## Dear Dr B: What do physicists mean by “quantum gravity”?

27 Sep 2016, 07:16 UTC[Image Source: giphy.com]“please could you give me a simple definition of "quantum gravity"?J.”Dear J,Physicists refer with “quantum gravity” not so much to a specific theory but to the sought-after solution of various problems in the established theories. The most pressing problem is that the standard model combined with general relativity is internally inconsistent. If we just use both as they are, we arrive at conclusions which do not agree with each other. So just throwing them together doesn’t work. Something else is needed, and that something else is what we call quantum gravity. Unfortunately, the effects of quantum gravity are very small, so presently we have no observations to guide theory development. In all experiments made so far, it’s sufficient to use unquantized gravity.Nobody knows how to combine a quantum theory – like the standard model – with a non-quantum theory – like general relativity – without running into difficulties (except for me, but nobody listens). Therefore the main strategy has become to find a way to give quantum properties to gravity. Or, since Einstein taught us gravity is nothing but the curvature of space-time, to give quantum properties to space and time. Just combining the quantum field theory with ...