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### Course: Physics library > Unit 16

Lesson 2: Minkowski spacetime- Starting to set up a Newtonian path–time diagram
- Visualizing multiple Newtonian path–time diagrams
- Galilean transformation and contradictions with light
- Introduction to special relativity and Minkowski spacetime diagrams
- Measuring time in meters in Minkowski spacetime
- Angle of x' axis in Minkowski spacetime

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# Introduction to special relativity and Minkowski spacetime diagrams

Including multiple observers in the "most obvious" way led to some problems. Let's see how we can start to solve those problems by introducing (what we'll later call) Minkowski spacetime diagrams.

## Want to join the conversation?

- how do we know that the speed of light is absolute?? how did we conclude that?? Sal has been saying that, observations proved that?? what kind of observations?(30 votes)
- Its a mathematical consequence of Maxwell's equations, which were deduced from experimental equations.

Yes, there are proofs. Pictures from stars during an eclipse show that a light ray does, indeed, bend near gravitational fields (a consequence of the general relativity theory). Nowadays, we also have pictures of gravitational lenses created by clusters of galaxies took by Hubble Telescope.

As for the special relativity theory, GPS systems and jets moving around the Earth with superclocks inside were used to show that it also holds.

Also, the Michelson-Morley experiment was repeated countless times, it intended to show how light speed wasn't absolute. The experiment never succeeds in doing so.

PS: The special relativity theory was built by 2 postulates, and one states that the speed of light is absolute. By proving this theorem's predictions (like what happened to the superclocks inside jets), we were able to make sure its a solid theory.(40 votes)

- At 10;10 ,how to we exactly know the point of reflection?(12 votes)
- Sal drawn the line from -1s prime at a positive 45 degree angle in relation to our point of view to stay consistent that light speed is absolute. Again he drawn a line at negative 45 degree angle from the +1s prime to meet with his previous line for the same reason that light speed is absolute.(15 votes)

- at about the tenth minute of the video, when we are drawing the photons movement in the graph with a forty five degree angle.. Angle from t' axis i guess. (Enlighten me if I am wrong on this!). And then he says we won't know where the photons get reflected, the point where the photon changes its direction! Why can't we have that point (point of change of direction of the photon) on the X axis of "our" reference?(9 votes)
- In a spacetime diagram like is being used the observers frame of reference O has the non-prime axes t and x and light will always move moving at a 45 degree angle and the observer will always be on their t axis.

When we look at the spaceship that is moving at 0.5c they are in O' and we are in O.

From the perspective of O the spaceship is moving in a line that is 30° from the t axis and since from the perspective of O' the position of the space ship traces out t' this is also t'. A photon of light emitted from the spaceship travels at a 45° angle with respect to t by definition. Since we know that we know that the photon was emitted at 0 seconds on t' and received at 2 seconds on t' we can find the bounce point by drawing the 45° up to the right from t' = 0 and an 45° down to the right from t' = 2. We know that the reflection point is on x' at 3*10^8 meters so if we draw a line through the reflection point and t' = 0 you get the x' axis which ends up being 30° from the x axis.

In general the angle that t' is from t will be the same angle in the opposite direction that x' is from x.

I am not sure if I made this more or less clear.(9 votes)

- OK.. I accept that time has to get dilated. But how do we see that practically?

Does a watch slow down? Should not be because the internal mechanisms with which the clock works are not dependent on speed. I mean the battery still provides the current and the clock rotates at the same speed.

Or,does our ageing process slow down. Well our biological clock should also not feel the time to slow down. Why will the pineal gland stop the circardian rhythm if we are travelling at half the speed of light?

In practice how can we feel the time to slow down??(6 votes)- From your frame of reference of being inside the spaceship, you experience time going at a normal rate (you always experience time going at the same rate for yourself). However you also experience space in the direction of your motion being contracted.

From the frame of reference of an outside observer watching you in the spaceship, they view your time as being slowed down, but they do not experience any spatial contraction.

In the end when the spaceship returns to the observer, you in the spaceship say you traveled for less time because the distance you travelled was reduced from the spatial contraction; and the observer also agrees with you that you travelled for less time because they saw your time slow down even though they also saw you travel a longer distance. So in the end, both people agree on the amount of time that has passed, but for entirely different reasons (spatial contraction vs time dilation).(9 votes)

- In the SpaceTime Graph for Sally at7:30Sal says that light will take 1 second to go from Sally to the space ship and another second for its reflection, so it means that Sally is also travelling at the speed of light which contradicts his previous saying and in the SpaceTme Graph in his frame of reference that Sally is travelling at half the speed of light, so I`m a bit confused as to what to take in to consideration.(5 votes)
- Sally is not travelling at the speed of light. The distance to the space ship with the mirror is exactly 1 light second, meaning it takes two seconds for the light to reach back to Sally. Sally and the spaceship with the mirror are both travelling at the same speed of
`c/2`

from Sal's point of view.(1 vote)

- During the video, Sal mentions 'spacetime' several times, to emphasize that they become a single entity. How does the 'relative time and space' coordinate system have space and time as a single entity, as opposed to the Newtonian system? I mean where do we derive that 'spacetime' part from?(5 votes)
- but clocks are made out of gears and stuff like that but these stuff don't have any sense of what Time dilation is and they should operate the same way in every situation.i really need help.pls som1 help me understand this(3 votes)
- What do you think makes gears and stuff operate in a way that things like bouncing photons or vibrating atoms don't? Gears are made of these simpler things.

Nothing has a sense of what time dilation is that "makes" it follow what special relativity says it should, special relativity is a theory that describes what we have observed.

The effects of time dilation and length contraction in special relativity have been observed and measures to extreme accuracy.(2 votes)

- Why is Non Eucidean Geometry not famous and is not known to many?(2 votes)
- It is much newer and many of its derivations ( AAA congruence ) seems very unorthodox. However, it is not inferior to Euclidean geometry.(3 votes)

- How do we know the speed of light is absolute?(2 votes)
- The The Michelson–Morley experiment was performed over the spring and summer of 1887. It measured the speed of light and proved that the speed is absolute for every frame of reference. Remember the earth is moving relative to almost ever other frame of reference. Since that time all experimental attempts to verify that the speed of light is constant regardless of your frame of reference have proven that it is indeed constant in a vacuum. In all frames of reference the speed of light is a constant in a vacuum. The speed of light in other substances like glass is somewhat slower than in a vacuum. This remains constant in all frames of reference and nothing can exceed the speed of light as proven in countless experiments since Einstein's time. Hope this helps.(3 votes)

- At10:27you draw the reflection line with a 45 degree angle. Why is this obvious that the reflection needs to happen with a 45 degree angle? Time and distance are not constants and we accept that, why do we assume that the reflection angle is a constant?(2 votes)
- In the space-time diagram the angle of the light rays have no relation to the reflection angles in space. Light always moves at a 45 degree angle in a Minkowski spacetime diagrams. So when you show a reflection of light in a Minkowski spacetime diagram the light ray goes from 45 degrees one way to 45 degrees the other so it will always be at a right angle.(3 votes)

## Video transcript

- [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.