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]