Is the quantum vacuum (the "zero particle" state) really empty? Is there an observer-independent notion of particle number? Surprisingly there is not! What feels to one observer like empty space may feel to another observer like a thermal bath of particles. The difference-maker is acceleration; accelerated observers in the vacuum will feel a non-zero temperature. This is known as the Unruh effect and is one of the fundamental and yet still untested predictions of quantum field theory. In this video, I will introduce the Unruh effect and overview a new experimental setup that may be able to detect it.