# Principle of Equivalence

Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field. One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass. One of the implications of the principle of equivalence is that since photons have momentum and therefore must be attributed an inertial mass, they must also have a gravitational mass. Thus photons should be deflected by gravity. They should also be impeded in their escape from a gravity field, leading to the gravitational red shift and the concept of a black hole. It also leads to gravitational lens effects.
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The perihelion of the orbit of the planet Mercury advances 2 degrees per century. 80 seconds of that advance was accounted for by perturbations from the other planets, etc., but the last 40 seconds of arc were unaccounted for. General relativity predicts an additional 43 seconds of arc and was one of the first triumphs of Einstein's theory.

The eccentricity of Mercury's orbit is exaggerated here to emphasize the effect.

### More numerical detail

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Reference Ohanian

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According to Roy D. North, more precise numbers are as follows (in arcsec per Julian century):
 5599 total advance with respect to to geocenter (our reference frame.) 5025 contribution of precession of Earth's equinoxes. 531 Classical or Newtonian contribution of the other planets. 43 General relativity correction (modern theory: 42.98)
 Greater perihelion advances with binary pulsars.
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Reference North

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# Gravitational Deflection of Light

 Einstein's calculations in his newly developed general relativity indicated that the light from a star which just grazed the sun should be deflected by 1.75 seconds of arc. It was tested during the total eclipse of 1919 and during most of those which have ocurred since.

This bending of light can produce a gravitational lensing effect if a distant galaxy or quasar is closely aligned with a massive galaxy closer to us. If one galaxy is directly behind another, the result can be a circle of light called an Einstein ring.

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Reference Kaufmann

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