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Quantum leap

November 18, 2011

Scientists at Chalmers University in Sweden have proved that a vacuum is not, in fact, empty, by capturing light in it. It is an experiment confirming a theory of quantum mechanics first proposed over 40 years ago.

Light particles can exist virtually, but only very brieflyImage: Universität Bonn

Physicists at Chalmers University in Gothenburg, Sweden, have managed to turn "virtual" light particles, flickering in and out of existence in a vacuum, into measurable, material particles.

The experiment was based on one of the more confusing, yet important principles in quantum mechanics: that a vacuum is by no means empty. In fact, empty space is a seething ocean of infinitesimal particles that fluctuate in and out of existence, defying the laws of classical physics because they only exist for the briefest of moments.

The Chalmers team succeeded in getting photons to leave their virtual state and become real photons, e.g. measurable light. They did this by effectively tricking the photons into thinking they were bouncing off a mirror spinning at close to the speed of light.

Their results were published in the journal Nature, on Thursday.

Copying a spinning mirror

In 1970, physicist Gerald Moore predicted that virtual photons would turn into material ones – in other words, measurable light - if they were bounced off a mirror moving at close to the speed of light. Since it is technologically impossible to do this, Moore's theory, known as the dynamical Casimir effect, was for forty years only a thought experiment.

Space, it turns out, is never really emptyImage: NASA, ESA, J. Richard, J.-P. Kneib

But the Chalmers scientists have now managed to replicate it by an ingenious trick.

"It's very hard to do, because anything that has a mass, you have to put in a lot of energy to make it move fast," said Per Delsing, a professor of Experimental Physics at Chalmers, in an interview with Deutsche Welle.

"So what we did, instead of moving something that is massive, we moved something that is mass-less. In layman's terms, you could say what we moved is a short in a circuit, and instead of changing where the short is, we change how much it's shorting – so we're changing the degree to which it shorts."

Timothy Spiller, a physics professor at the University of Leeds in the UK, was impressed with the experiment.

"I think it's a really beautiful demonstration," he told Deutsche Welle. "It increases our understanding and it's a breakthrough in physics."

The "mirror" of the Chalmers experiment is the SQUID (Superconducting quantum interference device) a quantum electronic component extremely sensitive to magnetic fields. By changing the direction of the magnetic field several billions of times a second, Delsing and his colleagues were able to make the "mirror" vibrate at a speed of up to five percent of the speed of light.

Give the vacuum a kick

It's important to make clear that the dynamical Casimir effect – creating light from nothing - is not a revolutionary new source of energy.

"If you take a mirror or a plate and wiggle it around very fast, then it ought to create photons, quanta of light, out of a vacuum," explained Spiller.

"Now, you're not getting anything for free. The energy comes from the moving plate being turned into light. In the vacuum you have all these little fluctuating things anyway, which kind of borrow energy on a very short time-scale and then give it back."

"If you whack the vacuum with this great big moving object, you're able to give some of those little fluctuations enough energy that they become real," said Spiller. "In short, if you give the vacuum a bit of a kick, stuff just appears out of it."

Werner Heisenberg
Heisenberg's uncertainty principle has withstood many experimental testsImage: DHM

This is, however, something only possible in quantum physics, first theorized at the beginning of the last century, but not in classical physics. It's possible because of the brief fluctuations in a vacuum.

"There's a very fundamental principle in quantum mechanics called the Heisenberg uncertainty relation," said Delsing. "Essentially it means you can make something which does not conserve energy if your timescale is short enough. And nature uses that to create particles and then quickly take them away again."

And when Delsing says "quickly," he means it: "It's a question of picoseconds."

A picosecond is one trillionth of a second, or – if you prefer - 0.000,000,000,001 seconds.

"People often call these fluctuations 'virtual particles,'" added Spiller.

"What they mean by that is they've borrowed energy to have this tiny, fleeting existence. Because of the uncertainty principle, you can have an uncertainty in energy and in time. These are complementary variables that between them have a bit of uncertainty - and quantum physics allows that uncertainty to exist. So these particles can borrow energy – their energy is 'uncertain' -provided they give it back. What these guys have done is give these virtual particles a kick to make them real."

Combining theory and practice

"Well, we think this is great," Delsing said of the work. "There were several experiments that led up to this. It was a long road. And it was a very intense collaboration between experiment and theory."

That collaboration is important, because while much of quantum mechanics is still theoretical, and in some cases impossible to demonstrate, it has stood the test of time. Increasingly the technology is being developed to test quantum theory.

This observation – like many others - goes some way to proving that quantum theory is very much on the right track. "It certainly confirms our quantum theory predictions," said Delsing. "There are many experiments that confirm the theory, and this is one of them."

"People keep trying to find new tests for quantum physics, in one sense in the hope that they find something unexpected and different," said Spiller. "Every time they come up with a new test, it seems that quantum mechanics meets that challenge and gives the results you would expect."

Author: Ben Knight
Editor: Cyrus Farivar