In "Space-Time Is Smooth, Not Foamy", a Space.com article, it is stated:

In his general theory of relativity, Einstein described space-time as fundamentally smooth, warping only under the strain of energy and matter. Some quantum-theory interpretations disagree, however, viewing space-time as being composed of a froth of minute particles that constantly pop into and out of existence.

What are these "quantum-theory interpretations" the article is talking about? Can you quickly summarize the physics behind this experiment?

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    $\begingroup$ Hi Sklivvz, they are talking about quantum gravities (which are theories unifying QM and GR), not quantum interpretations. That is why I exchanged a tag. The particular approaches to quantum gravity, which the article talks about, are LQG (loop quantum gravity) like theories, which try to directly quanzize Einsteins equations and have as a feature that there are some kind of space atoms which can dynamically created and destroyed (by creation and annihilation operators) in discrete time steps. $\endgroup$ – Dilaton Jan 11 '13 at 14:07
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    $\begingroup$ OMG, they are even wrongly talking about quantum interpretations in this popular article, even though it is well known by everybody interested in the topic that the Fermi Gamma-ray Space Telescope tries to measure Lorentz violeting quantum gravity effects such as they occur in every quantum gravity theory which assumes minimal length or time scales, such as LQG. These science journalists should be ashamed about this blatant mistake ! $\endgroup$ – Dilaton Jan 11 '13 at 14:32
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    $\begingroup$ We can ask @Qmechanic to have a look at the tags if you dont believe me. I am 200 % sure that these science journalists have used the wrong terminology too (this is not uncommon for science journalists, Prof. Strassler often complains about their blatant misrepresentations and mistakes too) and the topic of the article and the purpose of the experiment is in fact a test of certain quantum gravities and has nothing to do with quantum interpretations. $\endgroup$ – Dilaton Jan 11 '13 at 14:37
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    $\begingroup$ The tags should correctly represent the physics and not blindly represent any dumb mistakes of science journalist. Therefore it would be better if you could not repeat this mistake in the question either and more correctly change the term "quantum interpretations" to "quantum gravity" too. The issue is really about quantum gravity and has nothing to do with any quantum interpretations. These are two completely different things. $\endgroup$ – Dilaton Jan 11 '13 at 14:50
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    $\begingroup$ A funny aside, I actually ignore the tag "quantum- interpretations" but in "quantum-gravity" I am very interested. So I have only seen your quite nice question before logging in ... ;-) $\endgroup$ – Dilaton Jan 11 '13 at 14:59

The experiment in question is written up in Bounds on Spectral Dispersion from Fermi-detected Gamma Ray Bursts, by Nemiroff, Connolly, Holmes, & Kostinski. They've written it in Physics Review Letter format, which means that it's only 4 pages long.

Basically, they put bounds on the dispersion relation for photon propagation by studying EM wave pulses from distant gamma ray bursts.

The popular science article that OP references is simplified to the point of being misleading. For one thing, it gives the impression that the researchers were actually looking at just 3 photons. They were dividing the photons into bunches, starting with the 3 most tightly clustered, then 4, then...

Regarding interpretation: The word 'interpretation' is not being used in the technical/philosophical sense -- "what do these variables in our physical model mean?" -- but rather in the sense of "How can I translate this from science to pop-science?". They're referencing some vague ideas of Wheeler's; he speculated that in a quantum gravity theory, space might be "foamy" or "fuzzy". This language predates essentially all technical work on quantum gravity, and is dragged out when poetic description is required. In this case, the idea is that quantum gravity effects would change the dispersion relation for photons at extremely high energies. Apparently not.


It is not possible to find the paper on the web, just the description. Quantum foam is an effort to tie up the quantum regime with the classical General Relativity. Loop quantum gravity is an effort on these lines :

The main output of the theory is a physical picture of space where space is granular. The granularity is a direct consequence of the quantization

What has been announced in this conference is one specific observation that sets the probability of granularity of space at very low level.

A team of researchers came to this conclusion after tracing the long journey three photons took through intergalactic space. The photons were blasted out by an intense explosion known as a gamma-ray burst about 7 billion light-years from Earth. They finally barreled into the detectors of NASA's Fermi Gamma-ray Space Telescope in May 2009, arriving just a millisecond apart.

Their dead-heat finish strongly supports the Einsteinian view of space-time, researchers said. The wavelengths of gamma-ray burst photons are so small that they should be able to interact with the even tinier "bubbles" in the quantum theorists' proposed space-time foam.

If this foam indeed exists, the three protons should have been knocked around a bit during their epic voyage. In such a scenario, the chances of all three reaching the Fermi telescope at virtually the same time are very low, researchers said.

(italics mine).

Of course this is not a definitive experiment and has to be repeated many times. An opposite conclusion is taken by another astronomy group on different data.

The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes have detected that among gamma-ray photons arriving from the blazar Markarian 501, some photons at different energy levels arrived at different times, suggesting that some of the photons had moved more slowly and thus contradicting the theory of general relativity's notion of the speed of light being constant, a discrepancy which could be explained by the irregularity of quantum foam.

Italics mine.

  • $\begingroup$ Hi Anna, yep many physics blogs have already talked about these results refuting (or at least constraining) LQG like quantum gravities. Is it not striking that these science confusingly and wrongly talk about quantum interpretations since in fact the issue is quantum gravity? $\endgroup$ – Dilaton Jan 11 '13 at 14:43
  • $\begingroup$ @anna v: wow!.(+1). By the way, can you give some introductory reference to LQG? $\endgroup$ – Eduardo Guerras Valera Jan 12 '13 at 6:43
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    $\begingroup$ @EduardoGuerras user1504 managed to find the original paper, which modifies a bit the popular press description quoted above, but does not change the essentials. I am not conversant with LQG . This which I found in arxiv.org/abs/1001.1330 sets up the mathematical foundations, way over my head. In the last paragraph it hand waves the disagreement with Lorenz transformations, which is the main criticism I have heard and which is the main reason I have not given too much weight to LQG $\endgroup$ – anna v Jan 12 '13 at 7:43
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    $\begingroup$ @EduardoGuerras you might be interested to read this answer by Lubos , in the past here physics.stackexchange.com/questions/3967/… $\endgroup$ – anna v Jan 12 '13 at 9:16

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