Empty space is anything but, according to quantum mechanics: Instead, it roils with quantum particles flitting in and out of existence. Now, a team of physicists claims it has measured those fluctuations directly, without disturbing or amplifying them. However, others say it's unclear exactly what the new experiment measures—which may be fitting for a phenomenon that originates in quantum mechanics' famous uncertainty principle.
"There are many experiments that have observed indirect effects of vacuum fluctuations," says Diego Dalvit, a theorist at Los Alamos National Laboratory in New Mexico who was not involved in the current work. "If this [new experiment] is correct, it would be the first direct observation of the field [of fluctuations] itself."
Thanks to the uncertainty principle, the vacuum buzzes with particle-antiparticle pairs popping in and out of existence. They include, among many others, electron-positron pairs and pairs of photons, which are their own antiparticles. Ordinarily, those “virtual” particles cannot be directly captured. But like some spooky Greek chorus, they exert subtle influences on the “real” world.
For example, the virtual photons flitting in and out of existence produce a randomly fluctuating electric field. In 1947, physicists found that the field shifts the energy levels of an electron inside a hydrogen atom and hence the spectrum of radiation the atom emits. A year later, Dutch theorist Hendrik Casimir predicted that the field would also exert a subtle force on two closely spaced metal plates, squeezing them together. That's because the electric field must vanish on the plates' surfaces, so only certain wavelike ripples of the electric field can fit between the plates. In contrast, more ripples can push on the plates from the outside, exerting a net force. The Casimir effect was observed in 1997.
But now, Claudius Riek, Alfred Leitenstorfer, and colleagues at the University of Konstanz in Germany say they have directly observed those electric field fluctuations by charting their influence on a light wave. The experiment riffs on a technique they developed to study a longer light pulse with a much shorter one by shooting them simultaneously through a crystal (see diagram). The longer "pump" pulse is polarized horizontally, meaning that the electric field in it oscillates sideways. The shorter "probe" pulse starts out polarized vertically. However, the properties of the crystal depend on the electric field in it, so the pump beam causes the polarization of the probe beam to change and emerge from the crystal tracing an elliptical pattern. By adjusting the timing of the pulses, researchers can use the polarization effect to map out the wiggles in the electric field in the pump wave.
But vacuum fluctuations themselves will affect the crystal and hence the polarization of the probe pulse, Leitensdorfer says. So to measure the fluctuations of the vacuum field, "we only put in the probe pulse, nothing else.” On average the polarization of the lone probe pulse remained vertical. But over many repeated trials, it varied slightly, and that noise was the sign of the vacuum fluctuations, the team says.
Spotting the effect is no mean feat, as the polarization also varies because of random variation in the number of photons in each pulse, or "shot noise." To tease the two apart, the physicists vary the duration and width of the pulse, but not the number of photons in it. The shot noise should stay constant, whereas the noise from quantum fluctuations should shrink as the pulses become bigger. The researchers saw a change of a few percent in the noise, an effect they attribute to vacuum fluctuations.
Some physicists question what the new experiment actually measures, however. The researchers assume that fluctuating optical properties of the crystal reflect the vacuum fluctuations, says Steve Lamoreaux, a physicist at Yale University and one of the first to observe the Casimir effect. But the variations in the crystal's optical properties could have some other source, such as thermal fluctuations, he says. "The material properties will fluctuate on their own," he says, so "how does one attribute these fluctuations to the vacuum alone?"
Moreover, Leitenstorfer's group is not the first to directly probe such fluctuations. In 2011, Christopher Wilson, a physicist now at the University of Waterloo in Canada, and colleagues reported in Nature that they had pumped up vacuum fluctuations and turned them into real photons. In principle, that can be done by accelerating a mirror back and forth at near light speed. Wilson used a more practical analog: a system in in which the effective length of a small superconducting cavity could be changed electronically. Leitenstorfer notes that the new experiment differs from Wilson's in that it does not require amplifying the fluctuations. Wilson responds, "While I agree that that's a difference, I don't think that it's fundamental."
Leitenstorfer contends that the new work makes a qualitative advance over previous efforts. "We clearly have gone a significant step further in comparison to anybody else by directly measuring the electric field amplitude of the vacuum as it fluctuates in space and time," he says. Others seem less certain about that.