Main content

## Physics library

### Course: Physics library > Unit 16

Lesson 4: Einstein velocity addition# Time dilation

We'll start thinking about time dilation in special relativity and how a Loedel diagram can help up appreciate symmetry between inertial frames.

## Want to join the conversation?

- I keep seeing two different formulas for time dilation, one puts the initial time OVER the Lorentz factor, and the other MULTIPLIES it. So, which one is right? Or are they both right?(6 votes)
- You may be confusing Time Dilation and Length contraction

https://en.wikipedia.org/wiki/Length_contraction(7 votes)

- WAIT! So, if someone is moving at 0.9 or 0.99 percent the speed of light, their time relative to a stationary observer will slow down, but this says that the stationary observer will also slow down from the point of view of the person travelling at almost the speed of light. This makes sense because it agrees with the fact that everything is relative and someone moving can be treated as a stationary person with the universe moving around them. BUT, when the person travelling at almost the speed of light stops, then they must arrive further in the future as they have experienced time dilation, if throughout the journey they have seen the stationary persons clock slow down, how can they then arrive further in the future?(4 votes)
- You are describing what it usually called "The Twin Paradox". This apparent discrepancy between the two observers is because you are only using Special Relativity that doesn't deal with the effects of acceleration.

The observer on the earth is not experiencing acceleration where as the person in a rocket travels away and then had to accelerate to return to earth. This acceleration breaks the equivalences between the two observers frames of reference.(7 votes)

- In the end Sal says that the blue event happens BEFORE the yellow event, in B frame of reference, and that the blue event happens AFTER the yellow event, in A frame of reference. This reasoning looks right, but does it take in account the time it takes for a
*light ray*to be emitted and to come back, in order for the event to be actually seen (by observer in B, in the first case, or by observer in A, in the second case)? Or maybe is this all actually due to this very "waste of time" between emitting a light ray and receiving it back?(4 votes)- The order of the events will not change because the time it takes for the light to travel back and forth between the events. When the light from the event gets to the observer they can calculate when it was at the event and even with this correction there will still be a discrepancy in the order of events between the two reference frames.(4 votes)

- but when you travel at high speed time flow slower for the traveler but here in A's perspective delta t prime is larger than delta t.(4 votes)
- When someone travels at significant fractions of the speed of light, relative to themselves, time passes normally. Likewise, an observer would also say that time is passing normally. However, the observer would say that relative to himself, the traveler is experiencing time slower.(2 votes)

- if a person is moving in a space time at 20 m/s and another person is moving in same space time at 30 m/s then will the second person will move forward in time ?

does theory of relativity does not act here(2 votes)- as far as i know, for both of them, time will continue to 'move forward' within their own frames of reference at any speed.

But if they are moving relative to one another, then the time for one person will be moving more slowly when seen from the other persons point of view.

However, this difference will be VERY small, since they have such small relative velocities compared with maximum velocity. (c)(6 votes)

- Why special theory of relativity is only taken in inertial frame not a non inertial frame?(2 votes)
- I am pretty sure that this is why its called special:

the general theory involves accelerating frames whereas the special (special case of...) refers only to non-accelerating frames

ok?(5 votes)

- Correct me if i am wrong.

The faster we move and approach the speed of light the slower the time gets for us viewed from a stationary frame of reference and events occur later for us.

But from our frame of reference the stationary frame is approaching the speed of light so time goes slower for it. What is going on here? I am so confused(1 vote)- There's no such thing as "a stationary frame of reference". You can be stationary in a particular frame of reference, or moving in that frame of reference.

If you are zooming along in your spaceship, you are stationary in the ship's frame of reference, but the universe around you is moving. Inside the spaceship, everything is normal for you. Time passes just as you expect. Outside the spaceship, maybe you see your friend zooming past you. If you could watch her clock you would see it tick slowly. Your friend is stationary in her reference frame, and her clock ticks normally for her, but if she could see YOUR clock while you are zooming past her, she would say that your clock is slow. Both of you think the other one's clock is slow. How can it be? Because time is not absolute, it's relative.(6 votes)

- This is more challenging then I thought. Can anyone explain this in a more understandable way?(1 vote)
- I'll give a quick and dirty explanation first and see if you're interested in a longer one...

Inside the frame of reference of the spaceship (or whatever moving frame), time flows normally for you. You notice nothing different. But for an observer watching you from a stationary frame of reference relative to yours, they see time for you travelling more slowly.(6 votes)

- There's one thing I don't understand: the phenomenon described here is called 'time dilation', but why 'dilation' and not 'speeding up' or 'change'? In the examples given, some event that is judged to happen at a certain time in frame A, happens at a
**later**time in frame B, so doesn't this imply that time is passing**faster**in B than in A, and not slower? I suppose it all depends on the perspective from which you're looking at it, but that is why I don't understand why we always talk about time dilation, as if the only thing that ever happens is that time slows down, whereas it seems like in some comparison we may say that time actually goes faster in one frame than in another. Or have I misunderstood something?(3 votes)- The term dilation is used because it is a mathematical term for a transform/function that scales a set of coordinates. Since in special relativity there is a change in distances in space and time this is a mathematical scaling of coordinates.(2 votes)

- I haven't done any quantum physics yet, but. If you have two entangled electrons at time zero. Then they get split and one goes in frame A and the other in frame B. And then electron A gets its spin reversed 9 minutes later. Entanglement would say that electron B gets its spin reversed at the same time, violating time dilation and the absolute speed of light assumption. Right?(2 votes)
- yes. This is what happens but what is (or at least seems to be) interesting is that there is no violation of any laws.

why??

because even though the change happens 'at the same time', there is no exchange of information that travels faster than light. so you are no better off in practical terms...(3 votes)

## Video transcript

- [Voiceover] So let's revisit a scenario that we have seen in several videos, especially the last video, where we tried to find this
neutral frame of reference. Let's say we're in spaceship A. We are in an inertial frame of reference. And let's say right at time equal zero in our frame of reference, spaceship B is exactly where we are, but it's traveling in
the positive x-direction at eight tenths the speed of light. We've already seen that we can overlay the spacetime diagrams for each of these frames of reference. If we do our axes, the x is one direction of spacetime that we associate with
the x-direction of space and then the ct, this vertical axis, this is another direction of spacetime that we perceive as the passage of time. Well, you can then overlay
B's frame of reference, and you kind of have these skewed axes. And you could look at a change in time. So for example, if you were to compare the event right when B passes us up to let's say some amount of time later, some amount of, so let's
say this right over there is the change in ct. In our frame of reference, these two events are happening
at the exact same place. They're just separated,
in our frame of reference, we would perceive them as
being separated by time. But if you want to say, well how much time seems to separate these events
in B's frame of reference? Well then, you would wanna go
parallel to the x prime axis. We're viewing B's frame of reference as a primed frame of reference and see where we intersect the ct prime, the ct prime axis. So this is our change in ct while this is our change in ct, this looks like our change in ct prime. And it looks longer
but we have to remember that we haven't scaled these things. And actually the scales change depending on the relative velocities. But we can actually verify algebraically that our change in ct
prime is going to be larger than our change in ct. And we just have to look
at Lorentz transformations to realize that. So our change in ct prime is going to be equal to the Lorentz factor
times our change in ct minus beta times our change in x. We've seen that multiple times before. Well our change in x, our change in x is zero. It looks stationary in
our frame of reference so that term is zero. So our change in ct prime
is going to be equal to the Lorentz factor times, I don't have to use this
parentheses anymore, times our change in ct. And our Lorentz factor is
going to be greater than one. I could actually calculate
that, let's do it. So the Lorentz factor, the Lorentz factor here. So gamma is going to be equal to one over the square root of one minus, well B's relative velocity so 0.8c over the speed of light squared. Well what is this going to simplify to? The c's cancel out. 0.8 squared is 0.64. One minus that is 0.36. This is going to be, and then you take the square root of that, that's going to be one over 0.6 which is equal to one over six tenths which is the same thing as 10 over six which is equal to the
same thing as five thirds which is equal to one and two thirds. So you can see our change in ct prime is going to be one and two thirds times the change in ct. Now you might wanna just say, well do these two look
like one and two thirds? And it might look a little bit like that the way I draw it, but you can't just look at it purely on, you can't just take a
ruler for this length and a ruler for this length because the scales are different and I haven't marked off the scales. Well this at least helps us visualize. But let's think about
the other way around. Let's imagine the change in ct prime between right where the spaceships pass by and a little bit later. Now, I'll do this in a different color 'cause this is actually a
different event in spacetime than the one that we were
focusing on right now. And we're gonna be viewing it
from B's frame of reference so that's our change in ct prime. But what is going to be our change in ct? Well, to think about it, we could go parallel to the x-axis, the x-axis right over
there, so you'll parallel, parallel to the x-axis and
you get right over there and so our change in ct looks like it's more. And once again, we can
algebraically verify it by really doing the same thing. Our change in ct is going
to be equal to gamma times our change in ct prime minus beta times delta x prime. And if you were to actually, if we actually did have
a change in x prime here, the beta, the velocity now
has a different direction so it would all be the negative but our change in x prime
from B's point of view, these two events are
happening in the same place so our change in x prime is zero. So you have change in ct is
equal to gamma times change, once again I don't need my parentheses, times change in ct prime. And it's going to be the same gamma because remember, we're taking v over c and so whether it's
either v or negative v, when you square it, it gets the same value so once again, gamma is going
to be one and two thirds. So it's going to be one and two thirds. So it seems a little bit strange. You know, I have some passage of time in my frame of reference where it looks, you know, it's something
that looks stationary, two events that look
like they're happening in the same place but one after another. It looks like their change in time, it takes longer for those
two events to happen in the ct prime in the
moving frame of reference. But then, if we have some event that looks stationary in
B's frame of reference and they're separated
by a change in ct prime, it looks like the change in
ct between those two events is even larger. So it looks like this
somewhat bizarre phenomenon. And to help us reconcile these and to visualize a little bit better, actually even to be able
to put ct and ct prime on the same scale, we can look for this
neutral frame of reference which we did in actually
the last video I made. I don't know if it's the
last video you've seen. Or we could say look, if A and B are travelling with a
relative velocity of 0.8 times of the speed of light
relative to each other, B is travelling 0.8c in
the positive x direction relative to a stationary A or A is travelling 0.8c in
the negative x direction relative to a stationary B. You can find a frame of reference where A and B were in
that frame of reference to a stationary observer
in that frame of reference, A and B are both travelling outwards at half the speed of light. And we figured that out where we did those videos on a
neutral frame of reference. And what's neat about that is if you make what looks like
a Minkowski spacetime diagram in that neutral frame of reference then the ct and ct prime, A
and B's frame of references, get equally skewed to the, you know, if you think about the time
axis to the left and the right. And since they're equally skewed, the time dilation relative
to this rest frame is the scaling is going to be the same. So we could put both of these on, both of these on the same scale. And you could see that this
neutral frame of reference, this ct prime prime frame of reference, I drew them over here, that if you look at either of
A or B's frame of reference, they're going to be in between A and B's frame of reference. But here it's the neutral one. It's the one where we're drawing the time or we're drawing the two
axes being perpendicular to each other. And now if you look at these two events, so if you look at this first event, where you have our delta ct, so you look at this first event where you have a delta in ct between right when the spaceships
pass and right over there. You see that if you were to look at that, if you were to look at
the ct prime coordinate for that event, when you go
parallel to the x prime axis, you get right over there. So this is, that is
your change in ct prime. And likewise, if you have that other event that I drew in that blue-green
color, right over here. And this is your change in ct prime. This is your change in ct prime, well then if you think of it
from A's frame of reference, well we just follow the
x-axis not the x prime. We go parallel to it. We end up right over there. Now what's really interesting about this, what's really interesting about this is from A's frame of reference, the yellow event happens
before the blue event. But in B's frame of
reference, the blue event happens before the yellow event. So really, really, really
fascinating things going on but what I really like about this diagram is that A and B's frame of reference are going to have the same scale since they're both equally
skewed to the left and the right if we're thinking about the time axis. And this type of diagram
is called a Loedel diagram. Loedel, Loedel diagram which is really a variation of a Minkowski diagram but it lets us really
appreciate the symmetry between these frames of reference.