# Why do we use Schwarzschild solution as background when applying QFT to black hole?

Schwarzschild solution is classical, so it does not allow for violation of energy conditions to allow for Hawking radiations. So what justifies its use in QFT for a black hole?

• I'm not sure if I fully understand your question. If the backreaction from your QFT is very small, then it's valid to use the classical Schwarzschild solution. What I mean is that if the quantum fields have only a small amount of energy in them, then they won't deform the spacetime metric very much; to a good approximation one can use the classical background of choice. – WAH Feb 10 '18 at 7:04