Word: ideas
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Dates: during 1990-1999
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Soon, however, discrepancies with the idea of an all-pervading ether began to appear. You would expect light to travel at a fixed speed through the ether. So if you were traveling in the same direction as the light, you would expect that its speed would appear to be lower, and if you were traveling in the opposite direction to the light, that its speed would appear to be higher. Yet a series of experiments failed to find any evidence for differences in speed due to motion through the ether...
This required abandoning the idea that there is a universal quantity called time that all clocks measure. Instead, everyone would have his own personal time. The clocks of two people would agree if they were at rest with respect to each other but not if they were moving. This has been confirmed by a number of experiments, including one in which an extremely accurate timepiece was flown around the world and then compared with one that had stayed in place. If you wanted to live longer, you could keep flying to the east so the speed of the plane added...
...gravity and acceleration could work if there was some give-and-take in the geometry of reality. What if space-time--an entity Einstein invented to incorporate the three familiar dimensions of space with a fourth dimension, time--was curved, and not flat, as had been assumed? His idea was that mass and energy would warp space-time in some manner yet to be determined. Objects like apples or planets would try to move in straight lines through space-time, but their paths would appear to be bent by a gravitational field because space-time is curved...
...Grossmann, Einstein studied the theory of curved spaces and surfaces that had been developed by Bernhard Riemann as a piece of abstract mathematics, without any thought that it would be relevant to the real world. In 1913, Einstein and Grossmann wrote a paper in which they put forward the idea that what we think of as gravitational forces are just an expression of the fact that space-time is curved. However, because of a mistake by Einstein (who was quite human and fallible), they weren't able to find the equations that related the curvature of space-time...
Einstein continued to work on the problem in Berlin, undisturbed by domestic matters and largely unaffected by the war, until he finally found the right equations, in November 1915. Einstein had discussed his ideas with the mathematician David Hilbert during a visit to the University of Gottingen in the summer of 1915, and Hilbert independently found the same equations a few days before Einstein. Nevertheless, as Hilbert admitted, the credit for the new theory belonged to Einstein. It was his idea to relate gravity to the warping of space-time. It is a tribute to the civilized state of Germany...