Science Feature

Relativity's final test

Richard Webb

NIGHT in, night out, the rhythmic radio signals reach Earth. The slowest of them sound like a nail being hammered into wood, or a shoe being slapped against a post to rid it of mud. Others are more like a stuttering motor stopped at a traffic signal. Some make almost continuous tones, ripe to be combined into cosmic mood music.

Always the same signature tunes, always from the same points in the sky. Small wonder that when astronomers first heard them back in the 1960s, some thought they were messages from alien civilisations.

The signals aren't from ET, however; they are from pulsars. These extreme cosmic objects have been keeping us on our toes for over 40 years, and are poised for their greatest coup yet. Meticulous measurements of pulsars' timekeeping might just solve one of the biggest mysteries of modern physics: the whereabouts of gravitational waves.

The keystone of Einstein's general theory of relativity, gravitational waves are tiny ripples in the fabric of space-time. But they have proved frustratingly elusive, despite ever bigger and more expensive instruments being built to detect them. Pulsars could now pip these projects to the post - on a fraction of the budget. "We're already at the stage where we can start to rule out things," says George Hobbs of the Parkes Observatory in New South Wales, Australia, which is home to one of the pulsar-timing experiments. "We could make a detection next week," he says, depending on the nature of gravitational waves.

It was August 1967 when pulsars first made Earth contact. In a field on the outskirts of Cambridge, UK, graduate student Jocelyn Bell and her supervisor Antony Hewish were using a new antenna array to scan the sky for radio sources. Back then, astronomical observations were measured in miles - of paper. Mechanical pens traced radio signals onto long charts, and it was Bell's job to trawl through them.

In one chart, she spotted an odd bit of "scruff", as she described it: a train of pulses spaced 1.3 seconds apart. It did not fit with any astronomical phenomenon then known, and try as they might Bell and Hewish could find no explanation, natural or artificial. Stumped, they half-joked about who they should tell first that they had eavesdropped on little green men.

Relativity's predictions
By the time they published their findings the following year, the true culprit had been unmasked: a neutron star (Nature, vol 217, p 709). These extraordinarily dense bodies, left behind when a star many times the size of our sun explodes in a supernova, pack the mass of our sun and more into a sphere just tens of kilometres across. Besides being unusually dense, neutron stars also rotate rapidly and have huge magnetic fields. To turn a neutron star into a pulsar, its magnetic axis must be at an angle to its rotational axis. That way, the powerful jets of radiation erupting from the star's magnetic poles will sweep round as the star rotates, rather like the beam of a lighthouse. These jets are what regularly buzz our telescopes - although we still don't know exactly how they are formed (see "What makes a pulsar tick?").

The first pulsars to be discovered spin in a comparatively leisurely fashion, taking several seconds to complete one rotation. In 1982, however, a group led by Donald Backer of the University of California, Berkeley, upped the ante with a "millisecond" pulsar that whirls around a breathtaking 642 times a second, fuelled by matter and energy siphoned from a companion star. The pulses of millisecond pulsars are so fast and regular that they make fantastic cosmic clocks, rivalling the accuracy of any man-made atomic timepiece. From there, it is just a small mental leap to using them to spy out gravitational waves.

According to Einstein's general theory of relativity, mass distorts space and time around it, creating the force we know as gravity. Not even massless light is immune to its embrace. General relativity's most outlandish predictions have been confirmed, including the existence of black holes and the bending of starlight by massive celestial objects. But one remains elusive. If two massive objects are in orbit around each other, relativity says their accelerations will cause transient distortions in space-time that ripple out into the cosmos - gravitational waves.

So far, we have only one piece of circumstantial evidence that such waves exist. It comes, fittingly enough, from a pulsar. In 1974, astronomers Russell Hulse and Joseph Taylor discovered one pulsar circling particularly tightly around a companion, completing one orbit every 8 hours. They saw the distance between the two bodies steadily diminish as they spiral in towards each other - exactly what Einstein had predicted should happen if they were losing energy by radiating gravitational waves.

Source: New Scientist