The Relativity of Simultaneity: A Fundamental Concept in Special Relativity

[harrylin]

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

 

[mangaroosh]

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

 

[ghwellsjr]

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.

 

[harrylin]

[..] 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

 

[mangaroosh]

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?

 

[Dale]

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.

 

[Dale]

My interpretation of it would be that, if the clocks which give the time co-ordinates all ran at the same rate, then absolute simultaneity should prevail; and in order for RoS to prevail clocks would have to give different times (co-ordinates).

This interpretation is incorrect. dt'/dt=1 so all of the clocks run at the same rate.

That would be another thing that I don't understand, namely how, or why, the time co-ordinate would include a spatial term.

If the transform is linear (I.e. Both frames are inertial) then the transform in general includes a spatial term. It may be that the coefficient is zero, but that is an additional assumption.

 

[Naty1]

That would be another thing that I don't understand, namely how, or why, the time co-ordinate would include a spatial term.

in some sense, it makes no sense. But this may offer a way to begin to think about it:

A simple minded thought experiment: picture an x,y plot...as two particles move around there in different paths, they experience different rates of x and y distances covered,,,right? we don't normally think much about that...each would observe the
other moving differenty through space [distance] than themselves...

now consider a plot of time versus,say, x distance: observe two particles moving differently... say straight lines with different slopes as an example...one moves faster thru time and another faster thru distance [space]. We observe the particles and each observe's the other moving differently through time and the x direction than themselves...all the
observations are 'correct' but they are also 'different'.

 

[mangaroosh]

No, I explicitly showed that to be incorrect above, and I have stated it clearly multiple times.

Hi Dalespam, are you referring to another thread, because I only saw your response on the first page of this one and had a few questions about it?

This interpretation is incorrect. dt'/dt=1 so all of the clocks run at the same rate.

If the transform is linear (I.e. Both frames are inertial) then the transform in general includes a spatial term. It may be that the coefficient is zero, but that is an additional assumption.

I'm sorry, I don't really understand what dt'/dt=1; I am familiar with the notation from high school maths but I'm not sure how to interpret it in this context.


Where I am not clear is why there is absolute simultaneity according to Lorentzian relativity but RoS according to Einsteinian. Do both not use the same transformations?

 

[Dale]

No, I explicitly showed that to be incorrect above, and I have stated it clearly multiple times.

Hi Dalespam, are you referring to another thread, because I only saw your response on the first page of this one and had a few questions about it?

I explicitly showed it to be incorrect here:
https://www.physicsforums.com/showpost.php?p=3751522&postcount=2

And I stated it clearly here:
https://www.physicsforums.com/showpost.php?p=3747625&postcount=57
https://www.physicsforums.com/showpost.php?p=3747739&postcount=59
https://www.physicsforums.com/showpost.php?p=3752737&postcount=11

I'm sorry, I don't really understand what dt'/dt=1; I am familiar with the notation from high school maths but I'm not sure how to interpret it in this context.

It means that the ratio of the time in one frame to the time in the other frame is 1, i.e. time is not dilated.

 

[mangaroosh]

I explicitly showed it to be incorrect here:
https://www.physicsforums.com/showpost.php?p=3751522&postcount=2

And I stated it clearly here:
https://www.physicsforums.com/showpost.php?p=3747625&postcount=57
https://www.physicsforums.com/showpost.php?p=3747739&postcount=59
https://www.physicsforums.com/showpost.php?p=3752737&postcount=11

I appreciate you taking the time to reply DaleSpam, but it was the comments in "the constancy of c" thread lead to me starting this thread, to try and get a better understanding. You posted a few responses in the first page of this thread, the first being post #2; I replied to that explaining that I didn't fully understand the logic as represented by the maths; your subsequent reply was an expansion on that point (post #11); to which I posted some questions in reply. From there the discussion of my reply to your post was continued with George and Harry, where I outlined my rationale; your next reply was post #36 where you said that you had explicitly addressed the issue in a previous post (in the thread) and has stated it countless times. As mentioned, however, there were subsequent questions to that clarification which lead to the conversation with George and Harry.

I do appreciate your taking the time to entertain my questions, and I'm sure you're probably addressing countless other [perhaps equally naiive] questions, so it will hopefully reduce any frustration you might be feeling by highlighting why I still haven't got my head around the issue, despite your generous attempts to clarify it.

It means that the ratio of the time in one frame to the time in the other frame is 1, i.e. time is not dilated.

Thanks for the clarification.

My trouble with "getting it" lies in the difference, or lack thereof, between Einsteinian relativity and Lorentzian relativity; do I understand correctly that there is essentially no difference between the mathematics of both interpretations?

If so, the trouble I have is understanding how one interpretation includes RoS while the other includes absolute simultaneity. George suggested that it is down to the different assumptions about the propagation of light, but I don't see how that would lead to RoS without some intermediary step(s).

If my understanding is correct, and the mathematics is the same for both theories, then I'm not sure a mathematical explanation will address the issue; I would imagine it has to be more of a theoretical consideration of what the maths represents.

 

[James_Harford]

Hello Mangaroosh,

May I suggest yet another way of answering your two questions about Relativity of Simultaneity [RoS]? You might find it simpler to understand, and I am confident that DaleSpam, Ghwellsir, and other posters who have been answering your questions so tirelessly will vouch for its correctness:

Your OP question is,

this might seem like yet another basic question, but I was wondering about RoS. The impression that I got from reading about relativity was that relativity of simultaneity was a consequence of Lorentz contractions, primarily time dilation.

And your latest post question is (with [SR] & [LET] inserted for clarity):

... the trouble I have is understanding how one interpretation [SR] includes RoS while the other [LET] includes absolute simultaneity. George suggested that it is down to the different assumptions about the propagation of light, but I don't see how that would lead to RoS without some intermediary step(s).

Fortunately in this case, both questions can be addressed without Lorentz transforms, clocks or rulers. The price of using this approach is that we cannot quantify RoS, but we can demonstrate its existence.

Relativity of Simultaneity means simply that two events, separated by a distance from one another, and simultaneous according to one competent observer are not (generally) simultaneous according to another moving relative to the first at some constant speed.

The simultaneity of two events can be established using a shuttered lamp on a track between two distant mirrors positioned on the track to reflect any light pulses from the lamp back to it and yourself, the observer. The mirrors need not be at rest and may move independently at arbitrary speeds along the track. Suppose that you are very quick with the shutter and send an extremely brief pulse of light towards both mirrors. If the reflected pulses from both mirrors happen to return simultaneously, what can you legitimately infer? Since by Einstein's postulate, light moves at one speed only, you know these two things:
1. the mirrors were equidistant from the lamp at the time of the reflection, and
2. the two reflections occurred simultaneously.

Now suppose that a second observer happens to be moving along the track just as you send your light pulse from the lamp. Will he agree that the reflections were simultaneous? No. By the time the two reflected pulses meet at your lamp, observer #2 has moved some distance along the track, and correctly notes that the two pulses did not "return" simultaneously to his "here", and for that reason, by Einstein's postulate he must infer that the two reflections did not occur simultaneously.

The LET interpretation predicts exactly the same phenomena, but differs from SR only in that it insists that light depends upon a hypothetical medium, or aether, Therefore the method of "simultaneous arrival" for determining simultaneity is valid only for an observer who is motionless in the aether, and there is only one "true" definition of simultaneity -- that of the motionless observer. Einstein's great insight was that the aether hypothesis was completely superfluous.

- Regards

 

[harrylin]

[..]
Where I am not clear is why there is absolute simultaneity according to Lorentzian relativity but RoS according to Einsteinian. Do both not use the same transformations?

Hi I answered that in post #28 and also, indirectly, in post #31. RoS corresponds to the "local time" concept as interpreted by Poincare but Einstein interpreted it differently. Did you study the references? Is there something unclear about it?

 

[harrylin]

[..] My trouble with "getting it" lies in the difference, or lack thereof, between Einsteinian relativity and Lorentzian relativity; do I understand correctly that there is essentially no difference between the mathematics of both interpretations? [..]

Apparently you missed my post here, in which I explained this in detail and with a lot of precision:
https://www.physicsforums.com/showpost.php?p=3756233&postcount=54

Harald

 

[PhilDSP]

My trouble with "getting it" lies in the difference, or lack thereof, between Einsteinian relativity and Lorentzian relativity; do I understand correctly that there is essentially no difference between the mathematics of both interpretations?

It would probably be fair and correct to say that the mathematics going into and coming out of the Lorentz Transformation and their usage is very different comparing Einstein-Minkowski and Lorentz-Poincare relativity. While the Lorentz Transformation itself is obviously common. For the most part, Lorentz worked with vectors, differential equations and their initial and boundary conditions. Einstein and Minkowski built a platform where those things were abstracted away in which tensors and 4-vectors are preferred.

 

[Agerhell]

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

From wikipedia:

"A popular picture for understanding this idea is provided by a thought experiment consisting of one observer midway inside a speeding traincar and another observer standing on a platform as the train moves past. It is similar to thought experiments suggested by Daniel Frost Comstock in 1910[1] and Einstein in 1917.[2][3]

A flash of light is given off at the center of the traincar just as the two observers pass each other. The observer onboard the train sees the front and back of the traincar at fixed distances from the source of light and as such, according to this observer, the light will reach the front and back of the traincar at the same time.

The observer standing on the platform, on the other hand, sees the rear of the traincar moving (catching up) toward the point at which the flash was given off and the front of the traincar moving away from it. As the speed of light is finite and the same in all directions for all observers, the light headed for the back of the train will have less distance to cover than the light headed for the front. Thus, the flashes of light will strike the ends of the traincar at different times."

Is this helpful? According to SR the speed of light on Earth is not c in relation to the gravitational field of the Earth but relative to "any inertial frame". Thus the observer on the platform sees the front- and the backside of the wagon lit up at different times but the guy on the train will think that the front and the backside of the train are lit up simultanesously, because the light coming back from the rear and the front end of the wagon walls will hit him simultaneously.

I think this type of thought experiments are always what is meant when someone talks about "relativity of simultaneity", but I might be wrong...

 

[mangaroosh]

Hello Mangaroosh,

May I suggest yet another way of answering your two questions about Relativity of Simultaneity [RoS]? You might find it simpler to understand, and I am confident that DaleSpam, Ghwellsir, and other posters who have been answering your questions so tirelessly will vouch for its correctness:

Your OP question is,



And your latest post question is (with [SR] & [LET] inserted for clarity):



Fortunately in this case, both questions can be addressed without Lorentz transforms, clocks or rulers. The price of using this approach is that we cannot quantify RoS, but we can demonstrate its existence.

Relativity of Simultaneity means simply that two events, separated by a distance from one another, and simultaneous according to one competent observer are not (generally) simultaneous according to another moving relative to the first at some constant speed.

The simultaneity of two events can be established using a shuttered lamp on a track between two distant mirrors positioned on the track to reflect any light pulses from the lamp back to it and yourself, the observer. The mirrors need not be at rest and may move independently at arbitrary speeds along the track. Suppose that you are very quick with the shutter and send an extremely brief pulse of light towards both mirrors. If the reflected pulses from both mirrors happen to return simultaneously, what can you legitimately infer? Since by Einstein's postulate, light moves at one speed only, you know these two things:
1. the mirrors were equidistant from the lamp at the time of the reflection, and
2. the two reflections occurred simultaneously.

Now suppose that a second observer happens to be moving along the track just as you send your light pulse from the lamp. Will he agree that the reflections were simultaneous? No. By the time the two reflected pulses meet at your lamp, observer #2 has moved some distance along the track, and correctly notes that the two pulses did not "return" simultaneously to his "here", and for that reason, by Einstein's postulate he must infer that the two reflections did not occur simultaneously.

The LET interpretation predicts exactly the same phenomena, but differs from SR only in that it insists that light depends upon a hypothetical medium, or aether, Therefore the method of "simultaneous arrival" for determining simultaneity is valid only for an observer who is motionless in the aether, and there is only one "true" definition of simultaneity -- that of the motionless observer. Einstein's great insight was that the aether hypothesis was completely superfluous.

- Regards

Hi James, thanks a million for taking the time to reply.

I hope I'm using the right phraseology here when I say that, under Galilean transformations we would expect the moving observer to measure a different speed of light to the pulse operator, but under Lorentz transformations this isn't the case; is that accurate?

Lorentzian relativity maintains absolute simultaneity, while Einsteinian relativity incorporates RoS. I don't think we're too concerned with Lorentzian relativity in this particular instance, what we're looking for, or rather, what I'm trying to understand, is what phenomena must occur in order for the speed of light to remain constant for all observers, regardless of their motion realtive to the source of the light; which appears to be the reason given, why RoS prevails. That was essentially the point made by George, so I presume he would agree with your statement to a similar effect.


One thing he might disagree on is that Lorentzian relativity necessarily includes the superfluous concept of an aether.

nowadays, when we speak of LET, we have striped it of everything except the idea of a preferred rest frame.

 

[mangaroosh]

Hi I answered that in post #28 and also, indirectly, in post #31. RoS corresponds to the "local time" concept as interpreted by Poincare but Einstein interpreted it differently. Did you study the references? Is there something unclear about it?

Hi Harry, the conversation with DaleSpam is in reference to mathematical transformations, while your reply in post #28 was with respect to clocks; unfortunately I don't have the nous to make the connection between the two, so I find it more beneficial to engage with the point that is raised and see where it goes.

I had a quick glance at the reference in post #31, but statement immediately following the link was something you had mentioned before, with respect to detecting absolute simultaneity (or the time on a distant clock) and which I had addressed in #18; so that affected my judgement of the necessity to go through it in detail. Is there a specific part that I can jump to that would address the issue?

Just reading back over the exchange I realize that I didn't address reply in #20 directly. I tried reformulating the question, which lead to your reply in #28, which appears to just be a reformulation of the same reply that was questioned previously.

Post #20:

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?

The issue being raised appears to be the idea of detecting absolute simultaneity; but that isn't necessarily an issue that needs to be addressed. We don't need to figure out how to synchronise clocks to say that if all clocks remained synchronised then there would be absolute simultaneity; it's somewhat of a tautology. If any clock falls out of sync, and the change isn't due to mechanical processes, then RoS prevails; if the change is down to mechanical processes then absolute simultaneity prevails - that is assuming that a clock measures time, because if a clock doesn't measure time, then we have no way of measuring time, and it wouldn't necessarily be something that could be dealt with in physics - as someone mentioned in another thread on here a while ago, pertaining to the MMX.This is #28, which, as mentioned, appears to just reformulate the issue of detecting absolute simultaneity.

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.

 

[mangaroosh]

Apparently you missed my post here, in which I explained this in detail and with a lot of precision:
https://www.physicsforums.com/showpost.php?p=3756233&postcount=54

Harald

I didn't miss it, my reply was deleted because I used an invalid reference; although I didn't intend for it to be a reference per se, rather as a means of explaining a point that was made more eloquently than I could have done.

I'll PM you the reply, but I'd prefer to continue the discussion in an open thread. Perhaps if I PM you, you might be able to paraphrase the quote in your reply, for the posterity of the thread - that is if you deem it worthy of reply, of course.

 

[Dale]

I replied to that explaining that I didn't fully understand the logic as represented by the maths

That is fine. If you don't understand then ask new questions that will help me get at your root concern. But if you merely repeat the same incorrect claim that I answered already then all I can do is as I did above and refer back to my previous answers.

your subsequent reply was an expansion on that point (post #11); to which I posted some questions in reply. From there the discussion of my reply to your post was continued with George and Harry, where I outlined my rationale; your next reply was post #36 where you said that you had explicitly addressed the issue in a previous post (in the thread) and has stated it countless times. As mentioned, however, there were subsequent questions to that clarification which lead to the conversation with George and Harry.

You are correct, I was traveling for about two weeks with reduced time and access. However, I did find it frustrating to come back and find you repeating the same mistake that I already addressed weeks ago.

My trouble with "getting it" lies in the difference, or lack thereof, between Einsteinian relativity and Lorentzian relativity; do I understand correctly that there is essentially no difference between the mathematics of both interpretations?

I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.

 

[Dale]

my reply was deleted because I used an invalid reference

Try the reply without the invalid reference. Btw, I saw the reference, and if that is the kind of material that you are trying to learn relativity by reading then it is no wonder that you are confused.

 

[mangaroosh]

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

From wikipedia:



Is this helpful? According to SR the speed of light on Earth is not c in relation to the gravitational field of the Earth but relative to "any inertial frame". Thus the observer on the platform sees the front- and the backside of the wagon lit up at different times but the guy on the train will think that the front and the backside of the train are lit up simultanesously, because the light coming back from the rear and the front end of the wagon walls will hit him simultaneously.

I think this type of thought experiments are always what is meant when someone talks about "relativity of simultaneity", but I might be wrong...

Thanks Agerhall, I am familiat with the thought experiment alright, and it is essentially the same one that James posted, I think.

What phenomena occur that means the speed of light is c "in any inertial frame"? We would, ordinarily, expect the moving observer to measure a different speed of light - why is this not the case?

 

[harrylin]

I didn't miss it, my reply was deleted because I used an invalid reference; although I didn't intend for it to be a reference per se, rather as a means of explaining a point that was made more eloquently than I could have done. [...]

Then, evidently, you did not understand it. As Dalespam suggests, you can simply re-post the reformulation of your question without including a link to a crank(?) site.

 

[Agerhell]

Thanks Agerhall, I am familiat with the thought experiment alright, and it is essentially the same one that James posted, I think.

What phenomena occur that means the speed of light is c "in any inertial frame"? We would, ordinarily, expect the moving observer to measure a different speed of light - why is this not the case?

If you first assume that the speed of light is c in relation to the guy standing on the platform. The guy on the platform would classically expect that it takes longer time for the light from the guy on the train to reach the back and front wall and bounce back towards the guy on the train than if the train had not been moving. You can do the math, it is simple.

However, in Special Relativity the Time Dilation formula kicks in and it so happens that if the guy on the platform would expect it to take a factor k longer for the light to reach the back and front wall and get back to the guy on the train again, then the time for the guy on the train slows down with the same factor k so he thinks the two way speed of light has the same value, c, no matter what the velocity of the train relative to the ground is...

In this way the "two-way speed of light is the same in all inertial frames".

I believe this is how it is always explained...

 

[harrylin]

If you first assume that the speed of light is c in relation to the guy standing on the platform. The guy on the platform would classically expect that it takes longer time for the light from the guy on the train to reach the back and front wall and bounce back towards the guy on the train than if the train had not been moving. You can do the math, it is simple.

However, in Special Relativity the Time Dilation formula kicks in and it so happens that if the guy on the platform would expect it to take a factor k longer for the light to reach the back and front wall and get back to the guy on the train again, then the time for the guy on the train slows down with the same factor k so he thinks the two way speed of light has the same value, c, no matter what the velocity of the train relative to the ground is...

In this way the "two-way speed of light is the same in all inertial frames".

I believe this is how it is always explained...

The way you describe it, you probably mean the ceiling and the floor. Else you should add length contraction to the mix...
And to top it off (in view of the topic!), one usually makes the one-way speed equal to the two-way speed by means of the Poincare-Einstein synchronization procedure.

 

[James_Harford]

Your OP question:

this might seem like yet another basic question, but I was wondering about RoS. The impression that I got from reading about relativity was that relativity of simultaneity was a consequence of Lorentz contractions, primarily time dilation.

And your latest question about LET:

... the trouble I have is understanding how one interpretation [SR] includes RoS while the other [LET] includes absolute simultaneity. George suggested that it is down to the different assumptions about the propagation of light, but I don't see how that would lead to RoS without some intermediary step(s).

are answered in my earlier post. I guess I missed that you have a third question:

I don't think we're too concerned with Lorentzian relativity in this particular instance, what we're looking for, or rather, what I'm trying to understand, is what phenomena must occur in order for the speed of light to remain constant for all observers, regardless of their motion realtive to the source of the light...

May I suggest that we work on them one at a time? Bouncing back and forth between all of them and adding time dilation and lorentz contractions and LET makes all of this a rather poisonous mix of unrelated factors. Please go back to my earlier post and note that it fully answers your first two questions, as stated. The only valid questions about that post are
1. Is it a correct analysis?
2. Do you understand it?

If you do not, then, I would not go on to the third question until you do.
If you do, then you have just the third question.

That question, unlike the others, is about the ratio of a time and a space measurement, and this unavoidably requires some level of understanding of the Lorentz transforms themselves, which is bad news for you. Give up the notion that time dilations and Lorentz space contractions are sufficient. You will fail. You need a complete coordinate transformation, because only that gives exactly the differences in how the various observers make these measurements.

I would recommend that you start playing around with pencil and paper with various XT coordinate systems, and see if you can find (by trial and error) two coordinate systems in which the path of a photon (a slanted line) has the same speed. Whether you succeed or not, the exercise will get you past a psychological obstacle and improve your chances of grasping what your friends here are telling you.

One thing he might disagree on is that Lorentzian relativity necessarily includes the superfluous concept of an aether.

I doubt it.

- Regards

 

[Naty1]

I hope I'm using the right phraseology here when I say that, under Galilean transformations we would expect the moving observer to measure a different speed of light to the pulse operator, but under Lorentz transformations this isn't the case; is that accurate?

yes. Galileo would have EXPECTED to measure a different speed, but if he had the proper apparatus, would have been dumbfounded when he actually observed the fixed speed of light! And probably so would have Newton.

Dalespam:

I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.

YES! YES! ...I was just about to post similar when I realized there is this last page.

Harford:

May I suggest that we work on them one at a time? Bouncing back and forth between all of them and adding time dilation and lorentz contractions and LET makes all of this a rather poisonous mix of unrelated factors.

again, YES! Let's get the proper theory explained, then it
will become clear why older theories were inadequate.


The HISTORY section here explains why such confusion reigned before Einstein...nobody had Einstein's physical insights...

http://en.wikipedia.org/wiki/Minkowski_space

 

[mangaroosh]

If you first assume that the speed of light is c in relation to the guy standing on the platform. The guy on the platform would classically expect that it takes longer time for the light from the guy on the train to reach the back and front wall and bounce back towards the guy on the train than if the train had not been moving. You can do the math, it is simple.

However, in Special Relativity the Time Dilation formula kicks in and it so happens that if the guy on the platform would expect it to take a factor k longer for the light to reach the back and front wall and get back to the guy on the train again, then the time for the guy on the train slows down with the same factor k so he thinks the two way speed of light has the same value, c, no matter what the velocity of the train relative to the ground is...

In this way the "two-way speed of light is the same in all inertial frames".

I believe this is how it is always explained...

The way you describe it, you probably mean the ceiling and the floor. Else you should add length contraction to the mix...
And to top it off (in view of the topic!), one usually makes the one-way speed equal to the two-way speed by means of the Poincare-Einstein synchronization procedure.

Does this not just verify the point of the OP that RoS is just a consequence of Lorentz contractions, and isn't necessarily a separate, third aspect of Einsteinian relativity?

 

[mangaroosh]

That is fine. If you don't understand then ask new questions that will help me get at your root concern. But if you merely repeat the same incorrect claim that I answered already then all I can do is as I did above and refer back to my previous answers.

You are correct, I was traveling for about two weeks with reduced time and access. However, I did find it frustrating to come back and find you repeating the same mistake that I already addressed weeks ago.

Apologies DS, I tend to treat each conversation individually and try not to take it for granted that every poster reads every post, so I tend to restate certain points for the purpose of the conversation. A number of the times I have restated the point, however, has either been in somewhat different context or for the purpose of reformulating for clarity.

I would argue, though, that the times I have restated it it hasn't necessarily been superfluous to the two-way conversation. I can understand how it might be frustrating to read the same point repeated, particularly when you are under the impression that you have already answered it, but, as mentioned, I had subsequent questions which were taken up by someone else; this lead me to repeat and/or reformulate the point for the purpose of that discussion.

I don't think that anything productive will come from a discussion of Lorentzian relativity. It is a defunct interpretation which asserts the reality of undetectable entities. All of the confusion with LET comes from the simple fact that the aether is undetectable, so in LET you continually have to make careful distinctions between measurements and what is happening in (undetectable) reality. It is a rabbit hole not worth going down.

The purpose of this thread isn't necessarily to discuss Lorentzian relativity, I think it is sufficient to discuss it in the competing theories thread; the purpose of bringing up Lorentzian relativity here, was solely to juxtapose it with Einsteinian relativity for the purpose of clarification; namely that it appears that the same transformations can result in both absolute simultaneity and RoS. The question that is begged from that is, what is the difference between the two; why RoS under one interpretation, but absolute relativity in the other?

Please forgive the repetition there, I'm using it solely to emphasise that the question hasn't been addressed just yet.


I'm not sure the specific example of the transform you gave fully addresses the question, because the question is more general. It might be the case that length contraction and time dilation do not lead every single event to be relatively simultaneous, but if time dilation and/or length contraction never occurred at all, anywhere, would RoS still prevail?

Try the reply without the invalid reference. Btw, I saw the reference, and if that is the kind of material that you are trying to learn relativity by reading then it is no wonder that you are confused.

It's not so much that I'm confused, rather I haven't made the logical connection yet as to how RoS is separate, stand-alone aspect of Einsteinian relativity and not just a consequence of Lorentzian contractions.

It's probably also worth pointing out that I don't try to learn relativity from sources such as the one posted; I learn relativity largely from discussions with people on sites like this, from the references they post and from my own searches; however, I tend not to simply accept as gospel what I am told, and try to subject such things to critical inquiry, which usually serves the purpose of developing a better understaning; on that basis I will consider alternative viewpoints as long as they stand up to reason, and of course experiment.

The "reference" posted seems to make quite a reasonable point about the propagation of light, which I haven't seen considered elsewhere. Unfortunately I'm not in a position to subject it to the level of critical reasoning required so I can only present it as it is and see if there are arguments against it. However, as it strikes me as being quite reasonable, and conforms to [more reliable] information I have encountered previously, I would lean more towards accepting it (without accepting it fully).

Incidentally, I posted the reply without the reference in the other thread.

 

[mangaroosh]

Your OP question:



And your latest question about LET:



are answered in my earlier post. I guess I missed that you have a third question:



May I suggest that we work on them one at a time? Bouncing back and forth between all of them and adding time dilation and lorentz contractions and LET makes all of this a rather poisonous mix of unrelated factors. Please go back to my earlier post and note that it fully answers your first two questions, as stated. The only valid questions about that post are
1. Is it a correct analysis?
2. Do you understand it?

If you do not, then, I would not go on to the third question until you do.
If you do, then you have just the third question.

That question, unlike the others, is about the ratio of a time and a space measurement, and this unavoidably requires some level of understanding of the Lorentz transforms themselves, which is bad news for you. Give up the notion that time dilations and Lorentz space contractions are sufficient. You will fail. You need a complete coordinate transformation, because only that gives exactly the differences in how the various observers make these measurements.

I would recommend that you start playing around with pencil and paper with various XT coordinate systems, and see if you can find (by trial and error) two coordinate systems in which the path of a photon (a slanted line) has the same speed. Whether you succeed or not, the exercise will get you past a psychological obstacle and improve your chances of grasping what your friends here are telling you.


I doubt it.

- Regards

Hi James, if possible I'd like to change the first question; I didn't formulate it in reply to yourself, but did in reply to Agerhall.

Your explanation was based on the second postulate, the constancy of c in every reference frame, regardless of the motion relative to the source. The question that arises from that is, what phenomena have to occur to allow for this possibility? Ordinarily, with the addition of velocities we would expect the moving observer to measure a different speed of light; what phenomena occur that leads to him measuring the speed of light to be the same as the other observer?