IF A neutron star spins furiously in the vacuum of space, does it make a sound? Yes, apparently, but it is sound with a gravitational twist.
Peter Shawhan at the University of Maryland, College Park, calls such objects 鈥渟pace-time sirens鈥� and has been playing the noises they make to bemused colleagues at conferences for the last few months. He was inspired by the term 鈥渟tandard siren鈥�, used by cosmologists to describe a pair of massive black holes spiralling towards each other.
Listen to the gravitational-wave signal that should be produced when two black holes or neutron stars spiral inward and eventually merge
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Listen to the gravitational-wave signal that should be produced by a rotating asymmetric neutron star (if the Earth鈥檚 rotation and orbit were sped up by a large factor)
Einstein鈥檚 general theory of relativity predicts that such binary black-hole systems should generate gravitational waves 鈥� ripples in the fabric of space-time. If the rate of change of the frequency and amplitude of these waves could be measured, the distance to the black holes could be gauged. Because such binaries would generate powerful gravity waves they could, in theory, be detected by , even if they are more than halfway across the universe. Working out the distances to a number of these standard sirens could help us understand the history of the expansion of the universe.
鈥淕ravity waves could be detected halfway across the universe鈥�
鈥� can also generate gravity waves if they are asymmetric. Neutron stars are extremely dense remnants of massive stars that have collapsed, and such an object spinning at about 700 revolutions per second would generate gravity waves with a frequency of about 1500 hertz. Radio astronomers have identified about 150 pulsars whose spin frequencies are such that gravity waves emanating from them should fall within the frequency range detectable by that have been searching for them since 2002.
Gravitational waves have not yet been detected but Shawhan and others are optimistic. They are particularly interested in pulsars whose spin rates are falling more rapidly than most because these may be losing a lot of their rotational energy to gravitational waves. One example lies about 6000 light years away in .
We can鈥檛 hear gravitational waves directly, of course, but the LIGO frequency band runs from about 40 hertz to several kilohertz, which is within the range of human hearing. Because Earth is rotating on its axis while moving around the sun, LIGO is moving relative to any pulsar it might detect. So any gravitational wave signal it received would undergo a Doppler shift, increasing in frequency as LIGO approaches the pulsar, and dropping in frequency as it moves away. What would these pulsars sound like if they were propagated through air? 鈥淭o me, they sound like a siren,鈥� says Shawhan.
