Relativity of Simultaneity

by mangaroosh
Tags: relativity, simultaneity
PF Gold
P: 4,785
 Quote by mangaroosh There's only two possible scenarios: either the clocks do tell the same time, or they don't. If they do then absolute simultaneity prevails; if they don't RoS prevails; what would cause them not to tell the same time?
We have no way of knowing if a clock remote from us has the same time on it as our local clock. That's the problem. Once you recognize that there is no test, no measurement, no way to detect, no way to determine, etc., etc., etc., the time on a remote clock, then you can follow Einstein's process. He said unless you define the time on the remote clock, it is impossible to deal with the problem. And you can define it arbitrarily in many different ways. So rather than suppose, like everyone else did, that there is an absolute universal time that nature is ticking away at, he postulated that the time on a remote clock is equal to the time on a local clock when a light signal takes the same amount of time to get from the local clock to the remote clock as it does for a light signal to get from the remote clock to the local clock. Under this defintion, RoS prevails. Under the previous assumption of an absolute universal time, RoS is not a factor.
P: 3,188
 Quote by mangaroosh I mightn't be making the point very lucidly, but the intention isn't to set two distant clocks to the exact same time; the question is, how might it arise that they don't tell the exact same time? There's only two possible scenarios: either the clocks do tell the same time, or they don't. If they do then absolute simultaneity prevails; if they don't RoS prevails; what would cause them not to tell the same time?
We understood your intention, which appears to be based on an unfounded assumption. Clocks are man-made and when you put a battery in it you can set it at any time you want. Thus, in order to have two clocks tell the same time, you have to do that.

You seem to have already a difficulty with getting two distant clocks synchronized according to yourself, despite your suggestion that all clocks will be automatically synchronized with all other clocks according to everyone. Nevertheless it was only an introduction to the next question: how can you do that in such a way that everyone will agree?
P: 359
 Quote by ghwellsjr We have no way of knowing if a clock remote from us has the same time on it as our local clock. That's the problem. Once you recognize that there is no test, no measurement, no way to detect, no way to determine, etc., etc., etc., the time on a remote clock, then you can follow Einstein's process. He said unless you define the time on the remote clock, it is impossible to deal with the problem. And you can define it arbitrarily in many different ways. So rather than suppose, like everyone else did, that there is an absolute universal time that nature is ticking away at, he postulated that the time on a remote clock is equal to the time on a local clock when a light signal takes the same amount of time to get from the local clock to the remote clock as it does for a light signal to get from the remote clock to the local clock. Under this defintion, RoS prevails. Under the previous assumption of an absolute universal time, RoS is not a factor.
 Quote by harrylin We understood your intention, which appears to be based on an unfounded assumption. Clocks are man-made and when you put a battery in it you can set it at any time you want. Thus, in order to have two clocks tell the same time, you have to do that. You seem to have already a difficulty with getting two distant clocks synchronized according to yourself, despite your suggestion that all clocks will be automatically synchronized with all other clocks according to everyone. Nevertheless it was only an introduction to the next question: how can you do that in such a way that everyone will agree?
Thanks guys; it hasn't clicked for me yet. It might be easier for me to outline the question by contrasting Einsteinian relativity with Lorentzian relativity; under Lorentzian relativity there is absolute simultaneity, while under Einsteinian relativity there is RoS. If both theories are indistinguishable in terms of experimental data, why is it that there is RoS in one and absolute simultaneity in the other, if both theories include clocks which tick at different rates?
PF Gold
P: 4,785
 Quote by mangaroosh Thanks guys; it hasn't clicked for me yet. It might be easier for me to outline the question by contrasting Einsteinian relativity with Lorentzian relativity; under Lorentzian relativity there is absolute simultaneity, while under Einsteinian relativity there is RoS. If both theories are indistinguishable in terms of experimental data, why is it that there is RoS in one and absolute simultaneity in the other, if both theories include clocks which tick at different rates?
Only because LET believers believe it is so. Or maybe I should say "believed" it was so because I don't think there are any LET believers left in the world.
P: 359
 Quote by ghwellsjr Only because LET believers believe it is so. Or maybe I should say "believed" it was so because I don't think there are any LET believers left in the world.
Would it not be more accurate to say that the theory postulates it? If there were no theory, there would be nothing to believe.
 PF Gold P: 4,785 I don't think it takes a formal theory to believe that time is absolute, that just seems natural and normal, don't you think? But once you measure the speed of light to be constant, it's only natural and normal to come up with a theory that retains absolute time, don't you think?
P: 359
 Quote by ghwellsjr I don't think it takes a formal theory to believe that time is absolute, that just seems natural and normal, don't you think? But once you measure the speed of light to be constant, it's only natural and normal to come up with a theory that retains absolute time, don't you think?
I'd agree, it doesn't take a formal theory to believe that time is absolute, but LET appears to postulate absolute simultaneity, and if the experimental evidence doesn't distinguish between it and SR then there must be a reason why RoS prevails in SR but not LET. I'm just trying to understand what that reason is.

In the last sentence, do you mean, once you measure the speed of light to be constant, it's only natural and normal to come up with a theory abandons absolute time?
PF Gold
P: 4,785
 Quote by mangaroosh I'd agree, it doesn't take a formal theory to believe that time is absolute, but LET appears to postulate absolute simultaneity, and if the experimental evidence doesn't distinguish between it and SR then there must be a reason why RoS prevails in SR but not LET. I'm just trying to understand what that reason is.
I think I've asked you before--do you think if no one else had put forward the idea that time could be relative, that is, time actually progresses at different rates under different conditions, you would ever come up with that idea on your own? Even now, many people struggle with this concept even though the idea is so prevalent in our world at this time.
 Quote by mangaroosh In the last sentence, do you mean, once you measure the speed of light to be constant, it's only natural and normal to come up with a theory abandons absolute time?
I only meant that just because the speed of light was measured to be constant, why should anyone connect that with time being relative? Wouldn't you do what the LET scientists did and say that the clocks moving in the ether run slow for some mechanical reason rather than say that time itself was slowing down for them? It made perfect sense that light propagated at a constant speed with respect to the ether and they just couldn't detect that fact rather than claim that light propagated at a constant speed for each inertial state.
P: 359
 Quote by ghwellsjr I think I've asked you before--do you think if no one else had put forward the idea that time could be relative, that is, time actually progresses at different rates under different conditions, you would ever come up with that idea on your own? Even now, many people struggle with this concept even though the idea is so prevalent in our world at this time.
That's a philosophical question without an answer I would say; history might show that no one but Einstein could have come up with the idea, but if you or I had been born in his stead and had his life experiences, we both would have come up with it.

 Quote by ghwellsjr I only meant that just because the speed of light was measured to be constant, why should anyone connect that with time being relative? Wouldn't you do what the LET scientists did and say that the clocks moving in the ether run slow for some mechanical reason rather than say that time itself was slowing down for them? It made perfect sense that light propagated at a constant speed with respect to the ether and they just couldn't detect that fact rather than claim that light propagated at a constant speed for each inertial state.
Those appear to be two differing interpretations, both equally supported by evidence; is that a fair enough comment?

According LET clocks tick at different rates for mechanical reasons, and so absolute simultaneity prevails.

According to Einsteinian relativity, clocks tick at different rates becuse "time itself slows down"; is it because "time itself slows down" in certain reference frames that RoS prevails? I presume it must be, because if time didn't slow down, and slower ticking clocks were the result of the mechanics of the clock then, as per LET, absolute simultaneity would prevail. Alternatively, if time itself didn't slow down and clocks all ticked at the same rate, then absolute simultaneity would prevail.
P: 3,188
 Quote by mangaroosh Thanks guys; it hasn't clicked for me yet. It might be easier for me to outline the question by contrasting Einsteinian relativity with Lorentzian relativity; under Lorentzian relativity there is absolute simultaneity, while under Einsteinian relativity there is RoS. If both theories are indistinguishable in terms of experimental data, why is it that there is RoS in one and absolute simultaneity in the other, if both theories include clocks which tick at different rates?
In Lorentzian relativity there is absolute simultaneity that cannot be measured, as well as "local time". Poincare pointed out that clocks measure local time. This local time already had the characteristic of relativity of simultaneity before relativity, but until 1904 it was only approximate. Einsteinian relativity considers only what can be measured; consequently he calls "local time" simply "time".

A similar thing happened earlier in classical mechanics: Newtonian mechanics distinguishes absolute velocity that cannot be measured as well as relative velocity that can be measured. Classical mechanics only deals with relative velocity.
P: 359
 Quote by harrylin In Lorentzian relativity there is absolute simultaneity that cannot be measured, as well as "local time". Poincare pointed out that clocks measure local time. This local time already had the characteristic of relativity of simultaneity before relativity, but until 1904 it was only approximate. Einsteinian relativity considers only what can be measured; consequently he calls "local time" simply "time".
That's fair enough; but if all the local clocks registered the same time then absolute simultaneity would prevail; but local clocks register different times and so RoS prevails. Why do local clocks not register the same time?

 Quote by harrylin A similar thing happened earlier in classical mechanics: Newtonian mechanics distinguishes absolute velocity that cannot be measured as well as relative velocity that can be measured. Classical mechanics only deals with relative velocity.
At the risk of going off-topic, I think the idea of measuring absolute velocity is somewhat of a misnomer, because measurement is, by it's very nature, relative i.e. it is making a statement about one phenomenon by relating it to other phenomena.

Absolute velocity is a simple yes or no answer to the question, is there velocity?
PF Gold
P: 4,785
Quote by mangaroosh
 Quote by ghwellsjr I only meant that just because the speed of light was measured to be constant, why should anyone connect that with time being relative? Wouldn't you do what the LET scientists did and say that the clocks moving in the ether run slow for some mechanical reason rather than say that time itself was slowing down for them? It made perfect sense that light propagated at a constant speed with respect to the ether and they just couldn't detect that fact rather than claim that light propagated at a constant speed for each inertial state.
Those appear to be two differing interpretations, both equally supported by evidence; is that a fair enough comment?
Yes.
 Quote by mangaroosh According LET clocks tick at different rates for mechanical reasons, and so absolute simultaneity prevails. According to Einsteinian relativity, clocks tick at different rates becuse "time itself slows down"; is it because "time itself slows down" in certain reference frames that RoS prevails? I presume it must be, because if time didn't slow down, and slower ticking clocks were the result of the mechanics of the clock then, as per LET, absolute simultaneity would prevail. Alternatively, if time itself didn't slow down and clocks all ticked at the same rate, then absolute simultaneity would prevail.
Let me try it this way:

I'm going to stipulate, for the sake of argument, that LET is the correct understanding of the way nature works. That means that there truly is an immovable ether and light propagates at c only with respect to the rest state of that ether. Time and space are absolutes. And because of the mechanical properties of the ether and the way that matter interacts with it, when matter moves through the ether, it contracts along the direction of motion. Also, any physical clock made of matter will keep track of the absolute time correctly only if it is stationary in the ether. If it is moving, the operation of the clock makes it slow down and so it is no longer keeping the correct time. The Lorentz factor correctly describes how much a moving clock slows down and how physical objects are contracted along the direction of motion. This is the stipulated truth about nature.

Now let's suppose an observer who is stationary in that ether has some measuring rods and some accurate, stable clocks and a mirror. When he attempts to measure the round trip speed of light, he gets the correct answer because his rulers and clocks are normal since they are not moving. Now let's suppose that he gets in a spaceship and accelerates to a high rate of speed with respect to the ether. This will cause his clocks to slow down and his rulers to contract when aligned with the direction of motion. When he repeats his measurement of the speed of light, what will happen? Well we know if he aligns his experiment so that the light has to travel against the ether to get to the mirror, it will take longer than when he was stationary. After it hits the mirror and reflects back, we know that it will take a shorter time than before because it is being carried along by the ether. Furthermore, we know that when he measures the distance between the mirrors, they will be closer together. As long as his clocks and rulers are modified by just the right amount, he will get the same measurement of the speed of light as he did before. But we know why he gets the same answer and that's because of length contraction and time dilation for matter moving through the ether.

As a matter of fact, the moving observer will see everything exactly the same when he is moving as he did when he was stationary. He cannot tell that he is moving with repsect to the ether. Do you understand this?
P: 3,188
 Quote by mangaroosh That's fair enough; but if all the local clocks registered the same time then absolute simultaneity would prevail; but local clocks register different times and so RoS prevails. Why do local clocks not register the same time?
Originally (before SR) this was for practical reasons as you can read here:
http://en.wikisource.org/wiki/The_Measure_of_Time
It was found that even if we wanted to, we cannot detect absolute simultaneity. But if we wanted, we could define a truly "universal time" and synchronize all clocks accordingly.
 At the risk of going off-topic, I think the idea of measuring absolute velocity is somewhat of a misnomer, because measurement is, by it's very nature, relative i.e. it is making a statement about one phenomenon by relating it to other phenomena. [...]
Apparently Newton defined it to mean velocity relative to absolute space; it doesn't mean "absolute" in the secondary meaning that you think (and which probably resulted from it much later).
- http://gravitee.tripod.com/definitions.htm
(press "cancel" and scroll to "SCHOLIUM")

Harald
P: 359
 Quote by ghwellsjr Yes. Let me try it this way: I'm going to stipulate, for the sake of argument, that LET is the correct understanding of the way nature works. That means that there truly is an immovable ether and light propagates at c only with respect to the rest state of that ether. Time and space are absolutes. And because of the mechanical properties of the ether and the way that matter interacts with it, when matter moves through the ether, it contracts along the direction of motion. Also, any physical clock made of matter will keep track of the absolute time correctly only if it is stationary in the ether. If it is moving, the operation of the clock makes it slow down and so it is no longer keeping the correct time. The Lorentz factor correctly describes how much a moving clock slows down and how physical objects are contracted along the direction of motion. This is the stipulated truth about nature. Now let's suppose an observer who is stationary in that ether has some measuring rods and some accurate, stable clocks and a mirror. When he attempts to measure the round trip speed of light, he gets the correct answer because his rulers and clocks are normal since they are not moving. Now let's suppose that he gets in a spaceship and accelerates to a high rate of speed with respect to the ether. This will cause his clocks to slow down and his rulers to contract when aligned with the direction of motion. When he repeats his measurement of the speed of light, what will happen? Well we know if he aligns his experiment so that the light has to travel against the ether to get to the mirror, it will take longer than when he was stationary. After it hits the mirror and reflects back, we know that it will take a shorter time than before because it is being carried along by the ether. Furthermore, we know that when he measures the distance between the mirrors, they will be closer together. As long as his clocks and rulers are modified by just the right amount, he will get the same measurement of the speed of light as he did before. But we know why he gets the same answer and that's because of length contraction and time dilation for matter moving through the ether. As a matter of fact, the moving observer will see everything exactly the same when he is moving as he did when he was stationary. He cannot tell that he is moving with repsect to the ether. Do you understand this?
Sorry gh, it might be the way I'm phrasing the question; I understand the above (I think), but it isn't Lorentzian relativity I'm wondering about, it's RoS in Einsteinian relativity.

My understanding is that RoS prevails, or perhaps more accurately, RoS is a consequence of the fact that [local*] clocks tick at different rates - if they didn't then absolute simultaneity would prevail. According to LET, as you have outlined above, clocks slow down for mechanical reasons (presumably this is true even when LET is stripped of everything but the absolute rest frame). That much I understand.

The question pertains to Einsteinian relativity. My understanding is that RoS is what results when [local*] clocks tick at different rates - is that much correct? As mentioned, LET postulates that this is down to the mechanics of the clock (as outlined above) - what, according to Einsteinian relativity, is the reason that [local*] clocks tick at different rates?

*Just in case the term "local" isn't used in Einsteinian relativity, what I mean is the clock at rest in a given FoR
PF Gold
P: 4,785
 Quote by mangaroosh Sorry gh, it might be the way I'm phrasing the question; I understand the above (I think), but it isn't Lorentzian relativity I'm wondering about, it's RoS in Einsteinian relativity. My understanding is that RoS prevails, or perhaps more accurately, RoS is a consequence of the fact that [local*] clocks tick at different rates - if they didn't then absolute simultaneity would prevail. According to LET, as you have outlined above, clocks slow down for mechanical reasons (presumably this is true even when LET is stripped of everything but the absolute rest frame). That much I understand. The question pertains to Einsteinian relativity. My understanding is that RoS is what results when [local*] clocks tick at different rates - is that much correct? As mentioned, LET postulates that this is down to the mechanics of the clock (as outlined above) - what, according to Einsteinian relativity, is the reason that [local*] clocks tick at different rates? *Just in case the term "local" isn't used in Einsteinian relativity, what I mean is the clock at rest in a given FoR
The second postulate: that light propagates in both directions at the same speed of a round-trip measurement of its speed, is what results in RoS for SR. LET does not have that postulate. Instead, it claims that the one-way speed of light is constant only in the absolute ether.

The one-way speed of light cannot be observed and cannot be measured. That is why we are free to make any postulate regarding it. For example, let's say that we place a mirror 10 feet away from our light source and our timer. We turn on the light at the exact moment we start the timer. When we detect the reflected light and stop the timer it reads 20 nanoseconds. We have the option of dividing the times for the two trips any way we want. We can say that it took 0 time for the light to get to the mirror and 20 nanoseconds to get back or vice versa. Or we could say that it took 1 nanosecond to get to the mirror and 19 nanoseconds to get back. Or 2 and 18, 3 and 17, etc. Or we could say, like LET that we have to determine the division of the times based on how fast we think we are traveling with respect to ether when the round-trip measurement assigns the times as equal. That claim supports the idea of an absolute time. Or we could say, like SR, that those times are equal every time we make the measurement which leads to the concept of relative time which is just another way of saying Relativity of Simultaneity.

Please look again at Einstein's 1905 paper introducing Special Relativity. Look at the title of the first section. It's called "Definition of Simultaneity". Look at what he says in the third paragraph:

We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events. If, for instance, I say, “That train arrives here at 7 o'clock,” I mean something like this: “The pointing of the small hand of my watch to 7 and the arrival of the train are simultaneous events.”

Then read the rest of that section and see how he builds up a consistent definition of time in remote locations.

Please study the first half, Part I, of his paper. If you don't understand something there, please ask a question. You need to understand Einstein's presentation if you want to understand Einsteinian relativity. I don't want to entertain any more questions that aren't sourced from Einstein.
P: 3,188
 Quote by mangaroosh [..] My understanding is that RoS prevails, or perhaps more accurately, RoS is a consequence of the fact that [local*] clocks tick at different rates - if they didn't then absolute simultaneity would prevail. [..] The question pertains to Einsteinian relativity. My understanding is that RoS is what results when [local*] clocks tick at different rates - is that much correct? [..]
That is wrong, as demonstrated in the reference of post #31 (in the second half of that reference). RoS was already applied without accounting for time dilation. And also the "Voigt transformation" (although he didn't intend it that way) doesn't have time dilation.

In order to obtain the Lorentz transformations from the Galilean transformations, one has to assume time dilation and Lorentz contraction (done by nature) as well as RoS (should be done by the experimentalist, by means of clock synchronization).

Harald
P: 359
 Quote by ghwellsjr The second postulate: that light propagates in both directions at the same speed of a round-trip measurement of its speed, is what results in RoS for SR. LET does not have that postulate. Instead, it claims that the one-way speed of light is constant only in the absolute ether. The one-way speed of light cannot be observed and cannot be measured. That is why we are free to make any postulate regarding it. For example, let's say that we place a mirror 10 feet away from our light source and our timer. We turn on the light at the exact moment we start the timer. When we detect the reflected light and stop the timer it reads 20 nanoseconds. We have the option of dividing the times for the two trips any way we want. We can say that it took 0 time for the light to get to the mirror and 20 nanoseconds to get back or vice versa. Or we could say that it took 1 nanosecond to get to the mirror and 19 nanoseconds to get back. Or 2 and 18, 3 and 17, etc. Or we could say, like LET that we have to determine the division of the times based on how fast we think we are traveling with respect to ether when the round-trip measurement assigns the times as equal. That claim supports the idea of an absolute time. Or we could say, like SR, that those times are equal every time we make the measurement which leads to the concept of relative time which is just another way of saying Relativity of Simultaneity. Please look again at Einstein's 1905 paper introducing Special Relativity. Look at the title of the first section. It's called "Definition of Simultaneity". Look at what he says in the third paragraph: We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events. If, for instance, I say, “That train arrives here at 7 o'clock,” I mean something like this: “The pointing of the small hand of my watch to 7 and the arrival of the train are simultaneous events.” Then read the rest of that section and see how he builds up a consistent definition of time in remote locations. Please study the first half, Part I, of his paper. If you don't understand something there, please ask a question. You need to understand Einstein's presentation if you want to understand Einsteinian relativity. I don't want to entertain any more questions that aren't sourced from Einstein.
thanks gh, I'll have a look at Einstein's paper and try to base my questions on Einstein's paper.

Just one quick question though, to see if I am even in the ball park with understanding this: am I at least [some way] right in thinking that if all clocks ticked at the same rate then absolute simultaneity would prevail; but because clocks tick at different rates RoS prevails?
Mentor
P: 17,533
 Quote by mangaroosh am I at least [some way] right in thinking that if all clocks ticked at the same rate then absolute simultaneity would prevail; but because clocks tick at different rates RoS prevails?
No, I explicitly showed that to be incorrect above, and I have stated it clearly multiple times.

 Related Discussions Special & General Relativity 47 Introductory Physics Homework 1 Special & General Relativity 10 Special & General Relativity 16 Special & General Relativity 25