|The Crab Nebula.|
Image Source: Hyperphysics.
One way this could happen is by distorting paths so that photons of different frequency (energy) move at slightly different speeds. Such an effect is referred to as ‘dispersion’. Next to dispersion there is dissipation, which is basically energy loss into the background. While quantum gravitationally induced dispersion has received substantial attention during the last decade, dissipation hasn’t received as much love.
In a nice and straight-forward recent paper dissipation finally got some love from Liberati and Maccione
- Astrophysical constraints on Planck scale dissipative phenomena
Stefano Liberati, Luca Maccione
Then they look at observations of highly energetic photons from a distant source, the Crab nebula. If space-time was viscous, the photons would lose energy during their travel. Already the rather conservative estimate that the photons of the highest observed energies shouldn’t have lost more energy than they have left at arrival leads to very tight constraints. If the photons lose energy faster than that, the spectrum we receive on Earth would be highly distorted and pretty much incompatible with our knowledge of astrophysics.
This constraint from existing data clearly rules out Planck scale effects, ie effects that plausibly have a quantum gravitational origin, at first order. Better constraints can be obtained by drawing upon concrete astrophysical models for the typical energy of photons that are emitted, so it seems likely that in the future we will see even better constraints on this.
Much like with violations of Lorentz-invariance this is a case where nothing has been found. Yeah, Einstein was right, again. But this doesn’t mean that nothing has been learned. We’ve learned that any model for an emergent space-time that does not have a very small, almost vanishing, viscosity is clearly incompatible with observation.