A century after Albert Einstein unveiled a new concept to explain gravity, his so-called general relativity theory remains fertile ground for scientific observations and experiments.
Einstein's revolutionary idea stemmed from his special theory of relativity, published a decade earlier, which wed space and time into a single continuum known as spacetime. Observers at different locations, for example, would see the same star exploding at different times, depending on how far away they were from the event. What is constant is the speed of light.
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Special relativity did not take into account gravitational effects. Einstein toiled another 10 years to understand the physics and work through the math before unveiling his radically new idea in a four-part lecture at the Prussian Academy of Sciences that culminated on Nov. 25, 1915. Einstein published "The Field Equations of Gravitation" paper a week later.
"Before Albert Einstein came up with his general theory of relativity, we sort of pictured gravity as this magical force that connected different masses with one another," said NASA astrophysicist Ira Thorpe, with the Goddard Space Flight Center in Greenbelt, Md.
Under Issac Newton's theory of gravity, which had dominated physics for more than 200 years, if a mass in one part of the universe moved, all the other masses in the rest of the universe would instantly know and be affected by the motion.
That concept, however, ran counter to an implication of Einstein's special relativity theory, which established a universal speed limit -- the idea that nothing can move faster than the speed of light.
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Say one day the sun disappeared. Under Newtonian physics, the effect would be felt immediately on Earth. Under Einstein's theory, it would take roughly eight minutes – the time it takes both light waves and gravitational waves, which also travel at the speed of light -- to cover the 93 million miles between Earth and the now-vanished sun.
Rather than masses exerting gravitational forces on one another, Einstein realized that it was spacetime itself that was bending, similar to what happens when a bowling ball rolls across a trampoline.
One of the newest frontiers opened by general relativity is the search for gravitational waves, a rippling of spacetime caused by massive objects in motion.
"They're waves, much like water waves or light or any other kind of electromagnetic radiation, except here what's ‘waving' is space and time itself," Thorpe said during a webcast panel discussion hosted by DeepAstronomy.com.
Just like a bowling ball warps a trampoline more than a baseball, massive objects, such as black holes, bend spacetime more than relatively puny objects, like the sun.
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"There's a whole spectrum of gravitational waves, just like there's a whole spectrum of electromagnetic waves. So, just like you have radio and infrared and visible and ultraviolet and X-ray and all the way up through gamma ray, you have the same type of thing with gravitational waves," Thorpe said.
Though astronomers have not detected any gravitational waves yet, they know what frequencies and wavelengths different sources generate, thanks to computer modeling.
The longest gravitational waves were produced in the Big Bang explosion 13.8 billion years. "They get stretched out to the size of the universe as the universe expands. They kind of expand along with the universe," Thorpe said.
Some scientists are studying the remnant cosmic microwave background radiation for telltale fingerprints of gravitational waves. Others have been hunting for gravitation waves set off by massive, fast-moving objects, such as binary black holes.
Unlike most electromagnetic telescopes, which have to be pointed, gravitational wave detectors are more like microphones that you just stick out and see what's there, Thorpe said.
"You get sources from all directions and then you do data analysis to disentangle them," he said.
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General relativity has triumphed for 100 years, but the theory will soon face what may be its most challenging test. A global network of radio telescopes, linked together to form the Event Horizon Telescope, will attempt to home in on the edge of the supermassive black hole in the center of the Milky Way galaxy.
Black holes -- objects so dense that not even photons of light can escape the pucker of spacetime – provide a laboratory to test general relativity in one of gravity's most extreme environments.
Though the black hole itself, by definition, isn't observable, scientists hope to track matter as it spirals into the abyss to see if it behaves as Einstein's theory predicts. The results will help determine if general relativity theory prevails for another 100 years.