A century after the publication of Einstein's general theory of relativity—and just in time for the great man's birthday—evidence has emerged to support his theory, while constraining a more recent extension.
One of the central pillars of Einstein's work is that light travels at the same speed, irrespective of its energy. We know that this is not true in media such as glass, but in a vacuum Einstein's work requires radio waves, gamma rays and everything in between to keep pace.
While general relativity is regarded as one of the peak achievements of the human intellect, it remains subject to significant challenges. Although Einstein's work keeps successfully predicting the behavior of every cosmic laboratory we can find, uncertainty remains because it is incompatible with quantum mechanics. At least one of them requires some modification if we are to fully understand the universe.
So when Professor Tsvi Piran of the Hebrew University of Jerusalem tested the arrival times of photons from a gamma-ray burst (GRB) seven billion light-years away, he didn't know what he would find. According to one attempt to reconcile the success of general relativity and quantum theory on very different scales, spacetime has a “foamy” structure rather than being continuous. The hypothesized bubbles are billions of times too small to observe (of the order 10-35m), but it has been suggested that they would affect the transmission of light in a much more subtle version of what occurs in glass or water.
However, in Nature Physics, Piran and his co-authors report the opposite. Along with the gamma rays that give the burst its name, the burst emitted lower energy photons. Under the foamy model, the highest energy gamma rays would be expected to be most affected by the universal froth, and therefore arrive the latest. We don't notice this with continuously shining objects since the low energy photons released a little later arrive at the same time as the higher energy equivalents that preceded them away from the light source.
However, for something sudden like a GRB, Piran thought the difference might be detectable. If such a difference did occur, however, it was too small to be measured. Any cosmic fizz then must be smaller than the sensitivity of Piran's instruments, excluding some of the larger sizes predicted by theoretical models.
It's not the first time GRBs have been used to test the foamy theory of spacetime, but a previous effort relied on just three photons, somewhat less than a statistically significant sample. A study of gamma rays from a black hole 500 million light-years away found a four-minute delay for higher energy photons, but even the study's author admitted other explanations were possible.
“When we began our analysis, we didn't expect to obtain such a precise measurement," says Piran. "This new limit is at the level expected from quantum gravity theories and can direct us how to combine quantum theory and relativity."