Spacetime Is Smooth, Not Foamy
August 24, 2012

Unlike Beer, Spacetime Is As Smooth As Single Malt Whiskey

April Flowers for - Your Universe Online

Scientists have long believed that spacetime is foamy, like a freshly pulled draft beer, but new information from an intergalactic photo finish shows it might just be as smooth as a single malt whiskey, instead.

Robert Nemiroff, a physicist at Michigan Technological University, reached this conclusion after studying the tracings of three photons of differing wavelengths that had been recorded by NASA's Fermi Gamma-ray Space Telescope in May 2009.

Fermi is a powerful space observatory that will open a wide window on the universe. Gamma rays are the highest-energy form of light, and the gamma-ray sky is completely different from the one we can see with our naked eyes. The Fermi Gamma-ray Space Telescope, formerly GLAST, is opening this high-energy world to exploration.

The photons originated about 7 billion light years away from Earth in one of three pulses from a gamma-ray burst and arrived at the orbiting telescope just one millisecond apart, virtually a tie.

Gamma-ray bursts are short-lived bursts of gamma-ray photons. They originate far across the universe, and are believed to be caused, in many instances, by giant stars collapsing. These stars often collapsed billions of years before the Earth was formed.

“Gamma-ray bursts can tell us some very interesting things about the universe,” Nemiroff said. In this case, those three photons recorded by the Fermi telescope suggest that spacetime may not be as bubbly as some scientists think.

Theories of quantum gravity say that the universe is not smooth, but foamy (like that draft beer) — made of fundamental units called Planck lengths that are less than a trillionth of a trillionth the diameter of a hydrogen atom. Planck lengths are so small there's no way to detect them except via photons like those made up from gamma-ray bursts.

The wavelengths of these photons are some of the shortest distances known to science. They are so short they should interact with even the smaller Planck length. If this interaction occurs, the photons should be dispersed or scattered on their trek through Planck length-pixelated spacetime.

The photons should disperse in different ways if their wavelengths differ, just as a ping pong ball and a softball might take alternate paths down a gravely hillside. You wouldn't notice the scattering over short distances but across billions of light years the Planck lengths should disperse the light. If the spacetime is foamy from Planck lengths, then the three photons from the same gamma-ray burst should not have crashed through the Fermi telescope at the same moment.

They did, though, and that calls into question just how foamy spacetime really is.

“We have shown that the universe is smooth across the Planck mass,” Nemiroff said. “That means that there´s no choppiness that´s detectable. It´s a really cool discovery. We´re very excited.”

With graduate students Ryan Connolly and Justin Holmes and physics professor Alexander Kostinski, Nemiroff coauthored an article on the team´s results, “Bounds on Spectral Dispersion from Fermi-Detected Gamma-Ray Bursts,” published June 8 in Physical Review Letters.

A story on their work, “Cosmic Race Ends in a Tie,” appeared in the news section of the journal Nature on January 10.