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What Qm Might Say About Sr

Quantum mechanics Special relativity

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#69 JulianM

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Posted 23 January 2018 - 03:43 AM

[quote name="exchemist" post="354206" timestamp="1516696594"]Now you are re-introducing the rear of the platform again. I thought you had agreed we could leave that out (post 70).
 
If you want to reintroduce it, you need to define where it is in relation to the train carriage. Are you, for instance, suggesting the rear of the carriage is exactly in line with the rear of the platform when the flash of light is emitted?
[/quoste]

Not really, that was just an additional reference. The primary reference was the location of the rear of the train in the frame of reference of the platform.

Doesn't there need to be some comparison of some measurement in order to determine they see things differently? A simple measurement of - I saw a flash then t secs later I saw another flash tells us nothing as far as I can determine.

What comparison should we make?

#70 exchemist

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Posted 23 January 2018 - 04:45 AM

[quote name="exchemist" post="354206" timestamp="1516696594"]Now you are re-introducing the rear of the platform again. I thought you had agreed we could leave that out (post 70).
 
If you want to reintroduce it, you need to define where it is in relation to the train carriage. Are you, for instance, suggesting the rear of the carriage is exactly in line with the rear of the platform when the flash of light is emitted?
[/quoste]

Not really, that was just an additional reference. The primary reference was the location of the rear of the train in the frame of reference of the platform.

Doesn't there need to be some comparison of some measurement in order to determine they see things differently? A simple measurement of - I saw a flash then t secs later I saw another flash tells us nothing as far as I can determine.

What comparison should we make?

I pointed out right back at the start that you can only illustrate relativity of simultaneity if you compare what is seen by observers in 2 frames of reference in relative motion. The classic version of train scenario does this, by comparing what P and S observe.  

 

Having S make two observations, viz. rear of carriage and rear of platform, does not do that. 


Edited by exchemist, 23 January 2018 - 04:47 AM.


#71 JulianM

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Posted 23 January 2018 - 06:08 AM

I pointed out right back at the start that you can only illustrate relativity of simultaneity if you compare what is seen by observers in 2 frames of reference in relative motion. The classic version of train scenario does this, by comparing what P and S observe.  
 
Having S make two observations, viz. rear of carriage and rear of platform, does not do that.


So both P & S can only see the train, and neither can see the platform?

#72 exchemist

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Posted 23 January 2018 - 06:17 AM

So both P & S can only see the train, and neither can see the platform?

No. So that you have one observer in each of the two frames of reference.



#73 JulianM

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Posted 23 January 2018 - 10:03 AM

Can you please clarify. I am now confused.

As I understand it you have said we must ignore the platform, which is one of the frames.

I understand P & S, and I understand the train as a frame.

Why must we not take measurements on the platform since that is equally valid as a frame of reference.

#74 exchemist

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Posted 23 January 2018 - 10:58 AM

Can you please clarify. I am now confused.

As I understand it you have said we must ignore the platform, which is one of the frames.

I understand P & S, and I understand the train as a frame.

Why must we not take measurements on the platform since that is equally valid as a frame of reference.

The platform is not "one of the frames". A frame of reference is an attribute of an observer, not a physical thing. It is the coordinate system that an observer implicitly uses when making measurements from his own perspective. 

 

I'm saying you need to compare what S and P see when viewing light reflected to them from the same object. That object can be the front or rear of the train carriage. I suppose it could be the front or rear of the platform if you want. But as the platform is not mentioned in the version illustrated in the Wiki article you will need to define where it is if you want to introduce it into the proceedings. 


Edited by exchemist, 23 January 2018 - 11:00 AM.


#75 JulianM

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Posted 23 January 2018 - 04:15 PM

Well surely the platform would represent the x axis of the observer S.

You have eliminated the platform, the possibility of seeing any part of the train (only a theoretical reflection) and, of course, S cannot see what P sees because that would entail him having vision that acts faster than the speed of light.

No one in this scenario can do any comparisons, so I understand that when you say "you need to compare what S and P see" you are asking that I be the observer.

Let me think about this.

#76 exchemist

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Posted 24 January 2018 - 03:26 AM

Well surely the platform would represent the x axis of the observer S.

You have eliminated the platform, the possibility of seeing any part of the train (only a theoretical reflection) and, of course, S cannot see what P sees because that would entail him having vision that acts faster than the speed of light.

No one in this scenario can do any comparisons, so I understand that when you say "you need to compare what S and P see" you are asking that I be the observer.

Let me think about this.

The axes do not need to be represented by anything physical. But you can introduce the platform if you like. If you do, though, you need to specify where the ends of the platform are in relation to the train carriage.

 

P.S. I am NOT asking you to be the observer. I really fail to understand the problem you are having here. I keep telling you, you need to compare what S and P observe. Not that you are the observer, but you work out what each of them will observe, using the key fact that c is frame-independent.  


Edited by exchemist, 24 January 2018 - 10:28 AM.


#77 JulianM

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Posted 24 January 2018 - 07:23 PM

Actually it's not even necessary to specify anything about the ends of the platform, it's helpful to use it, or anything representing the axes of the "experiment".

I am not having a problem understanding what you are saying. You description and reference is very common and wisely used. It's not difficult to understand at all.

What I am describing is an alternate way of looking at this by considering what an observer actually does see.

I believe that you are not understanding my explanation of what happens when we use real world observations, and these are common everyday experiences.

Observers see things. They don't see photons or waves moving in space.

#78 exchemist

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Posted 25 January 2018 - 02:28 AM

Actually it's not even necessary to specify anything about the ends of the platform, it's helpful to use it, or anything representing the axes of the "experiment".

I am not having a problem understanding what you are saying. You description and reference is very common and wisely used. It's not difficult to understand at all.

What I am describing is an alternate way of looking at this by considering what an observer actually does see.

I believe that you are not understanding my explanation of what happens when we use real world observations, and these are common everyday experiences.

Observers see things. They don't see photons or waves moving in space.

Why did you say in post 80 that I was asking  you to be the observer? 



#79 JulianM

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Posted 25 January 2018 - 08:10 AM

Why did you say in post 80 that I was asking  you to be the observer?


Simply because you stated "you need to compare what S and P see".

This kind of communication, like email, can be confusing so perhaps I misunderstood your meaning. Where I was at was - both P & S are in identical situations, neither knows which is moving, etc., etc., you know the premises.

So I was at the point where, since we can determine they perceive the same thing then I assumed you were saying it takes a third, external, observer to see a difference.

#80 exchemist

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Posted 25 January 2018 - 02:11 PM

Simply because you stated "you need to compare what S and P see".

This kind of communication, like email, can be confusing so perhaps I misunderstood your meaning. Where I was at was - both P & S are in identical situations, neither knows which is moving, etc., etc., you know the premises.

So I was at the point where, since we can determine they perceive the same thing then I assumed you were saying it takes a third, external, observer to see a difference.

Oh dear. It looks as if you do not understand how the logic of a thought experiment works. You do not have to be a participant in the scenario to analyse it. 



#81 JulianM

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Posted 25 January 2018 - 05:34 PM

I understand the "experiment" for sure, but I am having trouble understanding your explanation of it.

The illustrations on Wikipedia are very simple, but quite incomplete if we are to consider this an "experiment". It does not consider how measurements would be taken or what would actually be seen.

For example it indicates a path of light, as understood by the platform, as just stopping. The platform observer cannot see that, so this is merely imagination.

We are left to assume that if there was a return path (not considered in the illustration) then the platform observer would be able to see two flashes, one from the light co0ming down the train and one from the light coming down the platform being reflected off of nothing, or if it is off the train he would see the rear of the train light up twice.


Now it's a fundamental fact of light, that we witness all day every day, that we don't see photons or waves. We see the effects of light striking an object - a train, a platform, a retina for example.

Clearly the rear of the train is illuminated by the flash (in my view only once) and this can be seen by both observers and imagined by a non-participant who is browsing Wikipedia. It is seen because the reflection travels back to the observers.

As soon as the platform axis is considered we see why, and this still assumes that the speed of light is c in both frames of reference.

I am assuming all the postulates of relativity and not challenging Relativity in any way, simply looking at this in a logical manner.

#82 exchemist

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Posted 26 January 2018 - 02:14 AM

I understand the "experiment" for sure, but I am having trouble understanding your explanation of it.

The illustrations on Wikipedia are very simple, but quite incomplete if we are to consider this an "experiment". It does not consider how measurements would be taken or what would actually be seen.

For example it indicates a path of light, as understood by the platform, as just stopping. The platform observer cannot see that, so this is merely imagination.

We are left to assume that if there was a return path (not considered in the illustration) then the platform observer would be able to see two flashes, one from the light co0ming down the train and one from the light coming down the platform being reflected off of nothing, or if it is off the train he would see the rear of the train light up twice.


Now it's a fundamental fact of light, that we witness all day every day, that we don't see photons or waves. We see the effects of light striking an object - a train, a platform, a retina for example.

Clearly the rear of the train is illuminated by the flash (in my view only once) and this can be seen by both observers and imagined by a non-participant who is browsing Wikipedia. It is seen because the reflection travels back to the observers.

As soon as the platform axis is considered we see why, and this still assumes that the speed of light is c in both frames of reference.

I am assuming all the postulates of relativity and not challenging Relativity in any way, simply looking at this in a logical manner.

Right, so you or I, considering this scenario and working out what each observer will see, are not, ourselves, observers. OK?

 

The platform is not mentioned, except as the place where one observer stands. You have an observer P in the centre of a train carriage and another S on the platform.

 

Both see light reflected from the two ends of the train carriage. Not from anything on the platform. The two ends of the train carriage.

 

P sees the front and rear of the carriage illuminated simultaneously. 

 

S sees the rear end illuminated before the front end, i.e. not simultaneously. 

 

That's it. 

 

None of this reasoning relies on seeing the light in transit.  At all. 


Edited by exchemist, 26 January 2018 - 02:17 AM.


#83 JulianM

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Posted 26 January 2018 - 04:14 PM

That's not quite what the wiki says. It does not deal with reflections, and it does make assumptions about the transit of light by drawing its vector, but that's ok.

The rest only works when you deny observer S of all knowledge of his surroundings. If we allow him to understand his frame of reference and measure along his axis he can determine that the illumination of the rear took place at a distance less than L/2 from him. This creates no problems since as the train passed he saw its length as L and; because the time of observation is based on distance if we use light as our measuring tool. If it's closer it is seen first which does not itself cause loss of simultaneity.

Now what is shown on the wiki is not a "thought experiment"; it is merely a "thought".

Still we can stay with the premise that S is ignorant of his surroundings as you wish but this gives a bigger problem. The portion of the train to the left of him behaves differently (time specifically in this example) to the portion on the right.

This logically can't be a result of movement since both ends of the train are moving at the same speed and in the same direction. We can get behind the idea that length and time can be affected by relative movement, but not the properties of left and right.
Are we to conclude that whenever a part of a train goes from left to right its properties change? I hope not.

So this is obviously not a proper analysis and we need to think a little deeper.

When you say - that's it it really only is that's it if you stop there.

#84 bangstrom

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Posted 27 January 2018 - 06:55 AM

The value of c is ~300,000 km/ sec and c has all the properties of a space/time dimensional constant but it acts nothing like a velocity.  I agree with JulianM. Stick to what is observed/detected and forget about what you think is happening. Light vanishes from a source and appears at a remote point with an observed time delay between the two events of one second delay for every 300,000 km of distance for all observers. This is what we observe.

 

Observers having different velocities should see the speed of light as a variable because velocities add but, in SR, velocities do not add to c because c is a constant distance/time ratio and not a speed and you can’t add a speed to a ratio any more than you can go faster than 1.6 kilometers per mile. We do not see photons and we do not see them traveling through space with an observable velocity and, when we analyze events in SR by plotting the path and speed of the photon, it fails every time.

 

The dilemma between QM and SR arises from Einstein’s second postulate for SR which is a conjecture not based on observation and not necessary for the math in either SR or QM. Einstein postulated that light travels with a measurable speed but how can this be true if, as JulianM said of QM, “ You cannot see, or even know, what is happening to the "bouncing ball" when it is travelling between the mirrors. All we can see is the events as it strikes the mirror.”

 

You can’t measure the speed of something you can’t observe or say that light even exists between source and sink so saying that light has a speed and travels through space is intuitive but not based on observation. SR makes sense when the interpretation of light as something traveling through space with a known velocity is dropped from our analysis of events.

 

The constant c works as a dimensional constant in both SR and QM giving us the interval of time found in every interval of space for all observers. Hermann Bondi said the value of c is essentially the length of a standard meter expressed in units of a second.

 

The observer on the moving train sees the light strike the front and the rear of the train simultaneously because he sees the two distances as equal and equal distances include equal intervals of time.

 

The observer on the platform sees the light strike the rear of the train first because the rear of the train has advanced towards him and the distance is now shorter. He sees the light strike the front of the train later because the front of the train is farther away when the light strikes but all observers calculate a time delay of one second for every 300,000 km of observed distance for light related events and that is the meaning of c.



#85 exchemist

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Posted 27 January 2018 - 10:57 AM

The value of c is ~300,000 km/ sec and c has all the properties of a space/time dimensional constant but it acts nothing like a velocity.  I agree with JulianM. Stick to what is observed/detected and forget about what you think is happening. Light vanishes from a source and appears at a remote point with an observed time delay between the two events of one second delay for every 300,000 km of distance for all observers. This is what we observe.

 

Observers having different velocities should see the speed of light as a variable because velocities add but, in SR, velocities do not add to c because c is a constant distance/time ratio and not a speed and you can’t add a speed to a ratio any more than you can go faster than 1.6 kilometers per mile. We do not see photons and we do not see them traveling through space with an observable velocity and, when we analyze events in SR by plotting the path and speed of the photon, it fails every time.

 

The dilemma between QM and SR arises from Einstein’s second postulate for SR which is a conjecture not based on observation and not necessary for the math in either SR or QM. Einstein postulated that light travels with a measurable speed but how can this be true if, as JulianM said of QM, “ You cannot see, or even know, what is happening to the "bouncing ball" when it is travelling between the mirrors. All we can see is the events as it strikes the mirror.”

 

You can’t measure the speed of something you can’t observe or say that light even exists between source and sink so saying that light has a speed and travels through space is intuitive but not based on observation. SR makes sense when the interpretation of light as something traveling through space with a known velocity is dropped from our analysis of events.

 

The constant c works as a dimensional constant in both SR and QM giving us the interval of time found in every interval of space for all observers. Hermann Bondi said the value of c is essentially the length of a standard meter expressed in units of a second.

 

The observer on the moving train sees the light strike the front and the rear of the train simultaneously because he sees the two distances as equal and equal distances include equal intervals of time.

 

The observer on the platform sees the light strike the rear of the train first because the rear of the train has advanced towards him and the distance is now shorter. He sees the light strike the front of the train later because the front of the train is farther away when the light strikes but all observers calculate a time delay of one second for every 300,000 km of observed distance for light related events and that is the meaning of c.

I shall leave Julian M in your capable hands. 





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