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- [Voiceover] I am about to do something

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that most of you will
probably find disconcerting,

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in fact, the first time
that I saw someone do this,

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I too found it disconcerting.

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So, just as a little bit of background,

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so far, when we've been thinking about

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our space axis, and we've really just been

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focusing on one dimension of space,

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we've been focusing on the x dimension,

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or the x prime dimension, depending on

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whose frame of reference
we are talking about.

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We measured that in terms of meters.

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We measured that in terms of meters,

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and then when we thought about time,

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well, we said, initially,
we said, "Well, time is

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"fundamentally something
different than space,"

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so we had a different set
of units called seconds,

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in fact, you know, this
goes well before we,

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well before the 20th century when we got

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special relativity, this
goes all the way back to well

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before even that, where we said
time is something different.

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We measure it in units
of time, in seconds,

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or minutes, or hours, or days.

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While distance, which
is different than time,

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we measured in meters, or feet, or miles,

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or kilometers, or whatever else.

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But as we started to
get into this world of

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special relativity and
we started to see that

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space and time are not each absolute.

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And, in fact, we can
actually think of all events

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as happening in this
continuum called spacetime.

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And I say it fast, "spacetime".

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Because it's not saying "space-time".

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Two different things.

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It's saying that they're really
just different directions

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in this continuum spacetime.

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And so, if they are all
the same thing why do we

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use different units for space and time?

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Why do we use seconds for
time and meters for space?

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Or at least in the examples
that I've been doing so far.

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And so to fix that,
instead of calling this

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the "t-prime axis" or the "t
axis," instead of labeling

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it in terms of seconds, what we can do

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and this is the part that many of you

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will find disconcerting, let's call this

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the "c times t-prime axis."

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And let's call this the "c times t axis."

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And the "c times t-prime axis."

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Well, what's that going to do?

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Well, we know that the speed
of light is an absolute.

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It is, if we are measuring it in,

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and I'll do approximately
because it's actually

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two point nine something.

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But, approximately three
times 10 to the 8th

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meters per second.

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For the sake of all of these
videos I'm just assuming

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it's three times ten to
the 8th meters per second.

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For simplicity it's roughly that.

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So if we were to take c times time

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instead of this being one
second, in terms of seconds,

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well, we multiply it times

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three times ten to the
8th meters per second.

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Well, the seconds cancel
out and we're left...

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If we want to measure
time in terms of meters,

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would be three times
ten to the 8th meters.

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So this is three times
ten to the 8th meters,

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instead of calling it one second.

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This over here is negative.

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So that over there is negative

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three times ten to the 8th meters.

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Along the "c t-prime axis."

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Likewise, this what we called one second,

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right over here, instead
we can call this as

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three times ten to the 8th meters.

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This we could call negative

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three times ten to the 8th meters.

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Now this will be counterintuitive to you

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because you've always
viewed time as fundamentally

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something different than meters.

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You haven't been thinking
in terms of spacetime.

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In fact, in our normal human
experience we don't experience

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the world in terms of space time.

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Time is something that we
are just falling forward into

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and space is something that
we feel like we have more

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agency and we can move in
the different dimensions

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of space more easily, while
time just feels like we're

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plummeting forward in that dimension.

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But now we're thinking
in terms of spacetime

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and this makes our units the same.

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Now if it helps you,
you could view this as

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three times ten to the 8th light meters.

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So you could think of this
as the time it would take

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for light to go three times
ten to the 8th meters.

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Likewise, if we wanted
everything in terms of what

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we traditionally conceive
of as our time dimensions,

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we could have kept this as one second,

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instead of calling this three
times ten to the 8th meters

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we could have called
this one light second,

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which would be the
distance that light travels

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in what we measure, or what
we consider to be one second.

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But the benefit of this
is now we're consistent.

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We're measuring different
directions in spacetime,

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which is kind of a continuum,

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there is no separate time and space.

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We're measuring them
all in the same units,

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which we will find is
very, very convenient.

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So, I know this is going to
take a little bit of time

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to get your head around,
and I'll maybe do a few

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more videos to make you
feel comfortable with

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this type of idea, but this will be a

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convenient thing for us.

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Especially as we start
to have a metric in our,

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what we would call, our
Minkowski spacetime.

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Because we are going to be
operating in the different

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dimensions as if they have the same units.

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So, hopefully, I encouraged
you to kind of sit and ponder

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and think about this a little bit,

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and hopefully it doesn't
bother you too much.