Einstein's gravitational waves remain elusive
Could the cataclysmic coming-together of two black holes produce fewer ripples in spacetime than we thought? Alan Duffy explains.
The cosmic do-si-do of two supermassive black holes spiralling towards each other is a cataclysmic dance of such intensity, it should ripple the fabric of spacetime itself – or so says Einstein’s general theory of relativity. One hundred years have passed since Einstein first proposed the existence of gravitational waves, but they are yet to be detected directly.
Astronomers in Australia have spent the past decade conducting the most thorough search yet for gravitational waves released when supermassive black holes circle each other, using the Parkes radio telescope in New South Wales. But as the researchers reported in Science in September, they could find no trace of them.
Could Einstein be wrong? Or have we misunderstood black holes?
Space is awash with gravitational waves but they’re extraordinarily weak. No doubt a gravitational wave is passing through you now, stretching you taller and thinner, then squashing you shorter and fatter. The reason you don’t notice is because your height is altered by less than the width of a proton (a fraction the size of an atom).
Astronomer Ryan Shannon, based at CSIRO and the International Centre for Radio Astronomy Research in Perth, and his team attempted to detect gravitational waves from black holes by measuring their effect on the pulses of radio waves coming from a neutron star more than 3,600 million billion metres away.
Neutron stars (another prediction of Einstein’s) were discovered in 1967. They are the crushed cores of large dead stars that, when they ran out of fuel, collapsed under their own immense gravity, squeezing as much mass as our Sun’s into the size of Sydney’s central business district.
And like an ice-skater who spins faster when she tucks her arms in, a neutron star rotates more rapidly as it collapses. As they spin, some emit a tightly focused beam of radiation that shines like a lighthouse. If the Earth lies in the rotating beams’ path, we detect this radiation as the pulses of radio waves, which earned these neutron stars the nickname pulsars.
A pulsar’s spin is so stable that the pulse it emits is as reliable as the super-accurate tick of an astronomical clock.
Over the past 11 years the CSIRO’s Parkes radio telescope has been timing the pulses from one such regular and bright pulsar. It spins at more than 300 rotations per second, and each of its 115,836,854,515 rotations over more than a decade has been right on time. But according to Einstein, this shouldn’t be the case.
We don’t fully understand the black hole mergers that generate gravitational waves
According to Einstein’s theory, the gravitational ripples emitted by countless pairs of circling black holes around the Universe should add up, sometimes stretching spacetime between Earth and the pulsar by 10 metres. This stretch should skew the arrival time of pulses from the pulsar by up to one ten-billionth of a second. The Parkes telescope’s timing equipment is accurate enough to detect such a minute change.
But it didn’t detect any delay.
The researchers didn’t doubt that gravitational waves exist. They have been detected indirectly. American astronomers Russell Hulse and Joseph Taylor won the 1993 physics Nobel Prize for doing this. They used a pair of neutron stars to measure the astoundingly tiny shortening of the stars’ year – about 30 seconds over three decades – as they spiralled inwards toward each other. Hulse and Taylor calculated that this amount of shortening followed Einstein’s predictions. Some of the energy that kept the stars rotating must have been emitted in the form of gravitational waves.
So the more likely explanation for the failure of the Parkes research is that we don’t fully understand the circling black holes that generate gravitational waves.
There are several possible reasons why, as they circle each other, pairs of black holes don't ripple spacetime by as much as we thought. Recent observations suggest every galaxy, including our own Milky Way, harbours a supermassive black hole at its core. For reasons still unclear, the mass of the black hole is directly related to the mass of its galaxy – in nearby galaxies where we have been able to make these measurements, at least.
But perhaps, when the Universe was younger, that relationship did not hold, and black holes were smaller that they are today. If so, their contribution to the cumulative rippling in spacetime Parkes was trying to pick up would have been smaller too - maybe too small for Parkes to detect.
Alternatively, early galaxies tend to be more gas-rich. This gas would act like treacle, slowing black holes down. Instead of dancing around each other for billions of years, they “fall in” toward each other much faster, creating a short sharp blast, but ultimately fewer gravitational waves.
All of which gives Shannon and his team plenty to ponder as they continue their search. Measuring gravitational waves directly would do more than confirm Einstein’s theory of general relativity. It would also be the first time astronomers have looked into the Universe with something other than light. All telescopes, regardless of their size and sophistication, use light waves (be they the long wavelength radio wave variety, visible light, or short wavelength X-rays).
The observation of gravitational waves would be the dawn of a new era of astronomy. Humanity would look outwards with gravity, and who knows what we might see.