About the Nobel Prize in Physics 1993
Russell A. Hulse Joseph H. Taylor Jr.
13 October 1993
The Royal Swedish Academy of Sciences has decided to award the Nobel Prize Physics for 1993 jointly to Russell A. Hulse and Joseph H. Taylor, Jr, both of Princeton University, New Jersey, USA for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation
The discovery rewarded with this year's
Nobel Prize in Physics was made in 1974 by Russell A.
Hulse and Joseph H. Taylor, Jr using the 300-m
radiotelescope at Arecibo, Puerto Rico, West Indies. Taylor, then
Professor at the University of Massachusetts, Amherst, and his
research student Hulse were searching systematically for pulsars
- a kind of rapidly rotating cosmic beacon with a mass somewhat
greater than that of the sun and a radius of about ten
kilometres. (A human being on the surface of a pulsar would weigh
some hundred thousand million times more than on Earth.) The
pulsar's "beacon light" is often within the radio wave
region.
The first pulsar was discovered in 1967 at the radioastronomy
laboratory in Cambridge, England (Nobel Prize 1974 to Antony Hewish). What
was new about the Hulse-Taylor pulsar was that, from the
behaviour of the beacon signal, it could be deduced that it was
accompanied by an approximately equally heavy companion at a
distance corresponding to only a few times the distance of the
moon from the earth. The behaviour of this astronomical system
deviates greatly from what can be calculated for a pair of
heavenly bodies using Newton's theory. Here a new, revolutionary
"space laboratory" has been obtained for testing Einstein's
general theory of relativity and alternative theories of gravity.
So far, Einstein's theory has passed the tests with flying
colours. Of particular interest has been the possibility of
verifying with great precision the theory's prediction that the
system should lose energy by emitting gravitational waves in
about the same way that a system of moving electrical charges
emits electromagnetic waves.
The significance of the discovery of the binary
pulsar
The discovery of the first binary pulsar is primarily of great
significance for astrophysics and gravitational physics. Gravity
is the oldest known natural force, the one we are most aware of
in daily life. At the same time it is in one sense the force that
is hardest to study since it is so much weaker than the other
three natural forces: the electromagnetic force and the strong
and the weak nuclear forces. The development of technology and
science since the second World War with rockets, satellites,
space voyages, radioastronomy, radar technology and the precise
measurement of time using atomic clocks has led to a renaissance
of the study of this earliest-known natural force. The discovery
of the binary pulsar represents an important milestone in this
historical development.
Relativity theory and gravitational
physics
According to Albert Einstein's general theory of relativity,
gravity is caused by changes in the geometry of space and time:
space-time curves near masses. Einstein presented his theory in
1915 and became a world celebrity when in 1919 the English
astrophysicist Arthur Eddington announced that one of the
predictions of the theory, the deflection of starlight passing
near the surface of the sun - "the light is drawn towards the
sun" - had been verified during solar eclipse expeditions. This
deflection of light. together with a small general-relativity
contribution to the perihelion motion of ercury (a slow rotation
of Mercury's elliptical orbit round the sun), was for several
decades the only, partly rather uncertain, support for Einstein's
theory.
For a long time the theory of relativity was considered
aesthetically very beautiful and satisfying, probably correct,
but of little practical significance to physics except in
applications in cosmology, the study of the origin, development
and structure of the universe.
Attitudes to the general theory of relativity changed, however,
during the 1960s when both experimental and theoretical
developments made gravitational physics a topical part of
physics. New opportunities for precise experiments, based on
satellite and radar technology, opened up. In particular, the
research of the Americans R. Dicke and I. Shapiro contributed to
this. Dicke performed precision experiments in which the sun's
gravitational field on the earth was used for verifying what is
termed the equivalence principle, the identity between
gravitational and inertial mass - one of the basic principles of
the general theory of relativity (and also of several alternative
gravitation theories). Important contributions were also
Shapiro's theoretical prediction and experimental verification,
using radar echoes from Mercury, of a new consequence of the
general theory of relativity - a time-delay effect for
electromagnetic signals passing through gravitational
fields.
All these experiments, however, were confined to our solar system
with its very weak gravitational fields and consequently small
deviations, hard,to measure, from the Newtonian theory of
gravity. Hence it was possible to test the general theory of
relativity and other theories only in the first post-Newtonian
approximation.
The discovery of the binary
pulsar
Hulse's and Taylor's discovery in 1974 of the first binary
pulsar, called PSR 1913 + 16 (PSR stands for pulsar, and 1913 +
16 specifies the pulsar's position in the sky) thus brought about
a revolution in the field. We have here two very small
astronomical bodies, each with a radius of some ten kilometres
but with a mass comparable with that of the sun, and at a short
distance from each other, only several times the moon's distance
from the earth. Here the deviations from Newton's gravitational
physics are large. As an example may be mentioned that the
periastron shift, the rotation of the elliptical orbit that the
pulsar (according to Kepler's first law from the beginning of the
17th century) follows in this system, is 4 degrees per year. The
corresponding relativistic shift for the most favourable example
in our solar system, the above-mentioned perihelion motion of
Mercury, is 43 seconds of arc per century (this is less than a
tenth of the very much larger contributions to the perihelion
motion caused by perturbations from other planets, chiefly Venus
and Jupiter). The difference in size between the shifts is partly
due to the orbital speed in the binary pulsar, which is almost
five times greater than Mercury's, and partly due to the pulsar
performing about 250 times more orbits a year than Mercury. The
orbiting time of the binary pulsar is less than eight hours,
which can be compared with the one month our moon takes to orbit
the earth.
A very important property of the new pulsar is that its pulse
period, the time between two beacon sweeps (0.05903 see) has
proved to be extremely stable, as opposed to what applies to many
other pulsars. The pulsar's pulse period increases by less than
5% during 1 million years. This means that the pulsar can be used
as a clock which for precision can compete with the best atomic
clocks, This is a very useful feature when studying the
characteristics of the system.
The very stable pulse period is in fact a mean of the pulse
period observed on earth over the time of one orbit of the pulsar
system. The observed period actually varies by several tens of
microseconds, i.e. by an amount that is much greater than the
variation in the mean value. This is a Doppler effect, and led to
the conclusion that the observed pulsar moves in a periodic
orbit, meaning that it must have a companion. As the pulsar
approaches the earth, the pulses reach the earth more frequently;
as it recedes they arrive less frequently. From the variation in
pulse period, conclusions can be drawn about the pulsar's speed
in its orbit and other important features of the system.
Demonstration of gravitational
waves
A very important observation was made when the system had been
followed for some years. This followed theoretical predictions
made shortly after the original discovery of the pulsar. It was
found that the orbit period is declining: the two astronomical
bodies are rotating faster and faster about each other in an
increasingly tight orbit. The change is very small. It
corresponds to a reduction of the orbit period by about 75
millionths of a second per year, but, through observation over
sufficient time, it is nevertheless fully measurable. This change
was presumed to occur because the system is emitting energy in
the form of gravitational waves in accordance with what Einstein
in 1916 predicted should happen to masses moving relatively to
each other. According to the latest data, the theoretically
calculated value from the relativity theory agrees to within
about one half of a percent with the observed value. The first
report of this effect was made by Taylor and co-workers at the
end of 1978, four years after the discovery of the binary pulsar
was reported.
The good agreement between the observed value and the
theoretically calculated value of the orbital path can be seen as
an indirect proof of the existence of gravitational waves. We
will probably have to wait until next century for a direct
demonstration of their existence. Many long-term projects have
been started for making direct observations of gravitational
waves impinging upon the earth. The radiation emitted by the
binary pulsar is too weak to be observed on the earth with
existing techniques. However, perhaps the violent perturbations
of matter that take place when the two astronomical bodies in a
binary star (or a binary pulsar) approach each other so closely
that they fall into each other may give rise to gravitational
waves that could be observed here. It is also hoped to be able to
observe many other violent events in the universe. Gravitational
wave astronomy is the latest, as yet unproven, branch of
observational astronomy, where neutrino astronomy is the most
direct predecessor. Gravitational wave astronomy would then be
the first observational technique for which the basic principle
was first tested in an astrophysical context. All earlier
observational techniques in astronomy have been based on physical
phenomena which first became known in a terrestrial
connection.
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| The radio waves from a pulsar are emitted in two bunches which sweep across space at the same rate as the pulsar rotates (upper figure). From a binary pulsar, gravitational waves are also emitted (lower figure). |
