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
05:09you
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05:15you
