List of Questions about Black Holes

1. Mar 6, 2012

MeLlamoLlama

So from what I understand, virtual particles that are created at the edge of a black hole can become the small detectable radiation (hawking radiation) if one of the particles falls into a black hole, becoming a real particle (its partner now no longer has to annihilate with it) and the second particle can either fall into the black hole w/ it or escape to infinity. My question is, if the black hole is in a very empty part of space (so it is not taking in much matter and energy), would its emission rate decrease and how so? If particles were just continually being created at the horizon of a black hole wouldn't that violate conservation of energy?

Edit: Considering a particle and anti particle are created, and one falls into the black hole (lets assume that it is a negative mass particle) then it would decrease the mass of the black hole, and the other particle would increase the mass/energy of the surrounding space. Is this why the law of conservation of energy holds?

Last edited: Mar 6, 2012
2. Mar 6, 2012

Mark M

Hi,

I'll start with the question about the conservation of energy. Remember, when a virtual particle pair is created, the two particles are opposites: a particle and it's anti-particle. Since the anti-particle has a hypothetical "negative energy" it will always be the one to get drawn into the black hole, because it is much weaker, leaving the particle to become a real particle. The negative energy of the anti-particle causes the black hole's surface area to shrink, continuing until the black hole totally evaporates. The energy added by the particles escaping is counterbalanced by the shrinking of the black hole, maintaining the CoE. The total Hawking Radiation at the end of a black hole's lifespan would be equal to the mass of everything it has taken in.

Secondly, the size of a black hole does have an effect on it's emission: the larger the black hole, the slower the emission. That's why micro-black holes created in a particle collider would evaporate very quickly.

Hope I helped.

3. Mar 7, 2012

Right.

4. Mar 7, 2012

Chalnoth

Anti-particles generally don't have negative energy. If that were the answer, then half the time the particle would fall into the black hole, while the other half of the time the anti-particle would fall into the black hole, so that overall the black hole would radiate but stay the same mass (as just as many negative-mass particles would fall in as positive-mass particles).

Instead, it is the curvature of space-time itself that forces the infalling particle to have negative energy, whether it is a particle or anti-particle.

5. Mar 7, 2012

Mark M

If the curvature of spacetime caused the particle falling into the black hole to receive a negative mass, wouldn't any particles, virtual or not, have negative energy upon descending into the black hole? Also, this would mean that both virtual particles have a positive mass, though I've always understood the anti-particle of the pair of virtual particles had to have a negative mass/energy so that the two particles remained with a net mass/energy of 0.

Also, if what I had originally posted was true, it wouldn't mean that the negative mass particles would fall in half the time and the positive mass particles fell in the other half, since the negative particles were much feebler and would not even stand a chance of escaping the black, unlike the positive particles which would could escape from a much closer distance.

Anyway, thank you for correcting my post, I'm just curious exactly why I was wrong.

6. Mar 7, 2012

Staff: Mentor

What exactly is "negative energy"?

7. Mar 7, 2012

sabanation12

Negative energy is hard to visualize.. maybe impossible :O

Last edited: Mar 7, 2012
8. Mar 7, 2012

sabanation12

hmmm that's a good question!

9. Mar 7, 2012

MeLlamoLlama

I read that the closer a particle is to a large body (in this case a black hole), the less energy the particle has because it would take a lot of energy to allow it to escape from the gravitational field of that body. Is this right, and if so is that what you mean when the curvature of space time "forces the in falling particle to have negative energy?"

So you are saying that generally anti-particles have positive energy? If that were the case, wouldn't the black hole generally gain more energy than it loses energy due to absorbing negative energy? I thought they were supposed to (theoretically) radiate away.

And to tell you the truth, the reason I have been having such a hard time wrapping my head around this is the part about having equal positive and negative energy particles falling into the black hole. In that case, wouldn't the black hole eventually gain mass (by random matter in space) and when it's not gaining this matter it would remain in an equilibrium.

It's so easy to skip over the small things without thinking :(

It will give me something to think about while falling asleep though :)

10. Mar 8, 2012

Chalnoth

Well, the best way to understand this is that it's largely just a heuristic device to get a rough picture of what's going on. I can't say I understand the entire effect, as I have never done the calculations myself. I will simply say that local conservation laws frequently force particles to have weird masses in quantum field theory. For example, when two electrons interact and exchange virtual photons, those virtual photons are forced to have negative mass squared (which is one reason why they're called virtual photons).

11. Mar 8, 2012

Mark M

Thank you for clarifying.

12. Mar 8, 2012

Mark M

It's a hypothetical concept, along with negative mass, that arise only in extreme situations in Quantum Mechanics.

http://www.concentric.net/~pvb/negmass.html [Broken]

Last edited by a moderator: May 5, 2017
13. Mar 8, 2012

Staff: Mentor

Interesting. Thanks for the link Mark!

Last edited by a moderator: May 5, 2017
14. Mar 10, 2012

MeLlamoLlama

I have another question: If a strong gravitational field causes a particle to lose energy, would a strong enough gravitational field cause a particle to have negative energy?

15. Mar 14, 2012

MeLlamoLlama

New Question:

When anti-particles fall into a black hole they supposedly become "real particles" allowing their partner to escape to infinity. Any idea how this happens?

16. Mar 15, 2012

Chalnoth

1. It is not necessarily the anti-particle that falls into the black hole. Of the pair, the particle will fall into the black hole with equal frequency to the anti-particle. So the particles the black hole emits are going to have about as many particles as anti-particles.
2. A particle which escapes to infinity is by definition a real particle. The physics forces any particle which escapes to infinity to behave like a real particle, not a virtual particle. Conservation laws similarly force the particle which falls into the black hole to subtract from the black hole's mass.