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100 years of relativity
Relatively successful Einstein’s general theory of relativity is an undoubted work of genius. Yet 100 years on, it still raises many questions – and physicists continue to look for something better
Quibe
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LBERT EINSTEIN’S contributions to modern physics are unmatched, but his stellar reputation rests above all on a theory he presented to the Prussian Academy of Sciences in Berlin in a series of lectures in November 1915. The general theory of relativity is at its heart a theory of gravity. But in explaining how the force that sculpts large-scale reality works, it revolutionised our world view. Ten years before, in 1905, Einstein had already shown how motion warps space and time. Starting with the assumption baked into James Clerk Maxwell’s theory of the electromagnetic force, formulated 40 years earlier, that light always travels at the same speed, the special theory of relativity demonstrated how two observers in motion relative to each other will not perceive ruler lengths and clock ticks the same way. 30 | NewScientist | 10 October 2015
For the general theory, Einstein combined this with another observation: that gravity’s effects on a body with mass cannot be distinguished from the effects of an acceleration. After a decade of calculation, he reached his conclusion: gravity is a product of warped space-time. The sun keeps Earth in orbit not by exerting a physical force on it, but because its mass distorts the surrounding space and forces Earth to move that way. In the words of physicist John Archibald Wheeler, “space tells matter how to move and matter tells space how to curve”.
A perfect theory? Observations led by the British astronomer Arthur Eddington during a solar eclipse in 1919 showed how the sun’s bulk bent the light reaching Earth from distant stars, just as the new theory predicted. In the past
century, general relativity has never failed an experimental test, and has become the foundation of a new picture of an expanding universe that began in a big bang 13.8 billion years ago (page 31). Yet for all its success, general relativity makes many physicists uneasy. Its prediction of black holes, monsters that suck in everything they come into contact with, is a perennial cause of discomfort – even though these bodies do seem to exist (page 34). The theory’s incompatibility with quantum theory, which explains how all the other forces of nature work, including electromagnetism, remains a problem (page 37). Then there is our failure so far to directly detect gravitational waves, ripples in space-time that are an essential prediction of the theory (page 40). It is time, after a century, to look back – and ask what the next century might bring.
Quibe
A
LBERT EINSTEIN’S contributions to modern physics are unmatched, but his stellar reputation rests above all on a theory he presented to the Prussian Academy of Sciences in Berlin in a series of lectures in November 1915. The general theory of relativity is at its heart a theory of gravity. But in explaining how the force that sculpts large-scale reality works, it revolutionised our world view. Ten years before, in 1905, Einstein had already shown how motion warps space and time. Starting with the assumption baked into James Clerk Maxwell’s theory of the electromagnetic force, formulated 40 years earlier, that light always travels at the same speed, the special theory of relativity demonstrated how two observers in motion relative to each other will not perceive ruler lengths and clock ticks the same way. 30 | NewScientist | 10 October 2015
For the general theory, Einstein combined this with another observation: that gravity’s effects on a body with mass cannot be distinguished from the effects of an acceleration. After a decade of calculation, he reached his conclusion: gravity is a product of warped space-time. The sun keeps Earth in orbit not by exerting a physical force on it, but because its mass distorts the surrounding space and forces Earth to move that way. In the words of physicist John Archibald Wheeler, “space tells matter how to move and matter tells space how to curve”.
A perfect theory? Observations led by the British astronomer Arthur Eddington during a solar eclipse in 1919 showed how the sun’s bulk bent the light reaching Earth from distant stars, just as the new theory predicted. In the past
century, general relativity has never failed an experimental test, and has become the foundation of a new picture of an expanding universe that began in a big bang 13.8 billion years ago (page 31). Yet for all its success, general relativity makes many physicists uneasy. Its prediction of black holes, monsters that suck in everything they come into contact with, is a perennial cause of discomfort – even though these bodies do seem to exist (page 34). The theory’s incompatibility with quantum theory, which explains how all the other forces of nature work, including electromagnetism, remains a problem (page 37). Then there is our failure so far to directly detect gravitational waves, ripples in space-time that are an essential prediction of the theory (page 40). It is time, after a century, to look back – and ask what the next century might bring.