Testing black holes in general relativity using their horizon areas.

The celebrated Hawking's law states that, assuming General Relativity and the null energy condition hold, the surface areas of black holes cannot decrease in physical processes. This law has already been tested to ~3.7 sigma significance using gravitational wave observations. In this talk, we show that it is possible to not just test the area increase law, but also make quantitative predictions for deviations from general relativity.