The idea that light should be deflected by passing close to a massive body had been suggested by the British astronomer and geologist John Michell in the 18th century. Optical determinations of the change of direction of a star are subject to many systematic errors, and far better results have been obtained of the directions of spacecraft with radio interferometers of very long baselines.
The effect comes from the decrease in the speed of light near a massive object the Sun.
The additional precession of the orbit of Mercury of 43 arc seconds per century was known before the development of the theory of general relativity. With radar measurements of the distances to the planets, similar anomalous precessions have been estimated for Venus and Earth and have been found to agree with general relativity.
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According to general relativity, the curvature of space-time is determined by the distribution of masses, while the motion of masses is determined by the curvature. In consequence, variations of the gravitational field should be transmitted from place to place as waves, just as variations of an electromagnetic field travel as waves. If the masses that are the source of a field change with time, they should radiate energy as waves of curvature of the field.
There are strong grounds for believing that such radiation exists.
One particular double-star system has a pulsar as one of its components, and, from measurements of the shift of the pulsar frequency due to the Doppler effect , precise estimates of the period of the orbit show that the period is changing, corresponding to a decrease in the energy of the orbital motion.
Gravitational radiation is the only known means by which that could happen. Double stars in their regular motions such as that for which a change in period has been detected and massive stars collapsing as supernovas have been suggested as sources of gravitational radiation , and considerable theoretical effort has gone into calculating the signals to be expected from those and other sources. Gravitational radiation is very weak. The changes of curvature would correspond to a dilation in one direction and a contraction at right angles to that direction.
One scheme, first tried out about , employed a massive cylinder that might be set in mechanical oscillation by a gravitational signal. The authors of this apparatus argued that signals had been detected, but their claim was not substantiated. In a more fruitful scheme an optical interferometer is set up with freely suspended reflectors at the ends of long paths that are at right angles to each other.
Shifts of interference fringes corresponding to an increase in length of one arm and a decrease in the other would indicate the passage of gravitational waves. Two black holes about 1. The black holes were 36 and 29 times the mass of the Sun and formed a new black hole 62 times the mass of the Sun. In the merger, three solar masses were converted to energy in gravitational waves; the amount of power radiated was 50 times more than that of all the stars shining in the universe in that moment.
As of , LIGO has made 10 detections of gravitational radiation. Nine were from mergers of a black hole binary, and one was the merger of a neutron star binary. As stated above, studies of gravity allow the masses and densities of celestial bodies to be estimated and thereby make it possible to investigate the physical constitutions of stars and planets.
Einstein's Theory of General Relativity
Because gravitation is a very weak force , however, its distinctive effects appear only when masses are extremely large. The idea that light might be attracted gravitationally had been suggested by Michell and examined by the French mathematician and astronomer Pierre-Simon Laplace. Predictions by classical physics and general relativity that light passing close to the Sun might be deflected are described above. There are two further consequences for astronomy.
Light from a distant object may pass close to objects other than the Sun and be deflected by them.
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In particular, they may be deflected by a massive galaxy. If some object is behind a massive galaxy, as seen from Earth, deflected light may reach Earth by more than one path. Operating like a lens that focuses light along different paths, the gravity of the galaxy may make the object appear multiple; examples of such apparently double objects have been found. Both Michell and Laplace pointed out that the attraction of a very dense object upon light might be so great that the light could never escape from the object, rendering it invisible.
Such a phenomenon is a black hole.
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The following image shows the deflection of light rays that pass close to a spherical mass. To make the effect visible, this mass was chosen to have the same value as the Sun's but to have a diameter five thousand times smaller i. The yellow area in the center indicates the spatial extent of the spherical mass.
An observer on the Earth can detect the deflection by the Sun of the light from a distant star by the change with time of year in the star's apparent position in the sky.
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The following animation shows light from a star on the left and an observer on the right, and the change in apparent position of the star caused by the presence of a mass in between:. The inset shows a magnified view of how the light rays reach the observer. In the absence of a mass, the light follows a straight line from the star to the observer.
In the presence of the mass, the light ray is bent, and the light reaches the observer from a slightly different direction. This direction defines a star's apparent position in the sky. Such shifts in position - although far smaller than in the images above - should be visible to a properly equipped optical observer on the Earth for a star near the Sun's path in the sky. However, under ordinary conditions sunlight causes the atmosphere to be so bright that it is not feasible to observe from the Earth with optical telescopes any star whose light passes near the Sun.
For the first tests of Einstein's predictions, astronomers therefore used solar eclipses, occasions on which the moon is between the Earth and the Sun, and blocks all sunlight from reaching the vicinity of the Earth and brightening its atmosphere. Each of the two groups took photographs of a region of the sky centered on the Sun during the May total solar eclipse and compared the positions of the photographed stars with those of the same stars photographed from the same locations in July when the Sun was far from that region of the sky. The results showed that light was deflected, and also that this deflection was consistent with general relativity but not with "Newtonian" physics.
The subsequent publicity catapulted Einstein to world fame, and led to his having the only ticker-tape parade ever held for a scientist on Broadway in New York City. With repetitions of eclipse measurements over the next half century, astronomers were able to improve on the accuracy of these first results by only about a factor two, yielding a confirmation of general relativity to within about ten percent.
The breakthrough came in with the realization that simultaneous measurements with a set of radio telescopes especially, "Very Long Baseline Interferometry" could be used to measure light deflection with much greater accuracy. In addition to providing the means to test general relativity to high accuracy, the fact that mass deflects light has been a great boon to studies of the universe.
Masses acting as gravitational lenses have now become a standard tool of astronomy. They allow astronomers to infer the masses of cosmic objects, and the structure and size scale of the universe with some caveats. Through their magnifying effect, gravitational lenses have also been used to observe the properties of very distant galaxies and quasars , as well as to search for planets around distant stars.
For the relativistic ideas behind this spotlight topic, check out Elementary Einstein , especially the chapter on General Relativity. Related Spotlights on relativity can be found in the section General Relativity. His research interests are in the dynamics of the Earth's upper mantle, seismology, and, most recently, precision measurements of light deflection near the sun.https://belgacar.com/components/espion-telephone/localiser-un-iphone-en-toute-discretion.php
How an eclipse proved Einstein's theory of relativity - Futurity
For Einstein Online , he has written the spotlight text Gravitational deflection of light with I. His research interests involve applications of radio and radar techniques to problems in geophysics, planetary physics, and astrophysics. He also devised and carried out precision tests of general relativity within our solar system; his contribution to Einstein Online is the spotlight text Gravitational deflection of light with S. Cite this article as : Steven S. Shapiro, Irwin I. Shapiro , " Gravitational deflection of light " in: Einstein Online Vol. Legal notice Impressum Datenschutzhinweis.
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