Gravitational Waves

The societies September lecture was given by Dr. Sathyaprakash from Cardiff University. It focused on general relativity – in particular gravitational waves.

Over the past 50 years cosmology has come along in leaps and bounds. Nowadays it is treated as a science in its own right rather than in the past when the origins and evolution of the Universe were strictly treated as a question of either philosophy or faith.

Dr. Sathyaprakash is working on a project called GEO600 that, hopefully, will take us back to the very dawn of time itself – when the Universe was born. Well, strictly speaking that should be the dawn of 'space-time' as Einstein was at pains to point out in his theories – space and time are indivisible. Just as in the lyrics of the popular song – "Try, try, try to separate them. It's an illusion". (Love and Marriage)

In his talk "Everything you wanted to know about Einstein's gravity – but were afraid to ask" Dr Sathyaprakash introduced the idea of gravity waves. These are distortions in the fabric of space-time that spread out through the Universe somewhat like the ripples on a pond. They are passing through the Earth and our bodies all the time. The reason we don't feel anything is that the ripples are so tiny and everything around us, including ourselves, is warped leaving us no static point of reference to measure against. It's rather like looking into a fairground hall of mirrors and trying to work out your true dimensions.

Back in 1908 Poincarre first came up with the notion of waves of gravity after studying Einstein's work and in the 1960's Joseph Weber tried, unsuccessfully, to observe the effect using a couple of suspended Aluminium bars. Einstein's General Theory of Relativity was used to explain why Mercury's orbit didn't quite follow the path predicted by Newton's equations. Then, in 1993, Russell Hulse and Joseph Taylor of Princeton University won a Nobel Prize for their discovery and subsequent study of a binary pulsar.

They observed a system of two neutron stars that orbited around each other. One of the neutron stars (the pulsar) was emitting radio pulses that could be detected back on Earth. These radio waves allowed the researchers to measure the stars separation as they circled around each other. They realised that the two bodies were creeping closer and closer together and eventually in the future would spiral in towards each other and collide. The actual decrease in their orbital period was tiny, a mere 75 millionths of a second a year! This means it will take around a hundred million years for the cataclysmic event to take place, which will send out a strong series of gravitational waves.

Their inevitable fate is due to the fact that as these two extremely massive bodies move around each other they are losing some of their inherent energy as gravitational radiation. The energy has to come from somewhere and so the stars slow down and drop into a closer orbit.

In 3 to 5 years time Dr Sathyaprakash hopes to be able to announce to the world that his experiment, aided by other worldwide detectors, has observed the ripples in space-time caused by explosion of a supernova or the merger of black holes. Future projects such as the space-based LISA set of NASA spacecraft should be able to observe the interactions of close galaxies, the merger of black holes and probe back in time to the era of the Big Bang itself.