A Fifth Force?

It may counteract gravity

In the famous anecdote, Galileo Galilei clambered to the top of the Leaning Tower of Pisa, simultaneously dropped cannonballs of different sizes and found that they all hit the ground at the same time. He thus convinced the world--and in the years to come, Sir Isaac Newton and Albert Einstein as well--that in a vacuum all objects, regardless of mass, fall at the same speed. Galileo's work went unchallenged until last week, when Purdue University Physics Professor Ephraim Fischbach, three of his graduate students and S.H. Aronson, a physicist at Brookhaven National Laboratory in New York, reported discerning a previously unknown...

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