Space. It's where things happen.

Time. It's when things happen.

We can measure where things are

and when things take place

but in modern physics

we realize when and where

are actually part of the same question.

Because when it comes to understanding the universe,

we need to replace three dimensional space plus time

with a single concept:

four dimensional spacetime.

We'll explore and explain spacetime

in this series of animations.

Animations?

Yeah.

Well, we're not very animated are we?

Sure we are! Look, I can go from here to here.

Whoah! How'd you get from here to there?

How fast did you go?

Did you run? Walk?

Did you even go in a straight line?

Ah! To answer that, you'll need to make our cartoon physics

look more like physics physics.

You'll need more panels.

More panels, please!

Okay, in each panel, Andrew's in a slightly different place.

So I can see each one records

where Andrew is at a different time.

That's great. But it would be easier to see

what's going on if we could cut out

all the hundreds of panels and stack them up

like a flipbook.

Right, now let's flip through the book

so that we can see one panel after another

getting through 24 in every second.

See! I told you it was an animation.

Now you can see me walking along.

Drawing all those panels and putting them into a flipbook

is just one way of recording the way I'm moving.

It's how animation, or even movies, work.

As it turns out, at my walking speed,

it takes two seconds to get past each fencepost

and they're spaced four meters apart.

So we can calculate my velocity -

how fast I'm moving thorugh space -

is two meters per second.

But, I could've worked that out from the panels

without flipping through them.

From the edge of the flipbook,

you can see all of the copies of the fenceposts

and all of the copies of Andrew

and he's in a slightly different place in each one.

Now we can predict everything that will happen to Andrew

when we flip through 24 pages every second,

including his speed of motion,

just by looking.

No need to flip through at all.

The edge of this flipbook

is known as a spacetime diagram

of Andrew's journey through - you guessed it -

space and time.

We call the line that represents Andrew's journey

his world line.

If i jog instead of walking

I might be able to get past a fencepost every second.

(He's not very athletic.)

Anyway, when we look at this new flipbook from the edge,

we can do the same analysis as before.

The world line for Andrew jogging

is more tilted over

than the world line for Andrew walking.

We can tell he's going twice as fast as before

without flipping the panels.

But here's the clever bit.

In physics, it's alway good to view things from other perspectives.

After all, the laws of physics

should be the same for everyone

or no one will obey them.

So let's rethink our cartoon

and have the camera follow Andrew jogging along

as the fenceposts approach and pass behind him.

Still viewing it as a flipbook of panels,

we don't need to redraw anything.

We simply move all of the cutout frames slightly

until Andrew's tilted world line

becomes completely vertical.

To see why, let's flip it.

Yes, now I'm stationery, just jogging on the spot,

in the center of the panel.

On the edge of the flipbook,

my world line was going straight upwards.

The fenceposts are coming past me.

It's now their world lines that are tilted.

This rearrangement of the panels is known as a

Galilean transformation,

and it lets us analyze physics from someeone else's perspective.

In this case, mine.

After all, it's always good to see things from other points of view.

Especially, when the viewers are moving

at different speeds.

So long as the speeds aren't too high.

If you're a cosmic ray moving at the speed of light,

our flipbook of your point of view falls apart.

To stop that from happening,

we'll have to glue panels togehter.

Instead of a stack of separate panels,

we'll need a solid block of space time.

Which we'll come to in the next animation.