So far, most effort in the field has gone into using requirements of mathematical consistency to construct a theory. It is impossible of course to construct a theory based on mathematical consistency alone, because we can never prove our assumptions to be true. All we know is that the assumptions give rise to good predictions in the regime where we’ve tested them. Without assumptions, no proof. Still, you may hope that mathematical consistency tells you where to look for observational evidence.
But in the second half of the 20th century, theorists have used the weakness of gravity as an excuse to not think about how to experimentally test quantum gravity at all. This isn’t merely a sign of laziness, it’s back to the days when philosophers believed they could find out how nature works by introspection. Just that now many theoretical physicists believe mathematical introspection is science. Particularly disturbing to me is how frequently I speak with students or young postdocs who have never even given thought to the question what makes a theory scientific. That’s one of the reasons the disconnect between physics and philosophy worries me.
In any case, the cure clearly isn’t more philosophy, but more phenomenology. The effects of quantum gravity aren’t necessarily entirely out of experimental reach. Gravity isn’t generally a weak force, not in the same way that, for example, the weak nuclear force is weak. That’s because the effects of gravity get stronger with the amount of mass (or energy) that exerts the force. Indeed, this property of the gravitational force is the very reason why it’s so hard to quantize.
Quantum gravitational effects hence were strong in the early universe, they are strong inside black holes, and they can be non-negligible for massive objects that have pronounced quantum properties. Furthermore, the theory of quantum gravity can be expected to give rise to deviations from general relativity or the symmetries of the standard model, which can have consequences that are observable even at low energies.The often repeated argument that we’d need to reach enormously high energies – close by the Planck energy, 16 orders of magnitude higher than LHC energies – is simply wrong. Physics is full with examples of short-distance phenomena that give rise to effects at longer distances, such as atoms causing Brownian motion, or quantum electrodynamics allowing stable atoms to begin with.
Next week, we will hold the 5th international conference on Experimental Search for Quantum Gravity, here in Frankfurt. And I dare to say we have managed to pull together an awesome selection of talks.
We’ll hear about the prospects of finding evidence for quantum gravity in the CMB (Bianchi, Krauss, Vennin) and in quantum oscillators (Paternostro). We have a lecture about the interface between gravity and quantum physics, both on long and short distances (Fuentes), and a talk on how to look for moduli and axion fields that are generic consequences of string theory (Conlon). Of course we’ll also cover Loop Quantum Cosmology (Barrau), asymptotically safe gravity (Eichhorn), and causal sets (Glaser). We’re super up-to-date by having a talk about constraints from the LIGO gravitational wave-measurements on deviations from general relativity (Yunes), and several of the usual suspects speaking about deviations from Lorentz-invariance (Mattingly), Planck stars (Rovelli, Vidotto), vacuum dispersion (Giovanni), and dimensional reduction (Magueijo). There’s neutrino physics (Paes), a talk about what the cosmological constant can tell us about new physics (Afshordi), and, and, and!
You can download the abstracts here and the timetable here.
But the best is I’m not telling you this to depress you because you can’t be with us, but because our IT guys still tell me we’ll both record the talks and livestream them (to the extent that the speakers consent of course). I’ll share the URL with you here once everything is set up, so stay tuned.
Update:Streaming link will be posted on the institute's main page briefly before the event. Another update: Lifestream is available here.