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| In ], the '''Shapiro effect''', or '''gravitational time delay''', is one of the four classic ] of general relativity. It says that a ] beam (or ]) which passes near a massive object as it travels from some observer's location to a target and returns to the observer, takes slightly longer to make the round trip (as measured by the observer) than it would if the object were not present. | |||
| The '''Shapiro Effect''', also known as the '''Gravitational Time Delay''' is the ] in the presence of a ] field--an effect predicted by ]. | |||
| More generally, the "travel time" of any signal moving at the '''local speed of light''' can be affected by the gravitational field in regions of spacetime through which it travels. In general relativity (and in most other gravitation theories), the local speed of light is a constant of nature, but the time delay effect implies that the effective '''global speed of light''' is ]. | |||
| Dr. Irwin I. Shapiro wrote in ] in 1964: "...according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path." In other words, the theory of relativity predicts that the speed of light is reduced when it passes through a gravitational field. | |||
| The time delay effect was first noticed in 1964, by ]. Shapiro proposed an observational test of his prediction: bounce radar beams off the surface of Venus and Mercury, and measure the round trip travel time. When the Earth, Sun, and Venus are most favorably aligned, Shapiro showed that the expected time delay, due to the presence of the Sun, of a radar signal traveling from the Earth to Venus and back, would be about 200 milliseconds, well within the limitations of 1960s era technology. | |||
| In his letter, Dr. Shapiro further suggested that a test of relativity theory could be made by observing delay of ] signals returned from the surface of a planet in our solar system. He estimated that the effect of the sun's gravitational field on the radar beam would delay the returning signal. The maximum delay would occur at the beam's closest approach to the sun. | |||
| ⚫ | The first test, using the ] ], was successful, matching the predicted amount of time delay. The experiments have been repeated many times since, with increasing accuracy. | ||
| His idea was to bounce radar beams off the surface of Venus and Mercury and measure the total time it would take for the beams to return. Since the relative positions of the planets, the earth and the sun are known quite accurately, the expected travel time of the radar beam could be computed with great accuracy as well. | |||
| Relativity theory predicts that the total time for the radar beam to go from the earth to the planets and back, at the closest approach of the radar beam to the sun, would be increased by 200 microseconds compared to what would be expected if the sun were not there. This is an easy time difference to measure. | |||
| ⚫ | The first test, using the ] Haystack radar, was successful, matching the predicted amount of time delay |
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| ==References== | ==References== | ||
| * {{Journal reference | Author=Irwin I. Shapiro | Title=Fourth Test of General Relativity | Journal=Physical Review Letters | Year=December 1964 | Volume=13 | Pages=789-791}} | * {{Journal reference | Author=Irwin I. Shapiro | Title=Fourth Test of General Relativity | Journal=Physical Review Letters | Year=December 1964 | Volume=13 | Pages=789-791}} | ||
| * {{Journal reference | Author=Irwin I. Shapiro, Gordon H. Pettengill, Michael E. Ash, Melvin L. Stone, William B. Smith, Richard P. Ingalls, and Richard A. Brockelman | Title=Fourth Test of General Relativity: Preliminary Results | Journal=Physical Review Letters | Year=May 1968 | Volume=20 | Pages=1265–1269}} | * {{Journal reference | Author=Irwin I. Shapiro, Gordon H. Pettengill, Michael E. Ash, Melvin L. Stone, William B. Smith, Richard P. Ingalls, and Richard A. Brockelman | Title=Fourth Test of General Relativity: Preliminary Results | Journal=Physical Review Letters | Year=May 1968 | Volume=20 | Pages=1265–1269}} | ||
| * {{Book reference | Author=d'Inverno, Ray | Title=Introducing Einstein's Relativity | Publisher=Oxford: Clarendon Press | Year=1992 | ID=ISBN 0-19-859686-3}} See '''Section 15.6''' for an excellent advanced undergraduate level introduction to the Shapiro effect. | |||
| * {{Journal reference | Author=Will, Clifford M. | Title=The Confrontation between General Relativity and Experiment | Journal=Living Rev. Rel. | Year= 2001 | Volume=4 | Pages=4-107}} See also A graduate level survey of the solar system tests, and more. | |||
| ] | ] | ||
Revision as of 03:12, 29 May 2005
In General relativity, the Shapiro effect, or gravitational time delay, is one of the four classic solar system tests of general relativity. It says that a radar beam (or light beam) which passes near a massive object as it travels from some observer's location to a target and returns to the observer, takes slightly longer to make the round trip (as measured by the observer) than it would if the object were not present.
More generally, the "travel time" of any signal moving at the local speed of light can be affected by the gravitational field in regions of spacetime through which it travels. In general relativity (and in most other gravitation theories), the local speed of light is a constant of nature, but the time delay effect implies that the effective global speed of light is path-dependent.
The time delay effect was first noticed in 1964, by Irwin I. Shapiro. Shapiro proposed an observational test of his prediction: bounce radar beams off the surface of Venus and Mercury, and measure the round trip travel time. When the Earth, Sun, and Venus are most favorably aligned, Shapiro showed that the expected time delay, due to the presence of the Sun, of a radar signal traveling from the Earth to Venus and back, would be about 200 milliseconds, well within the limitations of 1960s era technology.
The first test, using the MIT Haystack radar antenna, was successful, matching the predicted amount of time delay. The experiments have been repeated many times since, with increasing accuracy.
References
- . ISBN 0-19-859686-3.
{{cite book}}: Missing or empty|title=(help); Unknown parameter|Author=ignored (|author=suggested) (help); Unknown parameter|Publisher=ignored (|publisher=suggested) (help); Unknown parameter|Title=ignored (|title=suggested) (help); Unknown parameter|Year=ignored (|year=suggested) (help) See Section 15.6 for an excellent advanced undergraduate level introduction to the Shapiro effect.
- Template:Journal reference See also gr-qc/0103044 A graduate level survey of the solar system tests, and more.