Quantum gravity is notoriously slippery. While the Standard Model successfully describes three forces of nature, it doesn't include gravity, so gravity still has no consistent quantum theory. To make matters worse, gravity is so weak that it's difficult to probe at the sorts of energies where any minuscule quantum effects would pop out. However, some researchers predict that those tiny effects could accumulate over cosmological distances: light traveling from far-off quasars would be changed by the "quantum foam" of spacetime, producing blurry images in our telescopes—or even making objects seem to disappear.

A new report by E. S. Perlman and colleagues examines the disappearance hypothesis using gamma-ray data from quasars. In particular, they investigated a possibility suggested by the holographic principle, the idea that all the information in the cosmos can be encoded on the two-dimensional boundary that encloses it. Disappointingly for fans of quantum foam, the gamma ray data did not show any measurable fading or blurring of the quasars.

As the authors point out, these results don't rule out anything general regarding quantum gravity, quantum foam, or the holographic principle. But they do provide the tightest constraint yet on cumulative effects of quantum foam on light traveling across the Universe.

Our current theory of gravity is general relativity, which is a "classical" (meaning "non-quantum") theory. Under relativity, the structure of spacetime remains smooth on all length scales, from the whole cosmos down to subatomic distances. However, quantum mechanics suggests that, on the scale of the Planck length (1.6×10-35 meters), quantum effects should kick in. Theories disagree on exactly what those effects might be though. The "quantum foam" hypothesis, for instance, predicts that spacetime becomes a chaotic froth at the Planck length.

The Planck length is far too small to probe directly. But as light travels, the quantum foam would randomly shift the location of its peaks and troughs, something known as the phase of the light. Over long distances, the phase could shift enough to make the light interfere destructively with itself, blurring any image we pick up at Earth or even canceling the light from a distant object out entirely. The amount of phase shift depends on the particular version of quantum foam, but detecting any shift could be the first indication of quantum gravity, causing every researcher in the field to simultaneously wet their pants.

One particular version of quantum foam comes from the holographic principle, a concept that grew from the study of black holes. Like more ordinary objects, black holes have volume—they take up space. Unlike Earth or stars or other ordinary things, the main information about their contents is described by their surface area. (This is a horrible simplification, but go with it.) So, information on the contents of a three-dimensional volume is encoded on the two-dimensional surface that encloses that volume.

That's analogous to a hologram, which involves scanning a three-dimensional object and turning it into a two-dimensional picture. (Another horrible simplification.) Some researchers think that what holds true for black holes should be true for the Universe in general: the information needed to describe the entire Universe could exist on the two-dimensional surface that encloses it.

This could have a significant effect on quantum foam.

In whatever version of quantum gravity one prefers, the phase shift caused by quantum foam depends on the wavelength of the light: shorter wavelengths are closer to the length scale of the fluctuations. Short wavelength means X-rays and gamma-rays, which are more difficult for us to observe than less energetic forms of light. Fortunately, this high-energy radiation is produced in copious quantities by supermassive black holes at the centers of active galaxies, known as quasars and blazars. As a bonus, most quasars and blazars are very far from Earth: many are billions of light-years distant, meaning their light would travel through lots of quantum foam on the trip to Earth. That makes them ideal targets to test the quantum foam hypothesis.

Gamma-ray observations

Earth's atmosphere blocks gamma rays, which is lucky for us for health reasons. However, when a very high-energy gamma ray hits the atmosphere, it ends up moving faster than the speed of light in air, so it creates a shock wave, like an airplane moving faster than sound makes a sonic boom. The effect of the shock wave is a shower of blue light known as Cherenkov radiation. VERITAS and similar gamma-ray observatories are actually visible-light telescopes that look for these air showers. VERITAS consists of four 12-meter-diameter telescopes that triangulate on the showers to discover where they originated. I have

Earth's atmosphere blocks gamma rays, which is lucky for us for health reasons. However, when a very high-energy gamma ray hits the atmosphere, it ends up moving faster than the speed of light in air, so it creates a shock wave, like an airplane moving faster than sound makes a sonic boom. The effect of the shock wave is a shower of blue light known as Cherenkov radiation. VERITAS and similar gamma-ray observatories are actually visible-light telescopes that look for these air showers. VERITAS consists of four 12-meter-diameter telescopes that triangulate on the showers to discover where they originated. I have a selfie in one of those telescopes

The researchers used data from the orbiting Chandra X-ray telescope, the orbiting Fermi gamma-ray observatory, and the ground-based VERITAS (Very Energetic Radiation Imaging Telescope Array System) observatory in Arizona. They sampled a number of quasars and blazars, at distances ranging from hundreds of millions of light-years away to nearly 10 billion light-years distant. They then compared the number of and brightness of these supermassive black holes in gamma rays and X-rays to what is seen in visible light. If quantum foam was having a strong effect, there should be fewer quasars or blazars in the high-energy light data than in the visible data since visible light would not be measurably affected by quantum fluctuations.

Within experimental limits, the researchers found no difference in the quasar and blazar numbers, meaning no sign of quantum gravity effects. Their data placed an upper limit on the size of the phase shift due to quantum foam.

That doesn't rule out either the holographic principle or quantum foam (the original, simplest version of quantum foam is essentially undetectable for example), but it does eliminate some possible versions of these ideas. As the authors of the study point out, a more detailed prediction from the holographic principle could allow more subtle fluctuations. However, it does mean that—like quantum gravity itself—quantum foam is still an elusive fleck.

Astrophysical Journal, 2015. DOI: 10.1088/0004-637X/805/1/10 (About DOIs).