THE search for a quantum theory of gravity has often been derided as a
paradise for theoretical physicists because they cannot be proved wrong, however
wild their ideas are. But now a scientist in Switzerland has found a way to test
key predictions of rival theories, and some candidates are under pressure.
Physicists have been trying for decades to marry quantum theory with general
relativity鈥擡instein鈥檚 theory of gravity. In the process they have dreamt
up some of the most bizarre notions in science, from superstrings to
multidimensional space-times. Nearly all the theories predict that on tiny
scales, approaching the 鈥淧lanck scale鈥� of 10-35 metres, familiar notions
of space and time start to disintegrate, giving way to a seething melee of
quantum gravitational fluctuations known as 鈥渟pace-time foam鈥�.
But the magnification needed to see physics at work on this scale would be
equivalent to blowing an atom up to the size of a galaxy cluster. To probe such
scales you would need a particle accelerator 1016 times more powerful than any
we have now. No wonder cynics have dismissed quantum gravity as little more than
a parlour game.
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But Giovanni Amelino-Camelia of the University of Neuch芒tel in
Switzerland has found a loophole in this argument. He has shown that available
technology鈥攕ensitive laser devices known as gravitational wave
interferometers鈥攃an be used to test some rival quantum gravity
theories.
The interferometers were designed to search for gravitational waves. General
relativity predicts these ripples in space-time, but so far no one has observed
them directly. Gravitational interferometers consist of two heavy weights
monitored by laser beams. A gravity wave would move the weights and change the
interference pattern of the beams.
These instruments register movements of less than the width of an atomic
nucleus. While this is still huge compared to the Planck scale, Amelino-Camelia
points out that space-time fluctuations predicted by some quantum gravity
theories are affected by the duration of observation. This can lead to
fluctuations becoming big enough to register as 鈥渘oise鈥� in interferometers.
In this week鈥檚 Nature (vol 398, p 216), Amelino-Camelia works out
the expected size of this noise for various quantum gravity theories. He says a
theory based on 鈥渄eformed Poincar茅 symmetry鈥� is in trouble, according to
data from an interferometer at Caltech in Pasadena. More sensitive
interferometers now being built in Europe and the US should put other theories
under the spotlight within a few years.
鈥淭heorists are no longer free to say anything they want,鈥� says
Amelino-Camelia. For now, the much-vaunted superstring theory of quantum gravity
looks safe. 鈥淏ut this could change as theorists find out more about the
properties of the space-time fluctuations predicted by superstring theory.鈥�
Dharam Ahuluwalia, a theorist at the University of Zacatecas in Mexico, calls
the tests 鈥渇undamental and significant鈥�. 鈥淭hey show that quantum gravity is not
a science where humans cannot venture experimentally,鈥� he says.