Space & Innovation

Einstein's General Relativity Still Put to Test

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.

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

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.

The massive galaxy cluster Abell 2218 is filled with some stunning examples of gravitationally-lensed galaxies -- observational evidence of the effects of Einstein's theory of general relativity.

In November 1915, Albert Einstein published his famous theory of general relativity, a theory that began a revolution in physics and transformed our view on the entire universe. A key component of general relativity is that a massive object like a planet, star, galaxy or cluster of galaxies can have a dramatic warping effect on the "fabric" of the universe -- known as "spacetime." As light travels in straight lines through spacetime, should a mass cause an otherwise "flat" spacetime to curve, the path of light also becomes curved. Therefore, by this theoretical reasoning, we should be able to see the warped light of distant galaxies as that light travels past other galaxies on its way to being observed at Earth. And sure enough, there are countless examples in the cosmos of this warped light caused by a mechanism known as "gravitational lensing" -- like artifacts etched in ancient light, stunning arcs, misshapen orbs and even near-perfect circles have been observed in star fields. These artifacts are the lensed light from distant galaxies and these observations have been used to superboost some of our most powerful telescopes.

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In this dramatic observation by the Hubble Space Telescope and NASA's Chandra X-ray Observatory

, a cartoon "Cheshire Cat" seems to be looking back at us. In fact, this is a collection of galaxies over 4 billion light-years away in the constellation Ursa Major -- some of the galaxies' light has become warped and deformed on its way through the universe to our telescopes, creating what looks like a Cheshire Cat smile.

This diagram

provides a good description as to how gravitational lensing works. Light from a distant galaxy travels through spacetime as it curves around a cluster of galaxies in the foreground. Interestingly, the mass of the foreground cluster has a similar effect on this distant light as a glass lens would have if placed in front of a candle flame. Should the positioning be just right, the gravitational lens can amplify the distant light, creating a natural lens in space, magnifying the light from distant galaxies that would have otherwise remained too faint to be seen. It is this effect of gravitational lenses that is being leveraged by Hubble astronomers who have embarked on a project called "

Frontier Fields

" that is on the lookout for cosmic lenses to superboost Hubble's observing power.

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Of course, the alignment isn't always perfect between Earth, gravitational lens and distant galaxy. Also, the foreground object creating the lens is usually not regularly shaped. These factors cause most lensed galaxies to appear as broken arcs. Multiple instances of the same galaxy can also be projected as the distant starlight becomes warped and fragmented.

In this Hubble observation of the galaxy cluster Abell 370

, many galaxies are present, but several prominent arcs of galactic light can be seen. Often, for well-defined examples, these arcs can be reconstructed to reveal what that galaxy looks like without being warped.

This is another massive galaxy cluster called Abell 2218 filled with some stunning examples of gravitationally-lensed galaxies. These arcs are thought to be light from galaxies located 5 to 10 times further away from Earth than the galactic cluster. The cluster is credited with amplifying the weak light from galaxies that existed over 13 billion years ago, less than a billion years after the Big Bang. These arcs truly are artifacts from the beginning of time.

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Should the alignment be just right, and the lensing object be less complex than a cluster of galaxies, it's possible to see near-perfect circles of light or "horseshoe" shaped lenses where the light of a distant galaxy has been almost perfectly warped 360 degrees around the lensing object. The passage of an isolated massive black hole, for example, in front of a distant galaxy could create such a dramatic scene.

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As shown in this striking Atacama Large Millimeter/submillimeter Array (ALMA) observation

, the light of a distant galaxy has formed a complete circle aptly known as an "Einstein Ring." The light originated from an ancient "starburst" galaxy called SDP.81 and is the finest example of an Einstein Ring found to date.

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Occasionally multiple images of the same object can be seen around gravitational lenses. In this spectacular example, an ancient supernova in a distant galaxy has been magnified by the masses of galaxies contained within the MACS J1149.6+2223 cluster, located 5 billion light-years away. The supernova, located another 4 billion light-years behind the cluster, has been multiplied 4 times as the light from the same supernova took different paths around the lens. As this was a transient event, the different supernova images were detected at different times by Hubble. Such a configuration of lensed images is known as an "Einstein Cross."

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