So here it's t equals zero, so x equals three.

At t equals one,

it looks like it's about, well let's see, let's call it t equals two here.

It looks like it's moved over one light year to the left, okay?

And then at equals three, maybe it's moved over another light year,

just roughly speaking there.

I didn't draw it exactly to scale there, in terms of a certain slope and

certain velocity.

Therefore, but the key point here is it's an object moving to the left.

It's clearly going less

than the speed of light because the slope is greater than one.

Again you'll never have, if this is a slope one and this is actually slope

negative one, you'll never have any slopes here that are less than one.

You'll also have slopes for world lines of objects that are greater than one.

And just by inspection, we can see, yes, that's greater than one.

It's going pretty fast, but not as fast as the speed of light.

And it's moving in this direction, in the negative x direction.

Key point here, though,

that I want to point out is, look where these intersection points are.

And essentially what this is saying is this an object, at x equals zero,

starts out at three light years and

then starts moving towards the left in the negative x direction.

And if these green lines represent various things that at t equals zero started

off from the origin, in this case all three of these started in the x direction.

This one started in the negative X direction.

When you have the intersection of two world lines,

that means those two objects inhabit that spacetime point at the same time.

In other words, if we had shot a light beam off from the origin here

at t equals zero, and at the same time we had some object,

spaceship say, that starts at the x equal 3 light year point and

then starts moving essentially toward us along the x axis.

Somewhere in between, those two objects would meet.

The spaceship would run into the beam of light, so

spaceship is coming this way, as the red line indicates,

our beam of light is going that way and at a certain point they would meet.

This is the meeting point right here.

It tells us what time those two objects met, and at what x value in between there.

Same thing, say, for this object here, v equals 0.5c.

That means we shoot it off from the origin at 0.5c heading in that direction.

This object here at t equals zero started off heading towards us

at a certain velocity.

And the two objects met if they were two spaceships, hopefully not head on,

but pass by each other, right at this point here.

Because that means they inhabited the same spacetime point.

They were at the same location, at the same time.

And again, hopefully on slightly different tracks there.

And then suddenly up here.

So this object here, our third object started off at a slower velocity,

0.33c, not 0.5c.

So this object here will clearly catch the incoming spaceship first,

will meet up with incoming spaceship first.

And then the second outgoing spaceship,

say the one going 0.33c takes a little bit longer.

But eventually they meet up as well.

You can see, again, the intersection points tell you that that's where the two

objects met up, or where in this case, the light beam and the object met up.

At a certain point x, remember we are measuring along our lattice of clocks

here, our measuring system, and then at a certain time, t.

So when we have multiple world lines on the diagram and they intersect each other,

then we know those two objects essentially occupy the same point in space and

time at that given instant, and if they're spaceships or

something they could even collide with Each other in that case.

Technically, they would collide with each other.

But, we'll assume maybe our measuring system isn't so

fine that they couldn't just pass by each other very nicely, so

we don't have any accidents and things like that.

Okay, so a little bit more spacetime diagrams.

In the next video clip,

we're going to look at different ways we can actually do some plotting.

And sort of strange ways to do it.

It will be very unusual.

It'll seem strange.

But, we'll find later on that it'll make a lot of sense for us to do it this way.