General relativity and quantum mechanics offer very different descriptions of the structure of spacetime. General relativity features a smooth, flat Euclidean geometry, but at the quantum scale — specifically, the Planck length — spacetime bubbles and froths like a foam.

It would be really cool if scientists could also make a measurement of

the quantum foam, and even better if they could manage the feat in a

tabletop experiment.

ANALYSIS: Are We Living in a Hologram?

According to quantum mechanics, empty space isn’t really empty. It roils and boils with pairs of “virtual” particles and antiparticles that annihilate and disappear back into the quantum vacuum so quickly that the apparent violation of energy conservation incurred by their creation can’t be observed directly.

Alas, that means that scientists can’t measure spacetime at that tiny scale exactly. In order to do so, it would require enormous energies, because the smaller the scale one wishes to probe, the higher the energy one needs to do so. And the Planck scale is very small indeed.

But there is indirect evidence, both in tiny disturbances in the electron energy levels in a hydrogen atom, and also via a phenomenon known as the Casimir effect. Take two uncharged parallel metal plates and place them very close together. Normally there would be no movement because there would be no electromagnetic charge exerting a force to pull them together. But get the plates close enough, and it’s possible to measure the tiniest attractive force between them.

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That tiny effect comes from virtual particle pairs, which can’t get between the two plates, so there are more pairs popping into existence around the exterior of plates than there are between them. The imbalance pushes the plates together slightly. The smaller the separation between the plates, the fewer virtual pairs can get between them, and the greater the force of the inward attraction.

Now, physicist Jacob Bekenstein at the Hebrew University of Jerusalem in Israel believes he has figured out a method for directly measuring the structure of spacetime at the Planck scale, and it doesn’t involve enormous energies beyond even our most powerful accelerators. Instead, Bekenstein thinks all he needs is to zap a very cold block of glass with a single photon emitted by a laser.

The idea here is to exploit conservation of momentum and measure how the center of mass within the block of glass shifts position in response the emitted photons. And Bekenstein insists such an experiment won’t violate uncertainty because he would just be counting photons.

Even a single photon should impart a bit of momentum to the glass as it pushes through, moving the block a tiny bit. If that distance is smaller than the Planck length, then the photon won’t be able to pass through the block. That means Bekenstein could count the photons, and if the number that pass through the block is fewer than the number predicted by classical optics, he would have a measurement of the quantum foam.

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There may be other ways to search for evidence of quantum foam, such as looking in the radio signals emitted by black holes and pulsars — at least part of this “cosmic noise” might be due to quantum foam.

Alternatively, University of Maryland physicist Igor Smolyaninov thinks he could create an analog of quantum foam in a tabletop experiment involving metamaterials, exotic substances with unusual electromagnetic and optical properties that can be used to mimic the structure of spacetime.

The behavior of photons in a metamaterial should be identical to how they would behave in spacetime.

Essentially Smolyaninov proposed in a 2011 paper that it should be possible to create analog black holes that thermally fluctuate naturally in and out of existence within the metamaterial, much like the behavior of virtual particles, and the effect, he wrote in his paper, “should be large and easy to observe.”

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