Quantum foam -RIP, or needs new formulation?

[nomadreid]

A couple of questions about so-called quantum foam
A recent article (2013) stated that there was no delay of the sort that quantum foam would imply of photons coming from far galaxies: http://www.huffingtonpost.com/2013/01/10/quantum-foam-einstein-smooth-space-time_n_2449734.html (which includes the unfortunate typo of "proton" for "photon".) This appears to contradict earlier findings (2007), which did find a delay: http://www.news.ucdavis.edu/search/news_detail.lasso?id=8364. So, first question: were the 2007 results faulty? Second question: Given the fact that the Heisenberg Uncertainty Relation is not to be faulted, where could the faulty link be between the HUP and the existence of the quantum foam? Virtual particle pairs seem to be pretty solidly entrenched in the theory, so how could a quantum foam not exist (which, according to the article, it probably does not)? Finally, in that more recent article, sources were not given. Does anyone have a link to a more detailed (or even the original, if it is freely accessible on-line) paper on this?
Thanks.

 

[Chronos]

The Huffington article is based on the Nemiroff paper http://arxiv.org/abs/1109.5191, Bounds on Spectral Dispersion from Fermi-detected Gamma Ray Bursts. It confirms an early result reported by Schaefer in http://arxiv.org/abs/astro-ph/9810479, Severe Limits on Variations of the Speed of Light with Frequency. The apparent time lag of photons arriving from a flare in MK 501 in the reference article reported in the UC Davis release are based on the paper by Ellis, et al, http://arxiv.org/abs/0708.2889, Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope. The conclusions stated in the paper are considerably less sensational than those inferred from the article, so, I would not deem any conflict exists between the results of these studies.

 

[nomadreid]

Perfect answer, Chronos. Thank you very much. It is always preferable to have the original papers.

 

[bcrowell]

Ca. 5-10 years ago, Lee Smolin and others were pushing the idea that a specific theory of quantum gravity, loop quantum gravity (LQG), made specific, quantitative predictions about this sort of observation. They were in a big rush to publish the predictions before the satellites went up, because then it was possible to have a huge triumph for LQG: they would have predicted a nontrivial effect and then seen it confirmed.

The experiments actually came out negative, and I suspect the 2007 result you linked to was just wrong. (They appear to have been tentative about it.)

It then turned out that Smolin's calculations were wrong, and LQG did not generically predict any such effect, so the observations did not falsify LQG.

Even by Smolin's original, mistaken interpretation of LQG, this would not have been a test of quantum foam. It would have been only a test of one possible theory that could loosely be described as involving quantum foam. Quantum foam is not a term with an exact technical definition.

The Huffington Post article is just a cut-and-paste job from space.com, http://www.space.com/19202-einstein-space-time-smooth.html . The original paper is probably this: http://arxiv.org/abs/1109.5191 . These articles are popularizations, and IMO are significantly distorting and oversimplifying. They make it sound as if the experimental results disproved a viable scientific theory. Actually they don't. There is no viable scientific theory that unequivocally predicts a non-null result.

 

[nomadreid]

Many thanks for the additional background info, bcrowell. That is a great help, and much appreciated.

 

[Phys-excess]

You mentioned that the Heisenberg uncertainty principle is not to be faulted. It is my understanding the part of the HUP that relates to measurements has been invalidated. http://arxiv.org/abs/1208.0034 I realize that we still have a common sense UP because measuring speed requires more than one position. Either way, can you explain a little how the UP could affect these results? I'm trying to understand the issue you raised. Thanks.

 

[bcrowell]

It is my understanding the part of the HUP that relates to measurements has been invalidated. http://arxiv.org/abs/1208.0034

Not true. https://www.physicsforums.com/showthread.php?p=4065748#post4065748

So to summarize: the uncertainty principle is still valid, the experimental result is null, the null result is not new, the null result does not test any viable theory, and none of this tells us anything about the vaguely defined notion of quantum foam.

 

[Phys-excess]

What do you mean the result is null? As the author indicates, this experiment was done to test Heisenberg's theory of uncertainty. The refutation you linked me to seems to say that the HUP only applies to particles; however, I am quite certain the HUP applies to states rather than particles. The point being that a multi-particle state is no different than a single particle state when it comes to rules governing quantum states. Am I mistaken? I thought the reason Haroche and Wineland won the Nobel Prize last year was because they pioneered the method that led to this result.

 

[bcrowell]

What do you mean the result is null?

The result of the experiment the OP referred to, by Nemiroff, is null, meaning that the result was as expected according to standard theories. This experiment had nothing to do with the Heisenberg uncertainty principle.

 

[nomadreid]

To Phys-excess: If I understand the experiment that you referred to correctly (and I am willing to be corrected): As bcrowell rightly pointed out, the experiment you referred to has nothing to do with my question when you get down to it, but rather, to put it somewhat crudely, points out (again) that the Observer Effect is not the same as the Heisenberg Uncertainty Principle. The experiment minimized the uncertainty which one gets from the Observer Effect, but the uncertainty of the HUP is still there.

 

[julian]

There is no viable scientific theory that unequivocally predicts a non-null result.

In the context of LQG, I think it was Gambini and Pullin who made the first calculation of the effects of a quantum gravitational field on the propagation of light over a weave state (a weave state is a very large spin network state that is an eigenstate of the area and volume operators that approximates a given spatial geometry at length scales much larger than the Planck scale). For suitable weave states this may lead to modified dispersion relations, representing a Lorentz-violating effect.

Lorentz-violating effects may not exist in LQG: A single spin network state will not be Lorentz invariant but it is not clear that all weave states (generally a superposition of spin network states) break Lorentz invariance. Another reason given that LQG violates Lorentz invariance is the existence of a minimum length; the basic argument is that one should be able to reduce this minimum length further by performing a Lorentz transformation. The flaw in this argument is that we are not dealing with classical quantities, but rather quantum observables. The resolution then follows from the fact that the length operator, L, in the original frame does not commute with the length operator, L', in the transformed frame: they do not have simultaneous eigenstates - an eigenstate of L is not an eigenstate of L', rather it is a quantum superposition of eigenstates of L'. The eigenvalues of L' will be the same as the eigenvalues of L (including the minimum eigenvalue): it is the expectation value of L' that will be Lorentz contracted.

 

[bcrowell]

To Phys-excess: If I understand the experiment that you referred to correctly (and I am willing to be corrected): As bcrowell rightly pointed out, the experiment you referred to has nothing to do with my question when you get down to it, but rather, to put it somewhat crudely, points out (again) that the Observer Effect is not the same as the Heisenberg Uncertainty Principle. The experiment minimized the uncertainty which one gets from the Observer Effect, but the uncertainty of the HUP is still there.

I would say the reason the experiment wasn't that earth-shattering was that it dealt with averages over an ensemble of states.

https://www.physicsforums.com/showpost.php?p=4065760

See #2, for example.