One of the biggest challenges in physics – finding evidence for quantum gravity – could be tackled using a simple table-top experiment, according to Jacob Bekenstein from the Hebrew University of Jerusalem. Bekenstein, who is best known for studying the thermal properties of black holes, has come up with an interesting new proposal for using single photons to probe what is known as "quantum foam". The foam, which was introduced in 1955 by the US physicist John Wheeler, is believed to exist on length scales so small that quantum fluctuations affect space–time.

Bekenstein's proposal is the latest effort in the quest to understand how quantum mechanics can be unified with Einstein's general theory of relativity – a problem that has eluded physicists since they first began to understand the quantum and relativistic worlds in the early 20th century. One of the main reasons why physicists have struggled with developing a theory of quantum gravity is a complete lack of experimental evidence. The problem is that the effects of quantum gravity are only expected to be measurable over extremely small distances.

Some theories of quantum gravity suggest that experiments must probe distances smaller than the Planck length, which is 1.61 × 10–35 m. Probing this scale using an accelerator would involve colliding particles at enormous energies of more than 1016 TeV. This would be well beyond the capabilities of the Large Hadron Collider, which has a maximum collision energy of 14 TeV, or indeed of any conceivable future collider. Bekenstein's proposal, in contrast, is much more modest; he says it could be done in a small physics lab mostly using existing equipment.

Photons at the ready

The experiment would involve firing single photons at a piece of glass or crystal, suspended by a tiny thread. When the photon moves from the vacuum into the material, it loses speed because the material has a higher refractive index than that of the vacuum. The result is that a tiny amount of momentum is transferred to the material, causing it to move an extremely small distance. In the case of a blue photon with a wavelength of 445 nm, Bekenstein says it would cause a 150 mg piece of high-lead glass to deflect by about 2 × 10–35 m, which is on a par with the Planck length.

The bottom line is that if a photon is detected on the other side of the material, it means the mass was deflected by a distance greater than the Planck length. But if the energy of the photon is reduced (or alternatively the mass of the glass increased) until the deflection becomes equal to or smaller than the Planck length, then quantum gravity will affect how the glass responds to each photon.

In particular, Bekenstein believes that the presence of the foam would prevent the glass from recoiling in exactly the same way when struck by a succession of identical photons. Just as electromagnetic fluctuations can have measurable effects on much larger objects – an example being the Casimir force – space–time fluctuations should also affect how an object moves extremely small distances. In the case of Bekeinstein's proposed experiment, photons would not be able to travel through the glass, which would be observed as a drop in the number of photons detected on the other side.

The experiment is challenging but not beyond what experimental physicists can do today
Jacob Bekenstein, Hebrew University of Jerusalem

Bekenstein admits that the experiment is "challenging", but claims it "is not beyond what experimental physicists can do today". Indeed, creating and detecting single photons is a routine part of quantum-optics experiments that are done in many labs around the world. Minimizing the effects of thermal noise will also be a challenge, with Bekenstein calculating that the apparatus must be cooled to about 1 K and operated in an ultrahigh vacuum of about 10–10 Pa – both of which are achievable using existing technology.

Other table-top schemes

Bekenstein is not the only physicist to have proposed a table-top probe of quantum gravity. Earlier this year, for example, Igor Pikovski and colleagues at the University of Vienna and Imperial College London described a way of making optical measurements on a mechanical oscillator with a mass close to the Planck mass (about 22 μm). Indeed, Pikovski told that Bekenstein's plan seems feasible. "A big advantage is that physicists can control single photons very well and detect them extremely efficiently," he says.

Pikovski also points out that the technique could prove very useful even if experimental issues prevent it from probing distances down to 10–35 m. This is because some theories of quantum gravity predict that quantum foam or some other effect of quantum gravity could emerge at length scales as great as 10–25 m.

While it is still not clear whether the table-top experiments proposed by Bekenstein, Pikovski or others will succeed, Pikovski believes that laboratory measurements will provide important information about quantum gravity within a decade or so.

The research is described in a preprint on arXiv and Pikovski's proposal was published earlier this year in Nature Physics.