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Eclipsing Newton

I can't believe I almost let it pass me by, but today is the 90th anniversary of the most ground breaking experiment of the 20th century. On this day in 1919 Arthur Eddington observed the solar eclipse that would ultimately revolutionize our understanding of the universe.

A huge part of Einstein's General Theory of Relativity is how gravity itself warps the fabric of space-time itself. According to his then brand new theory, a ray of light passing near a massive object will bend around the object. The classic illustration of this is a heavy bowling ball suspended on a flexible rubber sheet. All around the bowling ball, the sheet bends in because of its mass. Now picture a small marble rolling across the sheet, its path taking it near to the bowling ball. It will curve in where the sheet is bent, deflecting its otherwise straight course.

Eddington, with the backing of the Royal Astronomical Society, set out to test this theory and embarked on an expedition to the island of Príncipe off the coast of western Africa to observe a total eclipse of the sun. His goal was to see if the light from stars passing near to the sun were bent the way Einstein predicted, just like the marble passing by the bowling ball. He needed a total eclipse of the sun because that was the only way to observe starlight without it being diluted by the blue sky of day.

After a long and peril filled venture to the tiny West African nation (I won't recount the details here, for a better and more hilarious account of his adventure and others, I strongly recommend Bill Bryson's A Short History of Nearly Everything), Eddington arrived to find a thick bank of clouds, obscuring the sun.

Seemingly the voyage had been in vain, but miraculously the clouds parted just minutes before the eclipse was to occur. He snapped a series of photographs on glass plates of the eclipse and set about analyzing them in great detail. When he finished, he found that the amount that the starlight around the sun bent around the sun, exactly matched Einstein’s predictions. There in black and white was proof that the theory of relativity was correct.

This is the textbook example of the success of the scientific method. Einstein's postulates made clear predictions that could be scientifically tested and measured to either prove or disprove the theory. In this case, the proof completely revolutionized the field of physics. The traditional Newtonian view of the universe was uprooted and Relativity became the predominant framework for understanding the universe.

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