Albert Einstein and the Theory of Relativity
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Albert Einstein
1879-1955 |
Newton's theory of gravitation was soon accepted without question, and it
remained
unquestioned until the
beginning of this century. Then
Albert Einstein
shook the foundations of
physics with the introduction of his Special Theory of Relativity in 1905, and
his General Theory of Relativity in 1915
(Here is an example of a thought experiment in
special relativity).
The first showed that Newton's
Three
Laws of Motion were only approximately correct, breaking down when velocities
approached that of light. The second showed that Newton's Law of
Gravitation was also only approximately
correct, breaking down in the presence of
very strong gravitational fields.
Newton vs. Einstein: Albert's Turn to Kick Butt
We shall consider Relativity in more detail
later. Here,
we only summarize the differences between Newton's theory of gravitation and
the theory of gravitation implied by the General Theory of Relativity. They
make essentially identical predictions as long as the strength of the
gravitational field is weak, which is our usual experience. However, there are
three crucial predictions where the two theories diverge, and thus can be
tested with careful experiments.
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The orientation of Mercury's orbit is found to precess in space over time,
as indicated in the adjacent figure (the magnitude of the effect is greatly
exaggerated in this figure). This is commonly called the
"precession of the perihelion", because it causes the position of the
perihelion to move. Only part of this can be accounted for by
perturbations in Newton's theory. There is an extra 43 seconds of arc per
century in this precession that is predicted by the Theory of General
Relativity and observed to occur (a second of arc is 1/3600 of an angular
degree). This effect is extremely small, but the measurements are very precise
and can detect such small effects very well.
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Einstein's theory predicts that the direction of light propagation should be
changed
in a gravitational field, contrary to the Newtonian predictions. Precise
observations indicate that Einstein is right, both about the effect and its
magnitude. A striking consequence is
gravitational lensing.
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The General Theory of Relativity predicts that light coming from a strong
gravitational field should have its wavelength shifted to larger values (what
astronomers call a "red shift"), again contary to Newton's theory. Once again,
detailed observations indicate such a red shift, and that its magnitude is
correctly given by Einstein's theory.
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The electromagnetic field can have
waves
in it that carry energy and that we
call light.
Likewise, the gravitational field can have waves that carry energy and are called
gravitational waves.
These may be thought of as ripples in the
curvature of
spacetime that travel at the speed of light.
Just as accelerating
charges can emit electromagnetic waves,
accelerating masses can emit gravitational waves. However gravitational waves are
difficult to detect because they are very weak and no conclusive evidence has yet
been
reported for their direct observation. They have been observed indirectly
in
the
binary
pulsar.
Because the arrival time of pulses from the
pulsar can be measured very
precisely, it can be determined that the period of the binary
system is gradually decreasing.
It is found that the rate of period change (about 75 millionths of a second
each
year) is what would be expected
for energy being lost to gravitational radiation,
as predicted by the Theory of General Relativity.
The Modern Theory of Gravitation
And there is stands to the present day. Our best current theory of gravitation
is the General Theory of Relativity. However, only if velocities are
comparable to that of light, or gravitational fields are much larger than those
encountered on the Earth, do the Relativity theory and Newton's theories differ
in their predictions. Under most conditions Newton's three
laws and his theory of
gravitation are adequate. We shall return to this issue
in our subsequent discussion of
cosmology.
