The fundamentals of space-time_ Part 1 - Andrew Pontzen and Tom Whyntie - video Dailymotion

TED-Ed

Space is where things happen. Time is when things happen. And sometimes, in order to really look at the universe, you need to take those two concepts and mash them together. In this first lesson of a three-part series on space-time, hilarious hosts Andrew Pontzen and Tom Whyntie go through the basics of space and time individually, and use a flip book to illustrate how we can begin to look at them together.  Lesson by Andrew Pontzen and Tom Whyntie, animation by Giant Animation Studios.

Transcript

00:00Space. It's where things happen. Time. It's when things happen. We can measure

00:13where things are and when things take place, but in modern physics we realise

00:17when and where are actually part of the same question. Because when it comes to

00:23understanding the universe we need to replace three-dimensional space plus

00:27time with a single concept. Four-dimensional space-time. We'll explore and explain

00:35space-time in this series of animations. Animations? Yeah. Well, we're not very

00:41animated, are we? Sure we are. Look, I can go from here to here. Oh, how'd you get

00:47from here to there? How fast did you go? Did you run? Walk? Did you even go in a

00:52straight line? Ah, to answer that you'll need to make our cartoon physics look

00:57more like physics physics. You'll need more panels. More panels please. Okay, in each

01:07panel Andrew's in a slightly different place, so I can see each one records where

01:12Andrew is at a different time. That's great, but it would be easier to see

01:19what's going on if we could cut out all the hundreds of panels and stack them up. Like

01:23a flip book. Right, now let's flip through the book so that we can see one panel after

01:30another, getting through 24 in every second. See, I told you it was an animation. Now you

01:37can see me walking along. Drawing all those panels and putting them into a flip book is

01:42just one way of recording the way I'm moving. It's how animation, or even movies,

01:47work. As it turns out, at my walking speed it takes two seconds to get past each

01:54fence post, and they're spaced four meters apart. So we can calculate my velocity, how

02:01fast I'm moving through space, is two meters per second. But I could have worked

02:07that out from the panels without flipping through them. From the edge of the flip

02:12book you can see all of the copies of the fence posts, and all of the copies of

02:15Andrew, and he's in a slightly different place in each one. Now we can predict

02:21everything that will happen to Andrew when we flip through 24 pages every

02:25second, including his speed of motion, just by looking. No need to flip through at

02:31all. The edge of this flip book is known as a space-time diagram.

02:36of Andrew's journey through, you guessed it, space and time. We call the line that

02:43represents Andrew's journey his world line. If I jog instead of walking I might

02:50be able to get past a fence post every second. He's not very athletic. Anyway,

02:56when we look at this new flip book from the edge we can do the same analysis as

03:01before. The world line for Andrew jogging is more tilted over than the world

03:05line for Andrew walking. We can tell he's going twice as fast as before without

03:11flipping the panels. But here's the clever bit. In physics it's always good to view

03:18things from other perspectives. After all the laws of physics should be the same for

03:23everyone or no one will obey them. So let's rethink our cartoon and have the

03:29camera follow Andrew jogging along as the fence posts approach and pass behind him.

03:34Still viewing it as a flip book of panels we don't need to redraw anything. We

03:40simply move all of the cutout frames slightly until Andrew's tilted world line

03:45becomes completely vertical. To see why let's flip it.

03:50Yes now I'm stationary just jogging on the spot in the centre of the panel. On the edge of the flip

03:57book my world line was going straight upwards. The fence posts are coming past me. It's now

04:03their world lines that are tilted. This rearrangement of the panels is known as a

04:09Galilean transformation and it lets us analyse physics from someone else's perspective. In this case mine.

04:17After all it's always good to see things from other points of view. Especially when

04:23the viewers are moving at different speeds. So long as the speeds aren't too high. If you're a

04:31cosmic ray moving at the speed of light our flip book of your point of view falls apart. To stop

04:38that from happening we'll have to glue panels together. Instead of a stack of separate panels

04:44we'll need a solid block of space-time. Which we'll come to in the next animation.

04:50animation.

04:57you

04:59you

05:01you

05:03you

05:05you

05:07you

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The fundamentals of space-time_ Part 1 - Andrew Pontzen and Tom Whyntie

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