Questions relating to time dilation

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The forum discussion centers on the complexities of time dilation and the relativity of simultaneity as described by Einstein's Special Relativity (SR). Participants explore whether time can be measured without light, concluding that mechanical clocks and the Earth's rotation can serve as alternatives. The conversation highlights that both moving and stationary observers perceive time differently, leading to apparent contradictions, particularly in the context of the twin paradox. Ultimately, the discussion emphasizes that the relativity of simultaneity resolves these contradictions, affirming that time dilation occurs independently of the measuring method used.

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  • Understanding of Special Relativity (SR)
  • Familiarity with the concept of time dilation
  • Knowledge of the relativity of simultaneity
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  • Research "relativity of simultaneity" for deeper insights into time perception in different reference frames.
  • Study the "twin paradox" and its implications in the context of Special Relativity.
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  • #31
salzrah said:
Erland, I don't get why you're trying to blow up what I said. Kind of ironic that you write all that and blame me for over complicating things. I was just trying to paint a visual of what happened and to make sure you saw the video; no where do I discuss the phenomena later as any justification for my argument...Either way, yes the way the lightning happened doesn't matter, only the flashes at the front and back are important...
I am glad we agree about that. I didn't mean to make you upset. Please, accept my apology if you were. I just wanted to point out that there is no point in considering the light originating up in the sky. Since we agree on this, let's continue:
salzrah said:
My answer to the two questions you posted is "same time." Can you explain to me why she sees the signal from the front detector before she sees the signal from the back detector?
If she sees the two signals from the detectors at the same time, mustn't she then also see the two flashes at the same time?
 
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  • #32
Nugatory-
In regards to the third signal C, you say that the light flash/signal is timed so that it reaches the lady's eyes at the same time as flash A. This inherently means that C and A did NOT happen at the same time. In the reference frame of the light flashes themselves, A occurs at a different time than C so they reach the lady's eyes at the same time (which she can calculate and find out didn't actually happen at the same time even though she sees them at the same time).
Because the light flashes are independent of the motion of the train, the man on the ground will see the light flashes at different times as well. He will see flash C, then flash A. When you say -
"a) Platform-guy says A and B happened at the same time, and C happened a bit later.
b) Train-lady says A and C happened at the same time, but a bit after B. "

a) is right, but in b) train-lady should NOT say A and C happened at the same time. Because just as you agreed with me, she can calculate that A and C did not happen at the same time but it was because of her motion she saw it that way. So they are BOTH right.
 
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  • #33
Erland said:
If she sees the two signals from the detectors at the same time, mustn't she then also see the two flashes at the same time?
Yes, but you said she sees the two flashes at different times, correct? I was wondering what your justification is to conclude that she sees the flashes at different times. I agree with this, I just want to see how you're looking at the situation.
 
  • #34
salzrah said:
Yes, but you said she sees the two flashes at different times, correct? I was wondering what your justification is to conclude that she sees the flashes at different times. I agree with this, I just want to see how you're looking at the situation.
Well, it follows from the original setup of the problem, and is explained in the video, that the passenger sees the front flash before she sees the back flash. I thought you agreed on this. Don't you?
 
  • #35
Erland, I think the underlying difference between us is that I am saying the lady/train has motion relative to the reference frame of the platform AND motion relative to the reference frame of the light flashes. I don't think you are accepting that she has motion relative to the origin of the light flashes. Or vis versa -- looking through the reference frame of the light flashes, you're not accepting that the lady is moving.
 
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  • #36
salzrah said:
Yes, but you said she sees the two flashes at different times, correct? I was wondering what your justification is to conclude that she sees the flashes at different times. I agree with this, I just want to see how you're looking at the situation.
Well, it follows from the original setup of the problem, and is explained in the video, that the passenger sees the front flash before she sees the back flash. If you agree on this, we now see that the assumption that she sees the signals from the detectors at the same time leads to a contradiction. How to resolve that?
 
  • #37
But I don't think she sees the signals from the detectors at the same time.
We can agree that the detectors and the light flashes themselves exhibit the same effects, correct?
(Basically, the signal from detectors = flashes of light)
You said she sees the flashes at different times, this means she must see the signals from the detectors at different times as well.
 
  • #38
That video is full of errors. Anybody who doesn't already understand Special Relativity is going to be totally confused if they pay attention to what that video shows and says. People who already understand SR are going to be mesmerized by the fantastic production quality of the video and think that its great, but if they would scrutinize it like a beginner does, they would reject it as having no value whatsoever.

I suggest that you do a search on this forum for the youtube code, and read my comments on the video.
 
  • #39
salzrah said:
But I don't think she sees the signals from the detectors at the same time.
We can agree that the detectors and the light flashes themselves exhibit the same effects, correct?
(Basically, the signal from detectors = flashes of light)
You said she sees the flashes at different times, this means she must see the signals from the detectors at different times as well.
I agree completely. She sees the signals from the detectors at different times (the front one first).

Now, wrt the inertial system of the train, the two signals travel equal distances with the same speed, c. Since they arrive to the passenger at different times, they must also have been sent at different times (relative to the train system). Right?
 
  • #40
But that's the thing, I don't think they travel equal distances because the train/lady is in motion relative to the reference frame of the flashes, which is like I said before the underlying difference between the way we view the situation.
 
  • #41
ghwellsjr said:
That video is full of errors. Anybody who doesn't already understand Special Relativity is going to be totally confused if they pay attention to what that video shows and says. People who already understand SR are going to be mesmerized by the fantastic production quality of the video and think that its great, but if they would scrutinize it like a beginner does, they would reject it as having no value whatsoever.

I suggest that you do a search on this forum for the youtube code, and read my comments on the video.

Will do. However, we aren't really using the conclusions of the video and I don't even remember what the video concludes for that matter. But we are using the situation that is described/presented in the video (and others that we created in this thread). Are you saying the train example has a problem in it itself so that it can't prove the relativity of simultaneity?
 
  • #42
The video is wrong all the way through, not just any "conclusions". It shouldn't be discussed except as a bad example.
 
  • #43
salzrah said:
But that's the thing, I don't think they travel equal distances because the train/lady is in motion relative to the reference frame of the flashes, which is like I said before the underlying difference between the way we view the situation.
When you talk about "the reference frame of the flashes" I assume that you mean the reference frame in which the light sources of the flashes are at rest. The light sources of the flashes would presumably be points in the air where the flashes originate, and these points will be at rest relative to the platform and in motion relative to the train. Let's agree about that.

But you can't say so about the signals from the detectors (which is the reason that I introduced the detectors and the signals). The detectors, which are the sources of the signals, are at rest relative to the train, let's say they are nailed to the front and back walls of the train. Therefore, you can't deny that the signals travel equal distances relative to the frame of the train.
 
  • #44
salzrah said:
Nugatory-
different times as well. He will see flash C, then flash A. When you say -
"a) Platform-guy says A and B happened at the same time, and C happened a bit later.
b) Train-lady says A and C happened at the same time, but a bit after B. "

a) is right, but in b) train-lady should NOT say A and C happened at the same time. Because just as you agreed with me, she can calculate that A and C did not happen at the same time but it was because of her motion she saw it that way. So they are BOTH right.

Why should train-lady not say that A and C happened at the same time? She's at rest so there's no "her motion" to use to calculate any other result. Furthermore, platform-guy agrees with her; he sees his movement towards the back of this at-rest train, and uses his motion to calculate that (using your argument) A and B did not happen at he same time but because of his motion he saw it that way. How is this any more right or wrong than (a) above, where we say that it's the train that's moving forward and the platform is at rest, so train-lady must make be the one who adjust for motion?

I did notice that you used the phrase "In the reference frame of the light flashes themselves" in your post, and this may be contributing to your confusion about how the relativity of simultaneity works. You'll hear people say "In the reference frame of something...", but you have to be aware that this is a just a convenient linguistic shorthand for what they really mean, which is "In a reference frame in which something is at rest...". Thus, there is no "reference frame of the light flashes themselves" because there's no frame in which they are at rest (light always travels at c, right?).
There are two things in this thought experiment that are moving, and can define reference frames: the train and the platform. Thus, we have the train frame, in which the train is at rest and platform is moving backwards; and the platform frame in which the platform is at rest and the train is moving forward. Above, (a) Is the platform-frame description and (b) is the train-frame description. There is absolutely no reason to consider one of them more "right" than the other.
 
  • #45
salzrah said:
But that's the thing, I don't think they travel equal distances because the train/lady is in motion relative to the reference frame of the flashes, which is like I said before the underlying difference between the way we view the situation.

Salzrah, you may find this thread to be helpful:
https://www.physicsforums.com/showthread.php?t=594424&highlight=scorch
 
  • #46
Erland- Even though the detectors are nailed to the train, as soon as the signals leave the detector the signals become independent of the train. As time passes, the train has MOVED to a new position relative to the origin of where the signal first leaves. It is no longer in the same position that it once was when the signals first leave the detectors, so the back light has to travel more than the front light. Yes, the detectors are nailed to the ends of the train but once it sends the signal, the signal propagates through space from where it was first sent.
 
  • #47
ghwellsjr said:
That video is full of errors. Anybody who doesn't already understand Special Relativity is going to be totally confused if they pay attention to what that video shows and says. People who already understand SR are going to be mesmerized by the fantastic production quality of the video and think that its great, but if they would scrutinize it like a beginner does, they would reject it as having no value whatsoever.

I suggest that you do a search on this forum for the youtube code, and read my comments on the video.

Hi
I may be a little slow today but I just watched the video and didn't see any immediate errors.
It appeared to be a straight forward if abbreviated visualization of the essential parameters and logic

What did I miss?

BTW i looked for your comments on the tube without success.
 
  • #49
salzrah said:
Erland- Even though the detectors are nailed to the train, as soon as the signals leave the detector the signals become independent of the train. As time passes, the train has MOVED to a new position relative to the origin of where the signal first leaves. It is no longer in the same position that it once was when the signals first leave the detectors, so the back light has to travel more than the front light. Yes, the detectors are nailed to the ends of the train but once it sends the signal, the signal propagates through space from where it was first sent.
The train has moved, relative to an observer at the platform, that's true. But the train has NOT moved relative to an observer in the train!

Somehow, you consider the reference system of the platform as more fundamental or correct than the reference system of the train. This means that you actually deny the Special Principle of Relativity, one of the two fundamental postulates of SR. Because by that principle, there is no way to distinguish between inertial systems making one more special than another.

You can of course deny this principle if you want to, but all experimental evidence and observations hitherto are in full agreement with it, and all attempts to find a particular "fundamental" inertial system have failed. The most famous experiment of this kind is the one by Michelson and Morley, who tried to detect the motion of the Earth through an assumed immobile "aether". They couldn't detect such a motion.
 
  • #50
Jimmy said:
The comments are in this thread:
https://www.physicsforums.com/showthread.php?t=614735&page=9
https://www.physicsforums.com/showthread.php?t=614735&page=11

Thanks Jimmy yes i see. it was an unfortunate use of the word "see" that was amiss.
Forgetting propagation time as it related to the perception of the track observer.
Since I naturally assumed, through familiarity, the strikes were simultaneous as observed by proximate track observers with watches this slipped by me completely. Certainly a faux pas on their part and I can see that someone encountering the scenario for the first time, and seriously analyzing it, could be confused if they noticed that the track guy couldn't actually receive the light from the strikes and see the train passenger at the midpoint between them simultaneously .
Ah Well
 
  • #51
Nugatory, the lady has motion relative to the origin of the light. And as I said before, this is the underlying thing you and I are disagreeing about. I understand your explanation of inertial reference frames, thanks for that. I will now speak of only ground-frame and train-frame. Let me go through my reasoning one last time leaving out no details.

The train-frame has motion relative to the ground-frame. The lights strike at two separate origins which are both in the train-frame and ground-frame. Now, let's say the lightning flashes are simultaneous in the ground-frame. The lightning flashes hit the origins and the lights propagate to the guy on the ground in the same time interval so he concludes they were simultaneous. The ground-frame has no motion relative to the origins and because the ground guy has no relative movement with the origins he can conclude that the lightning flashes did in fact occur at the same time and it is not because he just happened to SEE this. Now, let's assume for this argument's purpose that in the train-frame the strikes are also simultaneous. The train-frame DOES have movement relative to the origins of the light. So in the train-frame the train moves away from one origin and towards the other. So the distance the light has to travel is less from one origin and more for another, therefore the train lady sees the light flashes at different times but can calculate, using her motion relative to the lights origins, that the light strikes were actually simultaneous.
That was my argument-- that, in fact, BOTH reference frames can conclude the two lightning strikes were simultaneous even though the train-frame lady SEES them at different times.
I think the problem that you guys have with my argument, even though you don't explicitly state this, is that you are saying that the train-frame lady can NOT have motion relative to the origin of the light. Is that correct? If that is what you are saying, can you explain why? ... (although I feel like I already know the answer)
 
  • #52
salzrah said:
Nugatory, the lady has motion relative to the origin of the light.

The train-frame has motion relative to the ground-frame. The lights strike at two separate origins which are both in the train-frame and ground-frame. Now, let's say the lightning flashes are simultaneous in the ground-frame. The lightning flashes hit the origins and the lights propagate to the guy on the ground in the same time interval so he concludes they were simultaneous. The ground-frame has no motion relative to the origins and because the ground guy has no relative movement with the origins he can conclude that the lightning flashes did in fact occur at the same time and it is not because he just happened to SEE this. Now, let's assume for this argument's purpose that in the train-frame the strikes are also simultaneous. The train-frame DOES have movement relative to the origins of the light. So in the train-frame the train moves away from one origin and towards the other. So the distance the light has to travel is less from one origin and more for another, therefore the train lady sees the light flashes at different times but can calculate, using her motion relative to the lights origins, that the light strikes were actually simultaneous.
That was my argument-- that, in fact, BOTH reference frames can conclude the two lightning strikes were simultaneous even though the train-frame lady SEES them at different times.
I think the problem that you guys have with my argument, even though you don't explicitly state this, is that you are saying that the train-frame lady can NOT have motion relative to the origin of the light. Is that correct? If that is what you are saying, can you explain why? ... (although I feel like I already know the answer)

The train lady cannot calculate or consider the strikes as simultaneous for two reasons.

1) Regarding simultaneity any motion relative to the source of light is irrelevant because the speed of propagation is independent of any possible motion.

2) it is simply false because train passengers next to the locations of the light origins will say that by their clocks the lights originated at two different times.

SO in the train frame they are simply NOT simultaneous by any standard.
 
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  • #53
The basis she uses to consider herself in motion relative to the origins is looking at the origins, which we can say are on the ground/air where the flash occurred, and seeing that those origins are moving away from her.
 
  • #54
salzrah said:
I think the problem that you guys have with my argument, even though you don't explicitly state this, is that you are saying that the train-frame lady can NOT have motion relative to the origin of the light. Is that correct? If that is what you are saying, can you explain why? ... (although I feel like I already know the answer)
What IS this "origin of light"?

You cannot mean the light source(s), for in my example with the detectors and signals, the train-frame lady are at rest relative to the light sources (the detectors). No, you seems to mean that at the moment a signal leaves a detector, it somehow reaches its "origin", which, by some reason, is at rest relative to the ground-frame.

The ground-frame must therefore have the property that all "origins of light" are at rest relative to this frame, a property that no other inertial frame has. This blatantly contradicts the Special Principle of Relativity. So you deny this principle, as I wrote in my previous post.
 
  • #55
salzrah said:
The basis she uses to consider herself in motion relative to the origins is looking at the origins, which we can say are on the ground/air where the flash occurred, and seeing that those origins are moving away from her.

This is not the right way to look at it. Those locations relative to the track frame are moving away from her but they are not moving relative to her.
For instance ; Even if they did not occur right at the ends of the train but instead happened one foot in front and one foot behind the ends of the train. Those locations do not change in the train frame.
The front origin remains one foot in front of the train no matter how far from the location in the track frame.
Events have a singular location in space and time relative to any frame.
So a flash event. The origin of a light , has a single location t'=?,x'=?,y'=?,z'=? in the train frame whatever the specific coordinates may be, and this is fixed.
 
  • #56
The origin of light is the POSITION where the light comes out from. The detector source is at rest with respect to the train-frame BUT that is not the origin of the light. The origin of the light is a position, like x=4. The train-frame lady can see the lightning strike at x=4 and see that this position is moving away from her as her position in the x-axis increases. The train-frame lady is moving relative to this position of the origin at x=4.
 
  • #57
Austin0 said:
This is not the right way to look at it. Those locations relative to the track frame are moving away from her but they are not moving relative to her.
.

The positions ARE moving away from her because she is moving away from those positions. If there is a stationary ball at x=4, the train-frame lady will see this ball move away from her as the train moves in the positive x-direction.
 
  • #58
salzrah said:
The origin of light is the POSITION where the light comes out from. The detector source is at rest with respect to the train-frame BUT that is not the origin of the light. The origin of the light is a position, like x=4. The train-frame lady can see the lightning strike at x=4 and see that this position is moving away from her as her position in the x-axis increases. The train-frame lady is moving relative to this position of the origin at x=4.
But if we put an x-axis inside the train, which is at rest relative to the train, then the x-coordinate on that axis of the point where the light comes out from the detector does NOT move relative to train-lady.

We all the time come back to the fact that you consider the ground-frame as privileged compared to the the train-frame, and this contradicts the Special Principle of Relativity.
 
  • #59
salzrah said:
The origin of light is the POSITION where the light comes out from. The detector source is at rest with respect to the train-frame BUT that is not the origin of the light. The origin of the light is a position, like x=4. The train-frame lady can see the lightning strike at x=4 and see that this position is moving away from her as her position in the x-axis increases. The train-frame lady is moving relative to this position of the origin at x=4.

salzrah said:
The positions ARE moving away from her because she is moving away from those positions. If there is a stationary ball at x=4, the train-frame lady will see this ball move away from her as the train moves in the positive x-direction.

you are confused regarding coordinate systems.

Forget detectors. Someone sets of a flashbulb on the platform at x= 4 (track frame)as the lady is right next to it.
She is then moving away from the burnt out flash bulb at x=4 but the event of the flash itself remains right next to her at whatever coordinates she assigns relative to the train car.
This is the location of the flash event (origin of the expanding light sphere) in her frame.
Remember this is not the location of anything physical like your ball at x=4. Once the light begins propagating there is nothing there but an abstract coordinate location.
 
  • #60
Erland said:
But if we put an x-axis inside the train, which is at rest relative to the train, then the x-coordinate on that axis of the point where the light comes out from the detector does NOT move relative to train-lady.

We all the time come back to the fact that you consider the ground-frame as privileged compared to the the train-frame, and this contradicts the Special Principle of Relativity.

Yes, that is true. But the point of the light origin IS NOT MOVING with the train. It stays at the position where it was. So while the train moves relative to the point, even though the x-axis is at rest wrt the train, the x-axis is not at rest with respect to the point. Okay, can you tell me if you can represent the point of where the light originates as a ball? Or any object? It will help me in making an example if you agree with that...
 
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