Is space time smooth? A tale of 3 photons

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Is space time smooth? A tale of 3 photons

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1richardbsmith
Editado: Set 1, 2012, 3:55pm

This article seemed intriguing to me.

http://www.space.com/17399-gamma-ray-photons-quantum-spacetime.html

the photons' near-simultaneous arrival indicates that space-time is smooth as Einstein suggested, rather than pixilated as modern theories require — at least down to slightly below the scale of the Planck length, a smaller scale than has ever been probed previously.

What does the science community think of this analysis?

2richardbsmith
Set 1, 2012, 4:02pm

I also like the description of Planck length as "a smaller scale than has ever been probed previously."

Might that be bit understated?

3daschaich
Set 2, 2012, 3:30am

First and most importantly, I want to emphasize that this topic is well outside my area of expertise. While writing this response I stumbled across a relevant blog post that helped me understand some of the issues.

With that out of the way, let me begin by commenting on post #2: this is actually not understated. The paper in question notes that this sort of analysis has been carried out before, but claims to improve upon the previous result by about a factor of 10 or so (an "order of magnitude"). Technically, the order-of-magnitude improvement is a reduction in the time period during which they claim multiple high-energy photons were detected by the Fermi Gamma-ray Space Telescope. The actual operators of Fermi published the previous result in 2009, which concluded that the shortest observed time period was about 0.01 seconds. The newer paper instead reports a time period of 0.00155 seconds hiding in the same raw data.

The authors of the newer paper claim that the main reason for the difference in these two results is that they look only at high-energy photons, whereas the Fermi paper looked for time periods that contained at least one low-energy photon in addition to high-energy photons. However, the newer paper also plays some statistical games with the data that I can't pretend to understand, and don't particularly trust. (The statistical question is whether a short time period might just be a coincidence that isn't meaningful.)

At any rate, the newer paper claims that effects "expected in some versions of quantum gravity" can only appear at energies 525 times higher than the Planck mass, while the corresponding results from the Fermi paper were 1.2 to 102 times the Planck mass, depending on the assumptions going into the analysis. The relevant versions of quantum gravity are apparently those "in which the quantum nature of space-time on a very small scale linearly alters the speed of light" (quoting from the abstract of the Fermi paper). Given that (to my knowledge) we know nothing concrete about quantum gravity, I doubt this result worries very many people. (The blogger who wrote the post I linked to above apparently works on theories where no such effect is expected.)

So the main understatement I noticed in the space.com article is its comment (which I have to assume is tongue-in-cheek) that "not everyone is ready to jettison quantum gravity." To a first approximation, I expect that absolutely nobody is ready to jettison quantum gravity on the basis of this result.

As a final remark on the newer paper, I noticed that it was revised significantly between September 2011 and April 2012; its original version contained some errors flagrant enough for even me to spot, which doesn't increase my confidence in it.

As for what the science community thinks of this analysis, there doesn't seem to be much interest in the newer paper. Although it was published in Physical Review Letters, which is a rather prestigious journal (if not as prestigious as Nature, where the 2009 Fermi paper was published), it has only been cited three times since last September. (And to my eye, none of the three articles that cite it look particularly reputable.) By way of comparison, the 2009 Fermi paper has been cited almost 200 times, receiving roughly 60 citations per year, twenty times more than the newer paper.

4richardbsmith
Set 2, 2012, 7:46am

Thank daschaich,

Apparently then the biggest problem with the analysis is that their expected results are not necessarily predicted by theory, and in some cases are not at all predicted by theory.