One of the awkward aspects of modern physics is that its two most successful fields, relativity and quantum mechanics, are fundamentally incompatible, as things happen in the quantum world that relativity says should not be possible. That's left physicists looking for a way to harmonize the two, with two primary contenders: string theories, and quantum gravity theories. Testing either of them has been a bit challenging, but researchers have now managed to use a single, intensely powerful photon detected by the Fermi Telescope to significantly limit the number of viable quantum gravity theories.

The Fermi Gamma-ray Space Telescope has only been operational for about a year, but results from its observation have already been appearing in a number of significant publications. The observatory is designed to detect the highest energy radiation, which is only produced by the most energetic events in the universe, such as supernovae. In this case, the key observation was of a single photon produced by the gamma-ray burst GRB 090510, which came in at an extremely energetic 31GeV.

That photon allowed researchers to test some forms of quantum gravity against the predictions of relativity. According to relativity, the speed of light obeys Lorentz Invariance: it's the same for all observers and all energies of light. Some theories of quantum gravity, however, suggest that Lorentz Invariance may break down near the Planck length, 1.62 x 10-33cm, causing high-energy photons to travel at different speeds than their low-energy peers. Unfortunately, the effects are small, meaning you need something very high energy that's travelled for a very long distance before you could detect them.

That's why this gamma ray burst turned out to be so useful. GRB 090510 was found to have a redshift of z=0.9, which places it about 10 percent of the way across the observed universe, which gives it the sort of distance required. The 31GeV gamma ray has the sort of energy needed to see a difference between it and some of the lower-energy photons detected at the same time, and the event was short-lived, with most of the high-energy photons arriving within a single second of each other. If high energy photons moved at a different speed, we should be able to detect it.

We don't. There's a degree of imprecision when it comes to the time window in which the high-energy photons arrive, and the authors spend most of the paper considering different scenarios, like the probability that high-energy photons might be generated earlier than their low-energy peers (the authors' conclusion: this seems very unlikely). In the end, they conclude that if there is a quantum influence on the speed of light, it can only operate at distances of less than 1.2 times the Planck length.

A value this close to the Planck length means that quantum gravity models in which there's a linear relationship between photon energy and speed are "highly implausible." That leaves other quantum gravity options open, including those in which the the relationship is non-linear. Hopefully, theoreticians will be able to devise real-world tests for some of these.

Of course, it remains possible that some of the timing assumptions made by the authors are wrong. Fortunately, it looks like we may have other opportunities to test things at much greater distances. The same issue of Nature contains two papers that describe another gamma-ray burst, GRB 090423, which was detected earlier this year. Preliminary results suggested that it was the most distant item of this sort ever detected, and the papers confirm this: z = 8.2, which corresponds to an origin at the time when the Universe was only 630 million years old.

The discovery suggests that massive stars were being born and exploding in very short order after the birth of the universe. The similarity of the burst with more modern ones suggests that, on some levels, the early universe wasn't entirely different from its current state. It also raises the prospect that we can use further bursts of this age to study the Cosmic Dark Ages, when the gasses that make up much of the visible matter of the universe had cooled enough to form neutral atoms, absorbing much of the light. This period ended as the first stars started re-ionizing these atoms, allowing light to propagate across the Universe. This era started at about 800 million years post-big bang, so GRB 090423 may provide a window into the era.

Nature, 2009. DOI: 10.1038/nature08574

Nature, 2009. DOI: 10.1038/nature08459

Nature, 2009. DOI: 10.1038/nature08445