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What Qm Might Say About Sr

Quantum mechanics Special relativity

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#35 exchemist

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Posted 09 January 2018 - 07:50 AM

SR and GR are connected in the same fabric it is silly to include one without the other. It would be like saying model a orbital of an electron but do not include the force of electromagnetism. 

 

This thread is not about GR. It is about some kind of supposed conflict between SR and quantum theory that JulianM believes to exist and I am trying to find out what it is. Attempting to drag GR into the discussion is not to going to help in the slightest, let alone dark energy, of all things.

 

I'm out of this now, as far as you are concerned. 


Edited by exchemist, 09 January 2018 - 07:56 AM.


#36 JulianM

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Posted 09 January 2018 - 10:39 AM

Vmedvil,

I am not ready for a discussion of GR, Gravity, Black Holes, etc.

I thought my question was extremely simple and I'd like to keep it at the level at which it was posed.

#37 JulianM

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Posted 09 January 2018 - 11:45 AM

exchemist,

Please don't read anything too deep into my comparison of QM and SR, it is more of a whimsical reference which i will explain in a moment, and not is core to my posting.

In essence I am saying that when we examine an observable event in the "train experiment" it does not result in any lack of simultaneity of time. This is the point I am trying to throw some light on

Why the whimsical reference - because QM generally states that when you measure something you affect it, which is what we are doing when we see the object illuminated.

In the derivation of SR there is no attempt to examine observable events. We are simply asked to conclude, even though something is un-seeable (light moving away from us), that we can say what it is doing.

#38 exchemist

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Posted 09 January 2018 - 11:52 AM

exchemist,

Please don't read anything too deep into my comparison of QM and SR, it is more of a whimsical reference which i will explain in a moment, and not is core to my posting.

In essence I am saying that when we examine an observable event in the "train experiment" it does not result in any lack of simultaneity of time. This is the point I am trying to throw some light on

Why the whimsical reference - because QM generally states that when you measure something you affect it, which is what we are doing when we see the object illuminated.

In the derivation of SR there is no attempt to examine observable events. We are simply asked to conclude, even though something is un-seeable (light moving away from us), that we can say what it is doing.

OK then perhaps we need to focus on what you are saying about simultaneity. 

 

As you can gather, I think one can overstate what QM says in a case like this. If we turn on a beam of light, consisting of some billions of photons, QM does not have any objection to us plotting its trajectory to a reflector, and back to the eye of an observer. The uncertainty principle does not make c uncertain. So let's leave that for now. I'll need to think about your simultaneity issue as I am not an expert on SR.   



#39 exchemist

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Posted 09 January 2018 - 03:13 PM

exchemist,

Please don't read anything too deep into my comparison of QM and SR, it is more of a whimsical reference which i will explain in a moment, and not is core to my posting.

In essence I am saying that when we examine an observable event in the "train experiment" it does not result in any lack of simultaneity of time. This is the point I am trying to throw some light on

Why the whimsical reference - because QM generally states that when you measure something you affect it, which is what we are doing when we see the object illuminated.

In the derivation of SR there is no attempt to examine observable events. We are simply asked to conclude, even though something is un-seeable (light moving away from us), that we can say what it is doing.

OK. I see what you are saying I think, which is that the stationmaster can easily rationalise why the reflection from the rear of the train arrives before that from the end of the platform. So no weird concepts are needed. That's fine, but you are only considering one observer in one frame of reference, so that is what you should expect.

 

Relativity of simultaneity only arises when comparing the perceptions of different observers in different frames of reference that are in relative motion. Your thought experiment doesn't do this. 


Edited by exchemist, 09 January 2018 - 03:14 PM.


#40 JulianM

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Posted 13 January 2018 - 04:29 AM

OK. I see what you are saying I think, which is that the stationmaster can easily rationalise why the reflection from the rear of the train arrives before that from the end of the platform. So no weird concepts are needed. That's fine, but you are only considering one observer in one frame of reference, so that is what you should expect.
 
Relativity of simultaneity only arises when comparing the perceptions of different observers in different frames of reference that are in relative motion. Your thought experiment doesn't do this.


Yes, you have captured my thoughts quite well except that I am not considering as a reflection but rather as an event consisting of a sudden illumination of the rear of the train. The front also would work.

The stationmaster can easily rationalize it and this is very observable to the extent that it could be set up (though not by me) as an experiment.

I think that seeing the image generated by the flash is different from a reflected beam which is how SR treats it. The difference is which frame of reference is considered for the light arriving at the stationmaster. He sees it at speed = c in his frame but imagines it as coming towards him (c + v) when he thinks about the train.

In terms of the perception of different observers both see light at speed = c, both see the same length, and both agree on speed = v.
Both agree on observable events and can calculate, based on the location of events, that the time differences are equivalent to the distance from them.

So why the difference between an analysis of the observable events and the theory of SR?

#41 exchemist

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Posted 13 January 2018 - 05:45 AM

Yes, you have captured my thoughts quite well except that I am not considering as a reflection but rather as an event consisting of a sudden illumination of the rear of the train. The front also would work.

The stationmaster can easily rationalize it and this is very observable to the extent that it could be set up (though not by me) as an experiment.

I think that seeing the image generated by the flash is different from a reflected beam which is how SR treats it. The difference is which frame of reference is considered for the light arriving at the stationmaster. He sees it at speed = c in his frame but imagines it as coming towards him (c + v) when he thinks about the train.

In terms of the perception of different observers both see light at speed = c, both see the same length, and both agree on speed = v.
Both agree on observable events and can calculate, based on the location of events, that the time differences are equivalent to the distance from them.

So why the difference between an analysis of the observable events and the theory of SR?

But you have not made any reference to a second observer in your scenario. Without two observers in different reference frames, special relativity has nothing to say: you have "normal" classical physics. 

 

If you want to discuss two observers in different frames, you need to define where the second observer is, and then we will have a scenario in which SR can be brought to bear.  



#42 JulianM

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Posted 13 January 2018 - 08:52 AM

But you have not made any reference to a second observer in your scenario. Without two observers in different reference frames, special relativity has nothing to say: you have "normal" classical physics. 
 
If you want to discuss two observers in different frames, you need to define where the second observer is, and then we will have a scenario in which SR can be brought to bear.


Ok, let me think about that

#43 JulianM

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Posted 15 January 2018 - 04:21 PM

But you have not made any reference to a second observer in your scenario. Without two observers in different reference frames, special relativity has nothing to say: you have "normal" classical physics. 
 
If you want to discuss two observers in different frames, you need to define where the second observer is, and then we will have a scenario in which SR can be brought to bear.


I think we do have two observers in two different frames. We have a Stationmaster (S) on the platform and a Passenger (P) on the train. The train is moving relative to the platform.

Does that meet your criteria?

#44 exchemist

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Posted 16 January 2018 - 01:57 AM

I think we do have two observers in two different frames. We have a Stationmaster (S) on the platform and a Passenger (P) on the train. The train is moving relative to the platform.

Does that meet your criteria?

Well it would if you had referred to this passenger in your scenario. But you didn't: all you were comparing was two beams of light, both observed by the stationmaster. 

 

Why don't you now change your scenario to specify what a passenger in the train observes and compare that with what the stationmaster sees? I think you will find you the relativity of simultaneity reappears.



#45 JulianM

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Posted 18 January 2018 - 06:59 AM

I am trying to do that, which takes some thinking. I am not trying to prove or disprove any current theory, or argue for any particular idea.

What I find, though, is that when we restrict an observer to only what he can observe the outcome is different from what he concludes to be happening by imagining the other frame of reference.

I am 100% agreeing with all of Einstein's postulates but then examining his doubts about whether length shortening is real or visual in an attempt to understand his doubts.

If you are able to examine carefully and respond to what I am saying I would very much appreciate that. If you are already convinced I am wrong then the discussion is essentially over and we will just argue past each other.

Now I fully expect that I am headed for either an Aha or a Doh moment and I am very prepared to be wrong, but I still want to understand.

#46 exchemist

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Posted 18 January 2018 - 08:58 AM

I am trying to do that, which takes some thinking. I am not trying to prove or disprove any current theory, or argue for any particular idea.

What I find, though, is that when we restrict an observer to only what he can observe the outcome is different from what he concludes to be happening by imagining the other frame of reference.

I am 100% agreeing with all of Einstein's postulates but then examining his doubts about whether length shortening is real or visual in an attempt to understand his doubts.

If you are able to examine carefully and respond to what I am saying I would very much appreciate that. If you are already convinced I am wrong then the discussion is essentially over and we will just argue past each other.

Now I fully expect that I am headed for either an Aha or a Doh moment and I am very prepared to be wrong, but I still want to understand.

But why are you are preoccupied with comparing what an observer observes with what he might imagine could be happening in another frame of reference. For instance, he might imagine that he can treat the clocks on the train as running at the same rate as his watch - and he'd be wrong, but where does that get us? 

 

Relativity is concerned - like all science - with modelling what is observed, not what one observer might be (rightly or wrongly) imagining would be seen by another. 



#47 JulianM

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Posted 18 January 2018 - 01:01 PM

But why are you are preoccupied with comparing what an observer observes with what he might imagine could be happening in another frame of reference. For instance, he might imagine that he can treat the clocks on the train as running at the same rate as his watch - and he'd be wrong, but where does that get us? 
 
Relativity is concerned - like all science - with modelling what is observed, not what one observer might be (rightly or wrongly) imagining would be seen by another.


I agree with your last sentence and that is the point I am trying to make, so the key is to model, or measure what can be directly observed.

At the starting point, where we consider a train or similar thought experiment, Relativity does only deal with an imagined result. Basically it says that to reach a point in a moving frame either than one's own it travels a distance determined from (c + v) which we know to be not true therefore we conclude that time or distance must be different.

An actual observation, as I am trying to explain (not very well I guess) shows that there is no reason to assume lack of simultaneity.

The reason I am "preoccupied" is because there seems to be a difference.

He cannot see things in another frame of reference unless he receives information from that frame of reference and at this level he cannot yet presume that clocks are running at different speeds, even if we determine that later.

#48 exchemist

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Posted 18 January 2018 - 03:26 PM

I agree with your last sentence and that is the point I am trying to make, so the key is to model, or measure what can be directly observed.

At the starting point, where we consider a train or similar thought experiment, Relativity does only deal with an imagined result. Basically it says that to reach a point in a moving frame either than one's own it travels a distance determined from (c + v) which we know to be not true therefore we conclude that time or distance must be different.

An actual observation, as I am trying to explain (not very well I guess) shows that there is no reason to assume lack of simultaneity.

The reason I am "preoccupied" is because there seems to be a difference.

He cannot see things in another frame of reference unless he receives information from that frame of reference and at this level he cannot yet presume that clocks are running at different speeds, even if we determine that later.

No I have to disagree. Relativity does not speculate about what an observer might imagine. 

 

It deals in a logical manner with what would be observed

 

I wonder if you are confusing the idea of a thought experiment, i.e. us setting up an imaginary scenario (which we then analyse rigorously by ensuring the speed of light is the same in all reference frames), with some idea that the observer in the scenario is imagining something. He is not and this is never part of anything relativity analyses.  Re-read the descriptions of the classic thought experiments carefully. There is no reference whatsoever to anything imagined by any observer. The focus is 100% on he would observe

 

And I repeat, lack of simultaneity arises between observers in different frames. You have not attempted to do this, so it is not surprising you find no lack of simultaneity.


Edited by exchemist, 18 January 2018 - 03:29 PM.


#49 JulianM

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Posted 18 January 2018 - 06:55 PM

I thought I covered that way back in the thread.

The light flash travels down both the train and the platform at speed = c for both. Agree?

They are both the same length, by definition of the experiment, OK with that?

The rear of the train is illuminated by the flash and that image travels to S at speed = c in his frame of reference. A similar situation exists for P.

Now, because the light travels the same distance in each frame and at the same speed it must take the same time (more on this later)

Now simple math shows that when S sees the two illuminations and calculates how far away they were from him then the time difference is solely due to the distance in his time frame, i.e. they are easily determined as simultaneous. It can be determined from vt. Exactly the same occurs for P.

Now, about the time. We cannot start by assuming that time for the train is the same as for the platform if we are trying to unnderstand/show what is happening. That would be equivalent to saying - if we assume a time difference we get a time difference, so Relativity proves Relativity if you start with an assumption of Relativity.

Now how does classical relativity deal with this. The postulate is that instead of looking forthe observable illumination we consider the light travelling back in the train/platform and conclude that because light in the S frame reached a position in the P frame that is not coincident with the position of S then the distance travelled in S means that it could only reach that same coordinate in P if there was a time or distance difference. That is not observable by S.

I understand that you just don't believe this as it is counterintuitive based on Relativity theory but it takes some thinking.

Where is the flaw, and don't just say - well that's wrong. What specifically is the problem?

#50 exchemist

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Posted 19 January 2018 - 03:06 AM

I thought I covered that way back in the thread.

The light flash travels down both the train and the platform at speed = c for both. Agree?

They are both the same length, by definition of the experiment, OK with that?

The rear of the train is illuminated by the flash and that image travels to S at speed = c in his frame of reference. A similar situation exists for P.

Now, because the light travels the same distance in each frame and at the same speed it must take the same time (more on this later)

Now simple math shows that when S sees the two illuminations and calculates how far away they were from him then the time difference is solely due to the distance in his time frame, i.e. they are easily determined as simultaneous. It can be determined from vt. Exactly the same occurs for P.

Now, about the time. We cannot start by assuming that time for the train is the same as for the platform if we are trying to unnderstand/show what is happening. That would be equivalent to saying - if we assume a time difference we get a time difference, so Relativity proves Relativity if you start with an assumption of Relativity.

Now how does classical relativity deal with this. The postulate is that instead of looking forthe observable illumination we consider the light travelling back in the train/platform and conclude that because light in the S frame reached a position in the P frame that is not coincident with the position of S then the distance travelled in S means that it could only reach that same coordinate in P if there was a time or distance difference. That is not observable by S.

I understand that you just don't believe this as it is counterintuitive based on Relativity theory but it takes some thinking.

Where is the flaw, and don't just say - well that's wrong. What specifically is the problem?

First, can you define exactly what S and P are. 

 

I presume S is the stationmaster you described originally. But you have never described a second observer. Who is P, where is he located and what is the scenario for him?



#51 JulianM

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Posted 19 January 2018 - 06:06 AM

S is the Stationmasyer. He is standing in the middle of the platform.

P is the passenger. He is sitting in the middle of the train.

The train is the same length as the platform. Both have length L

The train is travelling at speed v relative to the platform (and vice versa of course)

At the moment that the Passenger is at the same position as the Stationmaster a flash of light occurs. Light travels along the train at speed c relative to the Passenger and at speed c along the platform relative to the Stationmaster.

This is exactly how a thought experiment would describe things and agrees with all postulates.

What I am now saying is that the light will travel along the platform/train and when it reaches the end (or front would work too) and as it reaches the end the rear of the train/platform is suddenly illuminated.

The question now is what does each person see, and of course because it is symmetrical and relative if both take measurements in the same way and take the measurements in their own frame of reference. They are allowed to assume the speed of light in their frame of reference and that at any given time events seen at the same time but at different distances took place at different times.

Now we agree that the flash of light is a single event, Yes?

We agree that anyone in this scenario measures lengths, speed of light, etc the same.

The two illuminations are therefore the same event, Yes? and both calculate the time interval form c and L/2, Yes?

I'll pause here for a moment to see if you disagree with any of this.



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