When Einstein saw that his mathematics for the general theory of relativity showed that the universe was expanding, he simply couldn’t believe it. So he “fudged” the mathematics, and stuck in an equation that suggested the universe could never change.
Years later, when astronomers discovered that the universe was, indeed, expanding, Einstein lamented that changing his mathematics had been his “biggest mistake.”
Source: Tom McGowen, Yearbooks in Science 1900-1919, (New York: Henry Holt, 1995), p.27.
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