Science News - Einsteins Gravity: One Big Idea Forever Changed How We Understand the Universe

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Nothing has such a grip on sciences quest to understand nature as gravity.

To the ancient Greeks, gravity was the property inherent in earthly matter causing it to seek the center of the universe, and therefore fall to Earth. Isaac Newton revolutionized science by identifying gravity as the mutual attraction between all masses. Newtons gravity united the motions of heavenly bodies with the physics of terrestrial falling objects. He explained the orbits of the planets, the reappearance of comets and the paths of wartime projectileswhile confirming Galileos insistence that all objects, regardless of their mass, fell to the ground at the same accelerating rate.

And then came Einstein.

A century ago, Albert Einstein rewrote Newtons law of gravity. Instead of massive bodies pulling on each other, mass distorted spacetime. Curves in spacetimes warped geometry in turn guided the movements of matter, dictating the same behaviors that Newtonian gravity describedwith subtle deviations. Tests of those departures from Newtonian predictions established Einsteins theory of gravitygeneral relativityas the new foundation for sciences understanding of the cosmos.

In the century since Einstein articulated his theory, it has passed every test. Starlight passing near the edge of the sun is deflected from a straight path precisely as much as relativitys spacetime curvature requires. Clocks in high-altitude aircraft (or in Earth orbit) tick more rapidly than those on the ground, a consequence of general relativity and an essential consideration for the accurate operation of todays GPS devices. Twists in spacetime around rotating bodies (like the Earth) have been detected by sensitive instruments, in agreement with Einsteins expectations. Imagination-defying oddities forecast by general relativitys equations, such as black holes and gravitational waves, have been established as actual cosmic phenomena. And objects in the faraway reaches of the universe are made visible (or their image multiplied) thanks to the way massive objects distort passing light much like a magnifying lensan effect of general relativity pointed out to Einstein by a guy who washed dishes for a living.

Einsteins general relativity also implied the possibility that the universe is expandingan idea unsuspected at the time, and one that Einstein resisted. He relented when, in 1929, Edwin Hubble demonstrated that the universe is indeed growing bigger all the time. It was general relativity that made it possible to understand Hubbles discovery, and to develop from it the modern Big Bang theory of the universes birth.

Today Einsteins general relativity is at the heart of exploring other cosmic mysteries that have so far eluded solution. Foremost among such mysteries is the identity of dark energy, a repulsive influence surmised to provide more than two-thirds of the total mass-energy content of the cosmos. Most experts believe that what people call dark energy is the cosmological constant, an odd sort of energy exerting negative pressure throughout all the vacuum of space. Einstein ironically believed that such a force was needed to prevent the universe from contracting; instead, it appears to drive the universe to expand at an ever accelerating rate.

But understanding dark energy and other perplexing mysteries will probably require a resolution of the longstanding conflict between general relativity and quantum mechanics, the math that rules the microworld as stringently as gravity commands the cosmos. For decades, physicists have sought a theory of quantum gravity, the continuation of Einsteins original quest to unify his general relativity with the theory of electromagnetism. So far, such efforts have failed to find any way to meld gravity with quantum physics seamlessly, but those efforts have led to promising new strategies for reformulating basic ideas of what spacetime is and where it comes from.

As the following selections from the Science News archive show, general relativity and the search for a theory beyond it have inspired many of the past centurys discoveries as well as posing key questions that occupy scientists today. They know that gravity has not yet revealed all its secrets.