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 ofgravitation 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.
- 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.
- 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.
- 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.
- 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.
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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.
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