Facts So Romantic

This Is How to Check Whether Spacetime Is Foamy

Photograph by ColorBlind Images/Getty Images

In 1944, John Wheeler received a haunting postcard. It was from his younger brother, Joe, who had written only two words: “Hurry up.” Wheeler was involved with the United States’ atomic weapons effort, and Joe wanted him to finish the bomb so he could come home from fighting in Italy. But by the time Hiroshima was hit, Joe had already been killed. Not being fast enough to save his brother led Wheeler to become obsessed with time and then General Relativity—Einstein’s theory linking space and time.

After Einstein, “spacetime” was conventionally understood to be like a smooth surface. But in the 1950s, Wheeler began to suggest that—just like matter and energy—space had to be made up of “quanta,” or particles. That, Wheeler wrote, “forces on space a foam-like structure.” Today, researchers across the globe are pursuing evidence for Wheeler’s quantum foam because it may be a harbinger of a post-Einsteinian physics, helping scientists understand how gravity and quantum mechanics are linked.

You might think LIGO (the Laser Interferometer Gravitational-Wave Observatory) may be of service in this quest. It bounces laser beams off of precisely positioned mirrors to detect the contractions and expansions of spacetime, the signatures of gravitational waves. Yesterday, LIGO scientists announced the observation of a gravitational wave emitted by the merger of two black holes 1 billion light-years away. But the minuteness of spacetime foam is well beyond its reach. Even with LIGO’s incredible sensitivity, “the fluctuations of spacetime are too small to be detected,” says Jack Y. Ng, a theoretical particle physicist at the University of North Carolina.

To spot spacetime foam, scientists are trying to see if it affects the clarity of powerful telescopic images of distant quasars—the massive, extremely bright objects in galactic cores. They’re useful in the effort to detect quantum foam because they’re incredibly luminous and far away, often brighter than the entire remainder of the galaxy they’re part of. According to some models of quantum foam, the farther quasar light has to travel to our telescopes, the more likely it is to have been delayed or shifted by intervening quantum foam. Thus, like fog on Earth, quantum foam should make pictures of distant objects appear blurrier or more distorted than they otherwise would.

“Spacetime foam is in a category—just like extra dimensions—that you cannot altogether rule out.”

Early last year, Eric Perlman, an observational astrophysicist at the Florida Institute of Technology, led a study to see if he could set limits on just how foamy spacetime could be. He looked at x-ray and gamma-ray emissions of distant quasars using observations from the Chandra X-ray Observatory, the Fermi Gamma-ray telescope, and the Very Energetic Radiation Imaging Telescope Array System (VERITAS).

The team was expecting to see some quantum foam-induced distortions, but didn’t find any. Instead, the x-ray and gamma-ray data showed that spacetime appears smooth even at distances shorter than the nucleus of a hydrogen atom, much less foamy than they and others had previously thought. “Spacetime foam remains theoretical,” says Perlman. “There has never been a direct detection of it thus far, much less whether its density varies over cosmic time.”

But it would be surprising if there turned out to be no quantum foam at all, says Steven Carlip, a theoretical physicist at the University of California at Davis. Most of his colleagues in the field agree, he says. More sensitive space telescopes, like NASA’s James Webb Space Telescope (JWST), may give physicists another chance of detecting the foam. With its larger aperture, the James Webb should have the sensitivity necessary to indirectly detect spacetime foam-induced blurring. Its mirror for collecting photons measures 270 square feet; the 26-year-old Hubble Space Telescope, by contrast, has only 48 square feet.

But even if the James Webb gets back blurry images, that still may not be definitive evidence that quantum foam exists, since there’s no clear way to ensure that the foam, and not some other factor, caused the blurriness. Eric Steinbring, an astronomer at Herzberg Astronomy and Astrophysics at the National Research Council in Canada, is trying to figure out how to tell the difference. He says intergalactic dust, or even gravitational lensing of distant objects, could cause blurring that might be confused with the effects of foam. “It is an observational problem, not really a theoretical one, but theory and experiment always go hand in hand,” he says. “You can’t know what to look for if you don’t look.”

If quantum foam continues to prove elusive, Wheeler’s idea won’t necessarily be easy to cast aside, says Giovanni Amelino-Camelia, a theoretical physicist at Sapienza University in Rome. “Spacetime foam is in a category—just like extra dimensions—that you cannot altogether rule out.” 

Bruce Dorminey, science journalist and author of Distant Wanderers: The Search for Planets Beyond the Solar System, is a Forbes.com tech contributor. Follow him @bdorminey.

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