Unruh effect

The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none. Unruh realized that the notion of vacuum depends on the path of the observer through spacetime.

Although the Unruh effect came as a shock, it makes intuitive sense. The vacuum is, by definition, the lowest energy state of the Hamiltonian, the Hamiltonian depends on the choice of time coordinate and, according to special relativity, two observers moving relative to each other must use different time coordinates. Consequently, from the viewpoint of the accelerating observer, the vacuum of the inertial observer will look like a state containing many particles in thermal equilibrium.

In some cases the vacuum of one observer is not even in the space of quantum states of the other. In technical terms, this comes about because the two vacua lead to unitarily inequivalent representations of the quantum field canonical commutation relations. This is because two mutually accelerating observers may not be able to find a globally defined coordinate transformation relating their coordinate choices. In fact, an accelerating observer will perceive an apparent event horizon forming (see Rindler spacetime). The existence of Unruh radiation can be linked to this apparent event horizon, putting it in the same conceptual framework as Hawking radiation. On the other hand, the Unruh effect shows that the definition of what constitutes a "particle" depends on the state of motion of the observer.

Just as the Rindler spacetime can be seen as a toy model for black holes and cosmological horizons, the Unruh effect provides a toy model to explain Hawking radiation.