Whenever one observer moves through the Universe relative to another, the observer-in-motion will experience time dilation: where their clocks run slower relative to the observer-at-rest. Based on this, Einstein suggested that we could make use of two clocks to put this to the test: one at the equator, which speeds around the Earth at approximately 1670 km/hr (1038 mph), and one at the Earth's poles, which is at rest as the Earth rotates about its axis. In this regard, however, Einstein was wrong: both clocks run at exactly the same rate relative to one another. It wasn't until 1971 that a proper test could be conducted, and it required a lot more than special relativity to make it so