Einstein's theory of General Relativity tells us that gravity is caused by the curvature of space and time. It is a remarkable theory that has been confirmed by countless observations, such as gravitational lensing, light deflection on the sun, redshift in the gravitational potential, black holes and their shadows, and gravitational waves. However, we already know that General Relativity cannot be quite right because it does not fit together with another well-confirmed theory that is quantum mechanics. To resolve this tension, physicists think that we need a theory of quantum gravity. In this video I explain the three key reasons why physicists think that we need a theory of quantum gravity. Support me on Patreon: https://www.patreon.com/Sabine