Einstein's general theory of relativity was published almost a century ago and now more proof has appeared to support his theory, while constraining a more recent addition of this theory. One of the crucial supports of Einstein's work is that light travels at the same speed, regardless of its energy. We now know that this is not accurate in media such as glass, but in a vacuum Einstein's work involves radio waves, gamma rays and everything in between to keep pace. Whereas general relativity is considered as one of the highest accomplishments of the human intelligence, it remains subject to major challenges. Even though Einstein's work keeps effectively forecasting the actions of every cosmic laboratory we can discover, uncertainty remains because it is irreconcilable with quantum mechanics. At least one of them needs some alteration if we are to completely understand the universe.
The different energies of light from a gamma ray burst reached at the same time. Image credit: NASA's Goddard Space Flight Center.
So when Professor Tsvi Piran of the Hebrew University of Jerusalem verified the detection of photons from a gamma-ray burst (GRB) about seven billion light-years away, he didn't know what he would actually find. According to one effort to merge the success of general relativity and quantum theory on very diverse levels, space-time has a “foamy” structure instead of being continuous. The theorized bubbles are billions of times too small to detect (of the order 10-35m), but it has been proposed that they would affect the transmission of light in a much more delicate form of what happens in glass or water. But in Nature Physics, Piran and his co-authors state the opposite. Along with the gamma rays that give the burst its name, the burst emitted lower energy photons. Under the foamy model, the peak energy gamma rays would be projected to be most affected by the widespread froth, and therefore arrive the latest. We don't notice this with uninterruptedly shining objects since the low energy photons unconfined a little later arrive at the same time as the higher energy equivalents that headed them away from the light source.
However, for something rapid like a GRB, Piran thought the variance might be noticeable. If such a variance did happen, however, it was too slight to be measured. Any cosmic fizz then need to be smaller than the sensitivity of Piran's tools, discounting some of the bigger sizes projected by theoretical models.
It's not the first time GRBs have been used to check the foamy theory of space-time, but an earlier work relied on just three photons, slightly less than a statistically significant sample. A study of gamma rays from a black hole 500 million light-years away create a four-minute postponement forhigher energy photons, but even the study's author admitted other descriptions were possible.
Piran says “When we began our analysis, we didn't expect to obtain such a precise measurement. This new limit is at the level expected from quantum gravity theories and can direct us how to combine quantum theory and relativity."