# Interaction of light with the quantum gravitational field

1. Sep 2, 2005

### marcus

I want to gather some links about this. If you know of some good articles and can add some links to the stash, please do. The gravitational field is the geometry of spacetime and can be roughly equated with spacetime itself, or a general idea of the vacuum----the ideas tend to overlap because in some sense spacetime is nothing besides its quantum geometry and because the other fields are defined on spacetime.

I became aware from a Turbo thread that in 1920 Einstein was recklessly speaking of the gravitational field as a sort of "Machian aether" which he defined different from the old aether in that it agreed with relativity and had no preferred rest-frame. But even if it is given a special definition and used in a logically OK manner, merely using this word runs a risk of precipitating much confusion and argument. So probably we should just say (quantum) "gravitational field" among ourselves and not use the AE word.

R. Loll and B. Dittrich recently posted an article that is very much about the paths of light through the CDT spacetime "counting a black hole"---
this paper is looking at the optical behavior of different simplex-assemblages, how beams are spread and concentrated
Also Loll and Westra were using optical criteria to determine how much and what kind of topology change they could allow.
http://arxiv.org/abs/hep-th/0507012
"Taming the cosmological constant in 2D causal quantum gravity with topology change"
http://arxiv.org/abs/gr-qc/0506035
"Counting a black hole in Lorentzian product triangulations"
So in these papers which are the most recent, save one, from Loll, light is playing an important and explicit role in quantum gravity. Maybe you think this is naive but I take it as a possible hint that I should learn about different interactions of light with the gravitational field including the flat-case vacuum so I am prepared in case some of these things come up in the CDT quantum spacetime.

So I want some links. If you have some good ones please volunteer them. I will mine Turbo's references first.

Last edited: Sep 2, 2005
2. Sep 2, 2005

### marcus

these are grubbed from something Turbo mentioned

http://arxiv.org/gr-qc/0107091 [Broken]
Faster-than-c signals, special relativity, and causality
Stefano Liberati (U Maryland), Sebastiano Sonego (U Udine, Italy), Matt Visser (Washington University in Saint Louis)
25 pages; 4 embedded figures. To appear in Annals of Physics
Annals Phys. 298 (2002) 167-185
"Motivated by the recent attention on superluminal phenomena, we investigate the compatibility between faster-than-c propagation and the fundamental principles of relativity and causality. We first argue that special relativity can easily accommodate -- indeed, does not exclude -- faster-than-c signalling at the kinematical level. As far as causality is concerned, it is impossible to make statements of general validity, without specifying at least some features of the tachyonic propagation. We thus focus on the Scharnhorst effect (faster-than-c photon propagation in the Casimir vacuum), which is perhaps the most plausible candidate for a physically sound realization of these phenomena. We demonstrate that in this case the faster-than-c aspects are `"benign'' and constrained in such a manner as to not automatically lead to causality violations."

http://arxiv.org/gr-qc/9807067 [Broken]
Superluminal propagation of light in gravitational field and non-causal signals
A.D. Dolgov, I.D. Novikov
Phys.Lett. B442 (1998) 82-89
"It has been found in several papers that, because of quantum corrections, light front can propagate with superluminal velocity in gravitational fields and even in flat space-time across two conducting plates. We show that, if this is the case, closed time-like trajectories would be possible and, in particular, in certain reference frames photons could return to their source of origin before they were produced there, in contrast to the opposite claim made in the literature."

http://arxiv.org/hep-th/9810221 [Broken]
The velocities of light in modified QED vacua
K. Scharnhorst (Humboldt University Berlin)
Comments: 7 pages, 5 figures... to appear as a special issue of the Annalen der Physik (v2: typos in the references corrected, minor stylistic changes)
Annalen Phys., 8. Series, 7 (1998) 700-709
"QED vacua under the influence of external conditions (background fields, finite temperature, boundary conditions) can be considered as dispersive media whose complex behaviour can no longer be described in terms of a single universal vacuum velocity of light c. Beginning in the early 1950's (J.S. Toll), quantum field theoretic investigations have led to considerable insight into the relation between the vacuum structure and the propagation of light. Recent years have witnessed a significant growth of activity in this field of research. After a short overview, two characteristic situations are discussed: the propagation of light in a constant homogeneous magnetic field and in a Casimir vacuum. The latter appears to be particularly interesting because the Casimir vacuum has been found to exhibit modes of the propagation of light with phase and group velocities larger than c in the low frequency domain omega<<m where m is the electron mass. The impact of this result on the front velocity of light in a Casimir vacuum is discussed by means of the Kramers-Kronig relation."

I should point out that I am not primarily interested in "FTL"-----I don't care whether the interaction with spacetime makes the signal travel faster or slower. I am happy if it travels SLOWER. this is not a "trekkie" thread. The only thing I care about is that there is discussed SOME INTERACTION of the light with the vacuum or with the gravitational field. this may reveal itself by making the light go a little faster or a little slower, or some other effect.

Also these references are all peer-ified JOURNAL publications. One is "Annals of Physics" and one is "Annalen der Physik" and one is "Physical Review Letters". So as not to pursue the goose, let us require this of our links.

Last edited by a moderator: May 2, 2017
3. Sep 2, 2005

Staff Emeritus
When I googled I found a lot of interest in the Schrnhorst effect, but nearly all of it, even the most sober scientific papers, was focused on "FTL". As your abstract from Scharnhorst's review article suggests, the QED interaction of light with the virtual vacuum goes back a long way, and Scharnhorst's own Casimir suggestion came in 1990.

This is all perfectly good physics, and stands to reason given the nature of the QED vacuum (I had a forehead hitting moment!). Apparently though, the easiest ways to observe the effect (like birefringence) won't work. To get a pair of plates flat enough around and 1 mm apart and actually measure the speed oof light between them sounds like heroic experimentation to me, but who knows? The capabilities of experimenters seem to expand at something like Moore's law rates.

4. Sep 2, 2005

### marcus

the impression I got was of general agreement that these effects aren't presently measurable or detectable, which agrees with what you say.

also I want to keep open to other kinds of effects besides Scharnhorst. would like if someone could supply links about effects on light of strong gravitational fields.

I have the nagging suspicion that very high energy GRB light should go slightly slower because it is interacting more with a chaotic uncertain-curved smallscale regime. this goes counter to what I have read and is not grounded on mathematical foundations, and it bothers me.

5. Sep 4, 2005

### hellfire

Could you please give some heuristic explanation of the effect? I always was under the impression that virtual particles arise only in case of interacting fields and that light does not interact with the modes of vacuum of a free field (so that their suppression leads to a faster propagation of light).

Last edited: Sep 4, 2005
6. Sep 4, 2005

Staff Emeritus
That was my impression too, but it's wrong. There are good reasons why a photon cannot induce real pair production; it violates conservation laws. But those laws are only true up to observability. For virtual particles which don't last long enough, or don't have enough energy, to be observed as real, non-conservational things can happen, and this is an echt prediction of relativistic quantum field theory, although I haven't been able to find it mentioned in Peskin and Schroeder.

So photons move through the q-vacuum pretty much the way they move through glass, making e^+e^- pairs instead of being absorbed by atoms, and slowing down. The Scharnhorst paper that Marcus linked to has a reference to an early paper (early in QED history) that developed this idea.

7. Sep 4, 2005

### Chronos

The Scharnhorst effect is, at best, difficult to measure, and deemed unlikely to cause superluminal propogation. To quote John Baez:
"However, further theoretical investigations have shown that once again there is no possibility of FTL communication using this [Scharnhorst] effect."
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#12

Several papers disputing the Scharnhorst result were published circa 1990, unfortunately, finding free sources of these papers has proven difficult.

Another source:
Quantum Noise and Superluminal Propagation
http://www.arxiv.org/abs/quant-ph/0004047

Regarding slips in the arrival time of high energy GRB photons:

Testing models for quantum gravity
http://www.cerncourier.com/main/article/42/7/18

Selected topics in Planck-scale physics
http://www.arxiv.org/abs/gr-qc/0305019

8. Sep 4, 2005

### Hans de Vries

There are a number of interesting publications in the field of very high energy
density lasers and the breakdown of the vacuum beyond Schwingers critical
field strength.

"The spontaneous breakdown of the vacuum"
http://arxiv.org/PS_cache/hep-ph/pdf/9805/9805507.pdf [Broken]

Interestingly puts an upper limit to the fine structure constant of 1/(16pi)
as a necessary condition for the spontaneous breakdown of the vacuum.

X-RAY lasers (free electron lasers):
"Pair Production from Vacuum at the Focus of an X ray laser"
http://arxiv.org/PS_cache/hep-ph/pdf/0103/0103185.pdf [Broken]
"Boiling the vaccum with an X ray free electron laser"
http://arxiv.org/PS_cache/hep-ph/pdf/0304/0304139.pdf [Broken]

Ultra high energy density lasers may also allow experimental tests of the
http://www-ssrl.slac.stanford.edu/lcls/workshops/12jan1999/hf_physics.html
http://www.aps.anl.gov/conferences/FLSworkshop/proceedings/papers/wgSum1I.pdf

Regards, Hans

Last edited by a moderator: May 2, 2017
9. Sep 5, 2005

### hellfire

This is my understanding from Peskin and Schröder, please correct me if I am wrong: The amplitudes for the fluctuations in the vacuum state of a free field can be computed as $<0|\phi(x_1) \phi(x_2) ... \phi(x_n)|0>$. According to Wick's theorem, an expression like this can be decomposed as a product of Feynman propagators, leading to Feynman diagrams with no loops. Only if an interaction exists, a term $$e^{-i\int dt H_I}$$ appears within the product <0|...|0> due to the fact that one does not consider $|0>$ anymore but $|\Omega>$, the vacuum state of an interacting field expressed in terms of the free vacuum state. Only this term with the integral leads to loops in the Feynman diagramms after expanding the exponential as a power series. Thus, I do not understand how Scharnhorst is considering "2-loop diagrams" at all. Basically, I still have problems to understand what roles these "virtual particles" are playing here and how do they arise.

Last edited: Sep 5, 2005
10. Sep 6, 2005

### Spin_Network

Here's one for a good start:http://arxiv.org/abs/astro-ph/0210124

another good thread marcus, I am a bit busy with things, but great postings, thanks.

11. Sep 6, 2005