Is Unitarity Limited by Real Clocks in Quantum Gravity?

[stevendaryl]

So if there is no ideal time---no time apart from observing correlations with the physical processes we designate as clocks---then there is no perfect eternal unitary evolution. Unitarity is only as good as the clocks are.

I'm not sure if this along the same lines, or not, but Julian Barbour has a program of removing time from physics and replacing it by relationships. His book "The End of Time" describes it.

 

[marcus]

I'm not sure if this along the same lines, or not, but Julian Barbour has a program of removing time from physics and replacing it by relationships. His book "The End of Time" describes it.

Hi Steven! I remember reading a beautiful essay of his where he uses classical mechanics and in effect turns a system like the solar system into a big clock. He extracts time from classical dynamics. The essay won first prize in a FQXi contest. You may have read it. He gives more attention to specific detail than Gambini Porto and Pullin do. But it is along similar lines in the sense that they too consider natural systems as clocks.

They use theoretical reasoning to get BOUNDS on how stable and precise physical clocks can be.
One thing they consider as a longterm frequency standard is a vibration mode of a black hole. (Harder to imagine how one would excite and observe said vibration, compared with Barbour's observation of a planetary system )

I neglected to say just now: if anyone hasn't read the two "Planck star" papers, just google
"planck star" and
"planck star phenomenology"

That is the basis of the problem that this thread is about. According to Loop QG, black holes bounce. This leads to the "Planck star" model of black hole. Extreme gravitational time dilation means that the rebound, which happens quickly from an inside perspective, takes billions of years seen from outside. The rebound results in a long-delayed energetic gamma ray burst at the end of the star's life.

Gambini et al would expect the outside observer to see no information writ in the final GRB (nothing that would allow one to reconstruct knowledge of what initially fell in.)
How does that square with the apparently quick (possibly unitary) evolution which occurs inside?
Is there something in the bounce itself which "resets" any possible clocks. Are any clocks that survive the bounce possible at all?

 

[erkokite]

I would like to submit for your inspection a paper which strikes me as relevant:
http://iopscience.iop.org/1367-2630/16/5/053041/article

 

[marcus]

Thanks! To me it seems like a fascinating idea for an experiment. Can the two systems remain entangled (in the usual sense) if time for the two is progressing at very different rates? It seems that their relative placement in the gravitational potential should have some effect. I'll get the arxiv.org and the inspire.org links in case anyone wants to look up author profiles or see if other researchers have cited this. I am glad you called attention to this:

http://arxiv.org/abs/1306.1933
Testing the effects of gravity and motion on quantum entanglement in space-based experiments
David Edward Bruschi, Carlos Sabín, Angela White, Valentina Baccetti, Daniel K. L. Oi, Ivette Fuentes
(Submitted on 8 Jun 2013)
We propose an experiment to test the effects of gravity and acceleration on quantum entanglement in space-based setups. We show that the entanglement between excitations of two Bose-Einstein condensates is degraded after one of them undergoes a change in the gravitational field strength. This prediction can be tested if the condensates are initially entangled in two separate satellites while being in the same orbit and then one of them moves to a different orbit. We show that the effect is observable in a typical orbital manoeuvre of nanosatellites like CanX4 and CanX5.
8 pages, 2 figures. I. Fuentes previously published as I. Fuentes-Guridi and I. Fuentes-Schuller.

http://inspirehep.net/record/1237905?ln=en
sample author profile:
http://inspirehep.net/author/profile/I.Fuentes.2

 

[marcus]

People seem to be accumulating a number of reasons to be suspicious of the standard Hawking BH evaporation story.
There is a sizable literature to the effect that assuming unitary evaporation leads to bizarre conclusions---lately the "firewall" and also I should mention Bianchi Smerlak finding that unitary evaporation implies a kind of "hiccup" where the BH temporarily gains mass.

There are competing scenarios where the BH bounces, turning into a gamma ray burst, or else a "white hole" the time reversal of collapse.

Bojowald recently posted a paper affirming his notion that the bounce (at least in cosmological case) involves a temporary change of signature.

And Erkokite just called attention to an experimental hypothesis that entanglement might decay between two systems experiencing different rates of time evolution due to different position in gravitational potential. (Is there a connection here with Gambin Pullin's reasoning about decay of unitarity during BH evaporation, as seen by outside observer/)

I think I'll have to leave this for the night and hope to see the situation more clearly in the morning.