General Relativity at 100: Einstein's Famous Theory Has Aged Well

The nature of gravity
12 things to know about Einstein's theories of relativity
2015 marks 100 years since the publication of Albert Einstein's General Theory of Relativity. Learn the basics of Einstein's theory of relativity in our infographic here.
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General relativity adds gravity to the theory of special relativity, which Einstein published in 1905. Special relativity posits that the laws of physics are the same for all nonaccelerating observers, and that the speed of light in a vacuum never changes, even if the observer or the light source is moving.

Special relativity laid out the relationship between energy and mass, in history's most famous equation: E = mc2. ("E" is energy; "m" is mass, and "c" is the speed of light in a vacuum — roughly 671 million mph, or 1.08 billion km/h). The theory also unified space and time into a four-dimensional "space-time."

General relativity expanded on this latter idea, explaining that matter warps space-time, much as a bowling ball set down on a bed creates a depression in the sheets. This monumental insight did not come to Albert Einstein easily; he earned his way to it, over a decade of intense thought and hard work.

"He had to retrace his steps. He proposed things which he subsequently retracted. But he kept on going," Blandford told Space.com. "He was guided not by mathematical ideas or mathematical techniques. He was guided first and foremost by physics intuition; that extraordinarily powerful physics intuition which had served him so well in the past did not let him down here."

General relativity characterizes gravity not as an innate force acting on objects but rather the consequence of space-time's curvature. (Imagine a marble rolling down the incline created by the bowling ball on the bed.)

It's a powerful, radical idea — and it has stood up to intense scrutiny for a century now, Blandford writes in a special review article published online today (March 5) in the journal Science.
Confirmation from many quarters

General relativity predicts that light will take a curved path around a massive object such as a galaxy cluster, which warps the fabric of space-time significantly. [The History & Structure of the Universe (Infographic)]

This has indeed been observed; astronomers routinely use "gravitational lenses" to study faraway light sources. In fact, on a smaller scale, the phenomenon even helps planet hunters search for worlds beyond Earth's solar system. (Exoplanets can sometimes be detected by studying how their star systems bend light from background objects.)

Peculiarities in the orbit of Mercury around the sun also back up general relativity.

"It explained the anomalous precession of Mercury's perihelion, or the rotation of the line joining the sun to the point of closest approach of the planet," Blandford writes in the Science review article. "Einstein used general relativity to explain a ~10 percent discrepancy in the precession attributable to the gravitational pulls of the other planets, ~43 arc sec per century. The agreement today is better than 10−4."

Other types of observational evidence have also helped put general relativity on firm footing, Blandford said.

"We've tested it in many, many different ways," he said. "I think it's fair to say that there's no credible measurement or observation that causes one to doubt it within its domain of applicability."
A dark universe
Albert Einstein at the Blackboard
[Pin It] Albert Einstein at the blackboard.
Credit: NASA
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General relativity also implies that the vast majority of the universe is composed of stuff that humans cannot detect directly or (at this point) even understand, David Spergel of Princeton University writes in another review article in the same issue of Science.

Careful study of the motion of matter and light throughout the universe has revealed that "normal" matter alone cannot explain space-time's curvature patterns, Spergel notes. Indeed, observations suggest that just 5 percent of the universe is familiar atomic matter, while 25 percent is dark matter and about 70 percent is dark energy.

Dark matter neither emits nor absorbs light, betraying its existence only through its gravitational effects. Dark energy, meanwhile, is a mysterious force that's associated with empty space and is thought to be responsible for the accelerating expansion of the universe.

In 1917, Einstein inserted a term called the "cosmological constant" into general relativity, as a repulsive force that would counteract gravity and achieve a static universe (which was the prevailing view of the universe's nature at the time). After astronomer Edwin Hubble's observations in 1929 famously showed that the universe is actually expanding, Einstein dropped the cosmological constant, allegedly deeming it the "biggest blunder" of his life.

But the constant looks prescient now that astronomers are grappling with the nature of dark energy.

"Why is the universe accelerating? The most studied possibility is that the cosmological constant (or equivalently, the vacuum energy of empty space) is driving cosmic acceleration," Spergel writes in the Science article. "Another possibility is that there is an evolving scalar field that fills space (like the Higgs field or the inflaton field that drove the rapid early expansion of the universe). Both of these possibilities are lumped together in 'dark energy.'

"Because all of the evidence for dark energy uses the equations of general relativity to interpret our observations of the universe's expansion and evolution, an alternative conclusion is that a new theory of gravity is needed to explain the observations," he adds. "Possibilities include modified gravity theories with extra dimensions."