Emergent Gravity IV - Learning & Questions

[John86]

interesting website Mat visser has. i really find his papers exciting.

http://arxiv.org/abs/0801.2673
Cosmological particle production in emergent rainbow spacetimes
Authors: Silke Weinfurtner, Piyush Jain, Matt Visser, C.W. Gardiner
(Submitted on 17 Jan 2008)

Abstract: We investigate cosmological particle production in spacetimes where Lorentz invariance emerges in the infrared limit, but is explicitly broken in the ultraviolet regime. Our specific model focuses on the boost subgroup that supports CPT invariance and results in a momentum-dependent dispersion relation. Motivated by previous studies on spacetimes emerging from a microscopic substrate, we show how these modifications naturally lead to momentum-dependent rainbow metrics. Firstly, we investigate the possibility of reproducing cosmological particle production in spacetimes emerging from real Bose gases. We have studied the influence of non-perturbative ultraviolet corrections in time-dependent analogue spacetimes, leading to momentum-dependent emergent rainbow spacetimes. Within certain limits the analogy is sufficiently good to simulate relativistic quantum field theory in time-dependent classical backgrounds, and the quantum effects are approximately robust against the model-dependent modifications. Secondly, we analyze how significantly the particle production process deviates from the common picture. While very low-energy modes do not see the difference at all, some modes "re-enter the Hubble horizon" during the inflationary epoch, and extreme ultraviolet modes are completely insensitive to the expansion.

Can anybody of you help me with following alinea, i am pondering with this one. must i see spacetime emerging from a quantum substratum like Volovik's of the view like a sort of Helium droplet like methaphor ?

We investigate cosmological particle production in spacetimes where Lorentz invariance emerges in the infrared
limit, but is explicitly broken in the ultraviolet regime. Thus these models are similar to many (but not all)
models of quantum gravity, where a breakdown of Lorentz invariance is expected for ultraviolet physics around
the Planck / string scale. Our specific model focuses on the boost subgroup that supports CPT invariance and
results in a momentum-dependent dispersion relation. Motivated by previous studies on spacetimes emerging
from a microscopic substrate, we show how these modifications naturally lead to momentum-dependent rainbow

 

[marcus]

I have a couple of questions, though not sure they can be answered yet.

Is there any word when and where audio/PDF files of the talks will be posted? I assume one would just click on "programme" and there will be a link beside the speaker's abstract.
So far, when I scroll down the abstracts I don't see any links.

The other question is about Loll's talk. Her abstract contains this:
"I will summarize the basic ingredients and construction method underlying this approach, and some of its achievements and outstanding challenges. Specifically, I will comment on what it teaches us about the role of time, its non-emergence, and the relation of the proper time present in CDT quantum gravity."

Non-emergence of time means that time is REAL and fundamental. It is not just an appearance. And it is not like where the universe is a static "block universe" and we have a merely psychological experience of the present moment. When she says time is non-emergent and indicates that the universe has a proper time. that means that there really is a "universe time", the causal layering in CDT refers to something real, the universe is causally ordered, it is evolving.

An emergent phenomenon, a superficial appearance, is sometimes called an "epiphenomenon" ----something that appears on top of or out of----like maybe the regular geometry of our world is just an epiphenomenon arising out of a microscopic ground that is much more chaotic.

OK so it seems that Loll is saying something that is quite controversial here---it looks like she is saying time is real and fundamental, and it is actually a strong point of CDT that it has this global foliation, this causal layer structure. It is not something to be mathematically wiggled out of or gotten rid of.

That is what it looks like. And it looks like she is being bold and up front about it, which I like. Although she might be wrong. Time might in fact not be real. Either way at least Loll is not timid (not what is called "risk-averse"). Or so it seems. I'd like clarification on this.

BTW I hope I didn't sound too jocular or irreverent in commenting on your group photo, Jal. I respect these people a lot, and expect a lot from them---just that group shots are always something of a hoot.

 

[jal]

Like you ... I keep checking the home page for the presentations.

Non-emergence of time means that time is REAL and fundamental.

OK so it seems that Loll is saying something that is quite controversial here

Either way at least Loll is not timid (not what is called "risk-averse"). Or so it seems. I'd like clarification on this.

It was not picked up by the participants during the conference.

I did have a couple of one on one conversations lasting some 10 min. and the subject did not come up.
jal

 

[atyy]

Question for Markopoulou: she related quantum graphity to Levin and Wen's string net condensation which produced photons. Gu and Wen now have a model that maybe has gravitons - is quantum graphity related to the later Wen model also?

http://pirsa.org/08110003
In this lecture at 57 minutes, Wen gives what I think is the answer to my question. The Gu and Wen models of gravity are not string-net models - unlike the Levin and Wen models of electromagnetism - however, he believes that the Gu and Wen models also involve "long-range entanglement".

 

[marcus]

So, in simple language since I don't know much about XGW's work, what do you conclude?

To me (knowing very little about it) it sounds from what you say as if there still could be some similarities between Wen-Gu and Markopoulou pictures.
An interesting aspect of her schema, that I have seen coming up repeatedly, is "disordered locality" which sounds like it might have something to do with "longrange entanglement".

 

[atyy]

So, in simple language since I don't know much about XGW's work, what do you conclude?

To me (knowing very little about it) it sounds from what you say as if there still could be some similarities between Wen-Gu and Markopoulou pictures.
An interesting aspect of her schema, that I have seen coming up repeatedly, is "disordered locality" which sounds like it might have something to do with "longrange entanglement".

I don't know. Wen's current picture is that there are many types of "long-range entanglement" of which Levin and Wen "string net condensation" is a type, while the Gu and Wen models seem to be "long-range entanglement" of a different type. However, he doesn't really understand long-range entanglement yet - ie. exactly how to distinguish between states with long-range entanglement and states without, or between different types of long-range entanglement.

Konopka, Markopoulou and Severini state that their quantum graphity model of "disordered locality" is related to Levin and Wen's "string net condensation" - if so, I presume that although quantum graphity appears related to "long range entanglement" of the "string net" sort, it is currently unknown whether it is related to other types of "long range entanglement" such as that apparently in the Gu and Wen model.

One partial tool that people have at the moment for studying "long range entanglement" is the "topological entropy". There seems to be a lot of interesting current work about this in condensed matter and quantum computing.

 

[John86]

http://arxiv.org/abs/0909.1044
Osmotic pressure of matter and vacuum energy
Authors: G.E. Volovik
(Submitted on 5 Sep 2009)
Abstract: The walls of the box which contains matter represent a membrane that allows the relativistic quantum vacuum to pass but not matter. That is why the pressure of matter in the box may be considered as the analog of the osmotic pressure. However, we demonstrate that the osmotic pressure of matter is modified due to interaction of matter with vacuum. This interaction induces the nonzero negative vacuum pressure inside the box, as a result the measured osmotic pressure becomes smaller than the matter pressure. As distinct from the Casimir effect, this induced vacuum pressure is the bulk effect and does not depend on the size of the box. This effect dominates in the thermodynamic limit of the infinite volume of the box. Analog of this effect has been observed in the dilute solution of 3He in liquid 4He, where the superfluid 4He plays the role of the non-relativistic quantum vacuum, and 3He atoms play the role of matter.

 

[John86]

http://arxiv.org/abs/0909.0160
Lorentz violation and black-hole thermodynamics: Compton scattering process
Authors: E. Kant, F.R. Klinkhamer, M. Schreck
(Submitted on 1 Sep 2009 (v1), last revised 7 Sep 2009 (this version, v2))
Abstract: A Lorentz-noninvariant modification of quantum electrodynamics is considered, which has photons described by the nonbirefringent sector of modified Maxwell theory and electrons described by the standard Dirac theory. These photons and electrons are taken to propagate and interact in a Schwarzschild spacetime background. For appropriate Lorentz-violating parameters, the photons have an effective horizon lying outside the Schwarzschild horizon. A particular type of Compton scattering event, taking place between these two horizons (in the photonic ergoregion) and ultimately decreasing the mass of the black hole, is found to have a nonzero probability. These events perhaps allow for a violation of the generalized second law of thermodynamics in the Lorentz-noninvariant theory considered.