Experimental evidence for the Unruh effect.

We examine the radiation reaction imparted on an electron accelerating and radiating in an optical medium. The radiation reaction of the electron is incorporated by modeling it as an Unruh-DeWitt detector and setting the energy gap equal to the recoil energy produced by the emitted photons momentum. In the subsequent analysis, we discover a vacuum Larmor formula and an in-medium power spectrum that are both thermalized by the acceleration. As such, these systems will explicitly exhibit thermalization of radiation reaction at the celebrated Fulling-Davies-Unruh temperature. Therefore, our theory proves that detections of radiation reaction associated with recoil are, in fact, simultaneous observations of the Unruh effect. Our derived power spectrum is then shown to have an excellent agreement with the recent experiment on quantum radiation reaction in aligned crystals and therefore provides evidence for the very first observation of the Unruh effect.