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The speed of light in a locale is always equal to c according to the observer who is there. That is, every infinitesimal region of spacetime may be assigned its own proper time, and the speed of light according to the proper time at that region is always c. This is the case whether or not a given region is occupied by an observer.
Formally, c is a conversion factor for changing the unit of time to the unit of space. [4] This makes it the only speed which does not depend either on the motion of an observer or a source of light and / or gravity. Thus, the speed of "light" is also the speed of gravitational waves, and further the speed of any massless particle.
The Gravity Probe B experiment [20] [21] was a satellite-based mission by a Stanford group and NASA, used to experimentally measure another gravitomagnetic effect, the Schiff precession of a gyroscope, [22] [23] [24] to an expected 1% accuracy or better. Unfortunately such accuracy was not achieved.
The Shapiro time delay effect, or gravitational time delay effect, is one of the four classic Solar System tests of general relativity. Radar signals passing near a massive object take slightly longer to travel to a target and longer to return than they would if the mass of the object were not present.
However, the predictions of Newtonian gravity do not match the observations, as discovered in 1859 from observations of Mercury. If the potential energy between the two bodies is not exactly the 1/ r potential of Newton's gravitational law but differs only slightly, then the ellipse of the orbit gradually rotates (among other possible effects).
The Pound–Rebka experiment says nothing about curvature of the space component of spacetime. But the theoretical arguments predicting gravitational time dilation do not depend on the details of general relativity at all. Any theory of gravity will predict gravitational time dilation if it respects the principle of equivalence.
An improved experiment was done by Pound and Snider in 1965, with an accuracy better than the 1% level. [31] A very accurate gravitational redshift experiment was performed in 1976, [32] where a hydrogen maser clock on a rocket was launched to a height of 10 000 km, and its rate compared with an identical clock on the ground. It tested the ...
During the Apollo 15 mission in 1971, astronaut David Scott demonstrated the theory of Galileo: acceleration is the same for all bodies subject to gravity on the Moon, even for a hammer and a feather. The paradox in this article considers the consequences of an experiment where one of the objects to release is electrically charged.