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Albert Einstein
Albert Einstein works at the blackboard during a lecture in Vienna in 1921. (F. Schmutzer via Wikipedia)

This week’s 100th anniversary of Albert Einstein’s general theory of relativity is a geeky cause for celebration, but what’s arguably the concept’s toughest test has just gotten under way.

General relativity was a follow-up to special relativity, Einstein’s big idea from a decade earlier. Back in 1905, he worked out a way to explain why the speed of light is constant, regardless of an observer’s point of view: It’s because space and time are not inflexible metrics, but interrelated dimensions that are measured differently depending on your perspective.

Special relativity explained a lot of the weirdness that physicists were puzzling over at the time, but the theory applied only to “special” conditions that didn’t involve acceleration – for example, how things fall in a gravitational field. On Nov. 25, 1915, Einstein laid out how the interplay of space and time gives rise to gravity and the fabric of the cosmos.

The theory passed its first big test in 1919, when observations during a total solar eclipse were found to be more consistent with Einstein’s view of gravity than with Isaac Newton’s. General relativity has been passing tests ever since. For example, if we didn’t take relativistic effects into account, our GPS readings would seem out of whack.

This week is prime time for centennial retrospectives on the theory and its implications. Here are a few to keep you entertained:

Although general relativity has passed every test so far, there’s still much about gravity that remains to be explained. One big question has to do with gravitational waves, a phenomenon that Einstein predicted but which has not yet been directly observed.

If such waves exist, today’s best instrument for detecting them is the Advanced Laser Interferometer Gravitational Wave Observatories, or Advanced LIGO, an MIT-Caltech operation that has facilities at Hanford in Washington state and near Livingston in Louisiana.

Advanced LIGO began its latest observing campaign in mid-September, and it’s due to finish the first run in January. There’s a chance that LIGO could pick up the waves generated by distant supernovas or black hole collisions. Such observations could confirm Einstein’s general relativity, or throw a monkey wrench in the works.

LIGO Hanford
The beamlines for the LIGO detector site at Hanford stretch out across the desert terrain of southeastern Washington. Each arm of the L-shaped detector is 2.5 miles long. (Credit: LIGO)

For example, there’s an alternate hypothesis called G4v that lays out a different way of looking at gravity, using four vectors of matter wave functions. The concept is championed by Carver Mead, a professor emeritus of engineering and applied science at Caltech.

If gravitational waves from a binary star system turn out to have an orientation perpendicular to the plane of the system’s rotation, that would confirm Einstein’s view. But if the orientation is edgewise, that would support the G4v concept instead.

If G4v wins out, that “would herald nothing less than a major revolution in theoretical physics,” University of Washington physicist John Cramer writes in his “Alternate View” column for Analog magazine.

“Black holes would become simply ultra-degenerate compact stars, with no singularity, naked or otherwise, lying in wait at the bottom of the gravity well,” Cramer says. “There would be no dark energy, because G4v explains the dimming of distant receding Type IIa supernovas as partially due to relativistic beaming, without the need for a non-zero cosmological constant.”

Chances are that Advanced LIGO’s results will either confirm general relativity or miss out on detecting gravitational waves altogether. But wouldn’t it be wild to contemplate a fresh conception of the cosmos, more than a century after Einstein rocked the physics world? To keep track of LIGO’s progress, check out the project’s official website and follow @livingligo (a.k.a. physicist Amber Stuver) on Twitter.

The original version of this story improperly referred to Mercury as the subject of the 1919 observations in support of general relativity. Thanks to Leonard Tramiel for pointing out the error. For more about the connection between relativity and Mercury’s orbit, check out Thomas Levenson’s recently published book, “The Hunt for Vulcan.”

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