Introduction to special relativity and Minkowski spacetime diagrams | Khan Academy

- [Voiceover] In the last
videos, we constructed

a little bit of a conundrum for us.

We had the situation where
I'm drifting through space,

and right at time equal zero,

one of my friends, she passes me by

in a spaceship going
half the speed of light

in the positive x
direction, relative to me.

And at time equal zero,

is exactly where she is at my position.

And then she keeps
traveling, and so I draw

the path that she is taking.

So after one second, she would be here,

after two seconds, she would be there,

and we set up our scales

so that on our time axis,

one second is the same length

as on our space axis, or on our path axis,

three times ten to the eighth meters.

And we did that so that at
least in my frame of reference,

the speed of light would
be at a 45 degree angle,

or would have a slope of one.

But the conundrum we hit was

is that the speed of
light would be perceived,

based on this model I set up,

would be perceived differently

based on which frame
of reference you're in.

The photon that I emit from my flashlight,

right at time equals zero,

well sure I'm going to see that as moving

at the speed of light in
the positive x direction.

But my friend, since she's already moving

in that positive x direction
at half of the speed of light,

if we assumed a Newtonian world,

she would see that photon going
at half the speed of light.

And likewise, if she emitted
a photon from her flashlight,

to her, it would look like it's
going at the speed of light,

but to me, it would look
like it's going faster

than the speed of light.

And the reason why that is a conundrum

is we know from observation
of the universe around us,

and this is profound, this
is a little mind-blowing,

this is counterintuitive
to our everyday experience,

but we know from observation

that the speed of light is absolute,

that it doesn't matter your
inertial frame of reference.

As long as you're in an
inertial frame of reference,

it doesn't matter your relative velocity

relative to another
inertial frame of reference.

You will always measure the speed of light

at three times ten to the
eighth meters per second.

But that's at a direct contradiction

with what our model just set up.

So it really forces us to
question all of our assumptions.

So what are the assumptions that we made

in this Newtonian world?

Well, we assumed that time is absolute.

Time is absolute.

And what do I mean by that?

Well, we assumed that
one second passing for me

is going to be one second
passing for my friend.

That's just our everyday notion.

If I'm in a car and you're not in a car,

and we both have watches, and
we synchronize our watches,

they seem to say synchronized.

But maybe we need to
loosen this assumption.

We also assumed that space is absolute.

Space is absolute.

And what do I mean by that?

Well in our everyday experience,

regardless of what our relative
frames of reference are,

we seem to agree that, well if
that meter stick over there,

that's riding on that train,

that is a meter stick, whether
I'm sitting on the train

or whether I'm not.

But maybe things start to
break down a little bit

as we think about higher velocities.

And maybe they break down
even at the slower velocities,

but we just don't notice them

because it's a very small error.

And maybe there's something
even more interesting.

In our everyday experience,

we just assume that time is somehow

something very different than space,

that you can move in the time direction

without moving in the space direction,

or you can move in the space direction

without moving in the time direction,

and that they're independent regardless

of which inertial frame
of reference you're in.

But maybe they aren't separate things.

In fact, maybe they're all
one continuum of spacetime,

and I don't mean space dash time,

I mean spacetime.

I mean the word spacetime,

where space and time really
aren't different things.

It's just one continuum, spacetime.

And I keep saying it fast like that,

because it's not space dash time,

like thinking about the
different dimensions,

or thinking about two different things.

We're renaming one thing called spacetime.

So these are the things
that we're going to start

to hold into question.

So what if we loosen these,
and we assume the thing

that we have observed in the universe,

that the speed of light is absolute,

regardless of your inertial
frame of reference.

So let's word from there, do
a little thought experiment,

and then think about what type
of a model we can construct,

in which case the speed
of light is absolute.

And to do that, we're still going to use

our little model here,

and we're going to focus on my
friend's frame of reference,

my friend who's on a spaceship,

who right at time equal
zero, is passing me by

at half the speed of light.

And we assumed that she's
in this train of spaceships.

So we assumed already that she's,

there are these spaceships
that are all moving

at half the speed of light
in the positive x direction,

relative to me,

and they're all three times
ten to the eighth meters apart.

So they're like that.

So let's say, and I
did these axes in blue,

because we're assuming this is my friend's

frame of reference, so you could say

this is the S prime frame of reference,

this is Sally's frame of reference

from a couple of videos ago.

Let's say a second before she gets to me,

and I'm also assuming, the
way I've drawn these axes,

and we're going to
modify them in the future

where we're actually going
to use the same units

for space and time.

Right now, I'm still sticking
to what we've classically done

where we use seconds for time

and meters for our path or our space,

but in the future, we'll
actually use meters for both.

But we'll get to that, I don't want to do

too many things at once.

But the way I've drawn it

along my space axis,

three times ten to the eighth meters

is going to be the exact
same length as one second,

and I'm doing that so
that the path of light

on my diagram

can be a 45 degree angle.

So this is three times
ten to the eighth meters

right over there.

So let's say a second
before Sally gets to me,

she releases a photon from her spaceship

in the direction of the spaceship
that is in front of her.

How far is it in front of her?

It's three times ten to the
eighth meters in front of her.

And let's say on the
back of that spaceship,

there is a mirror.

So she's pointing her
flashlight at that mirror.

So what's going to happen
from her frame of reference

after one second?

So for one second, from
her frame of reference,

she is stationary, so
this is Sally still here.

She is going to be still
at x prime equals zero

in her frame of reference,

and that ship is still going to be

three times ten to the eighth
meters in front of her.

They're all moving, relative
to me, at the same velocity,

but relative to each other,
they seem to be stationary.

And so what would be
the path of that photon?

Well that photon will have
gone from Sally's flashlight

from her headlamp, or whatever it is,

to that spaceship in front of her

would've just reached that mirror

on the back of that spaceship.

And so we can draw the path of light.

It will be, so let me,

so that path of light

will look like that on this diagram.

And then right at this moment,

right at t prime equals zero seconds,

the photon will be
reflected back to Sally.

Well how long will it
take to get back to Sally?

Well Sally's gonna receive
the reflection of that photon

after one second, because that's how long

it's going to take it to go

three times ten to the eighth meters.

So the path of that very first photon,

the path of that very first photon

is going to look like that.

All right, well hopefully this is pretty

straightforward here.

This is what would happen from
Sally's frame of reference.

A second before she reaches me,

at t prime equals negative one seconds,

emits a photon at t equals zero seconds,

and gets to the spaceship

that's three times ten to the
eighth meters in front of her,

essentially one light-second
in front of her,

and then a second later,

it's reflecting back, a second later,

she gets the reflection.

And so that's what this
diagram is describing.

But now let's draw it projected on top

of my frame of reference.

And this is when things are going to get

really, really, really,
really interesting.

So I've drawn my frame of reference here,

and I've intentionally
not marked off the seconds

or the meters on my frame of reference,

because once again,
I'm not going to assume

that a second in my frame of reference

is a second in hers, or a
meter in my frame of reference

is a meter in hers.

I have drawn her t prime
axis at the same angle

as I did before,

because for every second

we move into the future,

she's going to move half a
light-second in distance,

in the positive x direction.

So this slope right here,
one way to think about it,

the way I've drawn it,

this is a slope of two.

For every unit she moves
in the x direction,

she will move two in the time direction.

And what we're going to do again

is assume that on her axis,

I haven't drawn the x prime axis.

In fact, this is an
exercise to think about

where should the x prime axis be.

Should it be coincident with the x axis

like we assumed before?

Or is it going to be in a different place?

But we're going to assume
that the lengths I draw

for one second on, let's say,

in the S prime frame of reference,

is going to be the same
as the length I would draw

for three times ten to the eighth meters.

And we're also going to assume

that the speed of light is absolute,

so it's always going to
moving at a 45 degree angle

with respect to either frame of reference.

So that's where things are going
to get a little bit whacky,

but let's see what's going to happen.

So at negative one seconds,

we still have Sally
turning on her flashlight.

She wants to bounce it off of
the spaceship in front of her.

And so that photon's going to
move with the speed of light

in either frame of reference.

And so let me draw that.

Let me draw that,

so it's going to look like this.

I'm drawing at a 45 degree angle.

So actually, I don't know
where it gets reflected.

It's gonna get reflected where
it hits that x prime axis,

but I don't know where that is.

But we do know that it
then gets reflected,

and then it gets back to Sally
at one second in the future.

So the return path of that photon

is going to look something

is going to look something like this.

And that point that it changes direction,

I could have,

let me,

I could have done it like this,

whoops,

I could've done it like this.

But the interesting point is
where it changes direction

because that's where that
spaceship in front of her

must be at that point in spacetime.

Because now we're going to
start thinking of mixing up

space and time,

but I'm not going to get
too much involved in that.

Now why is this interesting?

Because from Sally's point of view,

from Sally's point of view,

this point here where the
light changes direction,

from Sally's frame of reference,

is happening simultaneously
with when she reaches me.

This is happening at t prime

is equal to zero for Sally.

So anything that is t prime
is equal to zero for Sally

must be on the x prime axis.

So this must sit on the x prime axis.

Once again, why do I know that?

Because everything on the x prime axis,

any event on the x prime axis,

let me do it in a different color,

I keep doing black.

Any event on the x prime axis

is going to be at t prime
is equal to zero seconds,

and from Sally's frame of reference

is going to be simultaneous
with when she passes me up.

So based on that, we know
that this is going to be,

we know that this point,

which is where Sally is,

that's going to be the origin
from her frame of reference.

And we know that this point
sits on the x prime axis,

so based on that, we can
draw the x prime axis.

You just need two points to define a line,

and so let me try to do it.

So let me try to draw.

I can do a better job than that.

Let me try to draw

the x prime axis.

It's going to look like,

it's going to look like this.

That right over there is the x prime axis.

Now at this point, you should
find this mildly mind-blowing.

Actually, even more than
mildly mind-blowing.

'Cause it's saying some
pretty, pretty crazy things.

First of all, let's just make
sure we know how to read this.

So for any event,

and now we're going to start thinking

in terms of spacetime,

although I'm still using different
units for space and time,

but we'll address that in the future.

If I want to read its coordinates
in my frame of reference,

well if I want to read its x coordinate,

I go parallel to the t axis,

and if I wanna read its t coordinate,

I go parallel to the x axis.

But for Sally's frame of reference,

well I essentially do the same thing.

If I want its x prime coordinates,

I go parallel to the t prime axis,

and if I want the t prime coordinates,

I go parallel to the x prime axis.

But what's really interesting,

and I'll go even deeper into
this into the next videos,

that moment,

that moment right over here,

that from Sally's frame of reference,

it looks like it's simultaneous
with her passing me up.

It looks like it's happening
at t prime equals zero.

In fact, it is happening
at t prime equals zero.

From her frame of reference,

it's happening at t prime equals zero.

From our frame of reference,

it is happening after Sally passes us up.

Notice, it is happening at t
equals some positive value.

It's not happening at t equals zero.

So this is starting to
get a little bit whacky.

One second, and simultaneous,
time, and space,

and things being simultaneous,

we're not going to agree on those,

depending on which frame
of reference we're in.

The thing we will agree
on is the speed of light.