Science

Empty Space Isn't Empty, And Quantum Researchers Now Have Direct Evidence

If there's one thing you need to know about the quantum world, it's that things aren't what you expect them to be. Nothing, for example. In classical physics, nothing is a space devoid of stuff. But according to quantum theory, nothing is chock-full of stuff. Scientists have had weak evidence of this nothing-stuff—or quantum vacuum fluctuations, if you want to get technical—since the 1940s, but new experiments may have given us direct proof of its existence. That could mean very, very big things for quantum research.

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The "squeezing" process has a sort of traffic-jam effect that lets scientists see fluctuations in the quantum vacuum.

A Churning Stew Of Nothingness

To get even more specific, classical physics defines nothing, or a vacuum, as a space devoid of matter in the lowest possible energy state. When you delve into the quantum realm, this definition poses a problem. You've probably heard of Heisenberg's uncertainty principle, even if you may not totally grasp it. In essence, it says that there's a limit to what we can know about quantum particles. Because everything in quantum mechanics is both a wave and a particle, if you know a particle's position you can't know its momentum, and vice versa. This boils down to the idea that the vacuum isn't really empty. It's actually churning with smatterings of particles that disappear and reappear at random, creating a fluctuating energy field.

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Of course, that's just because Heisenberg says so. We've never had actual proof of this so-called energy field. In the 1940s, scientists found indirect evidence of it by examining the radiation emitted by hydrogen atoms and the forces exerted on closely spaced metal plates, but that was it. Then in 2015, a team of German scientists led by Alfred Leitenstorfer announced that they had directly detected that fluctuating energy field by firing a super-short laser pulse into a vacuum and seeing tiny changes in the polarization of the light. Those changes, they said, were caused by the fluctuations in the quantum vacuum. Still, since many things could potentially cause that fluctuation, that result was up for debate.

Schematic sketch of the fluctuations in the vacuum. Notice how some "blips" actually drop below the ground energy state.

A Traffic Jam In Empty Space

Finally, in January 2017, Leitenstorfer and his team published what might be the smoking-gun evidence for quantum vacuum fluctuations. They again used a super-short laser pulse—specifically, a few femtoseconds long, which is half the size of a wavelength of light in the range they were studying—to generate what's known as "squeezed light," or light that has been slowed down in a certain segment of space-time. That squeezing, according to the press release, works sort of like a car causing a traffic jam: "from a certain point on, some cars are going slower. As a result, traffic congestion sets in behind these cars, while the traffic density will decrease in front of that point. That means: when fluctuation amplitudes decrease in one place, they increase in another."

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Here's the genius of it all: to detect fluctuations in a particle like a photon of light, you usually have to amplify that photon, which disturbs it and erases all of its quantum goodness. So instead of detecting the photons in space, they detected them in time. That sounds weird until you remember that Einstein's theory of special relativity says that space and time are the same in a vacuum. Combining that technique with their photonic "traffic jam," they got direct evidence of fluctuations in the quantum vacuum, including a cool phenomenon where some of the fluctuations actually dropped below the very lowest energy we thought possible.

But wait—that's not the best part. If these scientists actually found a way to detect particles without disturbing them, they may have unlocked a door that has been closed to scientists as long as quantum physics has existed. We've never been able to directly detect quantum particles before, and this new technique may be the way. Their findings need further verification, as all good science does, but if it's true, this could mean very big things.

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Written By Ashley Hamer March 9, 2017