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Speed of light for different observers

  1. Apr 1, 2010 #1
    The following is not a theory but only attempt to understand the principles of Special Relativity in connection with the speed of light.


    The idea that light waves require luminiferous aether to move through was refuted by Michelson-Morley experiment and that permitted Einstein to postulate that the speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.
    No one ever considered that the space itself appears as aether for the light waves.
    Space as distance between two objects is not any different for the light than a fiber cable.
    Imagine that you accelerate fiber cable through which travels light. The same way we can accelerate space.
    The only difference is the different speed in these two mediums.
    Can we accelerate space thus accelerating the light which travels through it?
    The answer “Yes” may sound stupid on a first though but here is a simple thought experiment, which can be actually performed with the predicted results.

    The Experiment


    The above graphic shows the principle of the experiment.
    A train-car with certain length is moving in right direction with certain constant speed.
    There is a light source attached at the back of the train-car which sticks out of it.
    In the front of the train-car there is a mirror, which for the purpose of the experiment we call “observer A”.
    That will be the frame of reference which we will call “frame A”.
    Outside this frame is located frame of reference B, which we will call “frame B”.
    While in motion (with constant speed) the light source on the train-car produces flash toward observer A. The light from the flash needs certain time to reach observer A, during which time the train-car advances in right direction.
    Knowing the speed of the train-car and the distance between the light source and observer A, we calculate at which place in frame B, observer A will see the light and on the same line in frame B we place observer B, so both A and B will see the light simultaneously.


    The above graphic is the position of the train-car at the moment of the simultaneously observed light by A and B.
    Since A is stationary to the source, the distance is measured from the source to the observer.
    However the distance covered by the light to observer B has to be measured from the place of the emission.
    In that case the result is two different distances, covered by the light from one emission, and perceived simultaneously by two different observers.
    Special Relativity would argue the above result with different running clocks in the two frames of reference.
    The different running clocks though, will not be favorable arguments since B will see the light shifted in the blue spectrum, due to its velocity in respect to the source at the moment of the emission.
    Let’s extend the experiment and put third observer (observer C) behind the train-car in frame B.
    Observer C will not see the flash, but only the reflection of the light beam for A and B.
    These reflections for C will be simultaneous, but also seen with different light frequencies.

    The above thought experiment shows two different distances covered by the light from one emission in simultaneous observation from three different observers.
    Since the light emission is one, the change of the frequency is due to the different speed of the light in respect to the observer.
    In other words, observer B measures the speed of light as higher than the measurements of observer A and the result will be seen in the different patterns left by the light.
    A question comes to mind after this experiment; can we consider the different light frequency as different speed of the light.
    The answer is very simple; if two observers perceive the light from one emission with two different frequencies, then their velocity is different not only in respect to the source, but to the light as well.

    And here is the my thought for the right understanding of the expression “speed of light for different observers”.
    Light is electromagnetic emission with certain frequency.
    One electromagnetic wave is not light. Even if we know the precise speed of one photon, that wouldn’t be the speed of light.

    The speed of light is characterized by the speed, with which every next wave due to the velocity at the moment of the emission approaches the observer.
    Imagine that the source and the observer are in approaching motion at the moment of the emission. The light waves will approach the observer with speed greater than c and that will shorten the time between the waves.
    In this case the light frequency is characterized not by wave length, which by my opinion is a wrong understanding about light frequency, but by the time between the light waves.
    Respectively, different time between the light waves is perceived as different speed of the light and will leave different light patterns when measured.

    Follows the question, “does the different pattern left by light from stationary sources with different frequencies shows different speed of the light”?
    No. Two stationary sources with two different frequencies will emit light with the same speed.
    All sources emit light with the same speed, but the velocity of the light should be measured as any other velocities.

    The above helps to understand better the events in the Universe and our place in it. We know that a star moves toward us if we observe its light in the blue spectrum, but we can never know the place of the star, since we don’t know the initial frequency of the light and its speed which depends on the velocity at the moment of the emission.
    Not knowing all this we can never determine the present place of the star whose light we observe.
    Considering the above, we cannot predict what happens in the Universe right now if we measure all events with same speed for the light.
    We see the past without knowing the present. In that regard it won’t help much if we do our measurements with different speed for the light, but it will definitely make a change.

    Attached Files:

  2. jcsd
  3. Apr 1, 2010 #2
    Last edited by a moderator: Apr 25, 2017
  4. Apr 1, 2010 #3
    Aaron, in that forum I presented the experiment like for kids ;)
    Here the presentation is like for students and teachers :)
    And by the way, try to be more respectful and decent, please.
    Last edited by a moderator: Apr 25, 2017
  5. Apr 1, 2010 #4


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    Very few question marks for someone seeking understanding.

    Ok, I'll give you the benefit of the doubt and help you with your attempt to understand.

    Draw two spacetime diagrams for your Gedankenexperiment. One in A's frame, the other in B's frame. (Better not use one single diagram for both points of view, I think)
    If you don't know how to do that: That's the first thing you must learn. Look it up, if you get stuck, ask here.

    Having done that, look at the time coordinate of the emission event in both frames. Different or not?
    Look at the space coordinate of said event. Calculate the speed of light in each frame. Different or not?
    If different: search for the error in your calculations. If you don't find it, ask. We'll help.

    That's how SR handles the situation. You have to understand it before you jump to conclusions.
  6. Apr 1, 2010 #5
    The question is actually one: is the speed of light the same for all observers.
    I built a thought experiment on the question and I expect someone to comment on it.
    Your patronizing attitude doesn't help much, Ich.
    Would you please tell me why do you think that the experiment is wrong or draws wrong conclusions?
    The experiment is as simple as Michelson-Morley experiment and if their experiment doesn't need your recommendations I wonder why my experiment would need them.
  7. Apr 1, 2010 #6


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    If you don't need recommendations, I wonder what you're doing here.

    Answer the questions, and you'll see what's wrong (Hint: has to do with the time coordinates of the emission event).
    Your only chance to understand is IMHO that you draw the diagrams yourself. If you try, we'll help. If not, that would be proof that you don't seek understanding, but want to promote fringe views. I don't think that's your intention.
  8. Apr 1, 2010 #7
    Not that I don't need recommendations but I wonder why such a simple experiment need to be complicated in such way :)

    There is one only light emission for both A and B and I don't see your point.
    I guess you are seeing the experiment from SR perspective, and you try to argue it from that point of view.
    Wouldn't it be correct to say that if this experiment is correct Special Relativity would stand incorrect and your argument would be invalid?
    Shouldn't you argue the experiment without using arguments drawn from SR?

    ONE QUESTION: do you agree that B will see the light as a blue-shifted (different light pattern from A if measured with photographic plate)?
    If yes, how would you explain it?

    Please don't mind my "aggressive" manner of discussion.
    I found out that this is the best way for me to understand something ;)
  9. Apr 1, 2010 #8
    I would suggest to look at the experiment as a proof that Space is actually medium through which the light propagates and if we accelerate the space, the speed of the light changes with the speed of the accelerated space.

    Thinking that way, we wouldn't need Michelson-Morley experiment because it would appear pointless.

    If you find the above not appealing to your logic, forget it and concentrate on the experiment.
  10. Apr 1, 2010 #9


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    I know that you don't see my point. That's why you must draw the diagrams.
    You analyze the thought experiment with SR. Either you find an internal inconsistency in SR, or not.
    So, will you, or won't you?
    And, please, make sure you've read the forum guidelines, especially https://www.physicsforums.com/showthread.php?t=17355".

    Doppler effect.
    Last edited by a moderator: Apr 25, 2017
  11. Apr 1, 2010 #10
    I read the forum rules and as I state in my opening post, I'm here to gain better understanding of SR.
    I guess that I have the right to say what is the obstacle for me to understand the theory.
    The obstacle for me is presented in the experiment.

    Pardon me, but there is no rule in your forums that say how fast must I agree with you or understand your explanation for that matter :smile:

    Now, about the Doppler effect: obviously it is, but why two different observers see the light from one emission, simultaneously, at one and the same point with two different frequencies?
    Would you help me to understand this?
    Last edited by a moderator: Apr 25, 2017
  12. Apr 1, 2010 #11


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    Then learn how to analyze the experiment.
    I didn't give an explanation. I offer to guide you in finding it. You may accept or refuse.

    Yes. Right after you've learned to draw and read said diagrams.

    If you don't want me to help you, just say so. No problem.
  13. Apr 1, 2010 #12
    I expected direct answer as a help :smile:
    Sorry, it is my misunderstanding.

    I'm afraid I don't understand what do you want me to do. Would you elaborate on that, please? Tell me your idea and I'll draw it.

    No, no!
    I would be happy to be helped by you.

    I'd like to ask something more on the Doppler effect issue in my experiment and I hope this time you'll answer me. That would help me great deal.
    Perhaps I am deceived by Michelson-Morley experiment, but please correct me:
    Michelson expected to register different speed of the light on the two light beams, because one of them was parallel to the flow of ether.
    That difference in the speed would be seen in the different interference.

    Why the different interference in my experiment wouldn't be seen as different speed of the light?
  14. Apr 1, 2010 #13

    Doc Al

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    Different observers measure different distances between emission at one end of the train (event 1) and arrival at the other end (event 2). They also measure different times.
    No. The change in frequency is due to the different speed of the source in the different frames. The speed of the light is the same.
    No. To measure the speed of the light, you'd need to measure the travel time. Do that and you'll find that everyone measures the same speed for the light.
  15. Apr 1, 2010 #14
    For observer C (from the extended part of the experiment) the distance between the emission and the two mirrors is the same, but C still sees the light with different frequency, which is my problem here - it would mean that the light for observer A was traveling slower.

    I don't understand how the source would change the frequency of the light which was already emitted before it change its position.
    And I tried to explain my view on the way the light changes it frequency when the source is in motion, but obviously (at least according to you) I am mistaken.
    Would you comment please the part in my post which is around this line:
    The problem for me is not the exact measurement of the speed, but the difference shown in the different light patterns.
    Because the way you explain it, we have to know the time measured in the frames.
    But in order to know the time we use the speed of light which is in argument :smile:
    Seems that SR does not allow gap to avoid this circle ;)
    That is why I am interested in the difference visible in the patterns.
    If you can help me with that I'll be very grateful.

    I think that it would be easy to start with observer C.
    Sorry, I am a bit slow but it has its advantages :wink:
  16. Apr 1, 2010 #15

    Doc Al

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    Staff: Mentor

    Since observers C and B are in the same frame, they agree on the distance between the two events. Observers in the train car frame disagree.
    You mean that C will see the light reflected from the moving mirror (at A) and the light reflected from the stationary mirror (at B) to have different frequencies? So?
    No it wouldn't. Why would you think that?

    You might want to learn about the Doppler effect.
    Not quite sure of the point you are trying to make, but frequency and wavelength are related by: speed = frequency X wavelength. The "time between the light waves" is the period, which is 1/frequency.
  17. Apr 1, 2010 #16
    This is incorrect, there are already multiple experiments that prove your above statement to be false.

    In addition to that, your gedank suffers from the following major flaws:

    1. It is virtually impossible to understand.

    2. The lack of mathematical support (there are no calculations) makes it unfalsifiable.

    3. You present ZERO experimental data, yet you conclude that light speed is dependent on the source speed.
  18. Apr 1, 2010 #17


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    Google spacetime diagram.
    http://physics.syr.edu/courses/modules/LIGHTCONE/events.html" [Broken] a site that seems to explain the concept.
    Make a diagram with the time axis upwards, the x axis (=direction of motion) to the right. Forget about y and z.
    For each interesting point (e.g. back of train, observer B...) draw its position as a function of time. These are all straight lines.

    This forces you to set up your experiment unambiguously, with concrete numbers.
    Ask if you get stuck.

    Because the source is moving. Moving sources must exhibit Doppler shift, relativity or not. This could be easily explained with a spacetime diagram.
    Last edited by a moderator: May 4, 2017
  19. Apr 1, 2010 #18
    I am fairly certain that consideration of such trains is what lead to SR.

    Lorentz certainly considered an eather, refining his eather until Einstein developed SR and it was shown the two were equivelent, without the need for the eather's preferred reference frame.

    Then Einstein went on to explain gravity in this context as a curvature of space-time. Curvature and flow are both mathematiaclly modeled as the differential of one variable with respect to another variable.

    The better question might be why no one ever thought to consider gravity as a flow of SR with time. (Eddington does talk about the equivelent medium representation.)
  20. Apr 1, 2010 #19
    That's the point with observer C - he sees the reflection of the light traveled in frame A, which is not the same length as in frame B.
    You would probably argue with the fact that the emission for B and C was at the same place, but it was at the same place for A as well :smile:
    Now if we measure the distance in frame A as different, the reflection from mirror A seen by C will be from the light traveled that different distance.
    Yet, it will still be seen simultaneously with the reflection from mirror B.

    That would mean different speed of light.

    Why would Michelson and Morley think it?

    I did.

    So, if the velocity is different then the frequency changes.
    My point was that the velocity between the light waves and the observer changes.
    The velocity between the source and the observer is important only as "speed giver" to the light waves. After the emission the speed of the source is unimportant. It may as well stop and move backwards but we will still see it at the place of the emission and the frequency will still be changed.
    I suppose that you will disagree with this, and if you do, please explain why.
    Explain please how do you see the change of the frequency due to different velocity.
    May be I'm missing something and I'll be thankful to you to correct me.
  21. Apr 1, 2010 #20
    Thank You very much.
    I greatly appreciate your help, and I promise that I'll check it and I'll see what I can do.

    I already said in my previous post that the speed of the source, by my opinion, doesn't matter after the emission.
    Please correct me if I am wrong but if I am right your answer from the above will be wrong.

    I am afraid that I may look double-fool giving the same wrong answers two times, so you may want to wait for the answer of Doc Al on the above :smile:
    Last edited by a moderator: May 4, 2017
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