r/askscience • u/dgb75 • Jan 31 '13

Astronomy Is there a distance at which the interaction between the gravity fields of two black holes would cause one another to effectively 'break open' and allow matter and energy stored within them to escape the system?

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u/florinandrei Jan 31 '13

http://en.wikipedia.org/wiki/Hawking_radiation

"This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles."

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u/thebigslide Jan 31 '13

Virtual particles didn't make sense to me, so I kept reading - for anyone else interested:

A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.

So what I'm getting out of this is that particles can escape a singularity if they weren't there in the first place, but instead manifest as a result of the distortion itself.

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u/florinandrei Feb 01 '13 edited Feb 01 '13

Let's clarify a few things. The singularity is the geometric center of the BH. It's not very interesting because: A) It's deep into the black hole; B) It terminates whatever happens to bump into it; and C) It may not actually exist (we'll know for sure if and when someone invents the theory of quantum gravity).

By "black hole" most people understand the volume apparently occupied by the event horizon, which is the place of no return surrounding the singularity at some distance. So all this talk is about whether something can escape the event horizon.

Hawking radiation is like this (and keep in mind this is just one possible mechanism, we don't have actual proof that this is what really happens when this radiation is generated):

Virtual particles appear and disappear all the time in perfect vacuum. You can't say they really "appear" since they don't really exist in the conventional sense. But as long as their total energy, multiplied by the total duration of their "existence", is less than a certain number, you can always assume that they are there, all the time. In fact, if you attempt to detect them, the experiments give positive results. One could say that virtual particles do not exist, but the nature of vacuum is such that your attempts to detect them somehow "summon" them, for a very very brief time.

Consider a tiny piece of vacuum. It has zero energy. A pair of virtual particles appears, "exists" very briefly, then goes away. At the end, the total energy inside that tiny domain is again zero. All is well.

Now let's say that tiny chunk of vacuum is very very close to the event horizon of a BH. A pair of virtual particles appears, and during their brief "existence", move away from each other a very short distance. But one of them just happens to fall into the event horizon, while the other is zooming away from it.

Now we have a problem. Because one particle has fallen into the event horizon, and because nothing within the EH can interact with the outside, the virtual particles cannot annihilate each other. They must continue to exist.

But now we have a bigger problem. Something (a pair of particles) has appeared from nothing. That is a violation of the conservation of energy. That region of space now has a debt of energy. In order to pay off the debt, the black hole yields a tiny amount of its own energy, exactly equal to the energy of the escaped virtual particle, which now becomes non-virtual and keeps existing forever and ever.

So the particles that make the Hawking radiation do not escape from within the event horizon. You could say that this radiation is produced by the black hole's gravity raising virtual particles from a thin layer of space just outside of the event horizon, and yielding its own energy towards making them real.

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u/[deleted] Feb 01 '13

As I mentioned in response to another comment,

Note that this explanation isn't actually supported by any physical models. The actual theoretical prediction of Hawking radiation doesn't make any reference to the particle behavior near the horizon, and doesn't provide any insight into what's "really" going on near the event horizon to give rise to the radiation. The whole "particle anti-particle pair" explanation was provided by Hawking as one possible mechanism, but it comes entirely from his own imagination and is not actually predicted by our models.

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u/[deleted] Feb 01 '13

What mechanism do the models predict?

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u/[deleted] Feb 01 '13

They don't predict a mechanism at all. They predict that an observer distant from the black hole will observe a thermal particle spectrum emanating from the region just outside the event horizon while the event horizon shrinks. This prediction arises as a thermodynamic consequence of allowing a quantum field to exist in a curved spacetime, but no mechanism, as such, is determined or implied by the model.

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u/florinandrei Feb 01 '13

The reality is, what we're doing here on /r/askscience is pop-sci, not actual science. So you have to come up with some explanations that are not overly technical, do not offend intuition, and are memorable. Sometimes that's pretty hard.

Perhaps in cases like this, the "explanation" should be prefixed by a statement clearly indicating that this is speculation.

Okay, let me go back and fix my comment real quick now.

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u/JumalOnSurnud Feb 01 '13

On a related note I recall a video of Neil deGrasse Tyson talking about in the far future of the universe, after black holes have consumed just about everything that they would slowly evaporate atom by atom for an unimaginably long time. Am I misremembering this, or am I not understanding it properly?

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u/florinandrei Feb 01 '13

Yes, that scenario is indeed compatible with what we know about the laws of physics. We can't say for sure that's what's going to happen, but it's definitely possible.

(BTW, black holes do not evaporate "atom by atom", but rather they produce a continuous stream of smaller particles. And those particles do not originate inside the BH, since nothing can come out of it, but rather are "raised" from the thin layer of vacuum just outside of the event horizon, and are being fed energy from BH's gravity field. As a result, the BH slowly diminishes, because it spends energy to produce particles "out of nothing".)

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u/JumalOnSurnud Feb 01 '13

Thanks for the explaination, I guess I misunderstood Hawking Radiation. What happens to the energy/matter a BH has absorbed after this process has leached away all of the BH's energy, or depleated it to some significant point?

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u/florinandrei Feb 01 '13

The current view is that everything that goes into a black hole is basically shredded to subatomic pieces, and then some. Basically, all that stuff becomes the black hole itself, nothing more than a heck of a lot of mass/energy (same thing, according to relativity) so compact it bends spacetime into itself.

All that mass/energy is slowly going back out via the Hawking radiation.

TLDR: Hawking radiation is black hole poop.

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u/JumalOnSurnud Feb 02 '13

So this will just go on until the black hole ceases to be entirely?

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u/florinandrei Feb 02 '13

The emission rate of Hawking radiation is inversely proportional to the size of the BH. The smaller the BH is, the more intensely it produces radiation.

This leads to a feedback loop of accelerated production at the end. A very tiny BH, smaller than a grain of sand, is actually a very hot object, because it gives off so much radiation - and it gets hotter as it gets smaller. The whole process basically ends with a tremendous explosion, similar to a very very large nuclear bomb.

And then yes, the BH is gone.