## Subatomic Spacetime

Posted by on Monday, January 14, 2013

One of the leading fields of research in theoretical physics at present is the search for a quantum theory gravity. I have written before in this blog about this unsolved problem, but here is a brief recap: Of the four fundamental forces of nature, all but gravity are explained by quantum theories. We know that gravity must also be compatible with quantum theory at high energies and small scales, but every attempt to combine the two most successful theories in science have met with either technical problems that render them useless, or at best they give no definitive phenomenon that can be tested.

However one aspect of quantum gravity is now within reach of astrophysics experiments, and some very preliminary data has just been released. Since gravity does not exhibit quantum effects on larger scales (ie distances and energies we experience in normal life) but does on subatomic scales, it is expected that space and time must behave differently at different energies and distances. In particular, it is believed that at distances far smaller than even an atomic nuclei, spacetime has bubbles and loops and a general foamy structure. This roughness leads to a energy dependence in the speed of light, while classical physics and the theory of relativity predicts energy independence.

As a result of this predicted difference, it is quite likely that high energy photons (such as gamma-rays) that are emitted simultaneously and that travel through large distances will arrive at slightly different times. Now a team of astrophysicist have captured three gamma rays from a burst that occurred seven billion light years away - and what they observed was that there was no difference in their speed (within the experimental limits of the detectors) indicating the the quantum effects are much smaller than originally thought (or that photons do not interact with the quantum foam in the way we expected).

Of course this is very preliminary, and three photons do not make a statistically significant result. There could be any number of problems with the detection and the processing of the data. Although there have been several claims made in the literature about what this means for competing candidates of quantum gravity, in truth it is too soon to really deduce too much from the data.

However it is an interesting achievement, and could be one of the first true experimental bounds on the spacetime quantum foam. With more data and more analysis, this experiment could also be the first to discover a positive sign of quantum gravity, even if it is at smaller distances than anticipated.

So although it is very preliminary, it is still quite interesting.

However one aspect of quantum gravity is now within reach of astrophysics experiments, and some very preliminary data has just been released. Since gravity does not exhibit quantum effects on larger scales (ie distances and energies we experience in normal life) but does on subatomic scales, it is expected that space and time must behave differently at different energies and distances. In particular, it is believed that at distances far smaller than even an atomic nuclei, spacetime has bubbles and loops and a general foamy structure. This roughness leads to a energy dependence in the speed of light, while classical physics and the theory of relativity predicts energy independence.

As a result of this predicted difference, it is quite likely that high energy photons (such as gamma-rays) that are emitted simultaneously and that travel through large distances will arrive at slightly different times. Now a team of astrophysicist have captured three gamma rays from a burst that occurred seven billion light years away - and what they observed was that there was no difference in their speed (within the experimental limits of the detectors) indicating the the quantum effects are much smaller than originally thought (or that photons do not interact with the quantum foam in the way we expected).

Of course this is very preliminary, and three photons do not make a statistically significant result. There could be any number of problems with the detection and the processing of the data. Although there have been several claims made in the literature about what this means for competing candidates of quantum gravity, in truth it is too soon to really deduce too much from the data.

However it is an interesting achievement, and could be one of the first true experimental bounds on the spacetime quantum foam. With more data and more analysis, this experiment could also be the first to discover a positive sign of quantum gravity, even if it is at smaller distances than anticipated.

So although it is very preliminary, it is still quite interesting.