Word: constant
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Dates: during 1990-1999
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Einstein's conclusions became known as the special theory of relativity. No matter how fast one is moving toward or away from a source of light, the speed of that light beam will appear the same, a constant 186,000 miles per second. But space and time will appear relative. As a train accelerates to near the speed of light, time on the train will slow down from the perspective of a stationary observer, and the train will get shorter and heavier. O.K., it's not obvious, but that's why we're no Einstein...
...apple because he and the surface of the earth were accelerating upward. This equivalence between acceleration and gravity didn't seem to work for a round earth, however; people on the other side of the world would have to be accelerating in the opposite direction but staying at a constant distance from...
...found that his equations didn't have a solution that described a universe that was unchanging in time. Rather than give up a static and everlasting universe, which he and most other people believed in at that time, he fudged the equations by adding a term called the cosmological constant, which warped space-time the other way so that bodies move apart. The repulsive effect of the cosmological constant would balance the attractive effect of matter and allow for a universe that lasts for all time...
...1920s with the 100-in. telescope on Mount Wilson. These revealed that the farther other galaxies are from us, the faster they are moving away. In other words, the universe is expanding and the distance between any two galaxies is steadily increasing with time. Einstein later called the cosmological constant the greatest mistake of his life...
...handle suggests that as of 50,000 B.C., during the Middle Paleolithic, the social brain was not humming very vibrantly. There were only 2 million or 3 million "neurons"--a.k.a., people--scattered across the whole planet, and lacking fiber optics or even postal service, they weren't exactly in constant contact...