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The story of gravity


When the apocryphal apple fell on Isaac Newton's head, it alerted the great scientist to one of the greatest mysteries of the universe - gravity.  His inverse square law of universal gravitation was published in his Principia Mathematica in 1687.   But Newton himself was far from satisfied with his own conclusions. Five years later, he wrote in a letter: "That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.\  When revised in 1713, he added the famous Latin phrase "hypothesis non fingo" or "I feign no hypothesis" about how gravity actually worked.

For Albert Einstein, gravity was a "fictional force" like the one that appears to slams you back into the seat of your Maserati when you punch the accelerator.   His general theory of relativity did away with the attractive force between massive bodies.  Instead, mass bends space/time.  So, when you fall off of a 50 story building, you may be comforted in knowing that the earth is not really pulling you down.  Rather, you have stepped into an inertial frame that is traveling on a geodesic or "world line" destined to intersect with the center of the earth (unfortunately, your own world line would be scheduled to end somewhat earlier).   At a local level, Newton's and Einstein's calculations coincide quite nicely.  It is only at cosmological distances or speed near that of light where Einstein's worldview is clearly superior.

So far, Einstein's general theory has passed all tests.  One of its first successes was in accounting for some minor perturbations in the orbit of Mercury.  Einstein himself predicted gravitational redshift and correctly calculated the bending of light around the sun. 

One prediction of general relativity that has yet to be confirmed is the existence of gravitational waves, ripples in space/time that propagate as a wave, analogous to light waves, radiating from various astronomical bodies, including bodies that orbit around each other.  

Several experiments have been set up around the globe to detect gravity waves.  One such attempt is the Laser Interferometer Gravitational Wave Observatory (LIGO), which aims to measure gravity wave-induced motion between "free masses" that are separated by 2 to 4 kilometers in length.  Because of the low signal to noise ratio, detecting a gravity wave requires massive amounts of computation.  Volunteers can get involved and contribute unused computer time through the Einstein@home program. 

Despite its successes, general relativity may not be the whole story.  One of the latest to take on Einstein is Erik Verlinde, who argues that rather than being a fundamental force, gravity is an emergent property that arises from entropy. "Changes in this entropy when matter is displaced leads to a reaction force," says Verlinde, "...this force, given certain reasonable assumptions, takes the form of gravity."  Whether he is right or wrong, a consideration of gravity has shaken up the physics community, once again.

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