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→Experimental and Observational Evidence Confirming Relativity: Another test
*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 from colliding [[black hole]]s. See [[Gravitational waves]]. Unlike the waves from orbiting pulsars, that could only be detected by the loss of rotation energy, the waves from colliding black holes were strong enough to be detected directly.
*In 2018 an observation of the 3-body system PSR J0337+1715, consisting of 2 white dwarfs and a pulsar, confirmed general relativity, under conditions of extremely high gravity, to enormous precision. A researcher stated "If there is a difference [between observation and the prediction of the Equivalence Principle], it is no more than three parts in a million."<ref>https://www.nbcnews.com/mach/science/einstein-s-theory-relativity-passes-its-toughest-test-yet-ncna889021</ref>
*An additional test of general relativity was performed with radio signals to the Cassini spacecraft.<ref>http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html</ref>
*An additional test of general relativity was performed with radio signals to the Cassini spacecraft.<ref>http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html</ref>