Over the last century, many experiments have confirmed the validity of both special and general relativity. In the first major test of general relativity, astronomers in 1919 measured the deflection of light from distant stars as the starlight passed by our sun, proving th...

• Second, light travels at an unvarying speed of 186,000 miles a second. No matter how fast an observer is moving or how fast a light-emitting object is moving, a measurement of the speed of light always yields the same result.

Starting from these two postul...

Albert Einstein. He published the first part of his theory — special relativity — in the German physics journal Annalen der Physik in 1905 and completed his theory of general relativity only after another decade of difficult work. He presented the latter theory in a series of lectures in Berlin i...

In 1971, scientists tested both parts of Einstein’s theory by placing precisely synchronized atomic clocks in airliners and flying them around the world. A check of the timepieces after the planes landed showed that the clocks aboard the airliners were running a tiny bit slower than (less than on...