What caused us to have relative time

[spaced-out]

If you had to give a physical reason for relative time, what would it be? (I will not be giving my view; just looking for yours.)

 

[dauto]

It's the same reason that my width is relative. (I look wider in front view than in profile)

 

[A.T.]

If you had to give a physical reason for relative time, what would it be?

The lack of reasons for the opposite.

 

[spaced-out]

What about the following:

"Different observers at rest in their respective frames disagree over the time interval between two events because they calculate the different in the readings of two clocks at rest relative to themselves. THE LACK OF AN ABSOLUTE SYNCHRONIZATION for these clocks causes the variation in delta-t from observer to observer."
[introduction to the theory of relativity by Sears & Brehme, Addison-Wesley, p. 87]

But this raises the question For what reason are clocks not absolutely synchronous? Anybody?

 

[Nugatory]

But this raises the question For what reason are clocks not absolutely synchronous? Anybody?

You're finding out why we sometimes become annoyed by "why...?" questions.

That's OK, we've all been there... In this case, the laws of electricity and magnetism predict, and experiment confirms, that light behaves in a way that is inconsistent with absolute simultaneity. But now we have to ask why the laws of E&M are that way, and so on, and on. Eventually we all figure out that the only way of terminating the infinite chain of "why..."" questions is by answering with some variant of "Because that's the way the universe we live in behaves".

Physics is about understanding, describing, and predicting how the universe works, not why it works that way. Even when someone gives you a fine mathematical explanation (for example, of how Newton's law of gravitation plus a bunch of math says that the planets will orbit the sun)... You have to remember that we chose the math because it matched how the universe was observed to work, so it's not really answering the "why" question.

 

[ghwellsjr]

But this raises the question For what reason are clocks not absolutely synchronous? Anybody?

Your question cannot be answered until and unless you are willing to define what a clock is. In Special Relativity, we define a clock to be an instrument that measures time (or time is what a clock measures). As a result, we find that since accelerating clocks results in them disagreeing about time, then we have to accept the fact that time is relative.

So, if you are unwilling to accept the definition of a clock according to Special Relativity, then you have to come up with another definition for which time can be absolute. Since you said that you will not be giving your view, then I think it is a little unfair for you to ask a question that most of us realize cannot be answered (because you won't give us your definition of a clock for which the question can have any meaning).

 

[Dale]

If you had to give a physical reason for relative time, what would it be?

The standard reason is that time is relative because the laws of physics are relative and the invariant speed is finite. I.e. the physical reason is that the two postulates are correct.

 

[Ookke]

Constant light speed (invariant speed) with all its consequences like relative time *is* weird, and it's perfectly legitimate question to ask "why".

Could it be so that an universe with more reasonable properties (e.g. where light behaves ballistically and inherits the emitters speed, as it maybe should by common sense) would lead to consequences that are impossible to formation or life. Simply put: we wouldn't be here, if light behaved more reasonable way?

Universe seems to be willing to twist space and time in very peculiar way, just to keep light speed constant in every inertial frame. Why it's so important?

 

[PAllen]

Universe seems to be willing to twist space and time in very peculiar way, just to keep light speed constant in every inertial frame. Why it's so important?

Because it's what we observe?

 

[Dale]

Constant light speed (invariant speed) with all its consequences like relative time *is* weird, and it's perfectly legitimate question to ask "why".

It is perfectly legitimate, but also usually non-scientific. What sort of experiment could you build to measure why the speed of light is invariant? All you can measure is whether or not it is invariant. There is no "why-ometer".

The reason that I said that it is usually non-scientific is that it is possible to have two different theories that each lead to the invariance of c in some limit. Then you could measure which theory is more accurate. Of course, as any 3-year-old child knows, you can then simply ask "why" again, this time about the postulates of that new theory.

 

[spaced-out]

why is the universal expansion increasing is a fair question and has been answered to a degree with the postulation of dark energy - the supposed physical cause. similarly, i am wondering about the direct physical cause for the relativity of simultaneity or relative time. To Wells: Why did you ask me what a clock is instead of asking Sears & Brehme? To Dale: what - in your opinion - are the 2 postulates? To Dale again: Why are you so sure that lightspeed invariance is the reason for relative time? Sears blamed it on asynchronous (nonabsolutely synch'd) clocks. How could an invariant light speed cause clocks to be out of synch? Just asking.

 

[PAllen]

What about the following:

"Different observers at rest in their respective frames disagree over the time interval between two events because they calculate the different in the readings of two clocks at rest relative to themselves. THE LACK OF AN ABSOLUTE SYNCHRONIZATION for these clocks causes the variation in delta-t from observer to observer."
[introduction to the theory of relativity by Sears & Brehme, Addison-Wesley, p. 87]

But this raises the question For what reason are clocks not absolutely synchronous? Anybody?

Because they can't be. Because there is no simultaneity built into the universe. Inability to synchronize clocks in an absolute way is just different words for "there is no such thing as absolute simultaneity".

 

[PAllen]

why is the universal expansion increasing is a fair question and has been answered to a degree with the postulation of dark energy - the supposed physical cause. similarly, i am wondering about the direct physical cause for the relativity of simultaneity or relative time. To Wells: Why did you ask me what a clock is instead of asking Sears & Brehme? To Dale: what - in your opinion - are the 2 postulates? To Dale again: Why are you so sure that lightspeed invariance is the reason for relative time? Sears blamed it on asynchronous (nonabsolutely synch'd) clocks. How could an invariant light speed cause clocks to be out of synch? Just asking.

Dark Energy is just a name for the observations. Whether it needs to be explained depends on your theory - specifically whether you have a theory which predicts vacuum energy that can be large and should act gravitationally. If you have a theory without either of these features, there is nothing to explain (e.g. in classical GR there is nothing to explain because the cosmological constant is a constant of integration and the theory gives no basis to select any particular value for it).

Similarly, for your question, you would have to have theoretical framework where space and time emerge from something more primitive. Without such a framework, there is no possible scientific answer.

To look at the issue another way, turn the why around. Why would you think "now" has a unique meaning across the universe? That would seem to need at least as much explanation as the converse. This shows that all such questions are meaningless unless you have theoretical framework that consistently encompasses both answers.

 

[ghwellsjr]

To Wells: Why did you ask me what a clock is instead of asking Sears & Brehme?

Because they are obviously accepting the definition of time being what a clock measures. They are pointing out that if you started with two colocated inertial clocks they would agree on all measured times intervals but if one was accelerated for awhile and became inertially moving with respect to the other clock, then they would no longer agree on measured time intervals. It's not because of any theory that this happens, it's a fact of nature.

Prior to Einstein, scientists were unwilling (or more likely it didn't occur to them) to accept the notion of relative time. They believed that time was absolute in the sense that there could exist in principle some clocks in an inertial state that would measure this absolute time, that is, there would be no deviation between the time intervals measured on these clocks and the time intervals that nature was operating on. Thus, when they detected deviations in the times measured by their clocks that were inertial but moving with respect to each other (or the same clocks but taking measurements at different times after having been accelerated) they attributed this phenomenon to the inability of their clocks to actually measure time. In other words, they did not believe that clocks were actually measuring time, they needed fudge factors applied to them to make their readings correspond to the parameter being measured, just like a clock might need a temperature coefficient applied to improve its accuracy over a wide temperature range.

But Einstein was apparently the first to realize that if we are willing to give up the notion of an absolute time, then we can define time in terms of what inertial clocks are actually measuring. Sears & Brehme accept Einstein's notion that all inertial clocks measure time correctly and so they agree that we have to give up the notion that there exists an absolute time. Since you are asking the question, "For what reason are clocks not absolutely synchronous", it's obvious that you are rejecting Einstein's definition of time as being what inertial clocks measure so you are using a different definition of what a clock is but you won't tell us what that definition is so how can we provide you with an answer to your question? Or to put it another way, if inertial clocks are not actually measuring time, then what do those clocks measure or what fudge factors are you applying so that they can actually measure time or how would you otherwise measure time?

 

[Dale]

To Dale: what - in your opinion - are the 2 postulates?

It isn't a matter of opinion. The two postulates of relativity are that the laws of physics are frame invariant and that c is invariant.

Why are you so sure that lightspeed invariance is the reason for relative time?

Einstein proved it in his famous paper in 1905. The two postulates imply the relativity of simultaneity, time dilation, length contraction, and all of the other relativistic effects.

The physical cause of any relativistic effect is therefore the fact that the laws of physics follow the two postulates.

Why are you so sure that lightspeed invariance is the reason for relative time? Sears blamed it on asynchronous (nonabsolutely synch'd) clocks. How could an invariant light speed cause clocks to be out of synch? Just asking.

Sears isn't wrong, nor is he disagreeing with me. The two postulates lead to asynchronous clocks, usually referred to as the relativity of simultaneity. As I mentioned already this was proved by Einstein in 1905.

 

[spaced-out]

... there is no simultaneity built into the universe. Inability to synchronize clocks in an absolute way is just different words for "there is no such thing as absolute simultaneity".

Evidence?
(& bear in mind that such a negative cannot be proved)

 

[spaced-out]

... But Einstein was apparently the first to realize that if we are willing to give up the notion of an absolute time, then we can define time in terms of what inertial clocks are actually measuring. Sears & Brehme accept Einstein's notion that all inertial clocks measure time correctly and so they agree that we have to give up the notion that there exists an absolute time. Since you are asking the question, "For what reason are clocks not absolutely synchronous", it's obvious that you are rejecting Einstein's definition of time as being what inertial clocks measure so you are using a different definition of what a clock is but you won't tell us what that definition is so how can we provide you with an answer to your question? Or to put it another way, if inertial clocks are not actually measuring time, then what do those clocks measure or what fudge factors are you applying so that they can actually measure time or how would you otherwise measure time?

How can asynchonous clocks measure time correctly?

 

[spaced-out]

It isn't a matter of opinion. The two postulates of relativity are that the laws of physics are frame invariant and that c is invariant.

Einstein proved it in his famous paper in 1905. The two postulates imply the relativity of simultaneity, time dilation, length contraction, and all of the other relativistic effects.

The physical cause of any relativistic effect is therefore the fact that the laws of physics follow the two postulates.

I have seen various versions of the postulates, so that is why i asked for your opinion.

As for those you listed, does the 2nd include the speed of light from point A to point B in an inertial coord. system? If so, then exactly how did this speed come to be c for any or all such observers?

 

[Nugatory]

Evidence?
(& bear in mind that such a negative cannot be proved)

Although experiment cannot prove a negative mathematical logic can, subject of course to the assumption that the postulates we start with are correct. (The proof that there is no largest prime is one of many such proofs of negatives).

It is a fairly straightforward exercise, using no more than high school algebra, to show that if Einstein's two postulates are correct, then there can be no such thing as absolute simultaneity.
(unless you have a different definition of absolute simultaneity, of course, in which case that's the conversation we should be having).

 

[Nugatory]

I have seen various versions of the postulates, so that is why i asked for your opinion.

Although there are various wordings floating around (it would be well to remember that anything you read in English is either a translation from the original German, or influenced by the subsequent development of the theory, or both) they are all equivalent enough to lead to the same conclusions. You don't have to take my word for it, you can start with Einstein's 1905 paper "On the electrodynamics of moving bodies" (google will find it), see how Einstein first worded them, compare with later versions.

As for those you listed, does the 2nd include the speed of light from point A to point B in an inertial coord. system? If so, then exactly how did this speed come to be c for any or all such observers?

Well, that IS one of the postulates [everyone else... please, please, please don't open up the one-way/two-way rathole here... please?], but there is a pretty good heuristic behind it:

We can calculate the speed of light just from the laws of electricity and magnetism, as Maxwell formulated them in 1861. Therefore, if different inertial observers measure a different speed of light because of their relative motion, they must necessarily be subject to different laws of electricity and magnetism because of their relative motion. Not only would this violate the other postulate (same laws of physics for everybody), it also goes against an enormous body of experimental evidence.

 

[Dale]

As for those you listed, does the 2nd include the speed of light from point A to point B in an inertial coord. system? If so, then exactly how did this speed come to be c for any or all such observers?

The postulates are the things which explain everything else in relativity. The postulates themselves are not explained, that is what it means for something to be a postulate.

So "what is the physical cause of relative time" is a question whose answer is that "the laws of physics follow the two postulates". But "what is the physical cause of the two postulates" is a different question whose answer is unknown at this time.

 

[Nugatory]

How can asynchonous clocks measure time correctly?

Easily, as long as you don't require absolute simultaneity. My wristwatch ticks off five minutes while the sand runs through the five-minute sandglass on the table in front of me, and I become five minutes older and grayer. You can do the same thing wherever you are with your wristwatch and sandglass. For both of us, our clocks "just work", we have a perfectly good notion of the passing of time, and our wristwatches tell us about it.

We only have a problem if we also insist on a notion of absolute simultaneity, by requiring that the start of my five-minute interval must be simultaneous with the start of yours, and likewise for the ends of those two intervals.

BTW, have you studied Einstein's train though experiment on the relativity of simultaneity? Understanding that one is essential to understanding and resolving the sort of logical contradictions that come with assuming absolute simultaneity and absolute time.

 

[Ookke]

It is perfectly legitimate, but also usually non-scientific. What sort of experiment could you build to measure why the speed of light is invariant? All you can measure is whether or not it is invariant. There is no "why-ometer".

The reason that I said that it is usually non-scientific is that it is possible to have two different theories that each lead to the invariance of c in some limit. Then you could measure which theory is more accurate. Of course, as any 3-year-old child knows, you can then simply ask "why" again, this time about the postulates of that new theory.

We need to eventually stop asking "why" and just take some postulates, that I can agree. Sadly, the postulate about light speed constancy is (to me at least) very far from being logical, natural or easy to accept. Maybe it "just is" the way how universe works and there is nothing more fundamental behind that, but I hope that some day there is a new theory, based on logical and obviously true sounding postulates, that also includes and explains the results of relativity.

 

[haael]

The simple philosophical answer to all the questions in this thread is: time is another dimension. Just like width and height and length.

We really should start with asking "what the heck is time". Then we get answers for all other problems, including the relativity of simultaneity and invariance of the speed of light.

It is really that simple: the universe is a 4-dimensional manifold, homogenous, isotropic, smooth, pseudo-Riemannian and curved. Everything else is just a consequence.

 

[PAllen]

The simple philosophical answer to all the questions in this thread is: time is another dimension. Just like width and height and length.

We really should start with asking "what the heck is time". Then we get answers for all other problems, including the relativity of simultaneity and invariance of the speed of light.

It is really that simple: the universe is a 4-dimensional manifold, homogenous, isotropic, smooth, pseudo-Riemannian and curved. Everything else is just a consequence.

That won't really satisfy anyone. Why psuedo-riemannian manifold? Why not Euclidean 3 space x time, a la Newton? Ultimately, the only answer is theories based on the former match observation, while theories based on the latter do not.

 

[ghwellsjr]

... But Einstein was apparently the first to realize that if we are willing to give up the notion of an absolute time, then we can define time in terms of what inertial clocks are actually measuring. Sears & Brehme accept Einstein's notion that all inertial clocks measure time correctly and so they agree that we have to give up the notion that there exists an absolute time. Since you are asking the question, "For what reason are clocks not absolutely synchronous", it's obvious that you are rejecting Einstein's definition of time as being what inertial clocks measure so you are using a different definition of what a clock is but you won't tell us what that definition is so how can we provide you with an answer to your question? Or to put it another way, if inertial clocks are not actually measuring time, then what do those clocks measure or what fudge factors are you applying so that they can actually measure time or how would you otherwise measure time?

How can asynchonous clocks measure time correctly?

Do you see the words in bold?

Please have a look at article 1 of Einstein's 1905 paper entitled "§ 1. Definition of Simultaneity". Notice how many times he uses the word defined or definition.

Inertial clocks are defined to measure time correctly according to Einstein in his theory of Special Relativity. It's no more complicated than that. As I said before, you don't have to accept Einstein's theory to explain what's going on in the universe, you can believe in absolute time and that our clocks don't measure that absolute time and maybe this is your view that you won't give us but you shouldn't keep asking the same question unless you tell us your definition of time and/or clocks and I hope it comports with reality.

 

[MuIotaTau]

What about the following:

"Different observers at rest in their respective frames disagree over the time interval between two events because they calculate the different in the readings of two clocks at rest relative to themselves. THE LACK OF AN ABSOLUTE SYNCHRONIZATION for these clocks causes the variation in delta-t from observer to observer."
[introduction to the theory of relativity by Sears & Brehme, Addison-Wesley, p. 87]

But this raises the question For what reason are clocks not absolutely synchronous? Anybody?

They simply aren't. If you create two clocks and you look closely enough, you'll see that they don't match up. That's simply a fact about our universe. You could also conclude this from other simple facts about our universe, such as that the speed of light is a constant in all reference frames, but then you could just as easily ask why the speed of light is a constant in all reference frames, and I would simply tell you once more that it simply is.

 

[Dale]

Sadly, the postulate about light speed constancy is (to me at least) very far from being logical, natural or easy to accept.

I agree that the second postulate is not easy to accept. However, there really is no reason to expect that the way the universe behaves should be easy to accept. So in the end that is simply something that we will have to struggle with.

It certainly is logical (no self-contradiction) and natural (nature behaves this way), so I don't know what would lead you to use those words in your objection.

I hope that some day there is a new theory, based on logical and obviously true sounding postulates, that also includes and explains the results of relativity.

I am fairly confident that there will eventually be another theory which can be used to derive the postulates of relativity in some limit and that theory will therefore scientifically explain "why" relativity. However, I would bet a rather large sum that the postulates of that theory will be even harder to accept than the second postulate, not easier. They will be accepted, not because they will be easy to accept, but because future experimental data will require us to accept them.

 

[spaced-out]

It is a fairly straightforward exercise, using no more than high school algebra, to show that if Einstein's two postulates are correct, then there can be no such thing as absolute simultaneity.

Well, the 1st post. does not pertain to simultaneity at all, it merely says that all inertial observers must find the same general laws. And the 2nd post. says that such observers must all get c for light's speed from Point A to Point B in their respective frames, but I fail to see this happening experimentally or even how it could happen.

OTOH, any pair of clocks in any inertial frame could possibly become absolutely synch'd by sheer accident (or by monkeys pushing buttons), and yet this would not in any way violate any physical law, so said clocks won't self-destuct or destroy the universe. They are not forbidden.

If you read Einstein, you will see that he merely recommended *discarding* absolutely synch'd clocks, but never said that they cannot actually exist.

 

[spaced-out]

Easily, as long as you don't require absolute simultaneity. My wristwatch ticks off five minutes while the sand runs through the five-minute sandglass on the table in front of me, and I become five minutes older and grayer. You can do the same thing wherever you are with your wristwatch and sandglass. For both of us, our clocks "just work", we have a perfectly good notion of the passing of time, and our wristwatches tell us about it.

We only have a problem if we also insist on a notion of absolute simultaneity, by requiring that the start of my five-minute interval must be simultaneous with the start of yours, and likewise for the ends of those two intervals.

BTW, have you studied Einstein's train though experiment on the relativity of simultaneity? Understanding that one is essential to understanding and resolving the sort of logical contradictions that come with assuming absolute simultaneity and absolute time.

Yes, I have indeed read and reread the train ex., and have the following complaint: It demands (with no justification) that we use clocks at events to report on their simultaneity status, with the knowledge that said clocks are not themselves truly synch'd., or that we must use light rays from the events while ignoring our motion toward or away from said rays. (Einstein even mentioned such motion in his train ex., but said that it is OK to ignore it. But it matters to me if diff. obs. move diff.ly wrt light rays whilst trying to use the rays to determine distant simultaneity.)

In short, E had no justification for his relative simultan. as far as I can see. Do you see any?

 

[Dale]

In short, E had no justification for his relative simultan. as far as I can see. Do you see any?

Of course he had justification. The two postulates. Not only do they justify it, they require it.

 

[ghwellsjr]

Well, the 1st post. does not pertain to simultaneity at all, it merely says that all inertial observers must find the same general laws. And the 2nd post. says that such observers must all get c for light's speed from Point A to Point B in their respective frames, but I fail to see this happening experimentally or even how it could happen.

The 2nd postulate does not say that observers must all get c for light's speed between two points, it defines it.

OTOH, any pair of clocks in any inertial frame could possibly become absolutely synch'd by sheer accident (or by monkeys pushing buttons), and yet this would not in any way violate any physical law, so said clocks won't self-destuct or destroy the universe. They are not forbidden.

What is your definition of "absolutely synch'd"? Are you going to continue to keep it a secret? How can we respond if you won't give us your view?

If you read Einstein, you will see that he merely recommended *discarding* absolutely synch'd clocks, but never said that they cannot actually exist.

I don't recall Einstein ever saying that. Could you please provide an online link and either specify the exact location in the material where he said those words and/or provide an exact quote?

 

[spaced-out]

Of course he had justification. The two postulates. Not only do they justify it, they require it.

Mr. Dale, can you tell me how the postulates justify using light rays from two events to determine their temporal order? Thanks!

 

[spaced-out]

The 2nd postulate does not say that observers must all get c for light's speed between two points, it defines it.

Exactly how?

What is your definition of "absolutely synch'd"? Are you going to continue to keep it a secret? How can we respond if you won't give us your view?

My def is the same as E's:
"[In classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

I don't recall Einstein ever saying that. Could you please provide an online link and either specify the exact location in the material where he said those words and/or provide an exact quote?

"Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e. that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light in vacuo and the principle of relativity (developed in Section VII) disappears."

http://www.bartleby.com/173/9.html

 

[Nugatory]

Mr. Dale, can you tell me how the postulates justify using light rays from two events to determine their temporal order? Thanks!

If you accept the postulate of the constant speed of light, then you can determine the temporal order from the distance and the light travel time: If a light signal is emitted from a point 5 light-seconds away from me and it reaches me at time N, I can infer that the emission event occurred at time N-5. This let's us make statements like "Emission event A happened at a point 10 light-seconds away and the light reached me at time 12; Emission event B happened at a point 4 light seconds away from me and the light reached me at time 7; Therefore B happened one second after A". That's temporal ordering, based on the postulate of the constancy of the speed of light.

[Edit - this is also the justification for using the light arrival times as the basis of the "which happened first" analysis in the train experiment on relativity of simultaneity. The train experiment is simplified a bit by placing the observers in the middle so that the distances between both events and the observer is equal, but you can go back and do the numerical calculations in terms of the separation distance, and you'll get the same outcomes]

 

[Nugatory]

And the 2nd post. says that such observers must all get c for light's speed from Point A to Point B in their respective frames, but I fail to see this happening experimentally or even how it could happen.

There are several arguments in favor of this constant speed of light postulate.

One is the argument from the laws of electricity and magnetism that I gave back in post #20. Constructing a sensible physics around the prediction of a constant velocity equal to c was the big unsolved problem of physics during the second half of the 19th century, and the reason why the title of the 1905 paper was "On the electrodynamics of moving bodies".

The second is the experimental evidence, starting from the Michelson Morley experiments: Try this section of the FAQ on experimental confirmation of special relativity.

The third is that when we carry these postulates to their logical conclusion, we find experimental testable predictions that have been tested, have been confirmed to the limits of experimental accuracy, and that cannot be explained by any other currently known theory. For that, you can refer to other sections of that FAQ.

 

[Integral]

We need to eventually stop asking "why" and just take some postulates, that I can agree. Sadly, the postulate about light speed constancy is (to me at least) very far from being logical, natural or easy to accept. Maybe it "just is" the way how universe works and there is nothing more fundamental behind that, but I hope that some day there is a new theory, based on logical and obviously true sounding postulates, that also includes and explains the results of relativity.

To address this you do not look to Einstein and relativity. Instead look to Maxwell. He predicted the constancy of the speed of light in the 1860's. Physicists spend the next 40yrs trying to show him wrong. They couldn't. That is why Einstein was able to postulate it in 1905.

 

[Dale]

Mr. Dale, can you tell me how the postulates justify using light rays from two events to determine their temporal order? Thanks!

If you know the speed of a signal and the distance it traveled to reach you then you know when it was emitted. The second postulate asserts that you know the speed of a light signal.

 

[ghwellsjr]

Well, the 1st post. does not pertain to simultaneity at all, it merely says that all inertial observers must find the same general laws. And the 2nd post. says that such observers must all get c for light's speed from Point A to Point B in their respective frames, but I fail to see this happening experimentally or even how it could happen.

The 2nd postulate does not say that observers must all get c for light's speed between two points, it defines it.

Exactly how?

If you had read the article that I referenced in post #26, you'd already know the answer to your question. Here's the process:

1) Measure the distance between the two points with your ruler.

2) Place a mirror at the second point so that a flash of light emitted from the first point will reflect back to the first point.

3) Observe the time on a clock located at the first point when a flash of light is emitted from the first point and aimed at the mirror located at the second point.

4) Observe the time on the clock when the reflection of the flash off the mirror at the second point reaches back to the first point where the clock is located.

5) Double the distance measured in step 1 and divide it by the difference between the two observed clock times from steps 3 and 4. This is the measured value of c.

6) Divide the measured value of c from step 5 into the distance measured in step 1 to get the propagation time for light to go from the first point to the second point in order to fulfill the defined propagation speed of c.

7) Replace the mirror at the second point with a second clock.

8) Observe the time on the clock located at the first point when a flash of light is emitted from the first point and aimed at the second point.

9) Observe the time on the clock located at the second point when the flash of light reaches it.

10) Subtract the observed time on the first clock from the observed time on the second clock and compare the difference to the propagation time determined in step 6.

11) If the comparison is not zero, adjust the time on the second clock to get it closer to zero and repeat steps 8, 9 and 10 until the comparison is zero.

Is that exact enough for you?

OTOH, any pair of clocks in any inertial frame could possibly become absolutely synch'd by sheer accident (or by monkeys pushing buttons), and yet this would not in any way violate any physical law, so said clocks won't self-destuct or destroy the universe. They are not forbidden.

What is your definition of "absolutely synch'd"? Are you going to continue to keep it a secret? How can we respond if you won't give us your view?

My def is the same as E's:
"[In classical physics] [t]he simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all [other] inertial systems. This is what is meant when we say that the time of classical physics is absolute." [Einstein's book _Relativity_, p. 149]

Here's a link to your quote (it's about three quarters of the way down in the section entitled "The Field"):

http://www.relativitybook.com/resources/Einstein_space.html

And the very next sentence is:

According to the special theory of relativity it is otherwise.

And why is it otherwise? Because classical physics doesn't comport with reality. In post #26, I asked you for your definition of time that would comport with reality and you say it's the same as what classical physics defined. Your comment that "any pair of clocks in any inertial frame could possibly become absolutely synch'd by sheer accident" is not true as Einstein points out in this article. Don't you read the whole thing or do you like taking quotes out of context? Don't you understand that in classical physics, all clocks in all inertial frames can be absolutely synch'ed, no matter what their state of motion or acceleration?

But now that we know that classical physics does not comport with reality precisely because that concept is not true. At best you could define or assert the existence of a single inertial frame in which all stationary clocks are absolutely synch'd but those clocks would not be synch'd in another inertial frame moving with respect to the original one. Do you disagree with this statement?

So if your definition of "absolutely synch'd" is the same as Einstein's, then you must realize that his definition is for the purpose of showing that it is not adequate.

Do you want to try again?

If you read Einstein, you will see that he merely recommended *discarding* absolutely synch'd clocks, but never said that they cannot actually exist.

I don't recall Einstein ever saying that. Could you please provide an online link and either specify the exact location in the material where he said those words and/or provide an exact quote?

"Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e. that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light in vacuo and the principle of relativity (developed in Section VII) disappears."

http://www.bartleby.com/173/9.html

This quote simply affirms the previous quote--that absolutely synch'd clocks are independent of motion in classical physics. But now we know this is not true. You cannot have all clocks absolutely synch'd. Do you disagree with this statement?

 

[spaced-out]

If you know the speed of a signal and the distance it traveled to reach you then you know when it was emitted. The second postulate asserts that you know the speed of a light signal.

But the one-way speed of light is based on E's def of simultan., which, in turn, is based on assuming that we can use light rays from events to determine their order.

 

[spaced-out]

At best you could define or assert the existence of a single inertial frame in which all stationary clocks are absolutely synch'd but those clocks would not be synch'd in another inertial frame moving with respect to the original one. Do you disagree with this statement?

(FYI: Your clock synch method unjustifiably assumes equal 1-way light speeds)
(FYI: The only reason E gave for discarding truly synch'd clocks was that they were "incompatible with the most natural def. of sim., but this def. assumes that it is OK to use light rays from events to determine their order, whilst totally ignoring different frame movement as the rays approach.)
(FYI: The overall purpose for the creation of SR was to work around the existing conflict btn the PR and the simple light law (the fact that light's speed in space is c constantly due mainly to light's source independency), but this conflict is bogus because the PR never denied absolute motion detect. since it alllows the law w = c - v for all observers.)

As for your above, just as with E synch., each frame's clocks could be independently truly synch'd (given a method for this), and they would remain that way unless a frame accelerated of course, but inert frames don't do this, so we are OK. It does not matter that one frame's clocks would not be synch'd with anothers because all that matters is that any given frame can use their truly synch'd clocks to measure its abs. spd. (as per the above simple equation that was given by E himself). After this, each frame can adjust for clock slowing and ruler contraction because the equations for this are known. And each frame can correctly meas. event order and temporal separation.

There was absolutely no reason whatsoever to abandon truly synch'd clocks (other than the current lack of a method for getting them that way, but this can change overnight).

 

[ghwellsjr]

At best you could define or assert the existence of a single inertial frame in which all stationary clocks are absolutely synch'd but those clocks would not be synch'd in another inertial frame moving with respect to the original one. Do you disagree with this statement?

(FYI: Your clock synch method unjustifiably assumes equal 1-way light speeds)

FYI: It's not my clock synch method, it's commonly attributed to Einstein. Einstein's postulate that all 1-way light speeds are equal does not need any justification other than that it comports with reality. Do you deny that it comports with reality?

(FYI: The only reason E gave for discarding truly synch'd clocks was that they were "incompatible with the most natural def. of sim., but this def. assumes that it is OK to use light rays from events to determine their order, whilst totally ignoring different frame movement as the rays approach.)

If you're going to talk about "truly synch'd clocks", you're going to have to define what you mean. And I don't know what you mean by "totally ignoring different frame movement as the rays approach".

(FYI: The overall purpose for the creation of SR was to work around the existing conflict btn the PR and the simple light law (the fact that light's speed in space is c constantly due mainly to light's source independency), but this conflict is bogus because the PR never denied absolute motion detect. since it alllows the law w = c - v for all observers.)

You're right, the conflict was bogus but not for the reason you state. There simply was no conflict, it was only apparent, as Einstein pointed out in his 1905 paper.

As for your above, just as with E synch., each frame's clocks could be independently truly synch'd (given a method for this), and they would remain that way unless a frame accelerated of course, but inert frames don't do this, so we are OK. It does not matter that one frame's clocks would not be synch'd with anothers because all that matters is that any given frame can use their truly synch'd clocks to measure its abs. spd. (as per the above simple equation that was given by E himself). After this, each frame can adjust for clock slowing and ruler contraction because the equations for this are known. And each frame can correctly meas. event order and temporal separation.

There was absolutely no reason whatsoever to abandon truly synch'd clocks (other than the current lack of a method for getting them that way, but this can change overnight).

Ok, let it change overnight. Let's hear your method. That's what I've been asking for from the very beginning. I expect an answer in 24 hours (based on GPS time). And make sure it comports with reality.

 

[Dale]

But the one-way speed of light is based on E's def of simultan., which, in turn, is based on assuming that we can use light rays from events to determine their order.

That isn't a big problem.

If you have a system of formal logic with some set of axioms, A, and some set of theorems, T, then it is almost always possible to pick some subset of T, A', such that from A' you can derive A. In other words, it is almost always possible to switch which statements you consider to be assumed postulates and which you consider derived consequences.

The usual approach to SR considers the speed of light to be a postulate, and the relativity of simultaneity follows from that. You certainly can construct equivalent alternate axiomatizations of relativity where the speed of light is a derived consequence of the simultaneity convention. They are logically equivalent, so all you have to do is to pick either one you prefer and use it.

 

[spaced-out]

FYI: It's not my clock synch method, it's commonly attributed to Einstein. Einstein's postulate that all 1-way light speeds are equal does not need any justification other than that it comports with reality. Do you deny that it comports with reality?

As I said, it has not happened experimentally, and I fail to see how it can happen. Indeed, I fail to see it happening even on paper. Can you show it on paper using a couple of clocks (that are mutually at rest, as in E's def.)?

If you're going to talk about "truly synch'd clocks", you're going to have to define what you mean. And I don't know what you mean by "totally ignoring different frame movement as the rays approach".

"If an observer sitting in the position M’ in the train did not possesses this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A."
http://www.bartleby.com/173/9.html

The only reason that the two observers see the light rays differently is their different motions as they view the rays. But E simply ignores this fact and declares that time is relative. In reality, the events happened in only one way, either truly sim. or not, so time is not relative merely because E decided to ignore diff observer motions during the experiment and making them equivalent and "both right."

You're right, the conflict was bogus but not for the reason you state. There simply was no conflict, it was only apparent, as Einstein pointed out in his 1905 paper.

Let me ask you this: OK, what was the apparent conflict?

Ok, let it change overnight. Let's hear your method. That's what I've been asking for from the very beginning. I expect an answer in 24 hours (based on GPS time). And make sure it comports with reality.

To say that there is no reason for discarding truly synch'd clocks does not demand a method for producing them. Also, to say that there is no physical reason why such clocks cannot exist does not call for a showing of how. BTW, if you were put to the test, how would you obtain absolute synch? Could you simply propel two mechanical objects equally toward the clocks from midway btn them?

 

[Sugdub]

If you had to give a physical reason for relative time, what would it be? ...

As already explained by some key contributors to this thread, your question cannot be answered if it is to be understood as “why is the world such that ...”. In the following, taking due account of the sound definition of “time” by ghwellsjr in # 6, I'll assume that the correct wording for your question is “how does the SR theory justify that clocks in relative motion in respect to each other do not tick at the same speed?” Please correct me if my interpretation of your question is wrong.

My answer to the newly formulated question is that the SR theory cannot provide such a justification, this answer being based on pure logical considerations.
If the clocks do not tick at the same speed, it means that one ticks faster than the other: the output of the theory is an order relationship. But logically speaking, an order in the conclusion of a rationale development can only be derived from some order relationship in the inputs to this development. Worded in a slightly different way, an asymmetry in the conclusion can only derive from an asymmetry in the inputs to the reasoning.
Well, let's now look at the inputs: two postulates which formulation aims at specifying equivalence relationships (physics laws are equally valid in all inertial reference frames, and the speed of light is the same in all such frames), plus one symmetrical problem-setting hypothesis stating that two identical clocks are in relative motion in respect to each other.
Three options: either one of the inputs hides an asymmetry / order relationship, or one input is missing, or else the expected conclusion cannot be arrived at from a pure logical standpoint.
I'll discard the first option on the ground that the principle of relativity of motion forbids that one of the clocks could be said moving objectively faster than the other. I'll discard the second option because I fail to see which input is missing. Hence my conclusion.

Let's be very clear: I'm not challenging experimental facts whatever they might show. I'm not rejecting the SR theory, far from it. But I can't see how it could justify that clocks in relative motion in respect to each other would run at a different speed. But may be this is not what the SR theory actually claims … may be somebody will make a very clear statement that will shed some light on this apparent deadlock.

Thanks

 

[Nugatory]

As I said, it has not happened experimentally, and I fail to see how it can happen. Indeed, I fail to see it happening even on paper. Can u show it on paper using a couple of clocks (that are mutually at rest, as in E's def.)?

As I said, it has not happened experimentally, and I fail to see how it can happen. Indeed, I fail to see it happening even on paper

The speed of light being the same for all observers has been confirmed by experiment many times. Indeed, some of these experiments were done before Einstein discovered SR in 1905, and one of the motivations for his postulate of the one-way speed of light was the total failure of these experiments to find any differences in light speed.

But I may be misunderstanding what you're trying to say here? You haven't replied to my post #36 in this thread, so I'm not sure.

 

[PAllen]

If the clocks do not tick at the same speed, it means that one ticks faster than the other: the output of the theory is an order relationship.

This is where you go wrong by not realizing the perfectly symmetrical claim made by SR and observed in experiment:

Each observer finds the other clock going slow. There is no observer independent order relationship. There is no asymmetry at all in what SR claims for a symmetric set up.

 

[PeterDonis]

But may be this is not what the SR theory actually claims

You're right; it isn't. What SR actually claims is that there is no invariant way to compare the "tick rates" of clocks in relative motion. The reason for that is the relativity of simultaneity: clocks in relative motion have different notions of which events happen "at the same time". Comparing the "tick rates" of clocks requires agreement between the clocks on which events happen at the same time, and clocks in relative motion don't meet that requirement.

 

[PeterDonis]

Each observer finds the other clock going slow.

More precisely: each observer finds that the other clock registers less elapsed time than his own between events, when he uses *his* standard of simultaneity. But the two observers use different standards of simultaneity.

 

[Dale]

Worded in a slightly different way, an asymmetry in the conclusion can only derive from an asymmetry in the inputs to the reasoning.

There is no asymmetry in the conclusion. If A and B are two clocks moving inertially wrt each other then A runs slow in B's frame and B runs slow in A's frame. The situations are completely symmetric. Any clock runs slow in any inertial frame where it is moving.

the output of the theory is an order relationship.

No, the output of the theory is a pair of order relationships. A is slower than B in B's frame and B is slower than A in A's frame. Because it is a pair of relationships the relationships are symmetric even though there is an ordering.