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   Prepare to be a bit confused.

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   It’s OK, partly because scientists are confused too, but usually on a deeper level, wrapping their ignorance with bigger words…

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      “Time,” of course is “relative,” says ………. Einstein.

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   For example¹ consider a tall skyscraper. But for that you’ll have to use the DOOR.

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[MORE]

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   “Time doesn’t move at the same speed for everyone…If you place extremely accurate clocks on every floor of a skyscraper, they will all tick at different rates. The clocks on the lower floors–closer to the center of the Earth, where gravity is stronger–will tick a little slower than the ones on the top floors. You never notice this because the variances are fantastically small, a spare billionth of a second here and there². Clocks on global positioning satellites have to be set to tick slightly slower than those on Earth’s surface. If they didn’t, GPS wouldn’t be accurate.”

   This is one of those reminders that in the big picture gravity, as a force, isn’t as simple as we first think it is. Practical Advice: Still hold on to those railings going up and down stairs. A fall in a stairwell is never going to kill or hurt you; it’s just your final landing that stops you at a some fixed point above the center of the Earth.

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   ¹ All quoted matter below is from “The Truth about Black Holes,” National Geographic (March 2014, p.102). Boldface is added. Understanding gravity is essential to moving up the ladder and–starting to–understand a “black hole,” a term first used in 1967, and that we’ve discussed earlier. Einstein, by the way didn’t believe in black holes.

   ² If this sounds puzzling, just remember that gravity and time are “related.” In general, as gravitational force lessens, time slows down. (If you’re still confused, don’t worry about it.)