Time reversal of gravity and Richard Feynman lecture on past and future

[ande4jo]

My question is on the assumption that there is nothing in laws of nature that says if gravity had time reversal then nothing would appear odd. The example professor Feynman gave was a system with objects rotating due to gravitational attraction that rotated in a clockwise manner with normal forward time would be allowed rotate counterclockwise with time reversed and nothing in the laws would forbid that. Thus, it was concluded from this example that gravity can and does allow time reversal. My question on this is what about the case of a black hole's event horizon which would appear to be a one way journey? Thus does a black hole (which is due to gravity) negate this assumption whereby gravity is no longer thought to obey time reversal (since nothing can return from an event horizon)?

 

[stevendaryl]

My question is on the assumption that there is nothing in laws of nature that says if gravity had time reversal then nothing would appear odd. The example professor Feynman gave was a system with objects rotating due to gravitational attraction that rotated in a clockwise manner with normal forward time would be allowed rotate counterclockwise with time reversed and nothing in the laws would forbid that. Thus, it was concluded from this example that gravity can and does allow time reversal. My question on this is what about the case of a black hole's event horizon which would appear to be a one way journey? Thus does a black hole (which is due to gravity) negate this assumption whereby gravity is no longer thought to obey time reversal (since nothing can return from an event horizon)?

Even though the laws of physics are time-reversible, that doesn't mean that the time-reversed versions of events look normal. If you smash a glass bottle with a hammer, the time-reversed version of events shows a random collection of glass fragments reassembling itself into a bottle. That is a possible sequence of events according to Newtonian mechanics, but it's so unlikely that we can dismiss it as never going to happen.

Similarly, the time-reversed version of events falling into a black hole is a possible solution of the equations of General Relativity, but it is considered so unlikely as to be dismissed. Running a black hole backwards produces a white hole, which is something that has objects and radiation streaming out of it, but nothing ever falls into it. White holes are possible in GR, but there is no plausible mechanism for one to be formed (just like there is no plausible mechanism to arrange glass shards that will reassemble into a bottle).

 

[Orodruin]

White holes are possible in GR, but there is no plausible mechanism for one to be formed (just like there is no plausible mechanism to arrange glass shards that will reassemble into a bottle).

As far as I have understood, there are some plausible mechanisms for producing white hole solutions in quantum gravity theories through bounce solutions, where an original forming black hole reverses due to the QG implementation and essentially white hole solution instead. Not my area of expertise though so others may have more input.

 

[ande4jo]

Even though the laws of physics are time-reversible, that doesn't mean that the time-reversed versions of events look normal. If you smash a glass bottle with a hammer, the time-reversed version of events shows a random collection of glass fragments reassembling itself into a bottle. That is a possible sequence of events according to Newtonian mechanics, but it's so unlikely that we can dismiss it as never going to happen.

Similarly, the time-reversed version of events falling into a black hole is a possible solution of the equations of General Relativity, but it is considered so unlikely as to be dismissed. Running a black hole backwards produces a white hole, which is something that has objects and radiation streaming out of it, but nothing ever falls into it. White holes are possible in GR, but there is no plausible mechanism for one to be formed (just like there is no plausible mechanism to arrange glass shards that will reassemble into a bottle).

Thanks for the quick reply.
As for the broken glass example of reassembly (and as I understand it), the reason that it is so very unlikely is that the huge number of particles involved make the possibility tend to zero, but any individual atom or molecule of the original glass can go back to its original location/state. I believe that this is fundementally different than the black hole example because not even one photon can come back from the event horizon (again, as I understand it). It is this idea that makes me still think that the law of gravity then would not be time-reversible.

 

[stevendaryl]

Thanks for the quick reply.
As for the broken glass example of reassembly (and as I understand it), the reason that it is so very unlikely is that the huge number of particles involved make the possibility tend to zero, but any individual atom or molecule of the original glass can go back to its original location/state. I believe that this is fundementally different than the black hole example because not even one photon can come back from the event horizon (again, as I understand it). It is this idea that makes me still think that the law of gravity then would not be time-reversible.

Applying time-reversal to a black hole produces a white hole. So you're right, no photon ever emerges from a black hole, but a photon can emerge from a white hole.

 

[timmdeeg]

To my understanding even though the laws of physics don't contradict time-reversal this is constrained by the second law of thermodynamics.

 

[pervect]

My question is on the assumption that there is nothing in laws of nature that says if gravity had time reversal then nothing would appear odd. The example professor Feynman gave was a system with objects rotating due to gravitational attraction that rotated in a clockwise manner with normal forward time would be allowed rotate counterclockwise with time reversed and nothing in the laws would forbid that. Thus, it was concluded from this example that gravity can and does allow time reversal. My question on this is what about the case of a black hole's event horizon which would appear to be a one way journey? Thus does a black hole (which is due to gravity) negate this assumption whereby gravity is no longer thought to obey time reversal (since nothing can return from an event horizon)?

The maximally extended Schwarzschild solution consists of a black hole region and a white hole region. Time reversal makes the white hole region into a black hole, and the white hole region into a black hole.

Realistic collapse is different, but it's not the laws of physics (at least as we currrently understand them) that makes the realistic collapse scenario have no white hole region. Rather it's in the initial conditions that were specified to create a realistic collapse scenario. This is rather similar to the way that time reversing works when you drop an egg. There's nothing that changes in the laws of physics when you time-reverse an egg drop, but it is unnatural for pieces of random debris on the floor to rise up off the floor and assemble themselves into an egg.

So with current theories, the apparent assymetry is assigned to the initial conditions. This may not be wholly satisfying, but it's the only answer we have at the current time, as there's no hint of an asymetry in the laws of physics themselves.

An example of a sort of physical law that would not have time symmetry would be if the evolution of the quantum wavefunction was not unitary. This would not, for example, conserve probability - probabilities wouldn't add to 100 percent. I'm not aware of any serious peer reviewed proposals for such a theory.

There are theories rather similar to GR in which black hole collapses each create new universes (resulting in a white hole in a different space-time), but these theories are not quite GR. They may be helpful in illustrating some of the ideas on what is, and is not, possible within the context of laws of physics which are perfectly reversible.

Google for Nikodem Poplawski if you want more details. The theory is Einstein-Cartan-Sciama-Kibble theory, and it's a theory that modifies GR very slightly to allow for better compatibility for spin 1/2 particles (fermions). These small and almost experimentally non-detectable modifications result in the "bouncing" of black hole collapse. However, it's impossible for the black hole collapse to "bounce" back into it's original space time. Instead, it creates a new, different space-time when it bounces. GR does not predict a bounce, GR predicts the creation of a singularity at which point it throws up its hands and says "we don't know what happens next".