Changes

Because the effects of relativity are so tiny, scientists have been devising sophisticated and sensitive tests ever since the theory was formulated in 1905.

Because the effects of relativity are so tiny, scientists have been devising sophisticated and sensitive tests ever since the theory was formulated in 1905.

The most famous experiment, and the one that is commonly cited in textbooks as the experiment that established the case for relativity<ref>Though relativity did not actually originate from this experiment</ref>, was the [[Michelson-Morley experiment]].  This showed that all observers will obtain the same measured value for the speed of light (3x10⁸ meters per second) no matter what their state of motion.  This is the first of the two fundamental principles:

The most famous experiment, and the one that is commonly cited in textbooks as the experiment that established the case for relativity,<ref>Though relativity did not actually originate from this experiment</ref> was the [[Michelson-Morley experiment]].  This showed that all observers will obtain the same measured value for the speed of light (3x10⁸ meters per second) no matter what their state of motion.  This is the first of the two fundamental principles:

#''The [[speed of light]] is constant for all observers, regardless of their velocities relative to each other.''

#''The [[speed of light]] is constant for all observers, regardless of their velocities relative to each other.''

#''The laws of physics are identical in all reference frames.''

#''The laws of physics are identical in all reference frames.''

*Another is ''gravitational time dilation''.  This is an effect separate from the time dilation of special relativity.  It was tested by the Pound-Rebka experiment in 1959.

*Another is ''gravitational time dilation''.  This is an effect separate from the time dilation of special relativity.  It was tested by the Pound-Rebka experiment in 1959.

*Later in the 20th century, even more subtle phenomena were tested.  One was the phenomenon of ''gravitational radiation'', or "gravitational waves".  These waves are incredibly difficult to observe, and had never been observed until 2015.  But extremely dense binary pulsars radiate gravitational waves with sufficient energy loss that, even though we can't detect the waves from Earth, we can see the effect of the energy loss from the radiation.  The extreme precision of the timing of pulses from pulsars makes it possible to observe their energy loss with great accuracy.  Observations by Hulse and Taylor of the pulsar pair known as B1913+16, if assumptions are made<ref>Similar to the way Kepler chose orbital parameters to fit observed planetary motion to his theory</ref>, could make the energy loss appear consistent with the predicted radiation.  The rotating pulsars have moved such that Earth is now out of the beams, so those observations have been discontinued.

*Later in the 20th century, even more subtle phenomena were tested.  One was the phenomenon of ''gravitational radiation'', or "gravitational waves".  These waves are incredibly difficult to observe, and had never been observed until 2015.  But extremely dense binary pulsars radiate gravitational waves with sufficient energy loss that, even though we can't detect the waves from Earth, we can see the effect of the energy loss from the radiation.  The extreme precision of the timing of pulses from pulsars makes it possible to observe their energy loss with great accuracy.  Observations by Hulse and Taylor of the pulsar pair known as B1913+16, if assumptions are made,<ref>Similar to the way Kepler chose orbital parameters to fit observed planetary motion to his theory</ref> could make the energy loss appear consistent with the predicted radiation.  The rotating pulsars have moved such that Earth is now out of the beams, so those observations have been discontinued.

*In late 2015 (and announced in 2016), the LIGO instruments directly detected gravitational waves.  See [[Gravitational waves]].

*In late 2015 (and announced in 2016), the LIGO instruments directly detected gravitational waves.  See [[Gravitational waves]].