Unruh effect Guide, Meaning , Facts, Information and Description
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, that is, the accelerating observer will find themselves in a warm background. The quantum state which is seen as ground state for observers in inertial systems is seen as a thermodynamic equilibrum for the uniformly accelerated observer.Unruh demonstrated that the very notion of vacuum depends on the path of the observer through spacetime. From the viewpoint of the accelerating observer, the vacuum of the inertial observer will look like a state containing many particles in thermal equilibrium -- a warm gas. Although the Unruh effect came as a shock, it makes intuitive sense if the word vacuum is interpreted appropriately, as below.
In modern terms, the concept of "vacuum" is not the same as "empty space", as all of space is filled with the quantized fields that make up a universe and the vacuum is simply the lowest possible energy state of these fields, a very different concept than "empty". The energy states of any quantized field are defined by the Hamiltonian, based on local conditions, including the time coordinate. According to special relativity, two observers moving relative to each other must use different time coordinates. If those observers are accelerating, there may be no shared coordinate system and, due to this, the observers will see different quantum states and thus different vacua.
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.
The equivalent energy kT of a uniformly accelerating particle is:
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