## Subtitles section Play video

• 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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# 【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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