杏吧原创

LAST YEAR, a British mathematician sent shock waves through the world of astrophysics by suggesting that the Universe may contain few objects that generate gravity waves 鈥� ripples in the fabric of space-time. Now, however, it seems that the researchers can breathe a collective sigh of relief.

According to a French physicist, the specific paradoxical situation highlighted by William Bonnor of Queen Mary and Westfield College, London can be resolved without ditching any cherished ideas about the waves. This is particularly good news for researchers who are hoping to observe gravity waves using giant detectors now being built in the US and Europe. They had feared that Bonnor鈥檚 ideas would make funding agencies rethink their plans to spend hundreds of millions of pounds on these projects.

Bonnor drew people鈥檚 attention to a hypothetical simple situation in which an accelerated mass produces no gravity waves (New 杏吧原创, Science, 17 September 1994). This was a paradox because, according to Einstein鈥檚 general theory of relativity, gravity waves are radiated by any mass that undergoes acceleration. If there were no gravity waves in a simple situation, argued Bonnor, then it could be that more complicated systems, such as coalescing neutron stars or black holes in the process of being born, also might not generate gravity waves. Bonnor鈥檚 simple situation is a mass which emits light mostly in one direction. Because photons of light carry momentum, the mass accelerates in the opposite direction 鈥� like a rocket.

Now, however, Thibault Damour of the Institut des Hautes Etudes Scientifiques near Paris has come up with an explanation of why such a 鈥減hoton rocket鈥� emits no gravity waves. The key, he says, is to realise that mass and energy are merely different faces of the same coin, as shown in Einstein鈥檚 famous equation, E = mc². 鈥淭his means that you have to look at all sources of energy as potential emitters of gravity waves 鈥� including photons,鈥� says Damour. 鈥淚f you do the calculations, you find that the gravity waves generated by the accelerated mass and the photons cancel each other out precisely.鈥�

Damour stresses that this cancellation happens only in the special case of the photon rocket because the photons are mostly emitted in one direction. 鈥淲hen photons are emitted in a more complicated pattern, there is no such cancellation and gravity waves are produced as expected,鈥� he says. Damour concludes that the situation highlighted by Bonnor therefore has no implications for the detection of gravity waves from objects in the Universe. In any case, he says, the energy in the motion of massive astrophysical bodies is usually much greater than that in the photons they emit. This would make it unlikely that gravity waves generated by photons could cancel out those generated by the accelerating mass.

Damour鈥檚 calculations will be published in a future issue of the journal Classical and Quantum Gravity. Bonnor has examined them closely and accepts that Damour has resolved the paradox of the photon rocket. But he points out that there are two other theoretical situations in which masses undergoing acceleration produce no gravity waves.

Both are 鈥渃osmological鈥� solutions of Einstein鈥檚 equations for universes in which matter or photons are not distributed evenly. One contains dust which, though it is accelerating, produces no gravity waves; the other contains photons which similarly emit no gravity waves. 鈥淚n neither case is it at all obvious why there are no gravity waves,鈥� says Bonnor. 鈥淭he moral of this story is that in every situation in which mass undergoes acceleration, theorists must check very carefully whether gravity waves really are radiated.鈥�