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  • Gravity. It controls the universe.

  • Everything attracts everything else.

  • Ouch!

  • Including you.

  • Ow!

  • In this final lesson,

  • we'll explore what gravity means for space-time,

  • or rather what space-time means for gravity.

  • Until now, we've been dealing with things moving at constant speeds,

  • with straight world lines in space-time.

  • But once you add gravity,

  • if you measure a speed at one moment,

  • then again a bit later,

  • the speed may have changed.

  • In other words, as I discovered, gravity causes acceleration,

  • so we need the world line to look different from one moment to the next.

  • As we saw in the last lesson,

  • the correct way to tilt an object's world line is using a Lorentz transformation:

  • Einstein's stretch and squash trick.

  • So, to map out what gravity is doing to Tom's motion,

  • we need to create a whole load of little patches of space-time,

  • each transformed by different amounts.

  • So that my world line is at a different angle in each one.

  • And then, we're ready to stitch everything together.

  • We assemble a cozy quilt of space-time

  • where world lines look curved.

  • Where the world lines join, the objects collide.

  • By making these connections between the patches,

  • a curvature gets built into space-time itself.

  • But Einstein's true genius

  • was to describe precisely how each patch is stretched and squashed

  • according to nearby mass and energy.

  • The mere presence of stuff curves the space-time,

  • and curving space-time moves the stuff around.

  • This is gravity, according to Einstein.

  • Previously, Isaac Newton had explained gravity using the ideas of force and acceleration,

  • without any wibbly wobbly space-time,

  • and that did pretty well.

  • But Einstein's theory does just slightly better at predicting,

  • for example, the orbit of Mercury around the Sun,

  • or the way that light rays are deflected by massive objects.

  • More importantly, Einstein's theory predicts things that simply don't exist in older theories

  • where space, time and gravity were separate.

  • The stitching can leave wrinkles in the space-time material.

  • These are called gravitational waves,

  • which should be detectable as tiny, repetitive, subtle squashes and stretches in space.

  • So we're building experiments to check if they are there.

  • In the meantime, indirect evidence,

  • most recently in the polarization patterns of light left over from the Big Bang,

  • strongly suggest that they are.

  • But despite Einstein's successes,

  • when too much stuff gets concentrated in too small a space,

  • like in a black hole,

  • the curvature of space-time becomes so large,

  • that his equations collapse.

  • We need a new picture of space-time that incorporates quantum mechanics

  • to unlock the secret at the heart of black holes.

  • Which means there's plenty more to be discovered about space, time, and space-time in the future.

Gravity. It controls the universe.

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B1 TED-Ed space time space gravity einstein curvature

【TED-Ed】The fundamentals of space-time: Part 3 - Andrew Pontzen and Tom Whyntie

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    稲葉白兎 posted on 2015/02/01
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