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 flip book.

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 flip book

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 fence post,

and they're spaced four meters apart.

So we can calculate my velocity,

how fast I'm moving through 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 fencepost,

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 flip book

is known as a space time 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 always 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 fencepost 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 bcomes

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 not their world lines that are tilted.

This rearrangement of the panels

is known as a Gallileon transformation.

And it lets us analyize physics

from someone 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 together.

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.