Pulsars: Gravitational Wave Detectors on the Cheap

The search for gravitational waves is one of the great scientific endeavors of modern times. Their discovery will allow astronomers to peer out into the cosmos in an entirely new way and to study exotic new phenomena such as collisions between black holes and neutron stars.

On Earth, physicists have built expensive detectors that can measure the way gravitational waves squeeze and stretch spacetime as they pass by. Despite years of study and millions of dollars of investment, these machines have found precisely nothing.

But there’s another way of hunting for gravitational waves: to look for the effect on pulsars as the waves race through the galaxy like ripples on a pond.

A pulsar is a rotating neutron star that emits a powerful beam of radio waves, which sweeps across space like the beam from a lighthouse. From Earth, pulsars look as though they are flashing with a period measured in anything from seconds to milliseconds and with a constancy more accurate than an atomic clock.

That’s given astronomers an idea. When a gravitational wave passes through the solar system, the stretching of spacetime will influence the arrival time of the pulsar signals. So the idea is for astronomers to watch many pulsars in different directions, looking for the telltale changes caused by gravitational waves. 

This so-called pulsar timing array should shimmer as gravitational waves wash over the solar system, like sunlight through thermals on a hot day.

On this blog, we looked at the idea a few years ago when it was little more than a twinkle in the eyes of the few ambitious astronomers. Things have moved on since then.

Today, George Hobbs at the Australia Telescope National Facility in Epping outlines progress on several fronts. The first task is to measure the arrival time of pulsar beams with the required accuracy. 

Next, these experimental results are compared with a computer model of the pulsars to work out the difference between the expected and measured arrival time. 

The difference can be the result of various factors, such as errors in the timing standard or turbulence in the interstellar medium which interferes with the signal. But once these have been taken into account, any remaining difference should be due to gravitational waves. 

Noise from various sources makes this task very challenging, but Hobbs is optimistic about the future. He says that the ongoing discovery of new pulsars is encouraging as is the construction of various new and highly sensitive radio telescopes, such as the 500-meter Spherical Telescope (FAST) in China and the Square Kilometre Array (SKA) in Australia and South Africa. 

There are still substantial challenges ahead. Nobody is quite sure what the signal from gravitational waves should look like. Astronomers disagree over whether it will be a kind of background hiss, a chirping, a burst, or some other signal. 

Neither are they willing to stick their necks out and say when the waves are likely to be finally discovered. Clearly, astronomers have learnt their lesson after several decades of false promises on that score, and Hobbs also keeps dutifully silent. 

The big unspoken fear is that gravitational waves will be much fainter and more difficult to detect than anyone had imagined. 

Astronomers assume the waves will travel unhindered through spacetime at the speed of light.  But any small damping effect would dramatically change this. 

We know the universe is filled with mind-boggling amounts of dark matter and dark energy, stuff that we know next to nothing about. So it’s not beyond the realm of possibility that this (or other) stuff could interact with gravitational waves in ways that are important.

If that turns out to be the case, they’ll be some very expensive paperweights some gathering dust in obsolete gravitational wave laboratories around the world. 

The pulsar timing array experiments, on the other hand, are much better value for money. They’re cheaper by far than dedicated detectors and also used for other valuable observations. In an age of austerity, that’s an approach worth applauding.

Ref: arxiv.org/abs/1210.2774: Pulsar Timing Arrays: Status and Techniques