JulianM

I'm lost, but I may try to read it.

Yes, I agree. I was really just asking Super Polymath to elaborate on what he is trying to say.

I don't disagree with this statement, and math is only useful when based on correct assumptions, but it is necessary to go beyond "more likely". This is off The original topic, but if you have something more than "it could be either" then I am happy to listen.

My understanding is exactly the same as exchemist

I get the impression from this that because the string is pinned to the axle it is stopping when all the string is wound off and tight. Is that what is happening?

OK thanks.

Yes, I understand. What I am trying to visualize is, if we examine events in a space-time diagram then we should presumably relate each cause and effect in it's proper frame of reference. For example, in the train experiment - - the original flash and the reflection can both be considered in the train's frame of reference - experiencing the reflection (illumination) of the rear of the train and the stationary observer should be considered in the frame of reference of the stationary observer and he experiences it at speed = c In a simple space-time diagram for this we would typically draw it for one frame of reference. This results in the "rays" of light being constructed at a 90deg angle. I don't think it would come out that way if we essentially overlaid one frame of reference onto the other, and I am not sure if that makes sense. An example of this would be the stationary observer, knowing the length of the train and being able to calculate how far the "illumination event" was from him calculating that the speed of transmission was something other than c. Of course this is how we come to know that time and length are dilated but I am trying to visualize how to draw the events.

Bangstrom, Thank you for allowing me to follow this line of reasoning. In my consideration of this I next populate the train with a large number of passengers, or Windows works too, and find that each one of them is a little closer than expected in the segment coming towards me, and the opposite in the segment moving away. This makes a kind of analogy to red and blue shift doppler effects if we think of a wave form drawn over their heads. The next step in thinking about this is that I can be fooled into thinking that light in the moving object looks like it is travelling faster than c from my reference point, when it isn't, it's speed is still c. So - when moving to a space-time diagram how do we represent this? I don't claim to know or to have any new theory, I am just trying to examine this in a broader sense than is traditionally done.

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.

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.

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.

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.

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.

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.

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

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.

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?

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?

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.

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.

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?

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 ?

Who are you and who are you talking to?

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.