Something Out Of Nothing? Virtual Particles, the Casimir Effect, and Aristotle.

In 1996, researchers at the University of Washington watched with bated breath. A conducting plate was suspended micrometres from a metal sphere, both objects submerged within a vacuum. This pair of conductors began to slowly drift towards one another, pelted by so-called ‘virtual particles’ which had just been popped into existence mere fractions of a second earlier. Through this meticulously designed experiment, Steven Lamoreaux and his team obtained some of the first evidence for a quantum mechanical effect predicted a half century earlier, one which seemed to fly in the face of conventional wisdom: something had been created out of nothing. [1]

This phenomenon, the Casimir effect, was postulated by Dutch physicist Hendrik Casimir in 1948. It hinges upon the idea that the elementary particles which constitute the basic building blocks of the universe – electron, muons, quarks, and so forth – each have an associated ‘quantum field’ which spans the universe. In this model, particles are simply manifestations of oscillations within their associated field. The lowest energy state available to these fields is known as their zero-point energy, but is always greater than zero. This means even the vacuum of space has an intrinsic non-zero energy. [2]

Particle interactions are explained as interactions between one or more fields. To analyse these interactions through the lens of quantum field theory, we consider a virtual particle popping into existence in the region between the particles. These virtual particles have properties distinct from their real counterparts. They may exceed the speed of light. In accordance with the Heisenberg Uncertainty Principle, a linchpin of quantum mechanics, the more energy contained by virtual particles the shorter duration they exist on average. To calculate the probability of any particle interaction, one must in principle sum over every possible virtual particle which may materialise between the real particles in question. [3]

 

In a Feynman diagram, interactions between real particles, which come enter and exit the diagram from the edge, are mediated by virtual particles, which begin and end within the diagram.

 

Casimir noted that by placing a pair of parallel metallic very close together (on the order of micrometres), the ways in which the field could vibrate (known as its normal modes) in the region between the plates would be constrained. As such, the set of virtual particles which can form there is restricted – the zero point energy between two such plates in a vacuum is actually reduced. Contrarily, no such restrictions exist on the outside of the plates – any virtual particles may exist, and the zero point energy is unaffected by the plates. As such, the plates are bombarded with more virtual particles on their outside than from within; there is a net movement of the conductors towards one another. [3]

When considering the Casimir effect and other phenomena explained through virtual particles, such as Hawking radiation, it’s tempting to conclude that no true vacuum may exist at all. After all, if these vacuums in space have some energy, if particles can materialise and vanish at random, how can they be called vacuums? After two thousand years, has Aristotle had the last laugh?

 

Not quite (at least, as far as the physicist is concerned). It’s important to remember that virtual particles do not actually exist. They are mere tricks, mathematical sleights of hand to compute the probability of particle interactions. It is a case of perturbation theory, where complicated systems in quantum mechanics are explained using similar, but much simpler, systems, which are then slightly tuned or ‘perturbed’ such that they behave like the actual, more complex system. The metals observed by Lamoreaux were not really bombarded with virtual particles, they were simply pushed together by virtue of the energy differential in the vacuum between and outside the conductors.

Some versions of quantum field theory, chiefly lattice field theory, eschew the model of virtual particles entirely. Nonetheless, virtual particles are the powerful tool granting physicists insight into otherwise difficult or downright elusive problems. [3]

Sources:

1. Lamoreaux, S. K. “Demonstration of the Casimir Force in the 0.6 to.” Physical Review Letters, vol. 78, no. 1, 6 Jan. 1997, pp. 5–8, https://doi.org/10.1103/physrevlett.78.5.