Yesterday I mentioned the Casimir effect, and how it could hypothetically be used to detect gravitons. But what exactly is the Casimir effect, and how do we know it’s real?

Fluctuations are limited by the conducting plates.

The Casimir effect is a great example the strangeness of quantum theory, and how even some of its strangest predictions turn out to be right. The effect was first proposed by Hendrik Casimir in the 1940s as a consequence of quantum fluctuations. The basic idea is that within quantum electrodynamics, a region of empty space actually contains quantum fluctuations of the electromagnetic field. These fluctuations are extraordinarily small, and in most cases we’d never notice them. But since they are electromagnetic, they are still affected by the presence of conducting materials. Specifically, they can be bounded by a conducting surface. So if you place two parallel conducting surfaces close to each other, the fluctuations are bound between the plates, but not outside the plates. As a result, there are less fluctuations between the plates than on either side. This means there is less pressure between the plates, and the plates are therefore pulled together.

This net attraction due to wave fluctuations is not particularly surprising. In fact you can demonstrate this effect with water waves. What makes is surprising is that according to classical electromagnetism, since the two plates are uncharged there should be no electric field between them and no force of attraction. Two plates in a vacuum are somehow attracted to each other simply because they are close together. When Casimir first calculated the effect, he used perfect “ideal” conductors. Later, more detailed calculations showed the effect for realistic conductors, and in 1997 the effect was confirmed experimentally. The most recent experiments get results to within 1% of the theoretical result. Strange as it is, the Casimir effect is very real.

Although the reality of the Casimir effect is not in doubt, its strangeness has led to much debate over what it actually means. Since it seems to show an extraction of energy from the “vacuum,” zero-point-energy fans have used it to support claims of “free energy” devices. Since the energy level between the plates is less than the average energy level outside the plates, the effect has been suggested as a solution for exotic physics such as wormholes and warp drive. It also raises difficulties in cosmology. If quantum fluctuations have real energy, then they should be affected by gravity, and that should affect the cosmological constant. According to QED the cosmological constant should be huge, but in fact it’s actually very small (assuming it’s the cause of dark energy).

But it’s important not to overstate the implications of the Casimir effect. It does raise some interesting questions about quantum gravity, but the main thing it does is demonstrate that our understanding of electromagnetism on a quantum scale is actually quite good.

Paper: H. B. G. Casimir and D. Polder. The Influence of Retardation on the London-van der Waals Forces. Phys. Rev. 73, 360 (1948)

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A recent article in Physical Review Letters proposes a new way to detect gravitons. The setup could be done in a lab, which is in stark contrast to the usual view that you’d need a Jupiter-mass detector orbiting a neutron star to detect gravitons. It’s one of those “if we pull this off we’ll eat like kings” experiments, so naturally we should be a little skeptical.

You might remember that gravitons are the hypothetical quanta for the gravitational field, just as photons are the quanta of electromagnetism, and gluons that of the strong. While we don’t have a complete quantum theory of gravity, it’s generally thought that gravitons likely exist, so it’s worth trying to look for them.

In this particular paper, the author proposes using the Casimir effect, which where two metallic plates placed very close to each other (fractions of a millimeter apart). According to quantum theory, there are less fluctuations between the plates than outside the plates. As a result the plates have a net attractive force between them. In practice this is due to electromagnetic quanta, but in principle it should also work for gravitons as well. The gravitational Casimir effect would just be much smaller. But the author claims that for superconducting materials the gravitational effect should be much stronger, and therefore be detectable. This claim is based on an earlier paper on gravitational waves and superconductors, and there are questions as to whether the earlier work is really valid.

Overall it’s an interesting idea, and the author is careful not to overstate the conclusions of the work. It might be an experiment worth trying, but I wouldn’t plan a banquet just yet.

Paper: James Q. Quach. Gravitational Casimir Effect. Phys. Rev. Lett. 114, 081104 (2015)

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