SPACE is smooth, say two astronomers after scrutinising images from a distant galaxy. That may sound like common sense, but in fact the claim could hardly be more controversial. According to theories that try to explain how gravity should act on the very smallest scales, space and time must be irregular, like constantly churning foam. If space-time were smooth, phenomena such as black holes or the big bang could not be explained, hence theorists鈥� scepticism over the new result.
The causes of the argument are astronomers Richard Lieu and Lloyd Hillman, from the University of Alabama in Huntsville. They looked at a distant galaxy called PKS1413+135, and saw a set of rings around it 鈥� an interference pattern called Airy rings (see Graphic). According to Lieu, this shouldn鈥檛 happen. Such interference patterns only form when light waves from a source overlap neatly, with all the crests and troughs coinciding. If space is a bumpy quantum foam, light passing through it should get distorted, speeded up in places and slowed down in others, so at the telescope the crests and troughs won鈥檛 coincide any more. Space should act like frosted glass, blurring the interference pattern.
The space-time fluctuations are supposed to be tiny, on scales below about 10鈭�35 metres, so any blurring is incredibly small. But, says Lieu, it should show up over long enough distances. 鈥淭he inspiration came when I was watching people racing in a field,鈥� he says. Two sprinters finish close together, but over a longer race slight differences in speed add up, and the runners usually finish far apart. Likewise, light waves travelling across vast reaches of space should get out of phase.
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Lieu and Hillman calculated that the 4 billion light years to PKS1413+135 should be far enough. So when they looked at the galaxy using the Hubble Space Telescope, they did not expect to see an interference pattern. But they did.
In a paper to be published in a future issue of Astrophysical Journal Letters, Lieu interprets this to mean that space-time must be smooth, or at the very least that the size of the irregularities 鈥� the 鈥淧lanck scale鈥� 鈥� must be even tinier than thought. The most extreme interpretation, that space-time is perfectly smooth, puts much of physics in serious trouble. For example, in smooth space-time there鈥檚 nothing to stop matter squeezing into infinitely small volumes, and therefore reaching infinitely high densities and temperatures 鈥� conditions impossible to describe. But if space-time is like an irregular foam, it can鈥檛 be squashed smaller than the Planck scale, allowing theorists to get a handle on the big bang or what might be going on inside black holes.
Not surprisingly, quantum gravity theorists aren鈥檛 convinced by Lieu鈥檚 claims. Assuming that distorting effects on the light will add up over long distances just isn鈥檛 valid, insists Gary Gibbons of the University of Cambridge. 鈥淐onventional quantum gravity theories say there is no effect,鈥� he says. Lee Smolin of the Perimeter Institute in Waterloo, Canada, agrees. But he does see some merit in Lieu and Hillman鈥檚 work. 鈥淎ny experimental probe of the Planck scale is good, and they are to be commended strongly for inventing a new one,鈥� he says.