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.