Music of the Spheres
In a distant galaxy, long ago, a pair of black holes, each about thirty times more massive than our sun, began to orbit one another. Over the next several hundred million years, gravitational waves generated by their motion caused them to spiral together, slowly at first but gathering speed as they came closer and closer, until they were whirling about one another at the same rate as the blades in a kitchen blender. They eventually slammed together at about a third of the speed of light, emitting a last burst of gravitational waves before settling down to the sedate life of an ‘ordinary’ black hole.
Those gravitational waves passed through our solar system on 14 September 2015. They were picked up by the Laser Interferometer Gravitational-Wave Observatory, only days after the new Advanced LIGO detectors had come into operation. Over the following months, more than 900 scientists worked together to study the event. On 11 February, they announced that we have, at last, detected gravitational waves. My colleagues and I have been celebrating ever since.
Einstein first predicted gravitational waves – ripples in gravity produced by rapidly accelerating masses – in 1916. He botched the first analysis. He corrected his mistakes in 1918, but remained confused, vacillating about the nature and even the existence of gravitational waves several times in his life. Indirect evidence for these waves finally came following the discovery in 1974 by Russell Hulse and Joseph Taylor of a binary system containing two neutron stars. Over many decades of observation, these neutron stars have been observed to slowly spiral towards one another at exactly the rate predicted by the laws of gravitational wave emission.
But gravitational waves were not directly detected until this month. The discovery event has been described as a ‘chirp’. ‘Hearing’ this signal is somewhat metaphorical. Sound is a pressure wave that propagates through a material medium like air or water. Gravitational waves are ripples of gravity propagating through space and time. The oscillation frequencies of the waves that LIGO measures correspond precisely to the range of pitch to which the human ear is sensitive. Although we did not directly hear the collision of two black holes, it is fair to say that LIGO converted the gravitational signal into an audio one, much as a radio converts electromagnetic signals into audio.
Astronomy has largely been a science of images: we point telescopes at objects in the sky and study the images we find. Astronomy with gravitational waves is a different endeavour. We cannot use gravitational waves to make an image of the source that produces them. Instead, the theory of general relativity tells us how the dynamics of a gravitational-wave source is imprinted on the waves that we measure. General relativity tells us how the pitch and strength of the waves change as their source wiggles here and wobbles there. It tells us how the properties of the two black holes – their masses, their spins, the shape of their orbits – influences the waves that come out.
When we listen to the waves that LIGO first played for us, we can tell that the system is quite heavy, since the signal ends a bit lower than middle C on the piano. If the system were lighter, the waves would have ended at a higher pitched note. We can tell that the two objects are about the same mass because the waves sweep across the band of frequencies that LIGO is sensitive to quite fast. If the masses were less equal, they would have moved through this band more slowly.
We can also tell that the two black holes appear not to be spinning very quickly, though we are less confident of this. If they were spinning rapidly and their axes were misaligned, each black hole would wobble in its orbit. The wobbling would imprint a distinct warble on both the strength and the pitch of the waves it produces. We don’t hear this warble, so either they aren’t spinning very fast or their axes are aligned quite closely.
However, there are gravitational-wave homonyms: two systems with slightly different properties might sound just the same if our detectors can hear only a few cycles of their waves. It reminds me of trying to understand a friend with a heavy accent in a noisy pub. Did he just say ‘Want another beer?’ or ‘Wanda’s mother’s fear’? ‘Beer’ makes a lot more sense; but Wanda’s mother does seem to be afraid of her own shadow.
We know we can hear these waves now, and we want to make our ears better. We want to reduce the noisy hiss inherent to our detectors as much as possible, and open up the range of pitches we can hear. We want to hear more of the wave packet, so we can listen for the wobbling warble of spinning black holes as they move through the last steps of their inspiralling dance. We want to hear binary neutron stars, and search for the explosion that is sure to follow in gamma rays and infrared. We want to hear the ghostly whispers of the earliest moments of the universe’s expansion. We want to listen without prejudice and to hear things that for now we can barely imagine.
Virgo, an antenna in Italy, will join the search later this year, to be joined by detectors in Japan and India in the next few years (both LIGO’s detectors are in the United States). A planned detector in space, eLISA, will extend our ears from the soprano and tenor of neutron stars and stellar mass black holes to the baritone of black holes with a million times the mass of the sun. The bass of even more massive black holes will be revealed using a network of pulsars, rapidly rotating neutron stars that emit regular radio pulsars. The basso profundo of the universe’s earliest moments has left its mark on the cosmic microwave background radiation. With sufficient care, effort and patience, we will disentangle it from the interference that currently masks it from our detectors. The universe has been talking to us for a long time. Until 14 September 2015, we didn’t have the ears to hear. Now that we do, it is time to listen to its story.