The news was a jaw-dropper: Astronomers studying photos from the Hubble Space Telescope had shown that a central tenet of modern physics--that space and time are fuzzy and foamy rather than smooth--could not be true. Media outlets ranging from The Times of London to Astronomy magazine reported that the entire notion of quantum gravity might be in trouble. Now, though, the conventional wisdom is striking back, as physicists say the astronomers' reasoning is fundamentally flawed.
The original argument, as Richard Lieu and Lloyd Hillman of the University of Alabama stated it in the 10 March issue of Astrophysical Journal Letters, ran like this: The linchpin of quantum mechanics, Heisenberg's uncertain principle, implies that space and time are riddled with "bubbles," which are far too small and fleeting for earthbound experiments to detect. The cumulative effects of the fluctuations, however, could alter light traveling across vast reaches of space in ways that should show up in images from the Hubble Space Telescope.
A Hubble image of a distant star or galaxy is ordinarily slightly blurred and may have a ring around it. This effect arises because light waves must travel slightly different distances from different parts of the telescope's main mirror to its detector. As a result, light waves get out of step, canceling each other out in places and reinforcing each other elsewhere, in a pattern that creates a ring. For that to happen, though, the light waves reaching different parts of the mirror must start out in step.
That shouldn't happen if space is as foamy as quantum gravity theories predict, Lieu said. Quantum uncertainties make it impossible to know the exact speed of a quantum wave of light, or photon, so some photons from a star should travel slightly faster than others. If so, they should get increasingly farther out of step as they travel, just as a pack of runners spreads out more in a marathon than in a 100-meter dash. This spreading would destroy the interference patterns in Hubble images of distant stars and galaxies. But the patterns are there. Ergo, no foam. Roberto Ragazzoni of the Italian National Institute for Astrophysics in Florence and colleagues report similar observations in a paper to be published in the same journal.
But others aren't buying it. Lieu's argument assumes that each photon maintains its speed all the way from a distant galaxy to telescope, says Y. Jack Ng of the University of North Carolina, Chapel Hill, so that photons get steadily more and more out of step in proportion to the distance they travel. But in fact, he says, foamy space and time should make each photon speed up and slow down randomly. In that case, the amount the photons are out of step increases only with the square root of the distance they travel. Over cosmic distances, that square-root sign produces an effect roughly a million billion times smaller than the one predicted by Lieu, Ng says, leaving the Hubble images unscathed and quantum gravity alive and kicking.