According to Einstein's theory of relativity, time and space are woven together to form a four-dimensional fabric called "space-time." Here, we see an illustration of the way Earth's gravity creates a depression in the fabric of space-time.
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According to Einstein's theories, as the speed of an object approaches the speed of light, that object experiences the passage of time more slowly. In this rendering of a disk of hot gas whirling around a neutron star, the gas in the inner part of the disk is moving at about 40 percent of the speed of light, experiencing the effects of relativity.
Image Credit: NASA/Dana Berry
This representation of the space-time continuum shows how matter changes the geometry of space-time. We know that heavy objects move other materials toward them through space, but it turns out that massive objects can also affect the passage of time.
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Albert Einstein, the German-born creator of the theory of relativity, was a physicist and mathematical genius who lived from 1879 to 1955.
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Einstein did much of his important work on special relativity while working as a technical assistant at the Swiss Patent Office. He published his paper on special relativity in 1905.
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In Einstein's famous equation of mass-energy equivalence, E stands for energy, m is mass and c is the speed of light in a vacuum. This equation shows, among other things, that matter and energy can be exchanged for one another -- the principle that brought us the atomic bomb.
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NASA sent an object known as Gravity Probe B into space in part to investigate the presence of gravitomagnetism, a phenomenon in which a massive spinning object (like a planet) causes disruptions in the curvature of space-time with its rotation. According to Einstein's theories, this type of disruption should take place.
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A Delta II rocket was launched in 2004 in California with Gravity Probe B on board to measure how the rotation of Earth causes a warp in space and time.
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So how can scientists test a theory like general relativity? One method involves sending radio signals from a point on one side of a massive object, like the sun, to a point on the other side of the object, so that the signal passes very close to the object. If the sun's mass really does bend space-time, the radio signal should take longer to fly nearby the sun than it would take to fly through empty space.
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This illustration depicts a 2002 experiment by Italian scientists, using NASA's Cassini spacecraft to perform the test discussed on the previous page. The test was a success. The sun's mass delayed the radio waves sent from the spacecraft to Earth, and Einstein's theory of general relativity was confirmed 50 times more precisely than ever before.
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But how can we determine the truth of Einstein's special relativity premises about the speed of light? Essentially, according to E=mc^2, no object can go faster than the speed of light in a vacuum, since its mass would become infinite, and the energy required to propel it would also become infinite. If Einstein was correct, even the photons sent by powerful gamma ray bursts like the ones shown above are constrained by this limit.
Image Credit: NASA/Swift/Stefan Immler
According to what we can observe, Einstein was correct about the speed of light -- no object with mass can exceed it. In fact, Einstein's universal speed limits even seem to apply to samples of photons launched toward us as gamma rays from massive explosions in space. The two photons depicted above may have incredibly different levels of energy, but they travel at essentially the same speed.
Image Credit: NASA/Sonoma State University/Aurore Simonnet
Other astronomical tests have confirmed the general theory of relativity with incredible precision. For one example, Einstein's theory was finally able to explain our observations of the elliptical orbit of Mercury, which were always off under the Newtonian model of gravitation.
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Einstein's theories, which centered on a variety of mathematical equations, were acknowledged by many awards, including the 1921 Nobel Prize in Physics.
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After discussing relativity with Einstein, an official of New York's Metropolitan Opera came up with this short definition of relativity -- "There is no hitching post in the Universe -- so far as we know" -- which Einstein deemed an accurate description.
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In 1933, Einstein spoke on relativity at an event at London's Albert Hall; attendants included Lord Rutherford and Sir Austen Chamberlain.
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This signed copy of Einstein's "On Special and General Relativity Theory" was part of a display at England's Science Museum Library and Archives that opened in 2008.
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In March 2010, Einstein's entire "General Theory of Relativity," originally published in 1915, was placed on display at the Israeli Academy of Sciences and Humanities in Jerusalem, Israel. This in the first time the work has been displayed in its entirety.
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This close-up shows some of the details of Einstein's 46-page handwritten manuscript containing his innovative theory of space and gravity.
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Einstein donated his entire manuscript to The Hebrew University of Jerusalem during its 1925 inauguration.
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Yuval Steinitz, Israel's finance minister, was present at China's Israel Pavilion Day in May 2010, where part of Einstein's manuscript was displayed temporarily.
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This statue of Einstein, at a science museum in South Korea in 2005, was part of a 100th-anniversary celebration of the physicist's theory of special relativity.
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For years, physicists have been trying to disprove Einstein. In September 2011, researchers at the CERN facility in Switzerland and Italy reported a Large Hadron Collider test may have produced one of the most revolutionary discoveries the past century of science: a particle with mass moving faster than light. If confirmed, this test could blow the lid off of Einstein's theory of special relativity.
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While many believe Einstein's theories are incomplete when it comes to describing quantum phenomena, for the most part, they have been confirmed again and again. The September 2011 CERN finding was no exception: Just a few months later, researchers rejected their original finding as an error. For the time being, Einstein remains on top.
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