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Gravity Probe A
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===Equivalence principle=== {{main|Equivalence principle}} The equivalence principle can be understood by comparing a rocket ship in two scenarios. First, imagine a rocket ship that is at rest on the Earth's surface; objects dropped within the rocket ship will fall towards the floor with an acceleration of {{val|9.8|u=m/s2}}. Now, imagine a distant rocket ship that has escaped Earth's gravitational field and is accelerating at a constant {{val|9.8|u=m/s2}} due to thrust from its rockets; objects in the rocket ship that are unconstrained will move towards the floor with an acceleration of {{val|9.8|u=m/s2}}. This example shows one way that a uniformly accelerating reference frame is indistinguishable from a gravitational reference frame. Furthermore, the equivalence principle postulates that phenomena that are caused by inertial effects will also be present due to gravitational effects. Consider a beam of light that is shined horizontally across a rocket ship, which is accelerating. According to a non-accelerating observer outside the rocket ship, the floor of the rocket ship accelerates towards the light beam. Therefore, the light beam does not seem to travel on a horizontal path according to the inside observer, rather the light ray appears to bend toward the floor. This is an example of an inertial effect that causes light to bend. The equivalence principle states that this inertial phenomenon will occur in a gravitational reference frame as well. Indeed, the phenomenon of [[gravitational lensing]] states that matter can bend light, and this phenomenon has been observed by the [[Hubble Space Telescope]], and other experiments.
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