B Is velocity the reason for the time dilation effect?

Mike_bb
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Hello!

I try to understand how in different frames clocks tick and stop simultaneously but show different time? I suppose that velocity is reason of time dilation effect but I'm not sure.

Thanks.
 
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A clock always shows its "proper time". For a clock moving along an arbitrary (necessarily time-like) world line the relation to the time of an arbitrary inertial frame, wrt. you consider the motion of the clock, you have
$$\mathrm{d} \tau=\mathrm{d} t \sqrt{1-\vec{v}^2/c^2},$$
i.e., time dilation, i.e., the fact that for a moving clock the proper time (i.e., the time as observed by an observer at rest relative to the clock) is always running slower than the coordinate time of the inertial frame, relative to which its motion is considered.

If the clock is moving with constant velocity you can immediately integrate the equation, i.e., then you have ##\vec{v}=\text{const}## and you get (assuming that we set ##\tau=0## for ##t=0##)
$$\tau=t \sqrt{1-\vec{v}^2/c^2} \; \Leftrightarrow \; t=\frac{\tau}{\sqrt{1-\vec{v}^2/c^2}}.$$
 
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Does velocity contribute to the time dilation effect?
 
Mike_bb said:
Does velocity contribute to the time dilation effect?
What do you think the "time dilation effect" is?
 
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jbriggs444 said:
What do you think the "time dilation effect" is?
Time dilation is the difference in elapsed time as measured by two clocks due to a relative velocity between them
 
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Mike_bb said:
Does velocity contribute to the time dilation effect?
Velocity does appear in the time dilation formula, the greater the velocity the greater the effect, so yes, velocity is pretty clearly involved. However, we have to be clear that it is relative velocity that matters here.

If we have two identically constructed clocks A and B, and they are moving relative to one another:
- Someone at rest relative to clock A will find that clock A is running normally while clock B is running slower than A.
- Someone at rest relative to clock B will find that clock B is running normally while clock A is running slower than B.
- They are both right.
 
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Mike_bb said:
Time dilation is the difference in elapsed time as measured by two clocks due to a relative velocity between them
So... simultaneous start on the two clocks? Simultaneous stop on the two clocks? Unaccelerated motion for each clock? Difference in their respective elapsed times upon being stopped?

This velocity of which you speak. Not frame relative. That is good. But... how do we know which clock is "slow" and which is "fast"?

This simultaneity of which you speak. Measured how?
 
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jbriggs444 said:
So... simultaneous start on the two clocks? Simultaneous stop on the two clocks?
This simultaneity of which you speak. Measured how?
For example, light clock and mechanical clocks. We can simultaneously start mechanical clocks at initial point and if light reach to end point we can stop simultaneously our mechanical clocks in different frames.
 
Mike_bb said:
For example, light clock and mechanical clocks. We can simultaneously start mechanical clocks at initial point and if light reach to end point we can stop simultaneously our mechanical clocks in different frames.
No. We cannot. Light clocks, mechanical clocks and all other clocks are subject to a slew in measured simultaneity based on frame of reference.
 
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jbriggs444 said:
No. We cannot. Light clocks, mechanical clocks and all other clocks are subject to a slew in measured simultaneity based on frame of reference.
I read that light reach to end point simultaneously in different frames.
 
  • #11
Mike_bb said:
I read that light reach to end point simultaneously in different frames.
If two events are simultaneous in one frame, they are in general not simultaneous in other frames, and that’s why you must be much more careful about saying that things are happening “simultaneously” - which frame matters.
This is the relativity of simultaneity, a crucial piece of relativity that is unfortunately skipped over in many popular books/articles on relativity.

There is no way to make sense of time dilation and length contraction without also understanding relativity of simultaneity. The three work together to keep everything consistent.
 
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  • #12
Mike_bb said:
I read that light reach to end point simultaneously in different frames.
Where exactly did you read this? It would help us to know if you are misunderstanding a good source or correctly understanding a bad source.
 
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  • #13
Nugatory said:
If two events are simultaneous in one frame, they are in general not simultaneous in other frames,
I mean another. I mean that in moving frame and in non-moving frame same event happens simultaneously.

Dale said:
Where exactly did you read this? It would help us to know if you are misunderstanding a good source or correctly understanding a bad source.
Ok. For example, light clock. Light reach to end point simultaneously in non-moving and moving frames.
https://en.wikipedia.org/wiki/Time_dilation#Simple_inference
 
  • #14
Mike_bb said:
Ok. For example, light clock. Light reach to end point simultaneously in non-moving and moving frames.
https://en.wikipedia.org/wiki/Time_dilation#Simple_inference
Ok, this is a good reference. Nowhere does it say:

Mike_bb said:
light reach to end point simultaneously in different frames.
”Simultaneous” is a comparison between two events with respect to one reference frame. If, in a given frame, the two events have the same ##t## coordinate, then we say that the events are simultaneous in that frame.
 
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  • #15
Dale said:
Ok, this is a good reference. Nowhere does it say:
Another example is the propagation of light from the middle of a moving train to its left and right
ends. From the train passenger's point of view, the light will simultaneously reach the right and left ends of the train, with point of view of an outside observer - at different times.

In this example, light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously. Is it true?
 
  • #16
The word "simultaneous" is tricky in SR. Suppose the light starts where each frame has a clock and those clocks are both reading time=0. Suppose each frame has clocks that are synchronized in that frame. The moving frame goes some distance while the light travels. Then the light hits the end where each frame has a clock. Those two clocks do not record the same time. It is tricky to call it "simultaneous" in those frames, since that usually means "at the same time" whereas the clocks indicate different times. This is called the "relativity of simultaneity".
Any time you use the word "simultaneous" in SR, you must indicate the inertial reference frame that considers those two events to be simultaneous.
 
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  • #17
Mike_bb said:
I mean another. I mean that in moving frame and in non-moving frame same event happens simultaneously.
"Simultaneus" is not normally an adjective used to refer to one event viewed from two reference frames.

Even if two frames are synchronous (share the same time coordinate) at one event along a world line, they will not normally be syntonous (sharing the same time rate) along that world line.
 
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  • #18
FactChecker said:
The word "simultaneous" is tricky in SR. Suppose the light starts where each frame has a clock and those clocks are both reading time=0. Suppose each frame has clocks that are synchronized in that frame. The moving frame goes some distance while the light travels. Then the light hits the end where each frame has a clock. Those two clocks do not record the same time. It is tricky to call it "simultaneous" in those frames, since that usually means "at the same time" whereas the clocks indicate different times. This is called the "relativity of simultaneity".
Any time you use the word "simultaneous" in SR, you must indicate the inertial reference frame that considers those two events to be simultaneous.
Is it true that in previous example light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously?
 
  • #19
You contradict yourself. Your first paragraph has it right: For the train passenger the light signal reaches the ends of the train at the same time ("simultaneously"), for the observer on the embarkment at different times, and indeed this is called the "relativity of simultaneity".

This is one of the few examples, where it really helps to draw a Minkowski diagram:

train-example.png

The primed reference frame is the rest frame of the train. The passenger in the train sits in the origin, i.e., in the middle of the train and sends a lightsignal at time ##t'=0##. His wordline is the ##t'## axis. From the point of view of the passenger light signal reaches the ends of the train (blue world lines) at times ##t_+'=t_-'##, i.e., simultaneously.

The observer on the embarkment is at rest in the unprimed frame, and clearly the light signal reaches both ends at different times ##t_-<t_+##. That's also intuitively clear, because also from his point of view the light signal propagates with the speed of light in vacuo, ##c##, although the light source is moving with respect to him. Now the one end moves towards the light source and the other away from the light source, i.e., the signal needs less time to reach the former than the latter end of the train.
 
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  • #20
Mike_bb said:
Is it true that in previous example light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously?
When you say it was "simultaneous" from both points of view, that usually means "at the same time". But the clocks of the two observers indicate different times.
 
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  • #21
Mike_bb said:
Is it true that in previous example light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously?
It is one event being recorded in two different ways.
 
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  • #22
Mike_bb said:
Another example is the propagation of light from the middle of a moving train to its left and right
ends. From the train passenger's point of view, the light will simultaneously reach the right and left ends of the train
Yes. This is two events, the light reaches the left end and the light reaches the right end. In the passenger’s frame they occur at the same ##t## so they are simultaneous.

Mike_bb said:
with point of view of an outside observer - at different times
Yes. The same two events have different ##t## coordinates in the ground’s frame. So they are not simultaneous in this frame.

Events which are simultaneous in one frame are not simultaneous in other frames.

Mike_bb said:
In this example, light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously. Is it true?
No, this is not true. As I said above ”simultaneous” is a comparison between two events with respect to one reference frame. If, in a given frame, the two events have the same coordinate, then we say that the events are simultaneous in that frame.

Here you have one event, light reaching the left end, and two frames the passenger’s and the ground’s. The concept of simultaneity doesn’t even fit.
 
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  • #23
FactChecker said:
When you say it was "simultaneous" from both points of view, that usually means "at the same time". But the clocks of the two observers indicate different times.
Yes. I used this term wrong. I don't mean about "time simultaneous" or "at the same time". I mean that when light reach to end of train with point of view of passenger then light reach to end of train with point of view of observer. Is it true?
 
  • #24
Mike_bb said:
I mean that when light reach to end of train with point of view of passenger then light reach to end of train with point of view of observer. Is it true?
Your question is not well-defined. The event where the light reaches the end of the train is a specific point in spacetime: it's the same point for all observers. But the time coordinate assigned to that event is different for the train passenger and the observer. It's not clear which of these you are asking about.
 
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  • #25
PeterDonis said:
Your question is not well-defined. The event where the light reaches the end of the train is a specific point in spacetime: it's the same point for all observers. But the time coordinate assigned to that event is different for the train passenger and the observer. It's not clear which of these you are asking about.
Yes. And I want to understand how is it possible that time coordinate assigned to that event is different for the train passenger and the observer. What is reason of this thing?
 
  • #26
Mike_bb said:
I want to understand how is it possible that time coordinate assigned to that event is different for the train passenger and the observer. What is reason of this thing?
Mathematically, the simplest way to see it is to take the coordinates of the event in one frame and Lorentz transform them into the other frame. The Lorentz transformation changes the time coordinate.
 
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  • #27
Mike_bb said:
Is it true that in previous example light reach left end of train with point of view of passenger and with point of view of outside observer simultaneously?
What does that even mean? What does it mean for one event to be simultaneous with itself?
 
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Thanks to all!
 
  • #29
FactChecker said:
Suppose the light starts where each frame has a clock and those clocks are both reading time=0. Suppose each frame has clocks that are synchronized in that frame. The moving frame goes some distance while the light travels. Then the light hits the end where each frame has a clock. Those two clocks do not record the same time.
I think both clocks will record the same time in all frames. The flash bulb is equidistant between the two clocks, and the clocks are synchronized*, so the reading on both clock faces will show that the light hit them at the same time.

* In the train frame
 
  • #30
David Lewis said:
I think both clocks will record the same time in all frames. The flash bulb is equidistant between the two clocks, and the clocks are synchronized*, so the reading on both clock faces will show that the light hit them at the same time.

* In the train frame
I think @FactChecker is considering two events, one when light is emitted and one when the light is absorbed at another location. The time between those events is different in different frames.

You, on the other hand, seem to be considering three events, one where light is emitted and two where the pulse is absorbed. If clocks at the two absorption events are synchronised in the frame where those events are equidistant from the emission event then they will record the same time, yes.
 
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  • #31
from another point of view: the cause and effect chain, time dilation is not the cause and it is not an effect either.
When we say that muons can lengthen their lives is an abuse of language, time dilation is not the cause, the cause is that we measure it from a special reference system. On the other hand, there is no cause why time dilates. If we get on a train and it accelerates we will see that the station clocks slow down, but the cause is a different measurement, the cause is not time dilation.
If this phenomenon (time dilation) does not belong to a chain of cause and effect, we can say that it is not a real phenomenon.
 
  • #32
Renato Iraldi said:
If this phenomenon (time dilation) does not belong to a chain of cause and effect, we can say that it is not a real phenomenon.
That seems like a strange definition of “real phenomenon”. But ok.
 
  • #33
Renato Iraldi said:
When we say that muons can lengthen their lives is an abuse of language, time dilation is not the cause, the cause is that we measure it from a special reference system.
The twin paradox with muons was experimentally verified by Bailey et al. (1977) in a storage ring. It is a real effect.

The distinction between twin paradox and time-dilation is analog to the distinction between two-way speed of light and one-way speed of light. The twin-paradox could also be called "two-way time dilation".

I think, therefore also time-dilation can be called a real effect, although it's magnitude in one direction depends on the clock-synchronization definition of the reference frame.
 
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  • #34
Sagittarius A-Star said:
I think, therefore also time-dilation can be called a real effect
A better term for the real effect is "differential aging".
 
  • #35
I can understand the importance of distinguishing frame independent effects from frame dependent effects. But one thing bothers me. Calling differential aging a "real" effect and time dilation not a "real" effect means that a "real" effect is caused, explained, and calculated using effects that are not "real". It seems like there could be some better terminology.
 
  • #36
FactChecker said:
Calling differential aging a "real" effect and time dilation not a "real" effect means that a "real" effect is caused, explained, and calculated using effects that are not "real".
No, it isn't. You are thinking of it backwards. The invariants are where all the actual physics is contained; and the invariants are also all you actually need to calculate answers. You do not need "time dilation" to either calculate or explain the actual physics.

For example, the explanation of differential aging is simple: different worldlines between the same pair of events can have different lengths. This is no more mysterious than the fact that different routes between two points on Earth can have different lengths. There is no more need to invoke "time dilation" to explain differential aging than there is a need to invoke "distance dilation" to explain why two different roads connecting the same pair of points on Earth have different lengths.
 
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  • #37
PeterDonis said:
For example, the explanation of differential aging is simple: different worldlines between the same pair of events can have different lengths.
Ok. We have a differential length. Do the Lorentz transformations give a formula for the differential aging in terms of the differential length?

Another question I have is whether a third IRF would agree that the differential aging has the same value? It seems that if the Earth twin and the traveljng twin agree on a differential age, then a third IRF observer would disagree on the value of the differential aging. Does that complicate the statement that differential aging is frame independent?
 
  • #38
FactChecker said:
Do the Lorentz transformations give a formula for the differential aging in terms of the differential length?
You don't use Lorentz transformations to compute the length of a worldline.

FactChecker said:
whether a third IRF would agree that the differential aging has the same value?
Differential aging is an invariant. It's the same no matter what frame you use--it doesn't even have to be an inertial frame. That's what "invariant" means.
 
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  • #39
FactChecker said:
Do the Lorentz transformations give a formula for the differential aging in terms of the differential length?
The "length" of the worldlines is the elapsed time along those worldlines (give or take a factor of ##c##). The Lorentz transforms don't really come into it.
FactChecker said:
Another question I have is whether a third IRF would agree that the differential aging has the same value?
Of course - if the twins zero their clocks when they separate and compare them when they meet again, all frames had better agree what the clocks say, since it's a direct observable.
 
  • #40
FactChecker said:
I can understand the importance of distinguishing frame independent effects from frame dependent effects. But one thing bothers me. Calling differential aging a "real" effect and time dilation not a "real" effect means that a "real" effect is caused, explained, and calculated using effects that are not "real". It seems like there could be some better terminology.
Personally, I consider the word “real” to be a philosophical term which is best avoided altogether. (Except as part of the mathematical term “real number”.) One of the main differences between interpretations is what they consider to be “real” and not, so if you avoid the word “real” it will help you to write interpretation-neutral statements
 
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  • #41
Dale said:
One of the main differences between interpretations is what they consider to be “real” and not, so if you avoid the word “real” it will help you to write interpretation-neutral statements
See also this thread Is time dilation a real effect?
 
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  • #42
PeterDonis said:
You don't use Lorentz transformations to compute the length of a worldline.


Differential aging is an invariant. It's the same no matter what frame you use--it doesn't even have to be an inertial frame. That's what "invariant" means.
Thanks! I see the difference. "Time dilation" is definitely frame dependent. "Differential aging" is a comparison between two proper times, which is not frame dependent. I was too casual (lazy?) to correctly understand the term "differential aging".
 
  • #43
I like the term “differential aging” as it does clarify the invariant vs frame variant distinction. As far as I know, however, it is a term that is just used here on PF
 
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  • #44
Dale said:
As far as I know, however, it is a term that is just used here on PF
Google turns up a number of papers by one E. Minguzzi which use the term, although it's not clear that their usage is the same as ours here. I have not seen the term in any textbooks or any of the classic papers in the field.
 
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  • #45
Dale said:
Personally, I consider the word “real” to be a philosophical term which is best avoided altogether. (Except as part of the mathematical term “real number”.)

The adjective "real" can also have the meaning, that time dilation is no accident of convention.
W. Rindler said:
This 'time dilation', like length contraction, is no accident of convention but a real effect. Moving clocks really do go slow. If a standard clock is taken at uniform speed ##v## through an inertial frame ##S## along a straight line from point ##A## to point ##B## and back again to ##A##, the elapsed time ##T_0## indicated on the moving clock will be related to the elapsed time ##T## indicated on the clock fixed at ##A## by the Eq. (21) ...
Source:
http://www.scholarpedia.org/article/Special_relativity:_kinematics#Special_relativistic_kinematics
 
  • #46
Isn't the above in post#45 just a restatement of twin paradox or "differential aging" ?
 
  • #47
Sagittarius A-Star said:
The adjective "real" can also have the meaning, that time dilation is no accident of convention.
As has already been pointed out, the term "time dilation" is ambiguous. The Rindler quote is referring to "time dilation" in the sense of what has been called in this thread "differential aging", and which is invariant and does not depend on any choice of convention. But "time dilation" can also be, and usually is, used to refer to a difference in the relationship of proper time to coordinate time, and that depends on your choice of coordinates, which is a convention.

cianfa72 said:
Isn't the above in post#45 just a restatement of twin paradox or "differential aging" ?
Yes. See above.
 
  • #48
Sagittarius A-Star said:
The adjective "real" can also have the meaning, that time dilation is no accident of convention.
Sabine Hossenfelder used this convention in her (rather poor) "I explain relativity" video. I had no idea it had been used by Rindler. I have to say I don't like it. "Time dilation" and "real time dilation" seems to me like "mass" and "relativistic mass" - an obvious source of confusion.
 
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  • #49
cianfa72 said:
Isn't the above in post#45 just a restatement of twin paradox or "differential aging" ?
Yes. But Rindler uses the twin paradox as argument, that time dilation in general is no accident of convention.
 
  • #50
PeterDonis said:
But "time dilation" can also be, and usually is, used to refer to a difference in the relationship of proper time to coordinate time, and that depends on your choice of coordinates, which is a convention.
In this case it's magnitude depends on a convention, but not it's existence at certain velocities.
 

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