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Intergalactic Race Shows That Einstein Still Rules
Ponca City, We love you writes "The NY Times reports that after a journey of 7.3 billion light-years, a race between gamma rays ranging from 31 billion electron volts to 10,000 electron volts, a factor of more than a million, in a burst from an exploding star, have arrived within nine-tenths of a second of each other. A detector on NASA’s Fermi Gamma-Ray Space Telescope confirmed Einstein’s proclamation in his 1905 theory of relativity that the speed of light is constant and independent of its color, energy, direction or how you yourself are moving. Some theorists had suggested that space on very small scales has a granular structure that would speed some light waves faster than others — in short, that relativity could break down on the smallest scales. Until now such quantum gravity theories have been untestable because ordinarily you would have to see details as small as the so-called Planck length, which is vastly smaller than an atom — to test these theories in order to discern the bumpiness of space."
The spread in travel time of 0.9 second between the highest- and lowest-energy gamma rays, if attributed to quantum effects rather than the dynamics of the explosion itself, suggested that any quantum effects in which the slowing of light is proportional to its energy do not show up until you get down to sizes about eight-tenths of the Planck length, according to the Nature paper, whose lead author was Sylvain Guiriec of the University of Alabama.
Granted they say it would have to be proven much smaller than a planck length for most people to accept this as empirical proof, it is empirical data backing Einstein. The 9/10s could be due to the explosion or a physical effect but the latter is now more unlikely given the many light year distance.
My work here is dung.
The event was approximately 2.2 seconds long. Thus it is plausible that these two photons left .9 seconds apart.
Warning: this article may contain humor, sarcasm, parody, and perhaps even irony. Read at your own risk.
It doesn't prove that the speed of light is constant, but it does reasonably prove that the speed of light is independent of wavelength, since they left from the same source at the same time.
This is actually just the latest in a series of measurements of this type. Since the Nature paper isn't free online, people may want to look at this similar paper from earlier this year that is available.
The article talks about testing "some theories" of quantum gravity. AFAIK the only theory of quantum gravity that makes anything like a prediction that could be tested in this way is loop quantum gravity (LQG). The two leading contenders for a theory of quantum gravity are LQG and string theory. String theory essentially assumes a background of flat spacetime (plus an xtra 6 rolled-up dimensions), so I don't think it's capable of addressing the issue of whether spacetime is frothy at the Planck scale. LQG doesn't assume a background of flat spacetime, and in fact one of the main research programs in LQG is focused on showing that flat spacetime can emerge as a solution to LQG in the appropriate limit. LQG unambiguously predicts that the vacuum is dispersive, i.e., that the speed of light depends on the energy of the photon. However, LQG does not unambiguously predict the exact form of the energy-dependence. The possible form that is usually assumed in order to evaluate observational tests is |v/c-1|~(E/E_P)^n, where v is the speed of the photon, c is the speed of cause and effect in relativity (often referred to as the speed of light), E is the energy of the photon, E_P is the Planck energy, and n=1 or 2. Previous observations, such as the one in the arxiv paper I linked to above, have pretty much ruled out n=1, so if LQG is right, we'd presumably have to have n=2. Some people have been saying that LQG is ruled out by these measurements, but I don't think that's really correct, it's just constrained by them. Here is a paper by LQG researchers discussing the empirical tests, and they don't seem to be saying "OK, we give up." It's actually very exciting for people in quantum gravity to have observations that even have some chance of disproving a theory (or some version of a theory); the whole field is a dead end if it can never be tested by experiment.
In a broader sense, the holographic principle gives strong, model-independent reasons for believing that spacetime is probably discrete, not continuous, at the Planck scale. Otherwise it's hard to imagine how there could be an upper bound on the information content of a given region of space. And any theory in which spacetime is discrete at the Planck scale will naturally give a dispersive vacuum. Therefore I'd say that either (a) we should eventually observe dispersion of the vacuum once the observations get sensitive enough, or (b) the holographic principle is telling us something that we don't yet understand.
Two good popular-level books that get into this kind of thing are Three Roads to Quantum Gravity by Smolen, and The Black Hole War by Susskind. Because Smolen and Susskind represent very different points of view on quantum gravity, anything that both books agree on is probably correct.
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