# Superluminal Gravitational Acceleration

1. Apr 19, 2014

### Terdbergler

Can gravitational attraction (generated by a super-massive black hole, for instance) accelerate you faster than c?

2. Apr 19, 2014

### Bandersnatch

No. You can't overtake light.

3. Apr 19, 2014

### Terdbergler

That it's impossible to go faster than light is common knowledge. It should be obvious that I'm asking for more information than just that. Does the body reach a terminal velocity in a vacuum? Or will the acceleration itself "decelerate" but never stop? There are a number of ways this could hypothetically play out. I'm asking for the way in which it would actually play out.

4. Apr 19, 2014

### Mordred

Infalling matter disintegrates into energy and becomes part of the accretion disk. Though not 100% of the infalling material will fall past the EH. Some of that energy may exit via the accretion jets. If you want the complete picture though technical you can find it contained in this lengthy and technical paper. There is a variety of influences on the angular momentum most of which are covered in this paper

http://arxiv.org/abs/1104.5499

5. Apr 19, 2014

### Mordred

you can't fault, Bandersnatch's answer considering the question and title you posted lol

6. Apr 19, 2014

### Staff: Mentor

This is actually a little more complicated than you realize. You are talking about incorporating an accelerating object with increasing gravitational time dilation. An observer stationary with respect to the black hole would need to compensate for the time dilation in order to properly calculate the acceleration of the infalling object. I honestly don't know exactly what the observer would "see", so I can't answer your question.

I don't think this is very accurate. Infalling matter is heated up within the accretion disk, which does emit radiation, but I wouldn't say it "disintegrates into energy". Matter can and does fall past the event horizon intact.

7. Apr 19, 2014

### Mordred

the disintegrates into energy is the terms used in the article, " the disintegration process must convert most of the rest mass energy of the infalling particle to kinetic energy, in the sense that, in the center-of-mass frame, the E particle must have velocity v > c/2. Its under the section described as the Penrose process. Essentially the energy of the particle splits into two particles one positive, and one negative, the negative particle falls in the positive particle escapes to infinity. page 44

though I may have this mistinterpreted for example I'm not sure what they mean by escapes to infinity lol (quantum tunnelling)? I would assume so as its similarity to Hawking radiation cannot be ignored
they do explain after that, a debate on this process

"The Penrose mechanism cannot serve as a useful energy source for astrophysical processes. In no case can one obtain energies which are greater by a signicant factor than those which already could be obtained by a similar breakup process without the presence of the black hole."

though this debate is more in line as to whether or not you can extract energy from a black hole.

so as you say the question is not easily answered

Last edited: Apr 19, 2014
8. Apr 19, 2014

### Staff: Mentor

Yes, but the penrose process is about stealing angular momentum from the black hole by using the ergosphere, not about the accretion process as a whole. The penrose process does not occur in a non-rotating black hole and is a little beyond the scope of the OP's question.

9. Apr 19, 2014

### Mordred

ah but my original reply is this

Infalling matter disintegrates into energy and becomes part of the accretion disk. Though not 100% of the infalling material will fall past the EH. Some of that energy may exit via the accretion jets. If you want the complete picture though technical you can find it contained in this lengthy and technical paper. There is a variety of influences on the angular momentum most of which are covered in this paper

you asked for the clarification on the use of the term "disintegrates into energy" let me ask you this in any calculation you've seen near or around the event horizon what form is the particles in? if I'm not mistaken its in the form of photons. ie relativistic. prior to reaching the EH particles reach thermal equilibrium. The temperatures involved are so extreme that hydrogen for example would not be stable. So do we term photons as matter? I chose the dintegrates term as it was used in the literature I provided.

And yes you are correct on the Penrose mechanism, relation to the ergosphere. However mass is already being stripped outside the EH in the region of the ergoshere. Once you remove mass from a particle its obviously not the same particle, which is defined by its mass spin etc. if you strip all the rest mass then and it becomes relativistic its essentially a photon. Now the Penrose process states this photon then decays into a particle pair the negative falls in the positive tunnels out. same with Hawking radiation at the EH itself

edit: in hindsite I should have stated converted to radiation not energy though black holes are often described as the most efficient matter to energy converters in nature next to direct matter-antimatter annihilation's

Last edited: Apr 20, 2014
10. Apr 20, 2014

### Staff: Mentor

I'm honestly not sure what the paper you linked is saying when they use "particle", as they talk of splitting particles into pieces. (As do other explanations of the Penrose effect I looked at) Perhaps they aren't referring to fundamental particles?

11. Apr 20, 2014

### Mordred

Yeah they don't specify in this article, any form of virtual particle production I've seen is usually
photons or electrons. However this article specifies electrons due to photon interactions

$$\gamma\gamma\rightarrow e^-e^+$$