Measurements made during an eclipse showed that the Sun’s gravitational field bends the path of starlight.
Sir Isaac Newton’s
Although this law was extremely successful, it seemed impossible that one body could exert a force on another body some distance away. To avoid this problem, scientists began to speak of a mass modifying the space around it by establishing a gravitational field. Another mass interacted with the field at its location but not directly with the first mass. How mass could establish a field was not explained.
Albert Einstein took gravitational theory a giant step forward. Taking a hint from the well-known result that all objects—no matter how massive—accelerate at the same rate when acted upon by gravity, Einstein deduced that gravitation and acceleration are equivalent in some fashion. He thought about how a beam of light might look to someone moving upward very quickly in an elevator. If the light beam were actually parallel with the floor as it entered the elevator, it would appear to bend downward slightly as the elevator accelerated. If acceleration and gravitation are equivalent, then gravity must also deflect light rays downward. Since light has no mass, as one normally thinks of it, this result was completely unexpected.
The Sun’s gravity causes space-time to curve, which bends the star’s light and makes it appear to be located where it is not.
Einstein was perplexed about why a light beam took a curved path when traversing a gravitational field in an otherwise empty space. After all, the path taken by a light ray in an empty space is the definition of a straight line. Yet when is a straight line also a curved line? The answer is clear when the line is drawn on a curved surface such as a globe. For example, one may travel in a straight line along the earth’s equator and eventually return to the starting point without ever turning around.
In 1915 and 1916, Einstein announced his general theory of relativity.
Einstein suggested three effects that could be measured to see if his theory was accurate: the gravitational “redshift” of light, the advancement of the “perihelion” of the planet Mercury (the part of the planet’s orbit that takes it closest to the Sun), and the deflection of starlight by the Sun. Einstein calculated that a ray of starlight just grazing the Sun should be deflected by only about 1.75 seconds of arc. Stars cannot normally be seen when the Sun is out, so Einstein suggested the measurement be made during a total solar eclipse.
Sir Frank Dyson, the British Astronomer Royal, sent out two expeditions to photograph the eclipse of May 29, 1919. Charles Rundle Davidson
On November 6, 1919, Dyson reported on the eclipse expeditions to a joint meeting of the Fellows of the Royal Society and the Fellows of the Royal Astronomical Society held at the Burlington House in London. Sir Joseph John Thomson,
After the confirmation of Einstein’s general theory of relativity, the public was eager to learn more about him and his theory. Within one year, more than one hundred books on the subject had been published. Leopold Infeld,
Einstein’s theory of gravitation will continue to be tested under any circumstance that can be devised, for that is the nature of science. The general theory of relativity has passed the three tests (gravitational redshift, perihelion advance of Mercury, and bending of starlight) Einstein suggested as well as many more tests using radar, radio telescopes, pulsars, and quasars.
Einstein has shown that space is not simply an empty place. Space and time are not independent but must be considered together; furthermore, they are curved by mass. Perhaps most exciting is the picture of the universe that the theory predicts. Ironically, when Einstein’s theory led to the conclusion that the universe was expanding, he rejected it at first, until 1929, when the American astronomer Edwin Powell Hubble
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Bergmann, Peter G. The Riddle of Gravitation. Rev. ed. Mineola, N.Y.: Dover, 1993. A clear, full presentation of gravitation from the ancient Greeks through special and general relativity and the consequences of quantum mechanics. Many illustrations and diagrams. Helpful bibliography, glossary, and index. -
Clark, Ronald W. Einstein: The Life and Times. 1971. Reprint. New York: Avon Books, 1984. Among the hundreds of books published on the subject, this one remains a classic. Offers a comprehensive story of Einstein’s life interwoven with his scientific theories. Accessible to lay readers. Includes many historical photographs. -
Eddington, A. S. Space, Time, and Gravitation. 1920. Reprint. Cambridge, England: Cambridge University Press, 1995. This delightful classic has been reprinted numerous times. Eddington was one of the original researchers of general relativity and was one of the first to understand it. For the well-informed reader, this book gives as much insight into general relativity as can be had without mastery of the mathematics of tensor calculus. -
Goldsmith, Maurice, Alan Mackay, and James Woudhuysen, eds. Einstein: The First Hundred Years. Elmsford, N.Y.: Pergamon Press, 1980. Collection of various papers on Einstein by experts. Contributions address Einstein the man as well as his impacts on science, society, world affairs, and the arts. Useful for the general reader. -
Hoffmann, Banesh, and Helen Dukas. Albert Einstein: Creator and Rebel. New York: Viking Press, 1972. Presents “the essential flavor of the man and his science.” Dukas was Einstein’s personal secretary for many years, and she offers some interesting insights. Includes many historical photographs. -
Pais, Abraham. Subtle Is the Lord: The Science and the Life of Albert Einstein. 1982. Reprint. New York: Oxford University Press, 2005. Meticulously referenced biography of Einstein. Features detailed chronology of Einstein’s life, good subject index, and exhaustive name index. -
Schilpp, Paul Arthur, ed. Albert Einstein: Philosopher-Scientist. 2 vols. Evanston, Ill.: Library of Living Philosophers, 1949. Collection of essays about Einstein and his work, most of which should be useful to the well-informed layperson. Of special interest are the autobiographical notes by Einstein in volume 1 and the extensive bibliography of Einstein’s works in volume 2. -
Will, Clifford M. Was Einstein Right? Putting General Relativity to the Test. 2d ed. New York: Basic Books, 1993. Fascinating work, geared for the informed layperson, reviews the classic three tests of relativity.
Einstein Describes the Photoelectric Effect
Einstein States His Theory of Special Relativity
Einstein Completes His Theory of General Relativity
Schwarzschild Solves the Equations of General Relativity