If you take two flat mirrors and place them very close together, the virtual particles that pop into existence between the mirrors will actually force them together. But that's nothing compared to when mirrors approach the speed of light.
We've talked about virtual particles before, but for our purposes all we really need to know is that, according to our understanding of quantum mechanics, pairs of particles and their corresponding antiparticles will pop into existence, then almost immediately annihilate each other. These are known as quantum fluctuations.
So how do the mirrors come into it? Basically, if the two mirrors are close enough together, the distance between them actually becomes smaller than the wavelengths of the virtual particles. This in turn creates an imbalance between the vacuum pressure inside the mirrors and that on the outside, creating an attractive force that brings the two mirrors together. This is known as the static Casimir effect, and it was experimentally demonstrated in 1998.
Still, that's downright normal compared to the dynamic Casimir effect. This can only come into play if a mirror is traveling at relativistic speeds, which is roughly speaking 10% of the speed of light or faster. Technology Review's arXiv blog explains what theoretically happens here:
At slow speeds, the sea of virtual particles can easily adapt to the mirror's movement and continue to come into existence in pairs and then disappear as they annihilate each other. But when the speed of the mirror begins to match the the speed of the photons, in other words at relativistic speeds, some photons become separated from their partners and so do not get annihilated. These virtual photons then become real and the mirror begins to produce light.
You'd think that something like this would be impossible to test, considering you need to get a mirror traveling close to the speed of light. But researchers at Sweden's Chalmers University have figured a rather ingenious way around that little conundrum. Again, Technology Review explains:
Instead of a conventional mirror, they've used a transmission line connected to a superconducting quantum interference device or SQUID. Fiddling with the SQUID changes the effective electrical length of the line and this change is equivalent to the movement of an electromagnetic mirror. By modulating the SQUID at GHz rates, the mirror moves back and forth. To get an idea of scale, the transmission line is only 100 micrometres long and the mirror moves over a distance of about a nanometre. But the rate at which it does this means it achieves speeds approaching 5 per cent light speed.
So having perfected their mirror moving technique, all Wilson and co have to do is cool everything down, then sit back and look for photons. Sure enough, they've spotted microwave photons emerging from the moving mirror, just as predicted.
This is, according to the researchers, the first ever observation of the dynamical Casimir effect. Now it's just a question of finding a sufficiently evil application for a light-producing mirror traveling at relativistic speeds. Seriously, something like that should be enough to conquer a small country at the very least.