of traveling through the hole into another universe. But that idealized solution has properties that make it physically unreasonable.)

Re your #5, I assume you are talking about the Alcubierre "warp drive". This is a valid solution to the equations of GR, but like the wormhole solution, it requires exotic matter. Also, even though the "warp drive" appears to allow FTL travel because of the way it warps spacetime, nothing ever actually travels outside the local light cones, which is what "faster than light travel" actually means in relativity. So, for example, if your warp drive ship emitted a light beam ahead of it when it set off on its journey, the light beam would arrive at the ship's destination before the ship itself did.

Re your #6, I don't understand what you mean by "as of yet, the universe hasn't expand enough to effect the state of matter".

 

[Dozent100]

I think the question displays a basic lack of understanding of Black Holes and Quantum theory. Rather than a point by point refutation, I will offer the following. The interior of a Black Hole is a mass so thoroughly compacted that the internal space between the atoms are compressed extremely small dimensions in atomic Terms. It is believed that a Neutron Star is the result of the compression of the atoms to the point that the electrons are forced into the protons and only Neutrons are left. What actually has happened inside a Black hole is only postulated, but the current theory is that a black hole is similar to a Neutron Star, but the compression to neutrons hasn't happened, because the collapse is much slower. A black hole with the mass of our Sun, which is actually too small to become one, would supposedly be the size of a small pea.
We also know that anything that passes the event horizon only comes out as radiation. That includes stars and solar systems. Most Galaxies are believed to have massive Black holes at their center. So they they grow. they can only die if there is no more mass for them to adsorb.
As far as time inside a Black hole, we just don't know. The effects of such a concentrated mass on dime dilation can be calculated, but truthfully but are unknown.
There are many discussions on the Speed of light, so I'll leave you to do your own research.
Gravity is a essential force. It doesn't grow or lessen, it is determined by the attraction of the bodies in the Universe and between Universes, assuming there are more than one. While the Universe is expanding, the effect of Dark Matter and Dark Energy remains constant through out, we think.

 

[Gaz1982]

I'd just add that the singularity almost certainly does not exist. It's an artifact in the math that occurs due to General Relativity, an artifact that makes no sense whatsoever.

One way to understand black holes is that we can talk, with pretty high confidence, about how black holes behave right up to their horizons (e.g. spinning black holes cause all sorts of interesting effects on their surroundings). But discussing anything inside the horizon is speculative at the moment.

I think a good way to understand a black hole is this:
Matter can only exert so much pressure. Once the pressure climbs above a certain limit, nothing can stop the collapse. In fact, for very high pressures, even if you postulated some hypothetical force that could raise the allowable outward pressure, that pressure itself acts as a source of gravity, so that it just becomes impossible for anything to withstand the collapse, and it necessary collapses in on itself.

The result of such a collapse is a region of space-time that is so tightly-curved that not even light can escape. This region is surrounded by a boundary (the event horizon) that demarcates the "point of no return". The behavior of this boundary encodes all of the behavior of the black hole. What happens on the other side of the boundary is largely speculative (we can calculate things with General Relativity, but we can't be very confident that those calculations are accurate), but it isn't necessary to understand how the black hole acts on its surroundings.

In fact, the "radius" of a black hole is not actually a meaningful concept. People talk about it, but the actually-meaningful parameter that describes the black hole is its surface area. The radius is typically taken as the parameter which gives surface area following the familiar equation $A = 4\pi r^2$. The "r" isn't a radius in any real sense (remember: we can't actually describe the interior of the black hole, and the radius is by definition inside the black hole). But it's useful to try to wrap our heads around the size of a black hole, because we usually don't think of objects' sizes in terms of surface area.

The singularity is a concept that bothers me. Am I right in assuming that Hawkins Radiation can explain why a singularity is not necessary, namely that the Black whole emits radiation to offset the mass it absorbs, and that these should be equal?

 

[Vagn]

The singularity is a concept that bothers me. Am I right in assuming that Hawkins Radiation can explain why a singularity is not necessary, namely that the Black whole emits radiation to offset the mass it absorbs, and that these should be equal?

It's generally assumed that the singularity is not a physical beast, more that general relativity is not complete and that any effective quantum theory of gravity will not have a singularity in a black hole.

 

[Gaz1982]

It's generally assumed that the singularity is not a physical beast, more that general relativity is not complete and that any effective quantum theory of gravity will not have a singularity in a black hole.

Sure, I get that. I know the singularity is a result of equations rather than a physical object, but am I right in presuming that the radiation output of the black hole by definition = to the mass absorbed?

 

[Vagn]

Sure, I get that. I know the singularity is a result of equations rather than a physical object, but am I right in presuming that the radiation output of the black hole by definition = to the mass absorbed?

No, otherwise you wouldn't get supermassive black holes forming in the galactic centre, these are generally agreed to reach such a large mass by accretion.

 

[Gaz1982]

No, otherwise you wouldn't get supermassive black holes forming in the galactic centre, these are generally agreed to reach such a large mass by accretion.

Sure, but that's not inconsistent with my question.

Such black holes may well absorb more matter than they radiate outwards, ensuring their growth, or at least stability. But the mass consumed most go somewhere in some form or another

 

[Vagn]

Sure, but that's not inconsistent with my question.

Such black holes may well absorb more matter than they radiate outwards, ensuring their growth, or at least stability. But the mass consumed most go somewhere in some form or another

The absorbed mass does go somewhere, it goes towards increasing the mass of the black hole.

 

[Gaz1982]

The absorbed mass does go somewhere, it goes towards increasing the mass of the black hole.

Yes sure. In the (relatively) short term. But black holes die right?

And my presumption is that they die from emitting their mass as radiation?

 

[Vagn]

Yes sure. In the (relatively) short term. But black holes die right?

And my presumption is that they die from emitting their mass as radiation?

The lifetime of a stellar mass black hole is much greater than the age of the universe, it must also have a temperature greater than the cosmic microwave background otherwise it will reach thermal equilibrium with respect to the CMB, this is reportedly equivalent to a black hole with a maximum mass similar to the moon.
Try having a read through of the wikipedia page on Hawking evaporation, it might help answer some of your questions.
http://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation

 

[Gaz1982]

The lifetime of a stellar mass black hole is much greater than the age of the universe, it must also have a temperature greater than the cosmic microwave background otherwise it will reach thermal equilibrium with respect to the CMB, this is reportedly equivalent to a black hole with a maximum mass similar to the moon.
Try having a read through of the wikipedia page on Hawking evaporation, it might help answer some of your questions.
http://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation

Thanks for that. But it rather speculates that information may indeed be expunged as radiation.

 

[PeterDonis]

The interior of a Black Hole is a mass so thoroughly compacted that the internal space between the atoms are compressed extremely small dimensions in atomic Terms.

No, this is not correct. The interior of a black hole is vacuum (except for a very brief period right after it forms, when the object that collapsed to form it is finishing its collapse). It is not a static object.

the current theory is that a black hole is similar to a Neutron Star, but the compression to neutrons hasn't happened, because the collapse is much slower.

The current theory is nothing like this at all. Where are you getting this from?

A black hole with the mass of our Sun, which is actually too small to become one, would supposedly be the size of a small pea.

Actually, the Schwarzschild radius of a black hole with the mass of the Sun is about 3 kilometers, so the surface area of its horizon would be about $36 \pi$ square kilometers. (The surface area of the horizon is a better way to think about the size of the hole, because it doesn't have a "radius" in the ordinary sense.)

We also know that anything that passes the event horizon only comes out as radiation.

Actually, we don't know this for sure, although I think most physicists believe it will turn out this way.

 

[PeterDonis]

Am I right in assuming that Hawkins Radiation can explain why a singularity is not necessary

No. Even a black hole that radiates Hawking radiation, and ultimately evaporates away completely because of it, can still have a singularity at its center. The singularity disappears when the hole finally evaporates away, but it's still there for the period of time that the black hole exists.

There are other speculations about quantum effects inside the horizon that could prevent the singularity from forming, but just Hawking radiation by itself is not enough.

am I right in presuming that the radiation output of the black hole by definition = to the mass absorbed?

Eventually, yes; if there comes a point where no more mass falls into the hole, then eventually all the mass that fell in will be radiated away. But as Vagn pointed out, this can take a long, long, long, long time.

Also, the condition of no more mass falling into the hole is much, much stricter than you might realize. For example, any black hole in our current universe, even if it is totally isolated from all other ordinary matter, is being bathed in the cosmic microwave background radiation. Since the temperature of that radiation is greater than the temperature of the hole (at least it is for any hole of stellar mass or larger), the hole will be gaining mass by absorbing the radiation. So no black hole in our universe will be able to start losing mass by Hawking radiation until the universe has expanded enough for the temperature of the CMBR to be lower than the temperature of the hole.

 

[Gaz1982]

No. Even a black hole that radiates Hawking radiation, and ultimately evaporates away completely because of it, can still have a singularity at its center. The singularity disappears when the hole finally evaporates away, but it's still there for the period of time that the black hole exists.

There are other speculations about quantum effects inside the horizon that could prevent the singularity from forming, but just Hawking radiation by itself is not enough.
Eventually, yes; if there comes a point where no more mass falls into the hole, then eventually all the mass that fell in will be radiated away. But as Vagn pointed out, this can take a long, long, long, long time.

Also, the condition of no more mass falling into the hole is much, much stricter than you might realize. For example, any black hole in our current universe, even if it is totally isolated from all other ordinary matter, is being bathed in the cosmic microwave background radiation. Since the temperature of that radiation is greater than the temperature of the hole (at least it is for any hole of stellar mass or larger), the hole will be gaining mass by absorbing the radiation. So no black hole in our universe will be able to start losing mass by Hawking radiation until the universe has expanded enough for the temperature of the CMBR to be lower than the temperature of the hole.

So you do believe in the physical existence of the singularity then?

 

[guywithdoubts]

The interior of a black hole is vacuum

Is this known for certain, or is it at least theoretically solid? I thought this was part of the problem regarding loss of information.

 

[PeterDonis]

Is this known for certain, or is it at least theoretically solid?

Classically, it's theoretically solid.

Quantum mechanically, the term "vacuum" is much more complicated than it is classically. First, because of the uncertainty principle, even a state that is "vacuum" according to quantum field theory can have effects that, from the standpoint of classical GR, arise from the presence of nonzero stress-energy, because of quantum fluctuations. Second, whether or not a particular state of the quantum field is "vacuum" turns out to be observer-dependent; for example, a state that is "vacuum" to an observer free-falling into a black hole is not "vacuum" to an observer hovering just outside the horizon. (This last fact is a key element in the standard derivation of Hawking radiation.)

However, even given the above, the various models that are used to investigate possible quantum corrections to the classical behavior all use "vacuum" states in the following sense: no other stress-energy is present besides that which arises unavoidably from the quantum issues I described above. In other words, there is still no ordinary matter, no ordinary radiation, no ordinary pressure or other stresses, etc. None of that "ordinary" stuff is required to predict the presence of quantum effects such as Hawking radiation. So in that sense, yes, the interior of a black hole being vacuum is theoretically solid even with quantum corrections included.

I thought this was part of the problem regarding loss of information.

No, the problem of loss of information is present even with a black hole whose interior is entirely vacuum (i.e., no "ordinary" stress-energy is present, in the sense I described above). All that's required for the information loss problem to be a problem is that a singularity forms at the center of the black hole (even if it ends up evaporating away eventually due to Hawking radiation). That's why models that involve quantum corrections large enough to prevent the singularity from ever forming are seen as desirable. (The only other way out of the information loss problem is to finesse it by saying that, even though quantum information gets destroyed in the singularity, it's OK because the singularity is always hidden behind an event horizon, so nobody outside the horizon will ever be able to tell.)

 

[PeterDonis]

So you do believe in the physical existence of the singularity then?

No. Remember that I'm describing physical models. There are various models which are not ruled out by any data we have at this stage of our knowledge; since some of those models include singularities (for example, the simplest Hawking radiation model has one, as I mentioned before), we can't rule out the possibility that singularities do in fact exist at the centers of black holes. But that's not the same as "believing" in their existence. Personally, I don't "believe" or "disbelieve" in them; I think we just don't know enough yet.

 

[Monsterboy]

So, for example, if your warp drive ship emitted a light beam ahead of it when it set off on its journey, the light beam would arrive at the ship's destination before the ship itself did.

Then how/why is it called faster than light travel ?

 

[PeterDonis]

Then how/why is it called faster than light travel ?

Because pop science is not always very careful about terminology.

 

[Monsterboy]

Because pop science is not always very careful about terminology.

So ,there is no question of traveling to nearby stars "faster than light" and reaching there in human time scales ?

 

[PeterDonis]

there is no question of traveling to nearby stars "faster than light" and reaching there in human time scales ?

If an Alcubierre warp drive could actually be built (which, since it requires exotic matter, it probably can't), it would allow you to, for example, travel to Alpha Centauri in much less than 4.3 years, even as seen by observers on Earth or Alpha Centauri. But the reason for this would be that the warp drive would drastically change the geometry of spacetime in between Earth and Alpha Centauri, such that the distance between them would be much less than 4.3 light years if you measured it along a path going through the warp bubble. So a light beam that passed through the warp bubble would get from Earth to Alpha Centauri even faster than the ship itself would.

(Btw, this description is heuristic only; it's the best I can do at translating what I understand of the math of the Alcubierre solution into ordinary English. There are a number of technical complications involved, so what I've said is certainly not a rigorous description of what the math actually says.)

 

[Gaz1982]

No. Remember that I'm describing physical models. There are various models which are not ruled out by any data we have at this stage of our knowledge; since some of those models include singularities (for example, the simplest Hawking radiation model has one, as I mentioned before), we can't rule out the possibility that singularities do in fact exist at the centers of black holes. But that's not the same as "believing" in their existence. Personally, I don't "believe" or "disbelieve" in them; I think we just don't know enough yet.

I don't know about you, but I always like to envisage what I think is there based on the facts I know.

And I find it hard to envisage a physical singularity

 

[PeterDonis]

I find it hard to envisage a physical singularity

So do I. But finding it hard to envisage it is not the same as having a good working model that replaces it with something else.

 

[Monsterboy]

If an Alcubierre warp drive could actually be built (which, since it requires exotic matter, it probably can't), it would allow you to, for example, travel to Alpha Centauri in much less than 4.3 years, even as seen by observers on Earth or Alpha Centauri. But the reason for this would be that the warp drive would drastically change the geometry of spacetime in between Earth and Alpha Centauri, such that the distance between them would be much less than 4.3 light years if you measured it along a path going through the warp bubble. So a light beam that passed through the warp bubble would get from Earth to Alpha Centauri even faster than the ship itself would.

(Btw, this description is heuristic only; it's the best I can do at translating what I understand of the math of the Alcubierre solution into ordinary English. There are a number of technical complications involved, so what I've said is certainly not a rigorous description of what the math actually says.)

Can Casimir effect be used for the Alcubierre drive if we can't find exotic matter ? Can low energy density vacuum allow "faster than light travel" ?

 

[Monsterboy]

Will super massive black holes eventually consume all the stars in their galaxy ?

 

[mfb]

All that do not escape from the galaxy (=most likely future of a star): probably. On a really, really long timescale.

 

[Drakkith]

Can Casimir effect be used for the Alcubierre drive if we can't find exotic matter ? Can low energy density vacuum allow "faster than light travel" ?

Not as far as we know.

Will super massive black holes eventually consume all the stars in their galaxy ?

No, many will eventually escape from their orbits due to gravitational interactions. There's also the possibility that the SMBH will evaporate before the majority of stars in the galaxy have time to decay in their orbits, but I don't know the timescales involved in either of these processes.

 

[mfb]

A black hole with a mass of 100 million solar masses emits about 10^-44 W of Hawking radiation.

A star with the mass of our sun, located 10000 lightyears away and orbiting this black hole, leads to the emission of many nanowatts of power as gravitational waves. The stars will get ejected or fall in long before the supermassive black holes evaporate.

Note that Hawking radiation scales with 1/M^2 while gravitational wave power scales with (approximately) M^3, so even with just 100 solar masses gravitational waves win by several orders of magnitude. In addition, the energy scale of the orbit is significantly smaller than the energy scale of the black hole.