 CSIRO's Parkes radio telescope. Photo: Shaun Amy
Astronomers have used a pair of pulsars orbiting
each other, found with CSIRO’s Parkes telescope in 2003, to show that
Einstein’s theory of general relativity is correct to within 0.05% –
the most stringent limit to date. They
also hope to be able to use the two pulsars to determine the exact
nature of the matter that pulsars and other neutron stars are made of.
Their results are to be published in the journal Science, and made available online in Science Express Science Express [external link] on 14 September 2006.
An international research team led by Professor Michael Kramer of
the University of Manchester's Jodrell Bank Observatory, UK, has been
observing the double-pulsar system since 2003 with three of the world’s
largest radio telescopes: CSIRO’s Parkes radio telescope in NSW,
Australia; the Lovell Telescope near Manchester, UK; and the Robert C.
Byrd Green Bank Telescope in West Virginia, USA.
The double-pulsar system, whose pulsars are called PSR J0737-3039A
and B, is the only known system of radio pulsars orbiting each other.
It lies 2000 light-years away in the direction of the constellation
Puppis.
The system consists of two massive, highly compact neutron stars,
each weighing more than our own Sun but only about 20 km across,
orbiting each other every 2.4 hours at speeds of a million kilometres
per hour.
Separated by a distance of just a million kilometres, both neutron
stars emit lighthouse-like beams of radio waves that are seen as radio
’pulses‘ every time the beams sweep past Earth.
By precisely measuring the variations in pulse arrival times, the
researchers found the movement of the stars to exactly follow
Einstein's predictions. "This is the most stringent test ever made of
GR in the presence of very strong gravitational fields—only black holes
show stronger gravitational effects, but they are obviously much more
difficult to observe,” Professor Kramer says.
Co-author Ingrid Stairs, an assistant professor at the University of
British Columbia in Vancouver, Canada, says it is possible to measure
the pulsars’ distances from their common centre of mass. "The heavier
pulsar is closer to the centre of mass, or pivot point, than the
lighter one and this allows us to calculate the ratio of the two
masses,” she says.
This mass ratio is independent of the theory of gravity, and so
tightens the constraints on general relativity and any alternative
gravitational theories.
Other relativistic effects predicted by Einstein can be observed:
the fabric of space-time around pulsar B is curved, and the other
pulsar’s “clock” runs slower when it is deeper in the gravitational
field of its massive companion. Each of these effects provides an
independent test of general relativity.
The distance between the pulsars is shrinking by 7 mm a day.
Einstein's theory predicts that the double pulsar system should be
emitting gravitational waves – ripples in space-time that spread out
across the Universe at the speed of light.
"These waves have yet to be directly detected,” says team member
Prof. Dick Manchester of CSIRO’s Australia Telescope National Facility
ATNF). "But, as a result, the double pulsar system should lose energy
causing the two neutron stars to spiral in towards each other by
precisely the amount that we have observed – thus our observations give
an indirect proof of the existence of gravitational waves."
The astronomers hope that over the next few years they can make even
more precise measurements of the characteristics of the system,
allowing them to measure the moment of inertia of a neutron star.
(“Moment of inertia” is a measure of how much a body resists a force
trying to rotate it.) “This measurement may be very difficult but if we
could do it to just a precision of 30 per cent, we could distinguish
between the many different ideas about the nature of the matter that
makes up neutron stars,” says team member Dr George Hobbs of the ATNF.
Source: CSIRO
Related Links:
M. Kramer et al., "Tests of General Relativity from Timing the Double Pulsar", Science
2006
DOI: 10.1126/science.1132305
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