At this blog there are extensive discussions about an anouncement of gravitational waves tomorrow, Thursday the 11th.
by "evidence for quantum gravity" I mean experimental results which don't agree with predictions of General Relativity or its classical generalizations.
Let us clear up some frameworks here. General Relativity is a classical theory, not quantized, and its application is important where there are large gravitational masses , deforming the flat four dimensional space of special relativity, or where great accuracy is needed. It has been validated in astrophysical observations many times, and is even necessary for the GPS system to run accurately.
They might be coming from string theory or loop gravity or from a completely different approach which we haven't discovered yet.
String theory is a theory within the framework of quantum mechanics, and it allows for quantization of gravity. The General Relativity framework emerges naturally when the dimensions are right from the string theoretical framework.
I want to understand what are the chances that new physics will show up during the upcoming gravitational wave observations.
The probability is very small. The observations are within the classical General Relativity predictions . They will validate the existence of classical gravitational waves. Classical gravitational waves, know nothing about gravitons, like classical electromagnetic waves know nothing about photons.
In the link at the top, there is a link where possible new physics might be found with LIGO like detectors, but not with the existing observation, as far as I know.
Gravitons exist in the effective Quantum Field theories for gravity that are being used in cosmological models , as the Big Bang model. And in string theories, which can quantize gravity. LIGO is studying the classical gravitational wave.
Also, I expect gravitational waves to be in a highly coherent state, thus making the individual gravitons unobservable. Is this true?
I do not know what the coherence has to do with whether gravitational waves can be broken down into individual gravitons, in the way electromagnetic waves can be broken down into photons. LIGO is not designed for single graviton detection, as the coupling constant of a graviton is so small that it will be undetectable