How does General Relativity contradict quantum mechanics?

[lizzie96]

How does General Relativity "contradict" quantum mechanics?

I couldn't work out exactly where to post this. I've heard several times that QM and General relativity "contradict" each other and in certain extreme conditions this becomes a problem. Is this right? I've only heard this in very vague terms and never read a technical explanation of why. Could anyone explain?

 

[mfb]

They do not contradict each other, we just don't know how physics looks like if both are important at the same time.
It is expected that gravity will show quantum-effects as well, but plugging gravity into the formalism of quantum field theory leads to things we cannot calculate (more technical: renormalization does not work).

 

[PeterDonis]

One particular case in which the predictions of GR appear to be inconsistent with QM is black holes. Briefly: GR predicts that a quantum system that falls into a black hole will be destroyed in the singularity at its center, along with the quantum information it contains; but this violates unitarity, a basic principle of QM, which says, basically, that quantum information can't be destroyed. This is called the "black hole information paradox"; the Wikipedia page has a decent overview:

http://en.wikipedia.org/wiki/Black_hole_information_paradox

The bottom line is that, as we know them today, both GR and QM are incomplete theories, so they don't so much "contradict" each other as fail to cover all possible physical scenarios.