The First Binary Pulsar, Einstein's General Theory of Relativity, the Nobel Prize, and Arecibo Observatory
J.M. Weisberg, Carleton College
The most exciting measurement in this system is the observation that the
two stars' orbits are shrinking at a rate of 1 cm/day. This shrinkage
is caused by the loss of orbital energy due to gravitational radiation,
which is a travelling ripple in spacetime that is predicted by
Einstein's General Relativity Theory but never previously verified (see
this
animation
showing gravity waves from a binary star system as
moving undulations in the spacetime grid). Arecibo observations show
that the pulsar orbit is shrinking at exactly the rate that general
relativity predicts it should, if gravity waves exist and are carrying
away the expected amount of energy.
Figure 1: The evidence that Binary Pulsar B1913+16 emits
gravitational radiation. As gravitational radiation carries energy away
from the binary system, the orbit loses energy, the stars spiral in
toward each other, and the pulsar runs "early" in its orbit. The dots
are measurements of how early the pulsar is in its orbit, while the
curve represents the expected behavior if gravitational waves are
carrying energy away from the system at the rate predicated by
Einstein's Theory of General Relativity. The excellent agreement
between observation and theory represents the strongest current evidence
for the existence of gravitational radiation. (Graph from J.M.
Weisberg, D.J. Nice, and J.H. Taylor.)
The first binary pulsar, i.e., a pulsar orbiting another star, was
discovered at Arecibo by Joseph Taylor and Russell Hulse in 1974. The
discovery earned Hulse and Taylor the Nobel Prize in Physics because the
object is so exotic and so well suited to testing modern theories of
gravitation such as Einstein's General Theory of Relativity. The pulsar
is so weak that it is very difficult to make meaningful observations of
it from any radiotelescope in the world except Arecibo.
The companion star is almost surely another neutron star. The two stars
orbit each other every eight hours at speeds up to 300 kilometers per
second, approaching each other to within a distance equal to the Sun's
radius! See this
animation of the life
of a double pulsar system created by
John Rowe Animations to
help visualize the processes leading to the rapidly spinning and
orbtiting pair of stars. (While this video shows two pulsars, we
actually only see one beam from the first binary pulsar system.) The
two stars come so close to each other that Newton's theory of
gravitation is not adequate to describe their motion. For example, the
stars' elongated orbits do not remain fixed, but rather rotate
themselves by over four degrees per year, in a process called "advance
of periastron." The rotation is some 35,000 times larger than a similar
effect in the orbit of the planet Mercury because the two stars' gravity
is so strong! The animation below shows a plan view of the orbit exhibiting this phenomenon.
Another interesting process first seen in this pulsar is a slow wobble
of its spin axis. The wobble, called "geodetic spin precession," is
caused by the curvature of spacetime induced by the companion. The
wobble enables us to observe different parts of the pulsar's "lighthouse
beam" than would ordinarily be seen. These measurements will produce a
two-dimensional map of the beam until it precesses away from Earth,
probably in a few decades.
These essential tests of general relativity are especially suited to
Arecibo Observatory, with its great sensitivity and advanced
instrumentation. Exciting measurements of this and other binary pulsars
continue to be made with the telescope today.
Sources of further information:
General level, early article on this pulsar and its use to show that
gravitational waves exist:
"Gravitational Waves from an Orbiting Pulsar," J.M. Weisberg, J.H.
Taylor, & L.A. Fowler, Scientific American, 245, 74 (1981).
Scientific Review Articles on Binary and (related) Millisecond
Pulsars:
"Testing General Relativity with Pulsar Timing," I.H. Stairs, Living
Reviews in Relativity , 6, 5 (2003);
http://www.livingreviews.org/lrr-2003-5
"Binary and Millisecond Pulsars," D.R. Lorimer, Living Reviews in
Relativity , 8, 7 (2005);
http://www.livingreviews.org/lrr-2005-7