Femtosecond Lasers Allow Physicists to Directly Observe Zero-Point Energy

All the way at the bottom of reality, there is still more than nothing. Physics requires it.

Michael Byrne

Michael Byrne

Image: University of Konstanz

This was a good week for nothing. That is to say that it was a good week for research into the vast strangeness that is zero-point energy—or vacuum fluctuations, the small quantum twitches of standard deviation that arise when every bit of matter and energy is removed from a unit of space. The unvoid.

On Wednesday, I reported on a study released by physicists at Chalmers University describing an experiment in which vacuum fluctuations are probed using artificial atoms reflected against a sort of mirror. The setup is based on using the atom's radiative decay (aka spontaneous emission) as an amplifier of sorts for the fizz of virtual particles that arise in vacuum conditions. Amplification has been a key requirement of most all zero-point energy detection experiments.

Now, a second study, released Thursday in Science, offers a new way into the zero-point world. Crucially, it's a method that doesn't require amplification and instead offers a means of direct detection using the electro-optic effect, in which the refractive index of a region of space alters the polarization of short laser probe beams. These pulses can then be analyzed, offering up new information about the true nature of the void.

Study co-author Claudius Riek. Image: Konstanz

Ah, yes, the void. There is no void. Instead, where all matter and energy have been removed from a space, there is always a slight fizz of virtual particles that pop in and out of existence. They are real in the sense that they have real effects, but they also usually disappear almost immediately. Virtual particles can be viewed as a consequence of quantum uncertainty, where properties of momentum and position can never be known completely, simultaneously. This forbids true emptiness, which is a very certain condition.

The catch is that vacuum fluctuations, the ground state of light and radio waves, are really, really faint. Hence the usual need for amplification. Direct observation was even considered to be impossible until now, which would leave only indirect means of detection, such as the artificial atom scheme (which is also really cool).

Key to the direct detection experiment are ultra-short pulses of light, on the order of single femtoseconds (millionths of billionths of a second). By probing "empty" space using pulses with lengths of less than one half of a cycle (or oscillation), the researchers were able to achieve very high temporal resolutions, finding only the slightest, most barely-there photons. At longer wavelengths, the positive and negative components of an oscillation or wave start to cancel out.

"Exactly this sub-cycle aspect is the essential novelty we are coming up with," Alfred Leitenstorfer, a professor of physics at the University of Konstanz and lead author of the new paper, told me. "This fact seems to be something that many colleagues from the quantum physics community simply did not have on their agenda, puzzling some of them at first. But we allowed us two years of time to think about it ourselves, also to exclude any possible alternative explanations. All colleagues we have discussed with during that period—and there were many distinguished individuals among them from the different communities involved—finally got that point and agreed that what we have accomplished is something new and fundamental."

So, yes, we are all of the time surrounded by the hum of a virtual but real world. We have direct evidence. There is no perfect darkness, nor is there perfect emptiness. There's something reassuring about that.