Pulsars as Gravitational-Wave Detectors

This is a follow-up to an earlier posting about gravitational waves, which relates a bone of contention between me and another blogger who insists that gravitational waves do not exist.

The following news release describes how pulsars can be used to detect the passage of gravitational waves.

The other blogger may not consider such detection as direct evidence, since he has disputed the interpretation of a Nobel Prize-winning set of measurements of an in-spiraling pulsar in a double star system, which most physicists consider highly convincing indirect evidence of gravitational waves.

In any case, this idea provides yet another way for astronomers to test Einstein’s prediction of gravitational waves. If the period of a pulsar is affected as predicted by an event that would be expected to produce detectable gravitational waves, then it will give the other blogger yet one more piece of evidence to explain away.

If, however, the predicted effect does not occur, then we will need to look for an alternative explanation of the data. Given the track record of General Relativity, it is far more likely that the theory will meet the challenge (and thus be supported and enhanced by the new evidence) than that it will fail.

Here’s the release:
New Tools for Gravitational-Wave Detection

Dave Finley
Public Information Officer
+1 (575) 835-7302
[email protected]


Teamwork between gamma-ray and radio astronomers has produced a
breakthrough in finding natural cosmic tools needed to make the first
direct detections of the long-elusive gravitational waves predicted by
Albert Einstein nearly a century ago. An orbiting gamma-ray telescope
has pointed radio astronomers to specific locations in the sky where
they can discover new millisecond pulsars.

Millisecond pulsars, rapidly-spinning superdense neutron stars, can
serve as extremely precise and stable natural clocks. Astronomers hope
to detect gravitational waves by measuring tiny changes in the
pulsars’ rotation caused by the passage of the gravitational waves. To
do this, they need a multitude of millisecond pulsars dispersed widely
throughout the sky.

However, nearly three decades after the discovery of the first
millisecond pulsar, only about 150 of them had been found, some 90 of
those clumped tightly in globular star clusters and thus unusable for
detecting gravitational waves. The problem was that millisecond
pulsars could only be discovered through arduous, computing-intensive
searches of small portions of sky.

“We’ve probably found far less than one percent of the millisecond
pulsars in the Milky Way Galaxy,” said Scott Ransom of the National
Radio Astronomy Observatory (NRAO).

The breakthrough came when an instrument aboard NASA’s Fermi Gamma-Ray
Space Telescope began surveying the sky in 2008. This instrument
located hundreds of gamma-ray-emitting objects throughout our Galaxy,
and astronomers suspected many of these could be millisecond pulsars.
Paul Ray of the Naval Research Laboratory initiated an international
collaboration to use radio telescopes to confirm the identity of these
objects as millisecond pulsars.

“The data from Fermi were like a buried-treasure map,” Ransom said.
“Using our radio telescopes to study the objects located by Fermi, we
found 17 millisecond pulsars in three months. Large-scale searches had
taken 10-15 years to find that many,” Ransom exclaimed. “Fermi showed
us where to look.”

“This is a huge help in our effort to use millisecond pulsars to
detect gravitational waves,” Ransom said. The more such pulsars
scientists can find and observe over time, the more likely they are to
detect gravitational waves, he explained. He said that astronomers now
have barely enough millisecond pulsars to make a convincing
gravitational-wave detection.

“With Fermi guiding the way, though, we can change that picture
quickly,” Ray said. “We’ve just started to follow up on the objects
located by Fermi, and have many more to go, with a great success rate
so far,” he added.

Ransom, along with his colleague Mallory Roberts of Eureka Scientific,
used the National Science Foundation’s Robert C. Byrd Green Bank
Telescope (GBT) to find eight of the 17 new pulsars. The scientists
announced their discoveries at the American Astronomical Society’s
meeting in Washington, DC.

Pulsars are neutron stars — the dense cores left after a massive star
has exploded as a supernova. About as large as a medium-sized city,
these neutron stars have strong magnetic fields that channel
lighthouse-like beams of radio waves that sweep through space as the
star rotates. When such a beam strikes the Earth, radio telescopes can
detect the strong radio waves.

As they age, pulsars slow their rotation rates. However, if the pulsar
is part of a binary-star system and can draw in material from its
companion, its rotation can be sped up. When the neutron star has been
sped up to rotate hundreds of times a second, it is called a
millisecond pulsar.

In addition to helping scientists detect gravitational waves, study of
millisecond pulsars also can yield important new information about
other effects of General Relativity and about fundamental particle

“This new ability to find many more millisecond pulsars really is a
treasure chest that can yield many valuable gems of scientific
discovery,” Ransom said.

# # #

This release, with graphics, is online:

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

Fred Bortz, author of Physics; Decade by Decade, Twentieth-Century Science set, Facts On File, 2007.