The Pound-Rebka experiment made use of the Mössbauer effect to confirm the gravitational redshift that had been predicted by Albert Einstein’s theory of general relativity. It represented the first experimental evidence that that aspect of Einstein’s theory was correct.

In 1916, Albert Einstein published the theory of general relativity. This revolutionary proposal was, in fact, an extended or generalized version of the theory of special relativity that had been presented in 1905. The essential difference between the two is that the theory of special relativity involves high velocity, uniform motion, and flat space-time; the theory of general relativity permits curved space-time and accelerated motion. The theory of general relativity was Einstein’s theory of gravity. Unlike Sir Isaac Newton’s concept of gravity—a force acting over a distance—it was Einstein’s contention that gravity is a consequence of the shape of space-time. Space-time, by Einstein’s definition, consists of three spatial dimensions and time as the fourth dimension. Mössbauer Effect
Redshifting, gravitational
Relativity, general
Pound-Rebka experiment[Pound Rebka experiment]
Gravity
[kw]Mössbauer Effect Is Used to Detect Gravitational Redshifting (1960)
[kw]Gravitational Redshifting, Mössbauer Effect Is Used to Detect (1960)
[kw]Redshifting, Mössbauer Effect Is Used to Detect Gravitational (1960)
Mössbauer Effect
Redshifting, gravitational
Relativity, general
Pound-Rebka experiment[Pound Rebka experiment]
Gravity
[g]North America;1960: Mössbauer Effect Is Used to Detect Gravitational Redshifting[06390]
[g]United States;1960: Mössbauer Effect Is Used to Detect Gravitational Redshifting[06390]
[c]Astronomy;1960: Mössbauer Effect Is Used to Detect Gravitational Redshifting[06390]
[c]Physics;1960: Mössbauer Effect Is Used to Detect Gravitational Redshifting[06390]
[c]Science and technology;1960: Mössbauer Effect Is Used to Detect Gravitational Redshifting[06390]
Pound, Robert Vivian
Rebka, Glen A.
Einstein, Albert
Mössbauer, Rudolf Ludwig
Adams, Walter Sydney
Greenstein, Jesse Leonard
Newton, Sir Isaac[Newton, Isaac]

In 1665, Newton had worked out the universal law of gravitation. This law and Newton’s laws of motion were extremely successful in predicting the motion of both falling bodies and orbiting bodies. One of the most spectacular successes of Newton’s laws occurred in 1846 with the discovery of the planet Neptune. The planet had been discovered within one degree of its predicted position. At about the same time, a detailed study was initiated on the orbit of the planet Mercury Mercury (planet) . Although Newton’s laws predicted that the orbit of Mercury would rotate very slowly around the sun, there was a 43-arc-second-per-century advance that was not accounted for by his theory. Some in the scientific community began to question the validity of Newton’s laws. Others speculated on the possible existence of a planet inside the orbit of Mercury, while still others proposed that the sun is not symmetric about its core. When the curved space-time of the theory of general relativity was applied to the problem of Mercury’s orbit, observations and calculations agreed.

To explain Einstein’s concept of gravity, one might imagine a sheet stretched out and held tightly by each of its four corners. A ball placed in the center of the sheet would cause a slight depression. The more massive the ball, the greater the depression. Another ball being rolled near this depression would naturally follow the curve or shape of this depression. It was Einstein’s contention that the sun warps the space-time around it, and Mercury follows this curvature in space-time. Other confirmations of the theory of general relativity were to follow in the years after its introduction. In 1919, Arthur Stanley Eddington Eddington, Arthur Stanley led an expedition to an island off the coast of Africa to photograph a total eclipse of the sun. The photographs revealed an apparent shift in the background field of stars that matched the prediction of the theory of general relativity. Einstein had explained that light from stars would bend in the curvature of space-time caused by the sun; Eddington’s work had proven this theory to be correct.

The theory of general relativity also predicted a phenomenon known as gravitational redshifting, or the Einstein shift. According to theory, as light escapes from the surface of a star, it loses some of its energy. This would cause the lengthening of the light waves. Since the longest visible light waves are red, lengthened waves would appear redder, hence the term “redshift.”

Rudolf Ludwig Mössbauer.

(The Nobel Foundation)

The detection of gravitational redshifting in a star is extremely difficult. First, redshifts are also caused by the velocity of moving objects compressing or expanding waves. This is known as the Doppler effect Doppler effect , and the motion through space and the rotation of a star itself would cause a Doppler redshift far greater than the redshift caused by gravitation. Second, convection currents operating on stellar surfaces would cause Doppler shifts ranging from blue to red. In spite of these difficulties, in 1925, the American astronomer Walter Sydney Adams studied the spectrum of the white dwarf companion of the star Sirius in search of evidence for gravitational redshifting. It was reasoned that since white dwarfs have gravitational fields thousands of times more intense than ordinary stars, gravitational redshifting associated with them might be observed more readily. Adams made observations that indicated the existence of gravitational redshifting, but this was not confirmed until the 1977 study by Jesse Leonard Greenstein and others. In this study, the spectra of more than twelve white dwarf stars were made with the Mount Palomar telescope. Analysis of these spectra proved the existence of gravitational redshifting.

All the tests conducted of the theory of general relativity were astronomical in nature. In 1960, Robert Vivian Pound and Glen A. Rebka designed an experiment to test the validity of the theory in the laboratory. This test made use of what is known as the Mössbauer effect, after its discoverer, Rudolf Ludwig Mössbauer.

To explain the gravitational redshift, suppose that a photon of light is fired upward from a source to a certain height where it impacts against a target. This collision would produce a positron and an electron by the process of pair production. The positron and electron would each fall back to the surface of the earth where, after the kinetic energy gained by the acceleration of gravity is extracted, they recombine into a photon identical to the original one. This sounds like a perpetual motion machine and the process, as stated, would indeed violate the law of conservation of energy. In reality, the photon climbing out of the earth’s gravitational field would lose energy or become redshifted. The amount of energy lost would be exactly equal to the amount of energy gained by the electron-positron pair as they fell in the gravitational field.

In 1958, the German physicist Mössbauer discovered a method whereby atomic nuclei could be used as extremely sensitive clocks. Since atoms emit light at specific wavelengths and frequencies, they may be considered as being a type of clock. It is known that when a radioactive isotope of some element emits a gamma Gamma radiation ray, it can absorb another gamma ray of exactly the same energy. This fact alone, however, does not allow the redshift to be measured. Thermal energy within a sample of any element will cause the nuclei of the atoms to oscillate. This motion will cause a Doppler shift in gamma-ray frequencies.

When an atomic nucleus gives off or receives a gamma ray, there is a slight recoil motion. This motion also causes a Doppler shift. It was Mössbauer’s discovery that if the nuclei in question were embedded in the correct type of crystal, then the forces exerted by the surrounding atoms would reduce the thermal oscillations and virtually eliminate the recoil during emission and absorption of gamma rays. The gravitational redshift experiment was only one of many applications of this discovery, for which Mössbauer was awarded the 1961 Nobel Prize in Physics Nobel Prize in Physics;Rudolf Ludwig Mössbauer[Mössbauer] .

The Pound-Rebka experiment of 1960 was the first accurate measurement of the gravitational redshift to be performed under laboratory conditions. The experiment was performed in the Jefferson Physical Laboratory Jefferson Physical Laboratory at Harvard University in Cambridge, Massachusetts. Gamma rays, which were emitted from a radioactive source located in the basement, traveled upward through holes drilled in the various floors to an absorber located in the penthouse. For the total distance of 22.5 meters, the calculated redshift was approximately two parts in a thousand trillion. If there was redshifting of the gamma rays from the emitter, these rays would not be absorbed by the absorbing crystal. It was found that gamma rays emitted at the bottom underwent a gravitational redshifting and were rarely absorbed at the penthouse level. The emitter was placed on a hydraulic lift that could be raised or lowered. By moving the emitter upward at a small velocity, a Doppler shift was set up that compensated for the gravitational redshift and allowed absorption.

About half of the total number of measurements were made with the emitter located at the top of the tower and the absorber at the bottom. This reversal in location of the absorber and emitter allowed the blueshift Blueshifting, gravitational to be measured. Again, when the platform was raised, a compensating redshift canceled out the amount of blueshift and made absorption possible. The results of the 1960 Pound-Rebka experiment agreed with the prediction of the theory of general relativity with an error of about 10 percent. An improved version of the experiment was completed in 1965 by Pound and an associate. The results of this experiment reduced the error to 1 percent.

Prior to the 1960 Pound-Rebka experiment, the gravitational redshifting predictions made by the theory of general relativity had not been verified under laboratory conditions. The problem with measuring the redshift was the infinitely small increments of time that had to be measured. With the discovery of the Mössbauer effect in 1958, the measurement of a gravitational redshift became possible. The precise frequencies of gamma rays emitted by radioactive nuclei such as cobalt or iron were within the range necessary to conduct an experiment.

Mössbauer’s discovery that thermal oscillations of nuclei and recoil from emission and absorption of gamma rays can be controlled significantly made it possible to detect the slightest amount of red- and blueshifting. Although the amount of redshift measured by Pound and Rebka was very small, it had significant implications. Not only did this experiment confirm the gravitational redshift predictions of the theory of general relativity but it also confirmed Einstein’s contention that there is no such thing as a universal time Time, relativistic .

Anything that oscillates or emits light on a periodic basis may be thought of as a clock. Examples of this might be a pendulum, an ordinary wristwatch operating on a mainspring, a quartz-controlled watch, or atoms that give off light or other forms of radiation. When atoms give off light, they do so with a definite wavelength and a definite frequency. By measuring this frequency, an accurate measurement of time can be made. Pound and Rebka found that radiation from atoms is redshifted as it travels upward against gravity. When any form of electromagnetic radiation is redshifted, its wavelengths become longer and its frequency becomes lower. In other words, Pound and Rebka had found that gravity causes the flow of time to slow. Thus, the concept of a universal time was shown to be invalid.

In his theory of special relativity, Einstein showed that time is relative to an inertial observer. Time differences between observers become apparent if there is a difference in motion between the observers. The time difference would no longer exist if both frames of reference were at rest relative to each other. In the theory of general relativity, Einstein had predicted that there would be a time difference for observers at rest if one were positioned “higher” than the other.

The data gathered by Pound and Rebka in their 1960 experiment verified the gravitational redshift predictions of the theory of general relativity with great accuracy. The prediction that the frequency of radiation traveling upward against gravitation is shifted toward the red end of the spectrum and radiation falling in a gravitational field is shifted toward the blue end of the spectrum was confirmed with an error of about 10 percent. The results of these experiments have influenced the acceptance of the theory of general relativity in the scientific community today. Mössbauer Effect
Redshifting, gravitational
Relativity, general
Pound-Rebka experiment[Pound Rebka experiment]
Gravity

• Asimov, Isaac. The History of Physics. New York: Walker, 1984. This volume is a well-written history of physics from the ancient Greeks to modern developments in particle physics. It is intended for the layperson.
• Calder, Nigel. Einstein’s Universe. New York: Viking Press, 1979. This volume concerns both the theories of special and general relativity. The reader should have an understanding of basic physics and astronomy and perhaps some background in relativity theory.
• French, A. P., ed. Einstein: A Centenary Volume. Cambridge, Mass.: Harvard University Press, 1979. This volume consists of a series of short articles by various authors on the life of Einstein and his contributions to science. The articles that pertain to the development of the theories of special and general relativity and quantum mechanics are quite technical.
• Gardner, Martin. The Relativity Explosion. New York: Random House, 1976. An excellent volume for the layperson. Both theories of special and general relativity are explained as well as an introduction to such topics as quasars, pulsars, and black holes.
• Hentschel, Klaus. The Einstein Tower: An Intertexture of Dynamic Construction, Relativity Theory, and Astronomy. Stanford, Calif.: Stanford University Press, 1997. Includes a chapter on initial attempts in the 1910’s to measure gravitational redshift. Bibliographic references and index.
• Kaufmann, William J. The Cosmic Frontiers of General Relativity. Boston: Little, Brown, 1977. This volume covers the theory of general relativity and its applications to such topics as stellar evolution, white dwarfs, pulsars, neutron stars, and the various types of black holes. Suitable for the informed reader.
• Keel, William C. The Sky at Einstein’s Feet. New York: Springer, 2006. Popular, comprehensive, and detailed introduction to general relativity for laypeople. Bibliographic references and index.
• Krane, Kenneth. Modern Physics. New York: John Wiley & Sons, 1983. A highly technical treatment of selected topics in physics such as the theories of special and general relativity, quantum mechanics, and nuclear physics. The volume was intended as a textbook for an advanced undergraduate course in modern physics.
• Will, Clifford M. Was Einstein Right? Putting General Relativity to the Test. 2d ed. New York: BasicBooks, 1993. A volume covering the theory of general relativity and the evidence for the theory. Such topics as curved space-time, gravitational redshifting, gravity waves, and the frontiers of experimental relativity are discussed. Suitable for the informed reader.

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