**1. Einstein relied on friends and colleagues to help him develop his theory. **

Though the theory of general relativity is often presented as a work of solo genius, Einstein actually received considerable help from several lesser-known friends and colleagues in working on the math behind it. College friends Marcel Grossmann and Michele Basso (Einstein supposedly relied on Grossmann’s notes after skipping class) were especially important in the process. Einstein and Grossman, a math professor at Swiss Polytechnic, published an early version of the general relativity theory in 1913, while Besso—whom Einstein had credited in the acknowledgments of his 1905 paper on the special theory of relativity—worked extensively with Einstein to develop the general theory over the next two years. The work of the great mathematicians David Hilbert—more on him later—and Emmy Noether also contributed to the equations behind general relativity. By the time the final version was published in 1916, Einstein also benefited from the work of younger physicists like Gunnar Nordström and Adriaan Fokker, both of whom helped him elaborate his theory and shape it from the earlier version.

**2. The early version of the theory contained a major error. **

The version published by Einstein and Grossmann in 1913, known as the Entwurf (“outline”) paper, contained a major math error in the form of a miscalculation in the amount a beam of light would bend due to gravity. The mistake might have been exposed in 1914, when German astronomer Erwin Finlay Freundlich traveled to Crimea to test Einstein’s theory during the solar eclipse that August. Freundlich’s plans were foiled, however, by the outbreak of World War I in Europe. By the time he introduced the final version of general relativity in November 1915, Einstein had changed the field equations, which determine how matter curves space-time.

**3. Einstein’s now-legendary paper didn’t make him famous—at first. **

The unveiling of his masterwork at the Prussian Academy of Sciences—and later in the pages of Annelen Der Physik—certainly afforded Einstein a great deal of attention, but it wasn’t until 1919 that he became an international superstar. That year, British physicist Arthur Eddington performed the first experimental test of the general relativity theory during the total solar eclipse that occurred on May 29. In an experiment conceived by Sir Frank Watson Dyson, Astronomer Royal of Britain, Eddington and other astronomers measured the positions of stars during the eclipse and compared them with their “true” positions. They found that the gravity of the sun did change the path of the starlight according to Einstein’s predictions. When Eddington announced his findings in November 1919, Einstein made the front pages of newspapers around the world.

**4. Another scientist (and former friend) accused Einstein of plagiarism. **

In 1915, the leading German mathematician David Hilbert invited Einstein to give a series of lectures at the University of Gottingen. The two men talked over general relativity (Einstein was still having serious doubts about how to get his theory and equations to work) and Hilbert began developing his own theory, which he completed at least five days BEFORE Einstein made his presentation in November 1915. What began as an exchange of ideas between friends and fellow scientists turned acrimonious, as each man accused the other of plagiarism. Einstein, of course, got the credit, and later historical research found that he deserved it: Analysis of Hilbert’s proofs showed he lacked a crucial ingredient known as covariance in the version of the theory completed that fall. Hilbert actually didn’t publish his article until March 31, 1916, weeks after Einstein’s theory was already public. By that time, historians say, his theory was covariant.

**5. At the time of Einstein’s death in 1955, scientists still had almost no evidence of general relativity in action. **

Though the solar eclipse test of 1919 showed that the sun’s gravity appeared to bend light in the way Einstein had predicted, it wasn’t until the 1960s that scientists would begin to discover the extreme objects, like black holes and neutron stars, that influenced the shape of space-time according to the principles of general relativity. Until very recently, they were still searching for evidence of gravitational waves, those ripples in the fabric of space-time caused (according to Einstein) by the acceleration of massive objects. In February 2016, the long wait came to an end, as scientists at the Laser Interferometer Gravitational Wave Observatory (LIGO) announced they had detected gravitational waves caused by the collision of two massive black holes.

**6. You can thank Einstein for GPS. **

Though Einstein’s theory mostly functions among the black holes and cosmic collisions of the heavens, or an ultra-small scale (think string theory), it also plays a role in our everyday lives. GPS technology is one outstanding example of this. General relativity shows that the rate at which time flows depends on how close one is to a massive body. This concept is essential to GPS, which takes into account the fact that time is flowing at a different rate for satellites orbiting the Earth than it is for us on the ground. As a result, time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds a day. This might not seem like a big difference, but if left unchecked it would cause navigational errors within minutes. GPS compensates for the time difference, electronically adjusting rates of the satellite clocks and building mathematical functions within the computer to solve for the user’s exact location—all thanks to Einstein and relativity.