Asked to name the most brilliant scientist of the last century, most of us would probably name Albert Einstein. And not without reason: he revolutionised our understanding of space, time and gravity, helped kick-start quantum theory, and found the most famous (and important) formula in all science - E = mc2 - revealing the equivalence of mass and energy. Even so, like many geniuses before him, including Isaac Newton, Einstein did make the occasional goof. Sometimes he caught them before they caused trouble. In 1911, while developing his theory of gravity, known as General Relativity, Einstein estimated the amount by which a beam of starlight would be deflected by the curvature of space-time as it passed through the gravitational field of the sun. He came up with a figure of around 1/5,000th of a degree - a tiny amount, but one that astronomers were keen to search for, as it offered them a chance to put Einstein's new ideas to the test. Fortunately, they did not succeed until 1919, by which time Einstein had spotted that his original theory was faulty. His new calculations pointed to a figure twice as big - enough to distinguish it from the figure predicted by Newton's old theory of gravity. When observations by astronomers confirmed the bigger figure, Einstein was shown to have overthrown Newton's view of gravity, and was propelled to international scientific stardom. It could have been very different had astronomers carried out their measurements sooner.
Astronomy was the focus of what Einstein himself regarded as his greatest blunder. In 1917 he applied General Relativity to the entire universe - and found his equations predicted an expanding universe. At the time everyone believed the universe was static and everlasting, so Einstein fiddled his equations to give the "right" answer. In 1929, astronomers announced that, despite what everyone thought, the universe really was expanding. Einstein's original equations had been right after all.
Among scientists, Einstein is widely regarded as having also been wrong about quantum theory. Despite having helped establish it as a description of the subatomic world, he refused to accept its probabilistic view of reality. Instead, he insisted that one day a more fundamental, common-sense theory would be found. Along with colleagues, he suggested an ingenious experiment that would prove him right. Not until the 1980s did scientists succeed in performing the experiment sufficiently well to reach a final decision. When it came, it showed that Einstein was fundamentally mistaken about the quantum world.
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