Fatal attraction: double pulsar tests Einstein's theories
Artist's impression of the double pulsar system.
Dr Michael Kramer, Professor Andrew Lyne, Dr Maura McLaughlin, Dr Duncan Lorimer, Professor Sir Francis Graham-Smith and Mr Tim Ikin.
University of Manchester, Jodrell Bank Observatory.
Last year researchers from Jodrell Bank Observatory of the University of Manchester announced they had identified a double pulsar system - one of the 'Holy Grails' of gravitational physics. The discovery of these two orbiting neutron stars has already enabled the testing with 'unprecedented accuracy' of several effects predicted by Einstein's theory of general relativity. Described as a' rare laboratory' for testing Einstein's 'mind boggling' theories, the discovery provides the staunchest test yet of general relativity.
A pulsar is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than the sun, yet only the size of a city, these incredibly dense objects produce beams of radio waves that sweep round the sky like a lighthouse, often hundreds of times a second. Radio telescopes, like the giant Lovell Telescope at Jodrell Bank, receive a regular train of pulses as the beam repeatedly crosses the Earth so the objects are observed as pulsating radio signals. The regularity of these beam sweeps makes pulsars exceptional clocks enabling very precise timing observations. The key feature of the double pulsar system is that you can measure the orbits of both stars in the system through the radio signals reaching Earth. Through some simple mathematics, the ratio of these orbits allows calculation of the ratio of the pulsar masses and the beauty is that this calculation is free of any assumed theory of gravity. This makes the system perfect for testing theories of gravity, such as Einstein's theory of relativity, which, if correct, should be able to predict the mass ratio of the pulsars as determined by their orbits. 'This is the first time we have a system where calculations can be made independent of the effects of gravity, providing the hardest test yet of Einstein's theories', says Michael Kramer of the Jodrell Bank Observatory.