Binary pulsar and gravitational wave

Clockwise from left: Artist's impression of the binary pulsar system PSR J0737-3039. Orbiting of binary pulsars and emission of gravitational waves. Pulse profile from binary pulsar.

A press release issued on 13 October 1993 says "The Royal Swedish Academy of Sciences has decided to award the Nobel Prize Physics for 1993 jointly to Russell A. Hulse and Joseph H. Taylor, Jr, both of Princeton University, New Jersey, USA for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation". An outline of this discovery and its relation to the study of gravitational waves, is presented here.
All stars are mostly composed of hydrogen- the lightest element in nature. Under high pressure and high temperature, the nuclei of hydrogen atoms fuse together to form a heavier nuclei like helium and produce tremendous amount of energy, which is actually the hydrogen bomb. The process is called nuclear fusion. All the stars including our sun give heat and light for million years through this process. But a time arrives when the fuel is finished. Then what happens? At this moment stellar evolution occurs. Some stars evolve to neutron stars and some stars to black holes. It depends on the initial mass of the star. The limit for initial mass for different ways of stellar evolution was given by an Indian Scientist, Subramanyan Chandrasekhar and the limit is called Chandrasekhar limit, for what he was awarded the Nobel prize in physics in 1983. At this stage, the external part goes outward giving tremendous pressure inside such that electrons in the atomic orbit enter into the proton of nucleus and make them neutrons, thus a neutron star is formed with a radius of ten kilometers having super dense materials and super strong magnetic field. The neutron star rotates several hundred times in second and radiate electromagnetic pulses with fixed interval of times, which was discovered by Antony Hewish and Jocelyn bell in 1967. Antony Hewish was awarded the Nobel Prize in physics in 1974, for this discovery.
During 1974 Joseph Taylor and Russell Hulse, two plasma physicists of Princeton University, were searching for new pulsars with the 300-m Arecibo telescope- the largest radio telescope in the world at Puerto Rico, West Indies. They discovered pulsar named PSR 1913 +16, from where pulses are coming sometimes more often and sometimes less. This was interpreted as pulses are coming from two orbiting around each other pulsars- called binary pulsar. For pulsars the year of orbiting is about eight hours. Their mass is about 1.4 times mass of the sun. When they are closing to each other they emit gravitational waves. For this discovery Russel and Hulse were awarded Nobel Prize in physics in 1993.
Discovery of binary pulsar and their existence is an excellent laboratory for testing the theory of gravity. Yet undiscovered binaries will provide even better opportunities to test general relativity. Astronomers are searching binary pulsars in our Milky Way Galaxy with increasingly better technology. There are about 1700 pulsars are known and 8 of them are binaries. one of the recent discovery in 2003 is the double pulsar system, PSR J0737-3039, in which both neutron stars are visible pulsars.
The discovery of binary pulsar and the decreasing of its orbital period is a proof of existence of gravitational waves and the manifestation of general theory of relativity which is an astonishingly beautiful theory. The Soviet physicists Lev landau and Evgeny Lifshits wrote in their text book 'The Classical Theory of Fields" that it is the most beautiful and profound of existing theories. The German physicist max Born said once he enjoyed general theory of relativity as he would enjoy an object of art. And the soviet physicist Vitaly Ginzburg wrote that " this theory makes an experience something akin to what one feels when contemplating the masterpieces of painting, sculptures, or architectures."
Gravitational wave is a concept of Einstein's General Theory of Relativity given in 1916. According to the theory, accelerating mass should radiate gravitational waves as accelerating charges radiate radio waves. Gravitational wave is a deformation of space-time. An object exposed to gravitational waves should become alternatively longer and thinner, shorter and broader. The variation however so small (10-17 in dimensionless) that it does not yet possible to detect. Several direct detectors of gravitational are being built in USA. The best known of these are Laser Interferometer Ground Observatory (LIGO) and Laser Interferometer Space Array (LISA). In Bangladesh, theoretical research on pulsar, gravitational waves and earthquake prediction from space are being done at the Department of Mathematics and Natural Sciences of BRAC University.

The author is a Plasma Physicist and Professor at BRAC University: Email: [email protected]