Quantum gravity is the holy grail of theoretical physics in the 21st century. The frustrating thing about the search for it is that the window in which we could experimentally access quantum effects of gravity is very far away from what we can reach. It would take particle energies as high as 1016 TeV to access them. That is 15 orders of magnitude higher than what even the Large Hadron Collider - The World's largest Microscope - will probe. Alternatively, one had to examine distances as small as 10-20femtometers!

Nevertheless, the situation is not completely hopeless, and over the last years there have been several proposals how one could test possible features that arise from quantum gravity. Here at Perimeter Institute, I have initiated a discussion group on the subject, that has a blog for discussions

Quantum Gravity in the Lab??

This week's topic was the possibility to detect an energy dependent speed of light as it arises in certain models:

Some scenarios of quantum gravity indicate a modification of the dispersion relation (that is essentially the relation between energy, momentum and mass of a particle.) In these cases, the speed of a photon depends on its energy. This means that a signal composed of different frequencies looses its shape while traveling. Roughly spoken, this is because higher energetic photons are faster than the lower energetic ones. The problem is that this difference in the time of flight is hard to detect, since as mentioned, quantum gravitational effects are notoriously tiny.

However, a difference in time of flight can add up given that the signal composed of different frequencies travels over a long distance. If one inserts the typical scales it turns out that γ-ray bursts provide a source that would make such an effect - tiny as it is - observable with the GLAST satellite. The bursts have a high energetic contribution that can reach up to 1 GeV, and a typical distance of a Gpc, and the accumulated difference in time of flight between the higher and lower energies becomes comparable to the typical duration of the burst itself (of the order milliseconds), and thus potentially detectable. For more information, see my summary of the discussion.