If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:9:38

- [Voiceover] In the last video, we started to construct
a space-time diagram for my frame of reference and I'm just drifting through space and I'm assuming that I'm in
an intertial frame of reference which means I'm moving
at a constant velocity relative to all other frames of reference and we set up a situation where I emitted a photon right at time zero, so after one second, it would have moved 3 times 10 to the 8th meters. After two seconds, it would have moved 6 times 10 to the 8th meters, and then we had a, we added a little bit of flavor to our little scenario where right at time zero,
a friend passes me up in her spaceship and she is traveling relative to me in the positive-x direction at half the speed of light, and so we plotted her
path right at time zero. Her spaceship is right there. I could draw the spaceship. It's at the origin. Then after one, after one second, she would have traveled 1 1/2 times 10 to the 8th meters. After two seconds, she would have traveled 3 times 10 to the 8th meters, so this blue line in the last video was her path. But now I want to make
it even more interesting. Let's assume that we have
actually a whole train of spaceships, all traveling
in the positive-x direction at the same velocity as her spaceship. So this is her spaceship right over here, at time equals zero, she is exactly where I am, but let's say that 3 times
10 to the 8th meters in front of her, there's another spaceship, traveling at exactly 1 1/2 times 10 to the
8th meters per second, so they're traveling at the
same relative velocity to me, but if you think about it, from
each of their point of view, they would seem to be stationary, because the distance between them, in this x-direction is
going to stay the same. So this person, at time
zero, is going to be 3 times 10 to the 8th meters
in the positive-x direction from me. Now, if I were to wait two seconds, they are going to be, so if I were to wait two seconds, they are going to be, they are going to be 6
times 10 to the 8th meters away from me, because they're going half the speed of light, and so I could draw their path and I'm going to do it
in a slightly more muted, thinner color, so that we, what I'm essentially
going to be doing here is I'm setting up gridlines for my friend's alternate frame of reference. I'm going to put that on
top of my frame of reference and to be clear, I'm not assuming special relativity. I'm assuming a Newtonian world, classical mechanics. Just to get familiar with these ideas and to see where the Newtonian
world is going to break down but let's say that's not the only ship, let's say there was another ship, that at time equals zero is
6 times 10 to the 8th meters away from me in the positive x-direction. Well, where are they going to be? Where are they going to be after two seconds? Well, after, let's see, after one second, they're going to be 1 1/2
times 10 to the 8th meters further. Then after two seconds,
they're going to be 3 times 10 to the 8th meters further, and then after three seconds, after three seconds,
so they're going to be 1 1/2 times 10 to the 8th
meters further than that, so their path is going to look like, so let me draw this. I'm really just doing these
to show what stationary objects in my friend's frame of reference, how they actually look to me, and these should be parallel lines. These should be parallel lines, so that's a pretty good attempt, and so I'll, let me, and so let me actually just set up that alternate frame of reference, so this is going to be not only my x-axis, I'm also going to call
this my x-prime axis and I'll do it in a
second of thinking about how we can read this. So, I can color this
in with the blue color. Actually, let me do it
in that light blue color that we're going to be
using for my friend's frame of reference, so this is also, this is also going to be
our x-prime axis overlaid, and this one, let me call
this the t-prime axis, so my friend's frame of reference, I'm going to call that the
prime frame of reference, or if I called, if I called my frame of reference
the S frame of reference, I can call my friend's frame of reference the S-prime frame of reference, and so, let me draw
horizontal lines to show the passage of each second, so, that's one second, two seconds, three seconds, so on and so forth. So let's make sure we're
comfortable with reading what's going on here. So, a point, let's say this point on our diagram. Let me just see. This point on our diagram, from my frame of reference,
in the S frame of reference, this is the point time is, or space, or my distance, my distance from the origin
in the positive-x direction is going to be, I just drop a, I just go parallel to my vertical axis, so I'm just going to do that, and I get 4.5 times 10 to the 8th meters, so 4.5 times 10 to the 8th meters, and what is our, so I can write the units there if I want, and our time is at one second, one second. So this is our coordinates from, this is the coordinates
from my frame of reference, from the S frame of reference, I guess S for Sal, and so, what would be the frame? What would be those same coordinates from my friend's frame of reference? Let's say her name is Sally, or S-prime. Well, from her frame of reference, our time is still one second. We see the horizontal line there. We're just going parallel
to our horizontal axis or our x-axis so the time is still one second, but her, the distance that the, or the x-coordinate is 3
times 10 to the 8th meters, 3 times 10 to the 8th meters, and I really want you to
sit and think about this and even take out a ruler
and draw this on your own paper and feel good about this. Now, why does this make sense? Well, this, you could
view this point as where that second spaceship was after a second, and from
Sally's point of view, from the S-prime frame of reference, at time equals zero, that person, that ship was
3 times 10 to the 8th meters in front of her, and after a
second, that spaceship is still 3 times 10 to the 8th
meters in front of her. After two seconds, it's still 3 times 10 to the 8th
meters in front of her, and so from her point of view
and her frame of reference, it is stationary, so the way to read this for any point, if you want my frame of reference, you, the time is easy, you just go horizontally to either one, so on her axis, we've essentially
just taken the coordinate axis and sheared it and skewed it. This is still one second, this is two seconds,
this is three seconds, but it turned all of these kind of a grid of squares if I were to draw
a grid for my coordinates into a bunch of, into a bunch of parallelograms which we see right over here, and so for any given point in, remember we're thinking of Newtonian space and time, to read it, we go horizontally for time, in either case, but for me, you would go straight down because my time axis is vertical. You go straight down and
you read where it intersects the x-axis to figure out our position but from her frame of reference we move parallel to her t-axis, or her t-prime axis, I should say, to get what her x, I guess we could say what
her x-prime coordinates are going to be, so this
is x-prime, t-prime. This is x and this is t. Now, what's clear, at any given point, at any given point, t is going to be equal to t-prime, or we could say t-prime is equal to t, either way, but what's the relationship
between x-prime and x? Well, let's think about that for a second, so when x is 4.5 times
10 to the 8th meters, x-prime is 3 times 10 to the 8th meters, and so it looks like, it looks like x-prime
is going to be equal to x minus, well what's it going to be minus? It's going to be minus 1.5 times 10 to the 8th meters but that difference is going
to grow as time goes on, meters per second times, times the amount of
time that has passed by, and you should see that because after one second, after one second, the difference
is 1.5 times 10 to the 8th meters, after two seconds, after two seconds, so
let's look at this point right over here, in my frame of reference,
that person is sitting at 6 times 10 to the 8th meters
in the positive-x direction, but in Sally's frame of reference, in the S-prime frame of reference, they're still at 3 times
10 to the 8th meters in the positive-x direction, so we'd multiply two seconds times 1.5 times 10 to the 8th so we have 3 times 10 to the 8th meters, so we take 6 times 10 to the 8th which would be that. We subtract out 3 times 10 to the 8th and we get the coordinates in S-prime, so this might get a little bit confusing and the key here is really
try to draw this yourself and really try to plot points and think about how they
are different coordinates in the different frames of reference, or different time-space
coordinates in the different frames of reference.