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  • Light: it's the fastest thing in the universe,

  • but we can still measure its speed

  • if we slow down the animation,

  • we can analyze light's motion using

  • a space-time diagram,

  • which takes a flipbook of animation panels,

  • and turns them on their side.

  • In this lesson, we'll add the single experimental fact

  • that whenever anyone measures just how fast light moves,

  • they get the same answer:

  • 299,792,458 meters every second,

  • which means that when we draw light

  • on our space-time diagram,

  • it's world line always has to appear at the same angle.

  • But we saw previously that speed,

  • or equivalently world line angles,

  • change when we look at things from

  • other people's perspective.

  • To explore this contradiction,

  • let's see what happens if I start moving

  • while I stand still and shine the laser at Tom.

  • First, we'll need to construct the space-time diagram.

  • Yes, that means taking all of

  • the different panels showing the different moments in time

  • and stacking them up.

  • From the side, we see the world line

  • of the laser light at its correct fixed angle,

  • just as before.

  • So far, so good.

  • But that space-time diagram represents Andrew's perspective.

  • What does it look like to me?

  • In the last lesson, we showed

  • how to get Tom's perspective moving all the panels

  • along a bit until his world line is completely vertical.

  • But look carefully at the light world line.

  • The rearrangement of the panels

  • means it's now tilted over too far.

  • I'd measure light traveling faster than Andrew would.

  • But every experiment we've ever done,

  • and we've tried very hard,

  • says that everyone measures light to have a fixed speed.

  • So let's start again.

  • In the 1900s, a clever chap named Albert Einstein

  • worked out how to see things properly,

  • from Tom's point of view,

  • while still getting the speed of light right.

  • First, we need to glue together the separate panels

  • into one solid block.

  • This gives us our space-time,

  • turning space and time into

  • one smooth, continuous material.

  • And now, here is the trick.

  • What you do is stretch your block of space-time

  • along the light world line,

  • then squash it by the same amount,

  • but at right angles to the light world line,

  • and abracadabra!

  • Tom's world line has gone vertical,

  • so this does represent the world from his point of view,

  • but most importantly,

  • the light world line has never changed its angle,

  • and so light will be measured by Tom

  • going at the correct speed.

  • This superb trick is known as

  • a Lorentz transformation.

  • Yeah, more than a trick.

  • Slice up the space-time into

  • new panels and you have

  • the physically correct animation.

  • I'm stationary in the car,

  • everything else is coming past me

  • and the speed of light

  • works out to be that same fixed value

  • that we know everyone measures.

  • On the other hand,

  • something strange has happened.

  • The fence posts aren't spaced a meter apart anymore,

  • and my mom will be worried

  • that I look a bit thin.

  • But that's not fair. Why don't I get to look thin?

  • I thought physics was supposed to be the same

  • for everyone.

  • Yes, no, it is, and you do.

  • All that stretching and squashing

  • of space-time has just muddled together

  • what we used to think of separately

  • as space and time.

  • This particular squashing effect

  • is known as Lorentz contraction.

  • Okay, but I still don't look thin.

  • No, yes, you do.

  • Now that we know better about space-time,

  • we should redraw

  • what the scene looked like to me.

  • To you, I appear Lorentz contracted.

  • Oh but to you, I appear Lorentz contracted.

  • Yes.

  • Uh, well, at least it's fair.

  • And speaking of fairness,

  • just as space gets muddled with time,

  • time also gets muddled with space,

  • in an effect known as time dilation.

  • No, at everyday speeds,

  • such as Tom's car reaches,

  • actually all the effects are much, much smaller

  • than we've illustrated them.

  • Oh, yet, careful experiments,

  • for instance watching the behavior of tiny particles

  • whizzing around the Large Hadron Collider

  • confirmed that the effects are real.

  • And now that space-time is

  • an experimentally confirmed part of reality,

  • we can get a bit more ambitious.

  • What if we were to start playing

  • with the material of space-time itself?

  • We'll find out all about that in the next animation.

Light: it's the fastest thing in the universe,

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B1 TED-Ed space time space line speed animation

【TED-Ed】The fundamentals of space-time: Part 2 - Andrew Pontzen and Tom Whyntie

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    稲葉白兎 posted on 2015/02/01
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