Gravitational waves – the Next Big Thing in astronomy
From post-doc to professor, Paul Davies has followed the search for gravitational waves – he even wrote the book on it. He describes his excitement when elusive ripples in space-time were finally found and looks ahead to a new age of exploration.
Some 36 years ago I wrote a little book called The Search for Gravity Waves. I wondered at the time whether these elusive entities would be detected in my lifetime.
But last week, I was stunned by the results unveiled by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US. Gravitational waves were at last detected, 100 years after Albert Einstein predicted their existence.
I was a young postdoc in Cambridge in 1971 when an American physicist, Joseph Weber, delivered a lecture on his search for passing gravitational waves using bars of metal, delicately suspended in a vacuum. Sensors around the cylinders could pick up the tiniest shudder, and clever methods were used to screen out mundane sources of vibration such as passing cars or students dropping pens on the floor.
These so-called “Weber bars” became the gold standard for researchers around the world, including Australia, where the young David Blair at the University of Western Australia built a state-of-the-art detector using a huge cylinder of the soft white metal niobium.
The proliferation of bar detectors was spurred by Weber’s claim that he had routinely detected bursts of gravitational waves. The sensational claim was greeted with widespread skepticism – the strength of the signal seemed too good to be true.
About the same time, calculations suggested that lasers – then a scientific novelty – might provide a more sensitive method of detection than shuddering bars. That was the beginning of the LIGO instruments that made history last week.
While we have only just detected them, we had indirect evidence of their existence since the 1970s based on observations of an object known as a binary pulsar. This is a pair of neutron stars that emits incredibly regular radio pulses enabling its orbital motion to be tracked to extraordinary precision.
Although physicists like me had anticipated this event – and this waveform – for decades, there is something truly liberating in seeing the reality.
Two American astrophysicists, Russell Hulse and Joseph Taylor of the University of Massachusetts Amherst, studied one such system, known prosaically as PSR B1913+16, for many years using a radio telescope.
Theory suggests that as stars orbit each other, they should emit copious gravitational waves. Using a formula derived by Einstein himself it is possible to work out the rate that the system loses energy in this way, and hence determine the expected rate of orbital decay.
Sure enough, the observations confirmed that the two neutron stars were locked in a death spiral that conformed to Einstein’s predictions.
After that, the race was on to detect such waves arriving on Earth.
So as I watched the press conference live last Thursday, I was skeptical. I wondered whether this might be another overhyped “maybe” discovery, such as the since-invalidated BICEP2 announcement in 2014.
But when I saw the textbook form of the wave, with the rise in amplitude and frequency and the sudden drop following the merger of the black holes, I knew I was witnessing a landmark in the history of science.
Although physicists like me had anticipated this event – and this waveform – for decades, there is something truly liberating in seeing the reality. I immediately began to think, “well, with that now in the bag, what's the next step?” – and a dozen ideas came to my mind.
Welcome though the confirmation of Einstein’s mathematical prediction may be, there is a bigger game at play here. Gravitational wave-based instruments look set to rival optical and radio telescopes as the Next Big Thing in astronomy. The same quality that makes gravitational waves so hard to detect – their extremely feeble effect on matter – also means they penetrate the gas and dust that blocks many electromagnetic waves.
Thus gravitational observatories should be able to peer inside supernovae, detect events near the surfaces of black holes and catalogue violent phenomena occurring billions of light years away that may otherwise never come to our attention.
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