7 General Relativity
137
were unable to reproduce his results. His anomalous findings were attributed
to over-enthusiasm and a lack of healthy scepticism. 9
Although over the next forty years no success was achieved in detecting
gravitational waves directly, indirect evidence of their presence soon began to
accumulate. A new astronomical object emitting regular pulses of radiation
was discovered in 1967. At first there was a flurry of excitement as it was
argued that the extreme regularity of the pulses indicated that they must be
artefacts produced by an extra-terrestrial intelligence. They were designated
by the acronym LGM, for “little green men”. Sanity soon prevailed, and the
radiation was explained as a beam projecting into space, focussed by intense
magnetic fields surrounding a mystery object. As the object rotated, the beam
swept across space, intersecting the earth regularly in an analogous fashion to
a lighthouse beam sweeping across a ship.
Similar objects were discovered regularly in the following years. They were
named pulsars. Their high speed of rotation indicated that they must be relatively small, about 20 km in diameter. It was suggested that they might be an
object that had long been predicted but not yet observed: a neutron star. 10
The mass of a neutron star is found to be about 1.4 solar masses, which leads
to a density so high that on earth one teaspoonful of neutron star material
would weigh about a billion tonnes.
According to General Relativity, such extremely compact objects revolving
in binary systems might be expected to perturb space–time significantly as
they rotate about each other. The result should be the emission of a gravitational wave, which would carry energy away from the system, and radiate
it throughout space. An example of such a binary pulsar system is discussed
in Appendix 7.4. The confidence of physicists in the reality of gravitational
waves therefore grew, and motivated the construction of even more sensitive
apparatus in an attempt to detect these elusive ripples in space–time on their
passage through the earth.
Besides binary pulsar systems, other stronger sources of gravitational waves
were known to occur, and offered more hope of detection. For instance,
nearby exploding supernovas might be detectable. However, such events are
very rare; the last two supernova explosions in our galaxy occurred four
centuries ago. Another possibility consisted of two black holes orbiting about
each other, and passing into the stage of a final merger. However, the question
was still open on whether such systems even existed.
9 “Sceptic” may be a pejorative term in some circles, but scepticism is important in science and is an
example of Occam’s Razor in practice.
10 Neutron stars were predicted by Walter Baade and Fritz Zwicky at the meeting of the American
Physical Society in 1933, less than 2 years after the discovery of the neutron.
137
were unable to reproduce his results. His anomalous findings were attributed
to over-enthusiasm and a lack of healthy scepticism. 9
Although over the next forty years no success was achieved in detecting
gravitational waves directly, indirect evidence of their presence soon began to
accumulate. A new astronomical object emitting regular pulses of radiation
was discovered in 1967. At first there was a flurry of excitement as it was
argued that the extreme regularity of the pulses indicated that they must be
artefacts produced by an extra-terrestrial intelligence. They were designated
by the acronym LGM, for “little green men”. Sanity soon prevailed, and the
radiation was explained as a beam projecting into space, focussed by intense
magnetic fields surrounding a mystery object. As the object rotated, the beam
swept across space, intersecting the earth regularly in an analogous fashion to
a lighthouse beam sweeping across a ship.
Similar objects were discovered regularly in the following years. They were
named pulsars. Their high speed of rotation indicated that they must be relatively small, about 20 km in diameter. It was suggested that they might be an
object that had long been predicted but not yet observed: a neutron star. 10
The mass of a neutron star is found to be about 1.4 solar masses, which leads
to a density so high that on earth one teaspoonful of neutron star material
would weigh about a billion tonnes.
According to General Relativity, such extremely compact objects revolving
in binary systems might be expected to perturb space–time significantly as
they rotate about each other. The result should be the emission of a gravitational wave, which would carry energy away from the system, and radiate
it throughout space. An example of such a binary pulsar system is discussed
in Appendix 7.4. The confidence of physicists in the reality of gravitational
waves therefore grew, and motivated the construction of even more sensitive
apparatus in an attempt to detect these elusive ripples in space–time on their
passage through the earth.
Besides binary pulsar systems, other stronger sources of gravitational waves
were known to occur, and offered more hope of detection. For instance,
nearby exploding supernovas might be detectable. However, such events are
very rare; the last two supernova explosions in our galaxy occurred four
centuries ago. Another possibility consisted of two black holes orbiting about
each other, and passing into the stage of a final merger. However, the question
was still open on whether such systems even existed.
9 “Sceptic” may be a pejorative term in some circles, but scepticism is important in science and is an
example of Occam’s Razor in practice.
10 Neutron stars were predicted by Walter Baade and Fritz Zwicky at the meeting of the American
Physical Society in 1933, less than 2 years after the discovery of the neutron.