# Can black holes die?

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1. Nov 10, 2014

### 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?

2. Nov 10, 2014

### 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!

Last edited: Nov 10, 2014
3. Nov 10, 2014

### Staff: Mentor

"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"?

4. Nov 10, 2014

### 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.

5. Nov 10, 2014

### Staff: Mentor

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.

6. Nov 10, 2014

### 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.

7. Nov 10, 2014

### Lyle

8. Nov 10, 2014

### Staff: Mentor

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".

9. Nov 11, 2014

### 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.

10. Nov 11, 2014

### Gaz1982

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?

11. Nov 11, 2014

### Vagn

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.

12. Nov 11, 2014

### Gaz1982

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?

13. Nov 11, 2014

### Vagn

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.

14. Nov 11, 2014

### Gaz1982

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

15. Nov 11, 2014

### Vagn

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

16. Nov 11, 2014

### Gaz1982

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?

17. Nov 11, 2014

### Vagn

The lifetime of a stellar mass blackhole 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.

18. Nov 11, 2014

### Gaz1982

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

19. Nov 11, 2014

### Staff: Mentor

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 nothing like this at all. Where are you getting this from?

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.)

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

20. Nov 11, 2014

### Staff: Mentor

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.