In the most precise astrophysics experiment ever made, Australian and U.S.
astronomers have used CSIRO’s Parkes radio telescope to measure the
distortion of space-time near a star 450 light-years (more than 4 000
million million kilometres) from Earth.

Their results, confirming Einstein’s general theory of relativity, are
published in the July 12 issue of the journal “Nature”.

The researchers were Mr Willem van Straten and Professor Matthew Bailes
(Swinburne University of Technology, Melbourne); Professor Shrinivas R.
Kulkarni, Dr Stuart Anderson, and Dr Matthew Britton (California Institute
of Technology); and Dr Richard N. Manchester and Mr John Sarkissian (CSIRO
Australia Telescope National Facility).

The research rests on the properties of one of Nature’s most bizarre
objects: a ‘pulsar’ called J0437-4715. A pulsar is a star made of highly
compressed matter. It spins, and in spinning gives off a stream of radio
pulses. J0437-4715 is one of the brightest and closest pulsars of its kind
and produces more than 170 pulses a second. It waltzes through space with
a companion — an old, shrunken star called a white dwarf.

The astronomers have been able to measure when J0437-4715’s pulses reach
us on Earth to within a tenth of a millionth of a second [100 nanoseconds],
thanks to sophisticated instruments developed by Caltech, leading-edge
computing at Swinburne University and the large collecting area of the
Parkes telescope.

This precise timing, and the closeness of the pulsar, has allowed the
astronomers to determine exactly how the pulsar’s orbit is oriented in
space — the first time this has been done.

Our right and left eyes see slightly different views of the world because
they are separated by a few centimetres. In the same way, two views of the
pulsar system made six months apart look slightly different, because the
Earth has moved from one side of the Sun to the other. The effect is
called parallax.

In the case of pulsar J0437-4715, the difference in the two views is
minuscule — about four millionths of a degree. But it’s enough to allow
the astronomers to construct a 3D model of how the pulsar orbits in space.

To do so, however, PhD student van Straten had to process more than 50,000
Gigabytes of data — as much as would fit on 77 000 CD-ROMs, or a stack
119 metres high.

Having worked out the orbit, the astronomers were able to test a subtle
effect predicted by Einstein’s general theory of relativity. A massive
object distorts the space-time around it. In the pulsar system, the
pulsar’s radio waves travel through the curved space-time around its white
dwarf companion, and arrive on Earth a little later than if they had
travelled through undistorted space-time. The effect, called the Shapiro
delay, was first proposed in 1964 by Irwin I. Shapiro, now Director of
the Smithsonian Astrophysical Observatory.

The data clearly showed the predicted delay, making this the first test
of general relativity in which the geometry of the system has been used to
predict a relativistic effect. An earlier test of a binary pulsar system,
made by Professors Joseph H. Taylor (Princeton University) and Joel M.
Weisberg (now Carleton College), used two general relativistic effects to
predict the value of a third, and so was a test of the self-consistency of
the theory. However, the observations of the pulsar in that system were
not precise enough for the geometry of its orbit to be checked.

“The precision of the current data is so good that we are now aiming to
use the pulsar to search for subtle ripples in the space-time continuum”,
says Professor Bailes. Astronomers think that such ripples would be
produced during the birth of the Universe or when ultra-massive black
holes coalesce. To search for them, experimenters are now designing and
building the next generation of pulsar instrumentation and supercomputers.

The pulsar team has a long history in the discovery and timing of radio
pulsars. In 1982, Kulkarni along with Professor D. C. Backer of the
University of California, Berkeley, discovered the first ultra-fast
(‘millisecond’) pulsar which led to a series of surveys in search of
similar pulsars. One such survey, made with the Parkes radio-telescope
by a team including Manchester and Bailes, led to the discovery of PSR
J0437-4715 in 1992.

This pulsar, with a period of 5.757,451,831,072,007 milliseconds, will
gain or lose no more than 1 millisecond over a period of one hundred
thousand years. “The precision of millisecond pulsars is comparable to
the atomic clocks that are used to keep time. It has long been recognized
that these natural clocks can be exploited for a number of tests in basic
physics,” says Professor Kulkarni.

The Swinburne group has dedicated an entire supercomputer, one of
Australia’s largest, to keep pace with the terabytes of data streaming
from Parkes.

More information:

Professor Matthew Bailes, Swinburne University of Technology,


Mr Willem van Straten, Swinburne University of Technology,


Dr Richard Manchester, CSIRO Australia Telescope National Facility,



Ms Rosie Schmedding


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