# B What happens to the matter inside a black hole when it dies?

1. Apr 1, 2016

### CrackerMcGinger

I was wondering about this for awhile and I still don't understand what happens to the enormous amount of mass stored in a black hole when it dies. I understand this question is based on a conclusion that may be incorrect, but if anyone could shed some light on this, please do so.

2. Apr 1, 2016

### phyzguy

What do you mean "when it dies"? Are you referring to evaporation by Hawking radiation? The time required for an astronomical black hole to evaporate through Hawking radiation is a ridiculously long time (see this link). So, for all practical purposes astronomical black holes don't "die", they just continue to exist.

3. Apr 1, 2016

### CrackerMcGinger

So they just fade away then? That actually makes since if I understood that right.

4. Apr 1, 2016

### phyzguy

You still haven't answered my question. What do you mean "just fade away". No, they don't fade away, they continue to exist.

5. Apr 1, 2016

### CrackerMcGinger

When I said "fade away", I meant to ask if they bleed away their mass by converting the matter contained within them into Hawking radiation, thus over a ridiculously long span of time lose it's mass and eventually the ability to be considered a black hole. If I picture such a thing happening and speed it up so it could occur in a few minutes I would think it would resemble something fading from existence, that's why I said "fade away".

6. Apr 1, 2016

### phyzguy

OK, so you are talking about evaporation due to Hawking radiation. So the answer to your original question of what happens to all of the mass inside the black hole is that the mass-energy associated with all of the mass is radiated away as Hawking radiation. The fact that there is such an enormous amount of mass in an astronomical black hole is why it takes such an enormously long time for this to occur, so long that it is pretty meaningless to contemplate.

7. Apr 1, 2016

### CrackerMcGinger

Would a black hole represent an area of massive entropy?

8. Apr 1, 2016

9. Apr 1, 2016

### CrackerMcGinger

Thank you for helping me with this.

10. Apr 1, 2016

### Stephanus

Did you really mean this black hole exist forever? I read about Hawking radiation, for a whale mass black hole it will evaporate in 1 sec. Surely not matter how massive is the black hole it can't exist FOREVER, can it?

11. Apr 1, 2016

### phyzguy

The time for a 1 solar mass black hole to evaporate is on the order of 10^67 years. The universe is approximately 10^10 years old, so this is about a trillion trillion trillion trillion trillion times longer than the current age of the universe. This is so long, that I consider it effectively 'forever'.

12. Apr 4, 2016

### CrackerMcGinger

I understand where you are coming from phyzguy, but in my mind, stating that because something lasts a ridiculously long time that that makes it last forever, is a little naive. No matter what it is, nothing lasts forever. As far as we know, it may just take on a new form, but nothing lasts forever. With the exception of gravity and time(and that might also end), we know nothing that could last forever. It lasts for a ridiculously long amount of time, but forever is an infinitely larger amount of time.

13. Apr 4, 2016

### phyzguy

Let's assume that you do a calculation of how long it takes a 1 solar mass black hole to evaporate due to Hawking radiation and come up with a number of 10^67 years. Actually, an astronomical black hole sitting out in space is growing with time, not shrinking, because matter and radiation are falling into it. How far in the future you would have to go so that it stopped growing and started shrinking due to Hawking radiation is very uncertain. Even if you knew this, if you believe the 10^67 years number, you are assuming that our current understanding of the laws of physics, which accurately describe the last 10^10 years or so, can be extrapolated forward by a factor of 10^57. Do you really believe this? In my mind, this is a very naive calculation. Maybe instead of saying that a black hole will last effectively forever, I should have said that a black hole will last as far into the future as our current understanding of the laws of physics can predict. Better?

14. Apr 4, 2016

### Staff: Mentor

That's not correct. As far as we know based on our current theories, any elementary particle that has conserved quantum numbers can only decay into a lighter particle with the same set of conserved quantum numbers (or a set of particles that have the same ones in aggregate). So any elementary particle that is the lightest one with a given set of conserved quantum numbers (for example, the electron is the lightest particle with a charge of -1) will last forever according to our current theories.

15. Apr 5, 2016

### Ilja

It is completely open if it dies. Hawking radiation is a prediction which is of extremely dubious character.

"Thus one second after a solar mass black hole forms,the radiation, produces by whatever the process is that produces Hawking radiation, originated from frequencies in the initial state of the uncollapsed system of order $e^{10^5},$ a number so absurdly large that any imaginable units would simply produce an insignificant change in that exponent. And the later the radiation one is considering is emitted, the larger and more absurd this factor becomes"
W.G. Unruh, Has Hawking radiation been measured? arxiv:1401.6612v1

The paper itself provides some argumentation in favor of Hawking radiation, despite this. But these arguments are quite weak, and I'm actually working on a paper showing this.

16. Apr 5, 2016

### Ilja

But this is not the really funny number related with Hawking radiation. Much more funny are the distances where an essentially special relativistic approximation has to remain valid to predict that Hawking radiation continues even a very short time (say, minutes) after the collapse.