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What Qm Might Say About Sr

Quantum mechanics Special relativity

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#52 JulianM

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Posted 19 January 2018 - 09:26 AM

You got what you wanted. Well done.
 
See how much of a disingenuous liar you are though?
 
''Oh I just found out he was banned at science forums?'' 
 
Pants on fire a bit maybe? It' ok, you did me a favor, I lasted one day at 'the science forums' - they didn't like my posting style. But really next time you pull a stunt like that somewhere, try and at least keep to the truth. Don't say ''oh I just found out he got banned'' when you've been sitting on that knowledge for months.


Who are you and who are you talking to?

#53 exchemist

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Posted 19 January 2018 - 09:41 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.

OK, yes. One question: are both of them calculating the time for light to reach the end of the train AND the end of the platform?  



#54 exchemist

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Posted 19 January 2018 - 11:00 AM

You got what you wanted. Well done.

 

See how much of a disingenuous liar you are though?

 

''Oh I just found out he was banned at science forums?'' 

 

Pants on fire a bit maybe? It' ok, you did me a favor, I lasted one day at 'the science forums' - they didn't like my posting style. But really next time you pull a stunt like that somewhere, try and at least keep to the truth. Don't say ''oh I just found out he got banned'' when you've been sitting on that knowledge for months.

You were banned on 4th Dec from that forum (science forums dot net). So I can't have been "sitting on that knowledge for months". I found out yesterday, as a matter of fact. 

 

You were banned under the pseudonym Geon from sciforums on 6th Sept. And I knew about that at the time.  

 

But I only reply briefly to defend myself, as this is off-topic. If you want to discuss it further, start a thread in the section about members. 



#55 exchemist

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Posted 20 January 2018 - 06:38 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.

Julian if you care to continue, now that the interruption is over, please do. :) 



#56 JulianM

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Posted 20 January 2018 - 01:59 PM

Julian if you care to continue, now that the interruption is over, please do. :)

 

Thank you, sir. I needed a break anyway to do some thinking and have a beer

 

Here is a link to a very similar scenario.

https://youtu.be/wteiuxyqtoM

 

There is no reference that I can see to who published it but it seems professionally produced and has 1.3 million views.

 

In this video clip  the voice over says that the Passenger will see the light from the front of the train first. Will They? Or would the light in the passengers frame travel at speed = c and therefore also arrive simultaneously ?



#57 exchemist

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Posted 20 January 2018 - 04:06 PM

Thank you, sir. I needed a break anyway to do some thinking and have a beer

 

Here is a link to a very similar scenario.

https://youtu.be/wteiuxyqtoM

 

There is no reference that I can see to who published it but it seems professionally produced and has 1.3 million views.

 

In this video clip  the voice over says that the Passenger will see the light from the front of the train first. Will They? Or would the light in the passengers frame travel at speed = c and therefore also arrive simultaneously ?

This looks like the classic train carriage thought experiment, in which indeed P sees the light reflected from the front of the carriage before S, because for S the front is receding, so by the time the light strikes it the distance  is >l/2, so the distance there and back it travels (at c) to reach his eye >l, whereas for P the distance there and back it travels (again at c) is obviously l exactly.  


Edited by exchemist, 20 January 2018 - 04:08 PM.


#58 JulianM

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Posted 20 January 2018 - 05:55 PM

This looks like the classic train carriage thought experiment, in which indeed P sees the light reflected from the front of the carriage before S, because for S the front is receding, so by the time the light strikes it the distance  is >l/2, so the distance there and back it travels (at c) to reach his eye >l, whereas for P the distance there and back it travels (again at c) is obviously l exactly.  

Yas, it is an example of typical train thought experiments.

 

Just a couple of clarifications -

 

1. In this clip there is no consideration of reflection or of the return path. The distance travelled is one way.

 

2. This clip does not preserve the speed of the light in the train at c. In the passengers frame of reference the strike occurred at the front of the train (L/2) away from him, must travel at speed = c and t is easily calculated for each. Yes, S sees the light strike P before it reaches him but that is easily explained in Newtonian terms.

 

Nevertheless we can put all that aside for the moment as we are not trying to prove or disprove SR.

 

The point I am making is that nobody sees a wave of light coming towards them. The only thing that is observed is an image created by light.

Now we have a common scenario I will use that video clip as my "experiment"

 

To make it easier to understand let's set this experiment in darkness in order to obscure other external events and consider only the flash at the front.

 

The passenger sees nothing whatsoever until the "flash" reaches him. As it reaches him he sees an image of the point where it ocurred, agreed?



#59 exchemist

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Posted 21 January 2018 - 08:39 AM

Yas, it is an example of typical train thought experiments.

 

Just a couple of clarifications -

 

1. In this clip there is no consideration of reflection or of the return path. The distance travelled is one way.

 

2. This clip does not preserve the speed of the light in the train at c. In the passengers frame of reference the strike occurred at the front of the train (L/2) away from him, must travel at speed = c and t is easily calculated for each. Yes, S sees the light strike P before it reaches him but that is easily explained in Newtonian terms.

 

Nevertheless we can put all that aside for the moment as we are not trying to prove or disprove SR.

 

The point I am making is that nobody sees a wave of light coming towards them. The only thing that is observed is an image created by light.

Now we have a common scenario I will use that video clip as my "experiment"

 

To make it easier to understand let's set this experiment in darkness in order to obscure other external events and consider only the flash at the front.

 

The passenger sees nothing whatsoever until the "flash" reaches him. As it reaches him he sees an image of the point where it ocurred, agreed?

Well yes, but now you have completely lost me. I no longer have any idea where you are going with all this. 

 

I have been maintaining from the beginning that what counts in all these theories is what is observed. Certainly, the classic form of this "train" thought experiment focuses on what is observed by S and P when the reflected light reaches them, rather than on anything they might imagine to be happening before it does so. (Light reflected from the front of the train to S will takes longer to arrive than it does to P, because for S the front of the carriage has moved away a bit before the light hits it, so the path it take is longer than the path back to P,  who is moving with the train, etc, etc. This is all standard.)

 

But carry on and let me see if I can follow where you go next. 



#60 JulianM

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Posted 21 January 2018 - 06:35 PM

Well yes, but now you have completely lost me. I no longer have any idea where you are going with all this. 

 

I have been maintaining from the beginning that what counts in all these theories is what is observed. Certainly, the classic form of this "train" thought experiment focuses on what is observed by S and P when the reflected light reaches them, rather than on anything they might imagine to be happening before it does so. (Light reflected from the front of the train to S will takes longer to arrive than it does to P, because for S the front of the carriage has moved away a bit before the light hits it, so the path it take is longer than the path back to P,  who is moving with the train, etc, etc. This is all standard.)

 

But carry on and let me see if I can follow where you go next. 

 

 

 

 

So, thank you for listening.

 

I will try to summarize. We know from Quantum Mechanics and various experients (e.g. Hiatachi) that by taking a measurement we "collapse" the wave front. Actually i don't  like that word, but we can move past that.

 

So we are looking at the train experiment (We could look at other experiments but this works well enough) from a perspective of who sees (measures) what and when.

 

I believe that a moving wavefront or photon cannot be observed and that the only observable event is the image created when that light illuminates something.

 

Now we have just agreed, I believe, that when the light strikes the front of the train both observers receive that image and that it is identical for them.

 

What happens next is your question. Well the next observable image in this scenario is that the light arrives at the Passenger, travelling a distance L/2 at speed = c. This also creates an image and that image travels from that position to the Stationmaster (S). It travels at the speed of light (it is light) and arrives at the Stationmaster.

 

Now the Stationmaster sees that image before he is illuminated by the same flash. I think that is what you just described.

 

The thing here is that he sees it at some distance from him. Since we were 8 years old, or so, we have been counting 1000 & 1, 1000 & 2, etc. In order to determine how far away a lightning strike from us, so our 8 year old Stationmaster can do the same thing and say - yes, that happened 3 seconds ago, but it happened x distance from me so it actually happened at the same time. In fact motion is not required. Given that all we have seen is two illuminations we can simply conclude that if someone were standing on the platform at the position where we see the Passenger illuminated then they would identically see the flash first, and we can calculate that, and that the difference would be a function of the distance travelled in time t at velocity v, and that the apparent difference can be determined by counting 1000 & 1, etc. just as we did as kids.

 

L is the same, c is the same and t is the same.

 

The fact is SR uses light as our measuring tool and our tool has a time dependency so taking a measurement of the position or behavior of a moving object is dependent on whether it moves during the time we take the measurement.

 

Is there somehing Here you disagree with?

 

If not I will continue.



#61 JulianM

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Posted 22 January 2018 - 05:29 AM

I know it seems like a paradox... but its the way relativity understands frame of references and moving observers relative to each other. The fact dilation occurs and that no true simultaneity exists, is part of experimental fact. 

 

I am not arguing whether Relativity exists or not. I think i made that clear in an earlier statement.This is a low key discussion of perceiving what happens by creating images, as they would be seen, based on a different thought process.

 

If you have something to add at that level, or if you see that I am making mistakes at a detail level, then I would be pleased to understand my errors.



#62 exchemist

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Posted 22 January 2018 - 05:43 AM

So, thank you for listening.

 

I will try to summarize. We know from Quantum Mechanics and various experients (e.g. Hiatachi) that by taking a measurement we "collapse" the wave front. Actually i don't  like that word, but we can move past that.

 

So we are looking at the train experiment (We could look at other experiments but this works well enough) from a perspective of who sees (measures) what and when.

 

I believe that a moving wavefront or photon cannot be observed and that the only observable event is the image created when that light illuminates something.

 

Now we have just agreed, I believe, that when the light strikes the front of the train both observers receive that image and that it is identical for them.

 

What happens next is your question. Well the next observable image in this scenario is that the light arrives at the Passenger, travelling a distance L/2 at speed = c. This also creates an image and that image travels from that position to the Stationmaster (S). It travels at the speed of light (it is light) and arrives at the Stationmaster.

 

Now the Stationmaster sees that image before he is illuminated by the same flash. I think that is what you just described.

 

The thing here is that he sees it at some distance from him. Since we were 8 years old, or so, we have been counting 1000 & 1, 1000 & 2, etc. In order to determine how far away a lightning strike from us, so our 8 year old Stationmaster can do the same thing and say - yes, that happened 3 seconds ago, but it happened x distance from me so it actually happened at the same time. In fact motion is not required. Given that all we have seen is two illuminations we can simply conclude that if someone were standing on the platform at the position where we see the Passenger illuminated then they would identically see the flash first, and we can calculate that, and that the difference would be a function of the distance travelled in time t at velocity v, and that the apparent difference can be determined by counting 1000 & 1, etc. just as we did as kids.

 

L is the same, c is the same and t is the same.

 

The fact is SR uses light as our measuring tool and our tool has a time dependency so taking a measurement of the position or behavior of a moving object is dependent on whether it moves during the time we take the measurement.

 

Is there somehing Here you disagree with?

 

If not I will continue.

There are a couple of things I disagree with or at least do not understand.

 

First, collapse of a quantum wavefunction seems to me to add nothing and to be distraction. When the observers observe the light pulse, the light is absorbed by the retinae of their eyes and the energy from absorption is converted to an electrochemical signal in the optic nerve. That's all we need. Can we leave QM out of it? 

 

Second, let's check we are using the same scenario. The one I am visualising is the one in the Wiki explanation, here: https://en.wikipedia...of_simultaneity

 

In this, light goes directly to the eyes of both observers from both ends of the carriage, and arrives at different times, due to the distance it has to travel being different for the two observers. 

 

 

But there is nothing about  an "image" going from P to S, as you are saying. where do you get that idea from?


Edited by exchemist, 22 January 2018 - 05:44 AM.


#63 JulianM

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Posted 22 January 2018 - 07:12 AM

OK, we will put QM aside, I am OK with that

 

Actually I was looking at a different scenario, but this one you refer to will do just as well so let's use yours.

 

I think we perfectly agree on the 3 depictions of the passengers view. We agree on L, c and t, so let's move to the 3 depictions of the Stationmasters view. If it's OK with you I'd like to keep the shorthand of P and S so we don't have to explain each observer each time.

 

 At the moment that the flash of light goes off in the center of the train it illuminates the area creating an image. That image, to S, is of P right next to him and the center of the carriage and that image, as you so correctly say, is absorbed into his retina  I think we agree, yes?

 

Now, in my opinion neither S nor P can see the wavefront/photon however you wish to think about it, because it is travelling away from both at the speed of light. It is un- seeable so we need to wait for a detectable, or observable event.

 

In the example you reference it would be the flash "arriving" at the rear of the train. At that instant it illuminates the rear of the train creating an image which will later be seen by S. That image travels from the rear of the train to his eye.

 

At this point S has nothing to compare it with, but he can wait for the rear of the platform to be illuminated and observe that image after it arrives at his eye. He now has something to compare.

 

He has 3 images. The first right beside him. The second of the rear of the train and the third of the rear of the platform.

 

The rear of the train can only be illuminated at time t calculated from the speed of light in the train and the length of the train. The same applies to the platform.

 

Are you ok with this?



#64 exchemist

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Posted 22 January 2018 - 09:20 AM

OK, we will put QM aside, I am OK with that

 

Actually I was looking at a different scenario, but this one you refer to will do just as well so let's use yours.

 

I think we perfectly agree on the 3 depictions of the passengers view. We agree on L, c and t, so let's move to the 3 depictions of the Stationmasters view. If it's OK with you I'd like to keep the shorthand of P and S so we don't have to explain each observer each time.

 

 At the moment that the flash of light goes off in the center of the train it illuminates the area creating an image. That image, to S, is of P right next to him and the center of the carriage and that image, as you so correctly say, is absorbed into his retina  I think we agree, yes?

 

Now, in my opinion neither S nor P can see the wavefront/photon however you wish to think about it, because it is travelling away from both at the speed of light. It is un- seeable so we need to wait for a detectable, or observable event.

 

In the example you reference it would be the flash "arriving" at the rear of the train. At that instant it illuminates the rear of the train creating an image which will later be seen by S. That image travels from the rear of the train to his eye.

 

At this point S has nothing to compare it with, but he can wait for the rear of the platform to be illuminated and observe that image after it arrives at his eye. He now has something to compare.

 

He has 3 images. The first right beside him. The second of the rear of the train and the third of the rear of the platform.

 

The rear of the train can only be illuminated at time t calculated from the speed of light in the train and the length of the train. The same applies to the platform.

 

Are you ok with this?

Not quite. Excuse me if I go slowly and pedantically here, but I want to make sure we are talking about the same things all the way through. 

 

The scenario I was referencing does not refer to the end of platform being illuminated. It concerns only the front and rear  ends of the carriage, as seen by P and S when the light reflected from the ends reaches their eyes. 

 

Do you have a reason for introducing the "rear" of the platform? 

 

And I prefer the term "reflection" for the light returning from the ends, rather than "image", as to me "image" suggests something qualitatively different from the outgoing light. 


Edited by exchemist, 22 January 2018 - 09:23 AM.


#65 JulianM

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Posted 22 January 2018 - 11:45 AM

The scenario you are describing is just as we find it in dozens, if not hundreds, of text books and articles. This method treats the "light" as if it were a tennis ball bouncing back and forwards in the train and that our eyes can follow it. 

 

My point is that no-one can see that. None of us have seen a ball of light moving away or coming towards us because it is travelling at the speed of light. What we can see is the effect of that light as it strikes an object and we see that all day and every day.

 

The reason for including the rear of the platform was to provide context for the observer S. If you want to ignore that for the moment we can also do that.

 

Now I deliberately choose the concept of an image, or snapshot, because it is qualitatively different from our bouncing ball and I agree with you, but treating it this way reveals new information. For example the observer now knows what happened, where it happened and can calculate when it happened. We can call this a measurement.

 

So, in my scenario I have a snapshot of the instant that the flash ocurred and another of the time that it struck the rear of the train. The obersver cannot see anything but this. He cannot see the bouncing ball, but he can see the center and the rear of the train as they are illuminated.

 

The image of the rear of the train arriving at S is what you are describing as "reflected light" and as I say provides the information that the light struck that point at that time and at that position in the frame of reference in which S is placed. Taking the speed of light and the distance he can also calculate the time when it ocurred, at least according to his clock.

 

Is there any issue with this, and can you bear with me for a little longer?



#66 exchemist

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Posted 22 January 2018 - 11:55 AM

The scenario you are describing is just as we find it in dozens, if not hundreds, of text books and articles. This method treats the "light" as if it were a tennis ball bouncing back and forwards in the train and that our eyes can follow it. 

 

My point is that no-one can see that. None of us have seen a ball of light moving away or coming towards us because it is travelling at the speed of light. What we can see is the effect of that light as it strikes an object and we see that all day and every day.

 

The reason for including the rear of the platform was to provide context for the observer S. If you want to ignore that for the moment we can also do that.

 

Now I deliberately choose the concept of an image, or snapshot, because it is qualitatively different from our bouncing ball and I agree with you, but treating it this way reveals new information. For example the observer now knows what happened, where it happened and can calculate when it happened. We can call this a measurement.

 

So, in my scenario I have a snapshot of the instant that the flash ocurred and another of the time that it struck the rear of the train. The obersver cannot see anything but this. He cannot see the bouncing ball, but he can see the center and the rear of the train as they are illuminated.

 

The image of the rear of the train arriving at S is what you are describing as "reflected light" and as I say provides the information that the light struck that point at that time and at that position in the frame of reference in which S is placed. Taking the speed of light and the distance he can also calculate the time when it ocurred, at least according to his clock.

 

Is there any issue with this, and can you bear with me for a little longer?

Not really, except that the light only reaches them after returning from the rear of the carriage, so the time delay they observe is after an out-and-back path, not just an outbound one.

 

Carry on. 



#67 JulianM

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Posted 22 January 2018 - 02:35 PM

Not really, except that the light only reaches them after returning from the rear of the carriage, so the time delay they observe is after an out-and-back path, not just an outbound one.

 

Carry on. 

 

Yes, agreed, it is an out-and-back path.

 

Now we can examine the image or snapshot that S received (the back path) of the rear of the train (created by the out path). What we can now see is the position of the rear of the train relative to the platform. Since this is in S's frame of reference he can measure how far away the event was when it took place.

When he calculates the time from the original event it took place, using the information he just gained from the return path, he also finds that at the place that it occurs it is the same time at which light in the Platform Frame would have reached the rear of the platform and returned to him. He needs to know the length of the platform, you have hidden the flash event from him, but that's a measurement he can easily make.



#68 exchemist

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Posted 23 January 2018 - 02:36 AM

Yes, agreed, it is an out-and-back path.

 

Now we can examine the image or snapshot that S received (the back path) of the rear of the train (created by the out path). What we can now see is the position of the rear of the train relative to the platform. Since this is in S's frame of reference he can measure how far away the event was when it took place.

When he calculates the time from the original event it took place, using the information he just gained from the return path, he also finds that at the place that it occurs it is the same time at which light in the Platform Frame would have reached the rear of the platform and returned to him. He needs to know the length of the platform, you have hidden the flash event from him, but that's a measurement he can easily make.

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?  





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