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Could moving observer be stationary vs moving and still give Albert the same results

  1. Dec 7, 2007 #1
    Could the moving observer be at some distance instead of moving and still give Albert the same conclusion?

    In IX. The Relativity of Simultaneity, Albert describes two simultaneous lightning strikes and two observers. One of the observers is stationary at a mid point between the simultaneous lightning strikes. The other observer is on a moving train. Albert says both observers are at the same mid point between the simultaneous lightning strikes. He also says the moving observer then moves towards the right in the diagram with the velocity v of the train. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A.

    Since the moving observer was at some point other than the mid point when they observed the lightning strikes, could that observer have just gone to that same point and been standing there when the lightning strikes occurred. Would the same following conclusion apply?

    Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.
     
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  3. Dec 7, 2007 #2

    Doc Al

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    Realize that the "moving" observer is in the middle of the train and remains there. So, from his frame of reference, he is in the middle between the two lightning strikes. (The lightning strikes took place at the ends of the train.) This is what enables him to conclude that the lightning strikes did not occur simultaneously in his moving frame.
     
  4. Dec 7, 2007 #3
    I think I am following you. I think you are saying the lightning strikes hit the train in the front and in the back. In that case, it seems you are saying that it would take longer for the light to travel from the back of the train to the front of the train. As a result, the speed of the train would be added to the speed of light which would violate the very problem Albert thought he was addressing by removing the meaning of time.

    Even if you are correct, I will ask the original question another way using your condition that the lightning hit the train. Suppose there was a third obserever who was in line with the person on the train when the moving observer and the third observer saw the lightning strikes? Is seems that the moving observer and the third observer would see the same delay in arrival time of the lightning strikes.

    We would have three points of reference,

    1 is the original observer on the embankment,
    2 is the person on the train.
    3 is the other person on the embankment that is in line with the 2nd observer when he observed the lightning strikes.

    Would observer 2 and 3 observere the same delay in the time the lightning strikes arrived at their indentical locations?
     
  5. Dec 7, 2007 #4

    Doc Al

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    I'm not sure I'm following you. In any case: From the view of the embankment observers, the light from the front of the train reaches the train observer before the light from the back of the train. It takes longer for the simple reason that the train observer moves towards the front light and away from the rear light. Of course, from the train observer's point of view the distance is the same, since he's in the middle of the train.

    This is a bit unclear, since observer 2 is in two different places (according to the embankment observers) when the light from each lightning strike reaches him. Let's say we put observer #3 on the embankment at the exact spot that observer #2 will pass when observer #2 sees the light from the front of the train. In that case the light from that lightning strike (at the front of the train/platform) will reach both observer #2 and observer #3 at the same time.

    Of course, this is no surprise to observer #3, since he knows he's closer to where the lightning struck. All observations made by stationary observers (no matter where they are along the embankment) will confirm that the lightning strikes occurred simultaneously.
     
  6. Dec 7, 2007 #5

    I think Albert had the ligtning hit the ground instead of the train. You may be correct that it hit the train.

    If the lighting hit the train, then the light is in the internal frame of train. All light in an internal frame travels at the speed of light.

    The Michelson and Morley experiment shows that the light should travel both forward and backwards through the train and arrive at the other ends at the same time.

    Are you saying that Michelson and Morley does not apply to the train? It applies to the earth which is a moving frame like the train. If it is as you say for the train, it should also be the same for the people on the embankment and they would see a difference in the timing of the arrival of the flashes.

    If the lightning did not hit the train, the light is not part of the train frame. In that case, the light would be chasing a frame and the frame would be headed toward the other light.
     
  7. Dec 7, 2007 #6

    JesseM

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    It doesn't matter whether it hit the ground or the train; all frames must agree on whether events in the same local region of spacetime coincide with one another, so either each end of the train was at the same position as the patch of ground the lightning hit at the moment the lightning hit it, or it wasn't. If it was, then it must be true that in the train-observer's frame, the spatial coordinate of each lightning strike is the same as the spatial coordinate of each end of the train.
    The light should travel from either end of the train to the middle of the train at the same speed, but that doesn't mean the light from each flash will reach the middle at the same time--not if the two flashes aren't simultaneous in the train's frame! To pick an exaggerated example, if you're sitting in the middle of a train, and on Jan. 1 2007 I turn on a light bulb at the left end, and then on Jan. 1 2008 I turn on a light bulb at the right end, do you think the fact that the light from each bulb approaches you at the same speed implies that you are going to see each bulb turn on at the same time?
    Again, different frames never disagree on local events. If a calculation in the track observer's frame shows that the light from each strike reaches the track-observer at the same moment but the train-observer at different moments, then this must be predicted in the train-observer's frame too (which means that in the train-observer's frame, the two strikes must have happened non-simultaneously, while they happened simultaneously in the track-observer's frame). Likewise, if a calculation in the track observer's frame shows that the light from each strike reaches the track-observer at different moments but the train-observer at the same moment, this must be predicted in the train-observer's frame too (meaning that in the train-observer's frame, the two strikes happened simultaneously, while they happened non-simultaneously in the track-observer's frame). These would be two entirely distinct physical scenarios.
     
  8. Dec 7, 2007 #7

    Doc Al

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    Think of the lightning as striking the ground just where the train was passing at that moment. (Imagine it making a burn mark on the ground and the end of the train.)

    The light from the flashes travels at the same speed in every frame.

    How do you figure that? In the train frame, the light does take the same time to travel from front to back as from back to front. But in the embankment frame, the ends of the train are moving towards (or away) from the light, so of course it takes different amounts of time to traverse the moving train--according to embankment observers.

    Sorry, I don't understand why you are bringing up MM and how you think it applies to the train. Or why you think anything that I've said contradicts MM in any way.

    The light doesn't belong to one frame. The light just exists. Any frame can observe the light. Imagine that the front and back of the train is transparent so that they can see the lightning strike. (If both frames can't see the light, there's not much to talk about!)
     
  9. Dec 7, 2007 #8
    I thought the first answer said the lightning strikes hit the train. Now it seems that it does not matter where the lightning strikes hit.

    If they hit the train, it seems that they must follow the rules of relativity and the results of the Michelson-Morley experiment for the person on the train. That person should observe the lightning strikes as simultaneous. Also, the person on the ground should observe them as simultaneous.

    If they hit the ground, it seems that they must follow the rules of relativity and the results of the Michelson-Morley experiment for the person on the ground. The person on the ground would observe simultaneous events but the person on the train would observe different times for the events for the reason Albert described.
     
  10. Dec 7, 2007 #9

    Doc Al

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    All that matters is that the lightning strikes near or on the ends of the train.

    No matter where the lightning strikes, the rules of relativity will be followed.
    Huh? That's not what relativity tells us.

    I think you're confusing the fact that the light takes the same time to reach the middle of the train (according to train observers) with the light reaching the middle at the same time. That would only be true if the lightning struck at the same time according to the train observers--which is not true.

    What matters is who observes the lightning strikes as being simultaneous. If the embankment observers see them strike at the same time, then the train observers will not.
     
  11. Dec 7, 2007 #10

    I am sorry we got side tracked with the other discussion. However in that area I have more questions now than before. I can get back to that later.

    I see what you are saying about person 2 being at different locations relative to when the observer 1 see both flases. I see what you are saying about observer 2 and 3 being at a common locations when the front strike arrives. The flashes for 3 would not be simultaneous. They would have a different delay than that observed by 2. Why can observer 3 know the flashes are simultaneous but observer 2 will not know the flashes were simultaneous?
     
    Last edited: Dec 7, 2007
  12. Dec 7, 2007 #11

    Doc Al

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    You must distinguish two things: When the lightning flashes happened versus when the light from those flashes reaches a particular observer. Everyone agrees that the light from both flashes will reach Observer #1 (in the middle of the embankment) at the same time. Everyone also agrees that the light from both flashes will reach Observer #2 (in the middle of the train) at different times.

    Observer #2 knows where the flashes happened in his frame (at the ends of the train, as far as he's concerned). Since he knows he's right in the middle of the train, the fact that the light reaches him at different times forces him to conclude that the flashes must have happened at different times. (If they flashed at the same time, the light would have reached him at the same time since the light from each flash travels the same distance in his frame.)

    On the other hand, Observer #3 also knows where the flashes happened in his frame of reference (at the ends of the platform, as far as he's concerned). So he can calculate how long they must have took to reach him. His calculations will confirm that the lightning flashes must have occurred simultaneously. (Sure the light from one flash reaches him before the light from the other flash. But that's just because he's closer to one of the flashes.)
     
  13. Dec 11, 2007 #12

    I think your's is one of the better descriptions of what and why Albert reached his conclusion.

    Why can other people on the ground know they are not at the mid point but the person on the train does not know he is moving? It sure seems that the person on the train is just as smart as the many people on the ground. It sure seems that the person on the train could also calculate and get the correct results.

    Suppose we use what will be referred to as Don’s Unified Simultaneous Relativity Experiment. In this experiment we will eliminate the simultaneous lightning strikes and substitute reflectors. Now, instead of observing simultaneous lightning strikes we will observe simultaneous reflections. We will also have 4 reflections with 2 on the ground and 2 on the train. We will have one flash of light that is split to travel on the ground and on the train.

    To setup and validate our conditions, we will have the train standing beside the companion points on the ground. The reflector at the back of the train will be next to the reflector on the ground. The same applies for the mid point and front point reflectors.

    In the middle of the train we have a single light source that is triggered when the train mid point and the ground mid point are beside each other. The light will be split with part going to a reflector on the ground at the mid point. The other part of the light will travel the same distance to a reflector on the mid point on the train.

    Those two simultaneous reflections will be split again. In both cases, part goes forward and part goes to the back. Since the train is not moving. We will have the single light event split to cause two simultaneous events. Those two events will cause 4 simultaneous events when the 4 light pulses arrive at the front and back reflectors. The front reflector on the ground and on the train will receive then reflect the light at the same time the back ground and train reflectors receive and reflect the light.

    The 4 simultaneous reflections will occur sending the original single light flash, that was split, back to the sending location. The four simultaneous reflections will be detected simultaneous by their respective midpoint ground and train reflector sensors. The mid point reflector sensor on the train will also be able to sense the light pulse reflections on the ground from the front and back.

    This scenario has one light pulse that causes 2 simultaneous events that cause 4 simultaneous events. The 4 simultaneous events fully replace and serve as substitutes for the two simultaneous lightning strikes in the Theory of Relativity paper. We have a hierarchy of relativity with the flash point at the beginning, then extending through the initial and secondary split of the light to the 4 simultaneous reflections before returning through the hierarchy for detection.

    Now we simply move the train back some distance and have it moving when the single light pulse occurs. Again, the single light source is triggered when the train mid point and the ground mid point triggers are beside each other. Again, the light is split with part going to a reflector on the ground at the mid point. The other part of the light will travel the same distance to a reflector on the mid point on the train. The single light that was split has unified the train moving frame and the ground stationary frame. Again, those two simultaneous reflections are split. In both cases, part goes forward and part goes to the back.

    Even though the train is moving, the 4 parts of the one pulse of light will cause 4 simultaneous events when the 4 light pulses arrive at the front and back reflectors on the ground and on the train. Again, the front reflector on the ground and on the train will receive then reflect the light at the same time the back ground and train reflectors receive and reflect the light. Again the two reflections on the ground will simultaneously arrive at the ground mid point reflector sensor. Also, the two reflections on the train will simultaneously arrive at the mid point of the train.

    However, since the train is moving, the reflection from the ground front and back reflectors will not arrive at the secondary train sensor, that is for detecting the ground light pulse, at the same time the train pulses of light arrive. That secondary train mid point reflector will measure the same timing results of the lightning strikes as observed in the Theory of Relativity paper.

    In Don’s Unified Simultaneous Relativity Experiment, three of the times that light from the front and back reflectors are detected confirms the timing results of Albert’s experiment. However, in Don’s experiment, there is a additional light detection event. We have the detection of the simultaneous return of the reflection from the train reflectors which is simultaneous with the detection on the ground. This fourth event reveals a problem with the partial results of Albert’s limited experiment.

    The one light pulse in Don’s Unified Simultaneous Relativity Experiment established the relationship between the 4 simultaneous events that are in different frames. While the 2 lightning strikes observed in Albert’s paper gives only an appearance of conflicting with a constant time, the inextricable unity of the simultaneous events in the different frames, in Don’s experiment, verifies that there is a unified complex hierarchy of relativity that precludes any false perception.

    The one observer who established the conditions of the experiment could pick any point of reference. The results are the same from any train reference point as well as from any ground or remote location.

    Conclusions from Don’s Unified Simultaneous Relativity Experiment

    1. Albert’s results related to the timing of the arrival of the light pulses are confirmed by Don’s experiment.

    2. The perception of time having no meaning established in Albert’s paper remains as a false perception that may misdirect observers.

    3. Don’s experiment shows that an event in a moving frame has a unified relationship with simultaneous events in other frames that may be stationary or moving relative to the original moving frame. The choice of frame of reference related to simultaneous events is irrelevant.

    4. Don’s experiment delivers the additional result that shows a unified relationship for the constant progression of time between and among different frames regardless of the position or movement or selection of a frame of reference.

    5. Don’s Unified Simultaneous Relativity Experiment shows that light from one source adopts a unified relationship with the frames the light enters. In the experiment I showed that the light from the single flash travels at the speed of light in different frames that are moving at different speeds relative to each other. The portion of the light on the ground travels at the speed of light relative to the ground. The portion of the light on the train also travels at the speed of light relative to the train. As a result, portions of the single flash of light simultaneously traveling at different speeds which are additive to the speed of the ground as well as additive to the speed of the train.

    When all data is known and considered, the unified simultaneous relativity in the constant progression of time enables observers to overcome the false perception from one moving frame to another.

    I think this experiment has profound impact. For that reason, I have a copyright on it. Reference to this experiment should include reference to me, the copyright owner.
     
  14. Dec 11, 2007 #13

    Doc Al

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    Of course the person on the train knows he's moving with respect to the ground (alternatively, the ground is moving with respect to him). But both train and ground observers have the perfect right to view themselves as being at rest and to measure times and distances with respect to themselves. You seem to think that the observations made by the ground are the only ones that are "correct". Not so.

    OK.

    I hope you realize that things will change dramatically when the train is moving. If the front and rear reflectors on the ground line up with the corresponding reflectors on the train when the train is at rest, they will not line up when the train is moving. For one thing, the train will be measured to be shorter when it's moving according to ground observers.

    None of this is particularly interesting since the train isn't moving. Things change when it moves.

    I don't know what you mean by "unifying" the train and ground frames--you've done no such thing. But OK, you have 4 flashes of light that leave the midpoint of train and ground at the same time.

    Nope. Let's consider the flashes on the train. According to train observers, they will reach the front and back train reflectors at the same time. But according to the ground observers the light will reach the rear reflector first. Similarly for the flashes on the ground: They reach the reflectors simultaneously according to ground observers but not according to train observers.

    Will the light reflected from the ground reflectors reach the ground midpoint at the same time. Yes! And will the light reflected from the train reflectors reach the train midpoint at the same time? Yes! So what?

    Yes, the light reflected from the front and rear ground reflectors is equivalent to the two lightning strikes that occur simultaneously according to the ground observers.

    It's hard to know what "times" you are talking about since you haven't defined your events.
    No it isn't. On what basis do you deduce that?
    To continue this you need to carefully define your events. For example, let's define your events as follows:
    Event 1 = arrival of light reflected from the rear train reflector at the train midpoint
    Event 2 = arrival of light reflected from the front train reflector at the train midpoint
    Event 3 = arrival of light reflected from the rear ground reflector at the ground midpoint
    Event 4 = arrival of light reflected from the front ground reflector at the ground midpoint

    Events 1 and 2 are simultaneous from any frame since they happen at the same place and time. Similarly, Events 3 and 4 are simultaneous. But that does not mean that all four events are simultaneous. (They aren't!)

    Don't worry. No one will mistake your convoluted (and incorrectly interpreted) "Unified Simultaneous Relativity Experiment" :rolleyes: with Einstein's crystal clear thought experiment.
     
  15. Dec 15, 2007 #14
    If you [like Einstein did] set the 2 flashes as simultaneous from the platform observer, then the train passenger must see the front flash before the rear flash.

    If you instead set the 2 flashes as happening at the same instant as seen from the train frame, then the guy on the platform doesn't. He must see the rear flash before the front flash.

    (I assume that's what the original question was getting at.)

    this video show it pretty clearly, though some fine details of the animation are a bit off
     
    Last edited by a moderator: Sep 25, 2014
  16. Dec 15, 2007 #15

    HallsofIvy

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    Yellow Taxi, you are right that is a very nice animation. Thanks.
     
  17. Dec 16, 2007 #16
    I have seen it written in many places that the entire content of the Theory of Special Relativity is: the Laws of Physics are the same in any inertial frame, and, in particular, any measurement of the speed of light in any inertial frame will always give 186,300 miles per second.

    There is one major consequence of that statement: The size or location or direction of motion or the speed of motion of any internal frame will not alter the speed of light inside that internal frame.

    Pick an internal frame from this list:

    A - The sun

    B - The earth

    C - A train on the earth moving very slow

    D - A train on earth moving very fast.

    Will the speed of light in all four of the above internal frames of reference be c? If not, which internal frame will not be included in the laws of physics? It sure seems to me that in each case, the speed of light inside an internal frame of reference must obey the laws of physics and the speed of light must be c in A, B, C, and D. .

    I have consistently argued in support of that point of view. However; it seems that people have been arguing against that point of view. In point of fact, it seems that part of The theory of relativity goes against that point of view.

    In the moving train thought experiment, if the lights at both ends of the train simultaneously strikes both the ground and the train at both ends, the lights simultaneously enter two separate internal frames of reference. The light in the ground frame will travel c in that frame. The light in the train frame must also travel c in the train frame.

    For the light to obey the laws of physics, it must obey the laws in both frames which will cause it to simultaneously arrive at the M and M’ in the experiment.

    As for "the train will be measured to be shorter when it's moving according to ground observers". That only applies if the ground observer does not include all the information. There is an optical illusion that makes the train only look shorter.

    I agree with the point of your statement that the observations made by the ground obseerver are NOT the only ones that are "correct". The observations from any location, be it the the ground or the train or from a GPS, may be correct it that observation has more correct data.

    How can anyone accept the conclusion that specifically excluding data can give a correct conclusion?
     
  18. Dec 16, 2007 #17
    Well, no, that does not answer what the original question was about.

    I do not understand why all the people on the ground are smarter than all the people on the train if the ground is the point of reference. The same applies in reverse.
     
    Last edited by a moderator: Sep 25, 2014
  19. Dec 16, 2007 #18

    Doc Al

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    This is true, but I don't think you understand that last part about the invariant speed of light. What it means is this: If someone on the train emits a beam of light, that very same beam of light will be measured as traveling at speed c with respect to any observer. If train observers measure the speed of the beam they get c; if ground observers measure the speed of that very same beam of light, they also get c. The light doesn't "belong" to any particular frame.

    This is true.

    This is truer than you think. It's not just the simple fact that if someone on the train measures the speed of a beam of light that they will measure the speed to be c. Of course that's true. But relativity is much stranger than that! As I said earlier, the speed of the very same beam of light will be measured to be c according to everyone, not just the train observers.

    No, you simply misunderstand relativity.

    The lightning strikes are only simultaneous in the ground frame.
    The light doesn't "belong" to a particular frame of reference.
    The light flash will be measured by the ground frame to travel at speed c with respect to the ground; the same light flash will be measured by the train frame to travel at speed c with respect to the train.
    Again, any light will be measured by any frame to travel at speed c with respect to that frame.

    Incorrect. The laws of physics dictate that the the light does not simultaneously arrive at the midpoint of the train.

    Wrong again.

    All observations are consistent and (done properly) are equally "correct".

    No data is excluded, certainly not by me.

    Please read the sticky posted at the top of this forum, from which I quote:
    On the off chance that you are actually interesting in learning something about relativity, I recommend these books:
    It's About Time by N. David Mermin. This is probably the easiest place to start.
    Space Time Physics by E. Taylor and J. Wheeler.
     
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