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Under the Radar

Gravitational waves: written in the stars?

About the author

Philip is a writer based in London. He writes on all areas of the sciences and its interactions with art and wider culture. He was previously an editor for the science journal Nature for two decades and is the author of many books on science, including The Self-Made Tapestry: Pattern Formation in Nature, H2O: A Biography of Water, Critical Mass (winner of the 2005 Aventis Prize for Science Books), and The Music Instinct. You can find out more at his website or blog.

Neutron stars creating gravitational waves

(Copyright: Nasa)

Examining the contours of burned-out remnants of stars could provide direct evidence for two of the most sought-after phenomena in fundamental physics.

One thing physics has taught us is that events at the smallest possible scales can have consequences of cosmic proportions. And, in turn, studying some of the universe’s most spectacular astrophysical phenomenon can reveal a lot about physics at its most elementary level.

The latest example of this has been proposed by a team of researchers in Europe – they say that studying the contours of burned-out remnants of stars thousands of light years away could provide concrete evidence for two of the most sought-after phenomena in fundamental physics.

The first phenomenon, gravitational waves, was predicted by Einstein’s theory of general relativity, which explained the force of gravity as a curvature in spacetime induced by mass. Einstein’s theory is typically illustrated by depicting spacetime as if it resembled a rubber sheet, which a heavy object (such as a star or planet) bends down into a dimple, into which other objects can roll. In this analogy, a disturbance of the heavy mass can produce a ripple in the sheet, radiating outwards like a splash in a pond. This is a gravitational wave, which carries away some energy from the source.

To create a ripple of any significant size, the disturbance has to be really big – for example, two stars colliding. Rather less dramatically, two star-like objects orbiting one another very fast and close are predicted to radiate energy as gravitational waves, so that they gradually lose energy and spiral in towards one another. This effect has been observed before: several times in so-called binary pulsars, and just recently in a binary white-dwarf system 3,000 light years away.

Although such discoveries provide indirect proof that gravitational waves exist, scientists would dearly like to see them directly. When a gravitational wave passes by, it distorts spacetime so that distances get very slightly shorter or longer in the direction of the wave. For a typical wave from some far-off astronomical paroxysm, the change in distance would be tiny: a kilometre would get shorter by about a quarter of a million-trillionth of a metre, or less than a thousandth of the diameter of a proton, one of the key subatomic particles in an atom’s nucleus.

Catching a wave

Nonetheless, scientists think this might be detectable. They have constructed tubular channels several kilometres long, down which they fire laser beams that bounce off a mirror at the far end. If a beam is split and sent down two channels at right angles in an L shape, the peaks and troughs of the two reflected waves interfere with each other when they bounce back and meet. A gravitational wave would change one channel length more than the other, depending on its direction, slightly altering the interference between the two beams.

Several such gravitational wave detectors now exist. One project, called LIGO and run by scientists in the US, operates two such detectors in Louisiana and Washington. The different arrival times of a wave at these two detectors over 3,000 km apart would enable the source of the wave in the sky to be roughly identified. However, neither LIGO nor any other gravitational-wave project has seen anything yet. The European Union is currently planning to build a new, more sensitive detector underground, called the Einstein Telescope.

Now astrophysicist Kostas Glampedakis at the University of Murcia in Spain and coworkers have suggested a new possible source of gravity waves: mountains on rotating neutron stars.

Neutron stars are old, burnt-out stars that have collapsed under their own gravity so that its atoms are mashed together into a sea of fundamental particles that are mostly, as you guessed, neutrons. In fact, neutron-star matter is so densely packed that a thimbleful would typically weigh hundreds of billions of kilogrammes, equivalent to a mountain range. Some of these stars spin incredibly fast (sometimes hundreds of times a second), resulting in a lighthouse-like flashing of a beam of radio waves streaming from the star’s magnetic poles, and these are called pulsars.

It’s long been recognised that if such stars had any bumps – or “mountains” – on its surface, this would create a wobble that could spawn a gravity wave. Because the gravitational field of a neutron star is so intense, one might expect the star to be pulled into a perfect sphere. But that’s not necessarily the case. Four years ago, Australian scientists showed that if a neutron star had an orbiting companion star from which it pulled off matter, this matter could accumulate at the poles, propped up by the star’s magnetic field to prevent it from flattening out. In that way, mountains several kilometres across but only a few tens of centimetres high could develop, having about the mass of the planet Saturn.

Extreme matter

Glampedakis and colleagues say that there is another way to build mountains on neutron stars which doesn’t rely on their having a companion star to cannibalise – and this points towards the second sought-after phenomena in fundamental physics. It supposes that the incredibly high density of the star could squash its atoms not into neutrons but into a sea of the still more fundamental particles of which atomic nuclei are made: quarks.

It’s not known if this “quark matter” can really exist. Some hope that it might be sighted in the Large Hadron Collider particle accelerator at CERN in Geneva, but a better bet could be to search for its signature in neutron stars – which would then in fact be quark stars, most probably with a core of quark matter coated with ordinary matter such as neutrons.

The researchers say that this quark matter will most likely form pairs, and as a consequence they can become stirred by vortices. The vortices will create something like a magnetic force that will make the star’s interior lumpy, with some bits more dense than others. In effect, these lumps would act like “internal mountains”, again producing a wobble that stimulates gravitational waves.

The calculations of Glampedakis and colleagues suggest that the Einstein Telescope might be able to detect gravitational waves produced this way. If, for example, the well-studied Vela and Crab pulsars – both produced by supernovae, the latter being the origin of the Crab Nebula – have cores made of paired quark matter, they should radiate strong enough gravitational waves to be just about observable. Depending on the details, they might even be spotted by upgrades of LIGO. In either case, this would not only vindicate Einstein but also offer a window on this weird, extreme form of matter.

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