The Unruh-ly behavior of particles in advanced quantum physics.
One of the strangest predictions of theoretical physics is the Unruh effect; named after Canadian physicist Bill Unruh (though Stephen Filling and Paul Davies also share the credit for its discovery). It is the effect whereby particles seem to exist for one observer, but not for another. It has yet to be definitively demonstrated experimentally and there is still much debate on how to interpret it, but it has offered valuable insights in to the complicated world of quantum field theory and is closely linked to Hawking radiation emitted by black holes. Interestingly it was on a list of things marked “to learn” written by legendary physicist Richard Feynman to himself that he left when he died in 1988.
Whilst unsurprisingly a non-accelerated observer observes empty space, to be just that i.e. empty, the Unruh effect predicts that an observer undergoing constant acceleration in ‘empty’ space, rather than observing the said space to be empty, will observe a thermal bath of so-called Unruh radiation. Generally Unruh radiation is thought of as consisting of photons, but due to the general nature of the Unruh effect can be just about any kind of particle. The non-accelerated observer, examining the same empty space, will not be able to see or detect the Unruh radiation that is visible to the accelerated observer. The Unruh temperature of empty space for an accelerated observer is given by T = ħa/2πck, where a is the acceleration of the observer, ħ is the Planck constant, c the speed of light in a vacuum and k is the Boltzmann constant. This means that, unless an observer’s acceleration is insanely large, the Unruh temperature will only just be above absolute zero- making Unruh radiation very difficult to observe (though feasible experiments have been designed to detect it, and some even claim it has already been detected). In simple terms this means that when you press down the accelerator in your car, particles will appear to you that someone standing still will not be able to see!
Quantum field theory is quantum mechanics more advanced progeny, for example: Quantum electrodynamics (the quantum theory of the electromagnetic force) and quantum chromodynamics (the quantum theory of the strong force) are both quantum field theories. In quantum mechanics the notion of a particle is very exact and the number of particles is always fixed- this is as particles are fundamental objects in quantum mechanics. In quantum field theory the picture is rather different as the fundamental objects are quantum fields and the number of particles is not set in stone and can be subject to quantum uncertainty (though this even this view is slightly simplistic). If a non-accelerated observer were to perform a measurement in order to count the number of particles contained within a region of empty space the expected value for the result of the measurement is (unsurprisingly) zero. The Unruh effect comes from the fact that when transforming the expectation value from a non-accelerated observer to an accelerated observer it can change from zero to a non-zero number. This is as in the mathematical setting of quantum field theory, the notion of “a particle” and in particular, the notion of “how many particles”, is actually quite weak.
The Unruh effect takes place in a set of spacetime coordinates called ‘Rindler coordinates’ which represent an accelerated observer and have some rather surprising properties. In special relativity due to the effect of length contraction (the effect whereby observers travelling at different speeds observe different lengths) in order to keep a constant distance from an observer undergoing constant acceleration a trailing observer must have an even greater acceleration How much more acceleration is a function of their distance from the observer and there is a finite distance where the acceleration required to keep up with an accelerated observer goes to infinity, this is called the Rindler horizon. The Rindler horizon is very much like the event horizon of a black hole and in in fact an event horizon is really just a Rindler horizon that is warped from a flat wall-like shape into the shape of a sphere by the curvature of spacetime. Anything beyond the Rindler horizon cannot reach the accelerated observer in a finite (proper) time, just as nothing can escape a black hole within a finite time. One thing to note though is that the Rindler horizon is not a physical horizon as it disappears as soon as the Rindler observer stops accelerating. Hawking radiation produced by a black hole can be seen as just a type of Unruh radiation brought about by the curvature of spacetime.-John Davis
Wiki link:
http://en.wikipedia.org/wiki/Unruh_effect
Sources:
Picture: http://www.extreme-light-infrastructure.eu/High-field_5_2.php
The Unruh effect and its applications, Crispino, Higuchi, Matsas
http://arxiv.org/abs/0710.5373
Hawking-Unruh radiation and radiation of a uniformly accelerated charge, McDonald
http://physics.princeton.edu/~mcdonald/accel/unruhrad.pdf
Quantum mechanics: Myths and facts, Nikolic
http://arxiv.org/abs/quant-ph/0609163
General Relativity, Wald
