# What caused us to have relative time

1. Nov 4, 2013

### 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.)

2. Nov 4, 2013

### dauto

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

3. Nov 4, 2013

### A.T.

The lack of reasons for the opposite.

4. Nov 4, 2013

### spaced-out

"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?

5. Nov 4, 2013

### Staff: Mentor

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.

Last edited: Nov 4, 2013
6. Nov 4, 2013

### ghwellsjr

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).

7. Nov 4, 2013

### Staff: Mentor

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.

Last edited: Nov 4, 2013
8. Nov 4, 2013

### 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?

9. Nov 4, 2013

### PAllen

Because it's what we observe?

10. Nov 5, 2013

### Staff: Mentor

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.

11. Nov 5, 2013

### 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.

Last edited by a moderator: Nov 7, 2013
12. Nov 5, 2013

### PAllen

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".

Last edited: Nov 5, 2013
13. Nov 5, 2013

### PAllen

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 vaccuum 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.

Last edited by a moderator: Nov 7, 2013
14. Nov 5, 2013

### ghwellsjr

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?

Last edited by a moderator: Nov 7, 2013
15. Nov 5, 2013

### Staff: Mentor

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.

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

Last edited by a moderator: Nov 7, 2013
16. Nov 5, 2013

### spaced-out

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

17. Nov 5, 2013

### spaced-out

How can asynchonous clocks measure time correctly?

18. Nov 5, 2013

### spaced-out

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?

Last edited by a moderator: Nov 7, 2013
19. Nov 5, 2013

### Staff: Mentor

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).

20. Nov 5, 2013

### Staff: Mentor

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.

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.

Last edited: Nov 5, 2013