136
R. Barrett and P. P. Delsanto
The effect is known as gravitational lensing, and is explained in more detail
in Appendix 7.3.
7.7 Gravitational Waves
We have mentioned earlier that Newton’s law of gravity contains no reference
to time; it therefore implies that if a mass distribution somewhere out in space
changes suddenly, the gravitational effect is felt instantaneously throughout
the universe. According to Einstein’s Theory of Relativity, however, the propagation of such effects cannot take place at a speed greater than that of
light.
One of the consequences of General Relativity predicted by Einstein is
that the effect of any sudden change in the distribution of mass is propagated through space in the form of a gravitational wave. This wave is a ripple
in the fabric of space–time. In our analogy with a heavy mass on a trampoline in Fig. 7.7, imagine the mass being suddenly displaced up or down by
a small amount. A wave induced by the movement in the elastic trampoline
surface will propagate across the trampoline at a speed determined by the
nature and tension of the material comprising the trampoline. In the case of
gravity, the gravitational wave is expected to propagate across the space–time
“fabric” at the speed of light. However, the magnitude of such space–time
distortions was predicted to be so small as to be unobservable with the technology available in the first half of the 20th Century (i.e., during Einstein’s
lifetime.) During this period, gravitational waves were generally considered
an intellectual curiosity of little relevance to mainstream physics.
This situation changed dramatically in the 1960s when Joseph Weber at
the University of Maryland announced the detection of gravitational waves
in a series of scientific papers. He used “antennas” made from aluminium bars
two metres long and one metre in diameter. They were located at two sites
separated by approximately 1000 km, and detections were triggered when
perturbations to the bars were detected simultaneously at the two locations.
The initial excitement of the physics world soon became tempered by a
feeling that all was not well. Weber’s detections raised more questions than
they answered. Theorists began examining what sort of event could produce
gravitational waves of a magnitude sufficient to register on Weber’s detectors.
Calculations indicated that all the stars in the universe would need to fall into
black holes to explain his detections. Over the next decade, other experimentalists designed and carried out more sensitive observations than Weber’s but
R. Barrett and P. P. Delsanto
The effect is known as gravitational lensing, and is explained in more detail
in Appendix 7.3.
7.7 Gravitational Waves
We have mentioned earlier that Newton’s law of gravity contains no reference
to time; it therefore implies that if a mass distribution somewhere out in space
changes suddenly, the gravitational effect is felt instantaneously throughout
the universe. According to Einstein’s Theory of Relativity, however, the propagation of such effects cannot take place at a speed greater than that of
light.
One of the consequences of General Relativity predicted by Einstein is
that the effect of any sudden change in the distribution of mass is propagated through space in the form of a gravitational wave. This wave is a ripple
in the fabric of space–time. In our analogy with a heavy mass on a trampoline in Fig. 7.7, imagine the mass being suddenly displaced up or down by
a small amount. A wave induced by the movement in the elastic trampoline
surface will propagate across the trampoline at a speed determined by the
nature and tension of the material comprising the trampoline. In the case of
gravity, the gravitational wave is expected to propagate across the space–time
“fabric” at the speed of light. However, the magnitude of such space–time
distortions was predicted to be so small as to be unobservable with the technology available in the first half of the 20th Century (i.e., during Einstein’s
lifetime.) During this period, gravitational waves were generally considered
an intellectual curiosity of little relevance to mainstream physics.
This situation changed dramatically in the 1960s when Joseph Weber at
the University of Maryland announced the detection of gravitational waves
in a series of scientific papers. He used “antennas” made from aluminium bars
two metres long and one metre in diameter. They were located at two sites
separated by approximately 1000 km, and detections were triggered when
perturbations to the bars were detected simultaneously at the two locations.
The initial excitement of the physics world soon became tempered by a
feeling that all was not well. Weber’s detections raised more questions than
they answered. Theorists began examining what sort of event could produce
gravitational waves of a magnitude sufficient to register on Weber’s detectors.
Calculations indicated that all the stars in the universe would need to fall into
black holes to explain his detections. Over the next decade, other experimentalists designed and carried out more sensitive observations than Weber’s but