Einstein's Relativity: Examining Simultaneity Theory

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Discussion Overview

The discussion revolves around Einstein's theory of simultaneity as presented in his "Relativity: The Special and General Theory," particularly focusing on the implications of relative motion on the perception of simultaneous events. Participants explore the train thought experiment and its interpretations, examining the effects of different reference frames on the observation of light and simultaneity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that an observer at the midpoint between two events can conclude that the events are simultaneous if they see light from both events arriving at the same time.
  • Others argue that while light speed is constant, the relative motion of observers affects their perception of simultaneity, leading to different conclusions about the timing of events.
  • A participant describes a scenario where an observer on a moving train synchronizes clocks and concludes that they are simultaneous, while a stationary observer on the platform sees them as non-simultaneous due to the relative motion of the train.
  • Some participants challenge the consistency of Einstein's explanations, particularly regarding the implications of moving observers and the definition of simultaneity.
  • A later reply acknowledges a misunderstanding regarding the definition of the midpoint and the applicability of simultaneity, noting that movement after the events does not negate the initial conditions for simultaneity.
  • One participant questions whether the observer on the train perceives the same effects of relative motion as the observer on the platform, suggesting a need for clarification on the nature of light's travel in different frames.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the interpretation of simultaneity and the effects of relative motion. The discussion remains unresolved, with differing opinions on the implications of Einstein's theory and the thought experiment.

Contextual Notes

Participants highlight limitations in understanding the definitions of simultaneity and the effects of observer motion on measurements. There is an ongoing exploration of how different reference frames influence the perception of time and events.

Who May Find This Useful

This discussion may be of interest to those studying relativity, observers in different reference frames, and the philosophical implications of simultaneity in physics.

  • #31
the problem is that einstein came up with a theory where he said C is constant.

then later on he realized that it was not and stated so in other papers. i believe it was from 1911 onwards but do not have the info to hand.

but when everyone reads his works they take everything he said as true - even after he said himself he was wrong.
 
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  • #32
solarflare said:
if you say that these are not relevant just because they do not fit into place then i guess we have gone back to the early days of science when philosophers told us how things were.

You've left out one important step - experimental verification. Einstein started out with the postulate of the constancy of the speed of light in vacuum, but the conclusions that follow from that postulate can be and have been experimentally verified.

solarflare said:
the problem is that einstein came up with a theory where he said C is constant.
then later on he realized that it was not and stated so in other papers.
If you're referring to the difference between special relativity and general relativity, all that's going on is that the former is a special case of the latter (which is, of course, why they're called what they are). An oversimplified but more accurate way of describing what happened would be:
Einstein came up with a theory where he said C is constant in a flat space-time.
Then later on he realized that this was the curvature=0 special case of a more general theory and stated so in other papers.
 
  • #33
in any case - even in just one instance if C is not constant then it is not constant.

there is a reason he says in vacuo - and that reason is that the speed of light changes as it moves through anything else. even things with his name in it are dismissed. bose einstein condensates are used to slow light down and even stop it. how can you stop light without reducing its speed?
 
  • #34
Relativity: The Special and General Theory by Albert Einstein, translated by Robert William Lawson
Part I - The Special Theory of Relativity

paragraph 22

In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) [ 22 ] is justifiably believed by the child at school. Who would imagine that this simple law has plunged the conscientiously thoughtful physicist into the greatest intellectual difficulties? Let us consider how these difficulties arise.

Of course we must refer the process of the propagation of light (and indeed every other process) to a rigid reference-body (co-ordinate system). As such a system let us again choose our embankment. We shall imagine the air above it to have been removed. If a ray of light be sent along the embankment, we see from the above that the tip of the ray will be transmitted with the velocity c relative to the embankment. Now let us suppose that our railway carriage is again traveling along the railway lines with the velocity v, and that its direction is the same as that of the ray of light, but its velocity of course much less. Let us inquire about the velocity of propagation of the ray of light relative to the carriage. It is obvious that we can here apply the consideration of the previous section, since the ray of light plays the part of the man walking along relatively to the carriage. The velocity W of the man relative to the embankment is here replaced by the velocity of light relative to the embankment. w is the required velocity of light with respect to the carriage, and we have
w=c-v\,

notice how he always refers to a vacuum when describing the scene. that is because he has to avoid the fact that the atmosphere will make the speed of light slower.
 
  • #35
solarflare said:
notice how he always refers to a vacuum when describing the scene. that is because he has to avoid the fact that the atmosphere will make the speed of light slower.
No, he does that to be clear. It's the speed that's special. Light just happens to go at that speed in a vacuum.
 
  • #36
just to clarify - i am not anti relativity - i just want the truth to be taught and the truth is that C is not constant - it may be constant within a set reference frame - or set of reference frames in a certain scenario but it is not constant with regards to how light propagates through the universe.
 
  • #37
Section 23 - Behaviour of Clocks and Measuring-Rods on a Rotating Body of Reference

Hitherto I have purposely refrained from speaking about the physical interpretation of space- and time-data in the case of the general theory of relativity. As a consequence, I am guilty of a certain slovenliness of treatment, which, as we know from the special theory of relativity, is far from being unimportant and pardonable. It is now high time that we remedy this defect; but I would mention at the outset, that this matter lays no small claims on the patience and on the power of abstraction of the reader.

here he admits that he has not stuck to his own rules of general relativity.
 
  • #38
solarflare said:
just to clarify - i am not anti relativity - i just want the truth to be taught and the truth is that C is not constant - it may be constant within a set reference frame - or set of reference frames in a certain scenario but it is not constant with regards to how light propagates through the universe.
You need to learn a bit about relativity before having an opinion about it. You are confusing the invariant speed c ≈ 3 x 108 m/s (which happens to be the speed of light in a vacuum) with the phase velocity of light through some medium.
 
  • #39
when we see him tell us that certain things he said in the past were not accurate - should we not listen?
 
  • #40
solarflare said:
when we see him tell us that certain things he said in the past were not accurate - should we not listen?
Do you realize how overwhelmingly that relativity has been supported by experimental evidence? (Despite Einstein's charming modesty.)
 
  • #41
solarflare said:
just to clarify - i am not anti relativity - i just want the truth to be taught and the truth is that C is not constant - it may be constant within a set reference frame - or set of reference frames in a certain scenario but it is not constant with regards to how light propagates through the universe.

The speed of light IN A VACUUM is constant in all local reference frames, regardless of relative velocities.

No one has seriously claimed, hinted, suggested, or said that the speed of light IN A NON-VACUUM MEDIUM is constant, and relativity doesn't claim that it will be. So what's the truth that's not being taught?
 
  • #42
Section 27 - The Space-Time Continuum of the General Theory of Relativity is not a Euclidean Continuum

In the first part of this book we were able to make use of space-time co-ordinates which allowed of a simple and direct physical interpretation, and which, according to Section 26, can be regarded as four-dimensional Cartesian co-ordinates. This was possible on the basis of the law of the constancy of the velocity of light. But according to Section 21 the general theory of relativity cannot retain this law. On the contrary, we arrived at the result that according to this latter theory the velocity of light must always depend on the co-ordinates when a gravitational field is present. In connection with a specific illustration in Section 23, we found that the presence of a gravitational field invalidates the definition of the coordinates and the time, which led us to our objective in the special theory of relativity.
 
  • #43
the speed of light changes due to gravitational forces
 
  • #44
form wikipedia

Some aspects of the data are statistically unusual for the Standard Model of Cosmology. For example, the greatest angular-scale measurements, the quadrupole moment, is somewhat smaller than the Model would predict, but this discrepancy is not highly significant. A large cold spot and other features of the data are more statistically significant, and research continues into these. ( WMAP )

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"This was the method I adopted: I first assumed some principle, which I judged to be the strongest, and then I affirmed as true whatever seemed to agree with this, whether relating to the cause or to anything else; and that which disagreed I regarded as untrue." ( plato )

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note the deflection away from parts of the information that do not fit the theory as being "not highly significant" or as plato would have said "untrue".
 
  • #45
cherry picked data that fits with a theory will of course make that theory seem correct
 
  • #46
solarflare said:
the problem is that einstein came up with a theory where he said C is constant.

then later on he realized that it was not and stated so in other papers. i believe it was from 1911 onwards but do not have the info to hand.

but when everyone reads his works they take everything he said as true - even after he said himself he was wrong.
That's not a problem but scientific progress; science is not dogmatic like religion.

However, nothing of these things have to do with the topic. So, start your own thread. Continuing here is against the rules (so I won't reply here anymore, and to others also: don't feed the trolls!):

"Do not hijack an existing thread with off-topic comments or questions--start a new thread."
 
  • #47
phyti said:
ghwellsjr said:
That's not Einstein's second postulate. He didn't say that the speed of light is measured to be equal to c, he said the propagation of light is defined to be c. So when you have two clocks and you measure how long it takes a flash of light to go from the one to the other, and you divide the distance between them by the measured time difference between the two clocks and you calculate the speed of light to be something other than c, you tweak one of your clocks and repeat until the calculation comes out to be c. You're not measuring the propagation of light, you're defining it.
This is what I read in the 1905 paper.
Par.1, he is defining time for the purpose of simultaneity by clock synchronization using light signals. He refers to experience as a reason for the speed of light in space as a universal constant. In par.2, he defines the reflected signal as composed of equal path lengths out and return, avoiding the current impossibility of timing light for a unidirectional path, which would require separated clocks, which leads back to par.1.
The clock synch procedure only makes it appear that the out and back signal paths are equal, producing a relative synchronization. At the end of par.2, using a second observer B moving at a different speed, he shows on the basis of absolute speeds (c±v),that B does not consider the first clocks as synchronized. What the observer does measure is the round trip time, and due to length contaction and time dilation, his distance and time are scaled by the same factor 1/gamma, thus light speed is constant.
This is in complete agreement with what I said. When Einstein refers to experience, he is talking about measuring only the round-trip speed of light as you can see by his equation for determining the value of c which he affirms is a universal constant. But he never says that we can measure the one-way speed of light, we can only define it. And that is what is second postulate does.

Keep in mind that when Einstein wrote his paper, no one believed that the propagation (one-way) speed of light was c for every inertial observer even though they all agreed that the measured round-trip value of the speed of light was the universal constant c. They believed that light propagated at c only in the absolute ether rest state. Einstein is stating something revolutionary when he postulates that light also propagates at c even for an inertial observer moving with respect to the presumed ether. Lorentz, et al, claimed that it was the contraction of the ruler and the dilation of time on the clock for the moving inertial observer that resulted in his measurement of the round-trip speed of light coming out the same as it would if he were stationary in the ether where there would be no length contraction or time dilation. It never occurred to them that even for a moving observer, they could define length to be what his ruler measures and they could define time to be what his clock measures and they could define the one-way speed of light to be equal to his measured two-way speed of light and come out with "a simple and consistent theory", as Einstein put it in his introduction.
 
  • #48
ghwellsjr said:
[..] he never says that we can measure the one-way speed of light, we can only define it. And that is what is second postulate does.
It does more than that, as explained in posts #23, 26. We discussed that earlier here:
https://www.physicsforums.com/showthread.php?t=517105
Keep in mind that when Einstein wrote his paper, no one believed that the propagation (one-way) speed of light was c for every inertial observer [..]
Astronomers already used the postulate that light has a constant velocity, and in particular that its velocity is the same in all directions. And the issues with that were already explained here: http://en.wikisource.org/wiki/The_Measure_of_Time

and we also had a similar discussion here:
https://www.physicsforums.com/showthread.php?t=518005
 

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