Robert Karl Stonjek
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 Posted: Fri Jul 04, 2008 11:01 am Post subject: News: Einstein was right - Unique stellar system provides 'l |
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Einstein was right: Unique stellar system provides 'laboratory' for testing
relativity
Pulsars are small, ultradense stellar objects left behind after massive
stars die and explode as supernovae. They typically have a mass greater than
that of our Sun, but compressed to the size of a city like Montreal. They
spin at staggering speeds, generate huge gravity fields and emit powerful
beams of radio waves along their magnetic poles.
These illuminate Earth-based radio-telescopes like rotating lighthouse
beacons as the pulsar spins. More than 1,700 pulsars have been discovered in
our galaxy, but PSR J0737-3039A/B, discovered in 2003, is the only known
double-pulsar system; that is, two pulsars locked into close orbit around
one another.
The two pulsars are so close to each other, in fact, that the entire binary
could fit within our Sun. PSR J0737-3039A/B lies about 1,700 light years
from Earth.
This new test of Einstein's theory was led by McGill astrophysics PhD
candidate René Breton and Dr. Victoria Kaspi, leader of the McGill
University Pulsar Group.
"A binary pulsar creates ideal conditions for testing general relativity's
predictions because the larger and the closer the masses are to one another,
the more important relativistic effects are," Breton explained.
"Binary pulsars are the best place to test general relativity in a strong
gravitational field," agreed Kaspi, McGill's Lorne Trottier Chair in
Astrophysics and Cosmology and Canada Research Chair in Observational
Astrophysics.
""Einstein's theory predicted that, in such a field, an object's spin axis
should slowly change direction as the pulsar orbits around its companion.
Imagine a spinning top when its slightly non-vertical: the spin axis slowly
changes direction, an elegant motion called 'precession.'"
The researchers discovered that one of the two pulsars is indeed
precessing -- just as Einstein's 1915 theory predicts. If Einstein had been
wrong, the pulsar wouldn't be precessing, or would precess in some other
way.
Pulsars are too small and too distant to to allow us to directly observe
their orientation, the researchers explained. However, they soon realized
they could make such measurements using the eclipses visible when one of the
twin pulsars passes in front of its companion. When this occurs, the
magnetosphere of the first pulsar partly absorbs the radio "light" being
emitted from the other, which allows the researchers to determine its
spatial orientation. After four years of observations, they determined that
its spin axis precesses just as Einstein predicted.
Even though spin precession has been observed in Earth's solar system,
differences between general relativity and alternative theories of gravity
might only shake out in extremely powerful gravity fields such as those near
pulsars, Breton said.
"However, so far, Einstein's theory has passed all the tests that have been
conducted, including ours. We can say that if anyone wants to propose an
alternative theory of gravity in the future, it must agree with the results
that we have obtained here."
Breton, Kaspi and colleagues in Canada, the United Kingdom, the U.S., France
and Italy studied the twin-pulsar using the 100-metre Robert C. Byrd Green
Bank Radio Telescope at the National Radio Astronomy Observatory in Green
Bank, WV.
"I think that if Einstein were alive today, he would have been absolutely
delighted with these results," said Dr. Michael Kramer, Associate Director
of the Jodrell Bank Centre for Astrophysics at Manchester University. "Not
only because it confirms his theory, but also because of the novel way the
confirmation came about."
Spin Precession:
Time-lapse animation displaying the evolution of pulsar B's geometry in the
double pulsar PSR J0737-3039A/B due to relativistic spin precession between
January 2004 and January 2029. The truncated dipolar magnetosphere of pulsar
B, shown as a colored dipolar structure, rotates about its spin axis,
pictured as a diagonal rod. The apparent orbital motion of pulsar A during
the eclipse corresponds to the horizontal black line intersecting pulsar B's
magnetosphere. Relativistic spin precession is similar to the wobbling of a
spinning top and induces a motion of the spin-axis orientation around the
orbital angular momentum, which is vertical in this movie. The theoretical
light curve corresponding to the eclipse animated in the upper panel is
drawn in the lower panel. The angle phi corresponds to the longitude of the
spin axis, with 0 degree being the direction coincident with the line of
sight. [(MOV, 5.9 MB) Credit: Rene Breton, McGill University]
Pulsar Sound:
This audio file is the sound that one would hear if the radio signal
detected from PSR J0737-3039A by the radio telescope was noise-filtered and
amplified into an audio device. While individual pulsations from the pulsar
are too fast to be distinguished, we can hear a mixture of F musical tones
harmonically related to 44 Hz (F1 tone), the spin frequency of the pulsar.
The sound is modulated in intensity as a result of the eclipse when the
pulsar passes behind its companion pulsar. [MP3 Version (1.1 MB), Credit:
René Breton]
Source: McGill University
http://www.physorg.com/news134313018.html
Posted by
Robert Karl Stonjek |
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