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SQUID calls 'virtual photons' into real existence

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While “vacuum fluctuations” – one of the stranger predictions of quantum physics – have been observed for some time, a team comprising scientists from Chalmers University of Technology in Sweden and the University of University of New South Wales have gone one step further, creating their own photons out of empty space.

Here’s how you do it: all you need to do is accelerate a mirror to a decent fraction of the speed of light. Virtual photons reflected by that mirror will leave their virtual state, and become measurable.

Since actually accelerating an object with any measurable mass to sufficient speed poses problems, some tricks are needed: instead of a mirror, use a device that creates a photon-reflecting field; and make that field controllable, so you can mimic the necessary acceleration.

That’s what the Swedish / Australian researchers claim to have done: a SQUID (superconducting quantum interference device) generates a field which acts as a “photon mirror”, and that field can be controlled by magnets. “Flip” the fields at gigahertz frequencies, and the effect is the same as a magnet moving at 25 percent the speed of light.

To understand how this happens, we need to work our way out from the Heisenberg uncertainty principle, which among other things predicts that empty space is never truly empty. Rather, it’s populated by “virtual particles” – particles and anti-particles in fact – that spontaneously come into existence and then annihilate each other, releasing a photon along the way.

It’s strange enough to know that such phenomena are not just a theory, but observable, as was described to El Reg back in September. To discover that virtual photons are not merely observable, but can be turned into real photons under lab conditions, with predictable behaviour, is a real head-turner.

The Casimir effect

A physicist called Hendrik Casimir proposed, in 1948, that virtual particles could be made to exhibit measurable physical properties, by a fairly simple (if difficult to conduct) experiment. If two parallel metal plates can be brought close enough together – very, very close – then virtual particles coming into existence between them would be squeezed out. The effect of “stuff” leaving the space between the plates would create a measurable but very small pressure bringing the plates together.

An odder manifestation of the Casimir effect was proposed in 1970 by Gerald T Moore, then of Brandeis University, the dynamical Casimir effect (DCE). As Professor Tim Duty of the UNSW School of Physics explained to Science Alert:

"The DCE was conceived as a kind of thought experiment, sort of like Schrödinger’s Cat. According to quantum theory, if one could accelerate a mirror very quickly to near the speed of light, the mirror would radiate light, as some of the mirror’s motional energy is imparted to virtual photons lurking in the vacuum, converting them into real photons.

“But it is practically impossible to accelerate a massive mirror to such high velocities. The required accelerations would be greater than the kind of shocks found in supernova or nuclear weapons explosions.”

Hence the use of the SQUID as the “mirror”, and since the SQUID is sensitive to magnetism, exposing it to a magnetic field that reverses a few billion times a second causes the “mirror” to vibrate at 25 percent of the speed of light.

Photons were observed emerging from the vacuum in pairs, at microwave energies – and the researchers say the observed photons have properties predicted by quantum theory. ®

Bootnote: I realize I didn’t win any science-scoops competition with this story. I’d rather understand what’s going on and explain it than be the first to run with the press release.

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