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A problem with light speed constancy

  1. Apr 13, 2008 #1
    I understand that the speed of light in a vacuum is supposed to be constant. However, I am having a problem reconciling this fact with the Doppler effects of light.

    Here's my problem...

    Let's correlate a propagating beam of light with a moving train.

    The faster that I move against the motion of the train, the quicker that each individual train moves past me. By saying that the "train car frequency" increases (has become blue-shifted), we can also understand that the speed of the entire train has increased relative to us.

    If you replace each individual train with a full oscillation of the EM field, then you can understand how I'm having trouble making sense of the fact that, by moving against a beam of light, its increase in frequency does not also mean that the entire beam is moving relatively faster.

    I understand that you can make an appeal to experimental "fact" in this case. That's all fine and well, but I just don't have access to the necessary equipment that will allow me to verify those claims. I'm trying to make philosophical sense of this conundrum. Can anyone help me out here?
  2. jcsd
  3. Apr 13, 2008 #2
    Unfortunately there is no law regarding the constancy of the speed of trains =)

    But anyway, if the frequency of the EM radiation increases, you'll find that it's wavelength has decreased. This is partly due to Lorentz contraction and partly due to good ol' fashioned Doppler.

    So for the train this effect is also true, but if you do the mathematics, you'll find that in the case of light, it all conspires to make c constant, whereas for the train, it doesn't quite.
  4. Apr 13, 2008 #3


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  5. Apr 13, 2008 #4
    Great... I try to ask a simple question about a simple physical law, and I'm once again faced with my arch-enemies: the relativists. (I posted in Classical Physics, and it was moved here.)

    No worries. You want to argue the theoretical basis for relativity, let's go right ahead...

    Let's go back to Einstein's train thought experiment: bartleby.com/173/9.html

    Okay, so the speed of light is supposed to be constant, no matter how fast, or in what direction the observer is moving.

    We have two observers, who are located at M (on the embankment) and M-prime (on the train).

    Now two strokes of lightning hit at locations A and B.

    Presumably, all of these distance relationships hold true:

    |A->M| = |B->M| = |A->M-prime| = |B->M-prime|

    Now, given that v=d/t, we have t=d/v.

    Since, at the moment that the lightning strikes, all of the distances are the same and all of the relative velocities are the same, it holds that all of the times are the same. Both observers, in other words, should see both lightning strikes simultaneously.

    But then, you say, these distance relationships do not in fact hold true because the train's length should be compressed due to the Lorentz effect. If this is the case, when point A on the train lines up with point A on the embankment, then M-prime should be just slightly to the left of M, and point B on the train should be even more to the left of point B on the embankment.

    In this case, when the two strikes hit points A and B on the embankment, these relationships hold:

    |B->M-prime| > |A->M| = |B->M| > |A->M-prime|

    The movement of the observer on the train will now compensate for the greater distance that light must travel from B->M-prime and the lesser distance that light must travel from A->M-prime. Both beams, therefore, should hit the observer on the train simultaneously.

    It is only when you think of this thought experiment "unrelativistically" (classically) that it is true that the two observers will not witness the strikes simultaneously. Here, Einstein is using an example based on classical physics in order to make some kind of point as to why classical physics is incorrect!

    Einstein's logic is filled with problems like this. Relativity is a badly argued theory.
    Last edited: Apr 13, 2008
  6. Apr 13, 2008 #5
    In that case there is little reason for you to post here. The purpose of this sub forum is for people to understand the theory better, not to argue whether this theory is valid or not.
  7. Apr 13, 2008 #6
    Ill argue the mistakes in your thought experiment when I have more sleep and time. However, this entire post suffers the same problems you claim relativity has

    Your asking a question about the constancy of the speed of light. Since this phenomena doesnt exist in classical physics, and only exists in relativity you are implicitly assuming relativity is true in your question. It makes no sense to ask this question in the realm of classical physics. Put in simpler terms:

    The speed of light being constant implies SR.
    Of course there are problems with the speed of light being constant in classical mechanics because of the above statement.
  8. Apr 13, 2008 #7
    Hello meatwad.

    Aceptance of the theory of SR is not compulsory. If you accept the basic postulates then logic predicts certain outcomes. If you accept the postulates you must accept the logical outcome. If you do not accept the postulates there is no more to be said.

    I suspect your arch enemies may not object to your non acceptance of the postulates of SR but to your not understanding the basics of the arguments following these postulates, and to your criticism of the logical consequences such as the simple thought experiment which you claim not to have grasped. If you do not accept the postulates of SR why bother to criticise the logical outcome as to you these reasonings should be irrelevant, just an excercise in logic like a Sudoko puzzle. If you do not accept the postulates then surely your line of attack should be to disprove these postulates.

    I have no wish to be your enemy, arch or otherwise. On a personal level i do not object to what you say or do but i do not understand why you object so strongly when others differ from you with regards to SR.

  9. Apr 13, 2008 #8
    Yes, that's correct. The increase in frequency is due to the relative seperation decreasing as a function of time, i.e. because of the relative speed. In your particular case you are rushing towards the train and as such you are encountering the sound waves at a higher rate than if you were at rest. The same thing happens if you are at rest and the train is moving. With light it is the same thing. However there is an additional modification which must be made due to the fact the the clocks in the different frames are running at different rates. I.e. the frequency of the oscillator as measured by the observer who is at rest in S (observer's frame) when compared to observers in S' (train observers) the S observers measure a different rate as the S' observers. However even if there is no time dilation there will still be a doppler effect.
    That's correct. What it means is that you're passing the waves at a higher speed and thus you measure a higher rate (if approaching the train, a lower rate if you're moving away from the train).

    I hope that helps.

  10. Apr 13, 2008 #9


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    Everyone's been very tolerant, but I think you're unbelievably arrogant.

    If you think SR is wrong, come up with a better way to fix gravity and electrodynamics in moving frames. Do you understand why SR is necessary ?

    Do you think this is false ?
  11. Apr 13, 2008 #10
    I find it interesting how a novice to relativity can spend a little while trying to understand relativity and come up against a point of misunderstanding and all of a sudden it's "zomg Einstein was wrong !!11". The man was one of the greatest minds of the previous century, not only because of the great stuff he came up with, but because of his perseverance when it came to things he didn't understand. Lesson to be learnt.

    Also, I think that he was fairly pedantic sometimes, and maybe you just need to give it a second reading. (I'll admit that he had a lot of confusion with GR, but my god man... )

    Sometimes it seems like popular science or even mainstream education have given people the idea that relativity is simple or easy. It's not. Mull over it until you understand it. When it becomes your bread and butter, go ahead and criticize it.

    If I have offended, my apologies.
  12. Apr 13, 2008 #11
    I left something out in my previous post. Even if the frequency of the oscilator on the train remained invariant there would still be a blue shift. In fact this is what happens with a train and its whistle. In Newtonian mechanics both the train observers and the train station observers measure the same exact frequency. However the moving train motion allows the sound waves to "bunch up." To understand what this means consider a train which, instead of a whistle, emits short pulses of sound. The velocity of the sound waves, as measured by the station observers, is not a function of the trains speed. However, since the train is in motion the train emits these sound pulses at closer an closer distances to the station observers then it takes less and less time to reach the station observers. The result is that the frequency of the pulses arive at shorter time intervals. If we adjust the pulse frequencies to be the same as the frequency of a sound wave then it is obvious even to the most casual observer :smile: that the station observers measure an increased sound frequency.
    There are a lot of things that you probably accept without verification. Have you ever actually measured the electric field around a charged object? If not then how can you assume that such a field exists? I've never actually been to Paris but I'm pretty sure it exists. Some people even believe that astronauts never landed on the moon, never mind that radio waves were recieved from the moon by many people on Earth, including amateurs. That kind of consiracy can't exist. There are far too many people who would have known the truth and with that many people secrets would not remain secrets.

    These experiments were carried out by many different experimentalists over many decades and which were consistent with the predictions of relativity. As such one to correctly gains confidence that relativity has been experimentally verified. Consider the motivation of the experimentalist. Put yourself in their place. An experimentalist creates an experiment to detect predictions of a theory. As such a negative result (i.e. one that proves that the prediction is wrong) would imply that one ore more of the postulates of relativity were wrong. A person who'd prove such a thing would make a place for himself in history. People would love to find errors in relativity for that reason. However even the most brightest of experimentalists have never constructed an experiment which ended up resulting in a negative result. Since the same holds for all experimentalists who do work in relativity it would be very confusing for each experimentalist to have the same odd notion of trying to misinterpret the results of all these experiments and try to claim that relativity is wrong.
    Huh? :confused: Why are they your enemies? If its because they disagree with you then perhaps you can tell me why they'd agree with you if they didn't believe in it? And if they believe in it then don't you think that they have good reason to? And that includes hundreds of thousands (if not millions) of people who have studied relativity and none of which found an error in it. I'm fairl sure that every single student, who is worth their weight in salt, would blindly accept something merely because someone said so. There is no motivation to do so. There is more motivation to find an error in it. Most of the students learning relativity go through all sorts of scenarios in their mind in order to satisfy themselves that there is no logical flaw in relativity. That includes myself and most physicists.

    Before you try to perform experiments yourself, or reject all the papers written to describe such experiments, I recommend that you consider what would motivate people to either lie in their papers, to report false results or for all the readers to misinterpret the reported results. Note that such lies have occured in physics. But the motivation was to make a name for themselves and to gain notariety. But such exeriments have never been duplicated with the same reported negative results.
    I beg to differ. Relativity is not argued badly in any way shape or form. Quite the opposite in fact. Einstein's logic is outstandingly perfect in nearly all instances I've ever read or heard of. If you think you've found a paradox then you can rest assured that you made an error somewhere along the line. In this case it appears that you've ignored the shortening of the distance between emitter and observer and the resulting increase in measured frequency which results from the shorter and shorter distances which the sound has to travel. Have you ever read the physics of the Doppler effect? Have you ever seen a proof that the observed frequency is in accordance with the predicted freuquncy? If not they why would you accept the Doppler effect which is a derived result which is consistent with observation. If you accept it merely because you've heard a higher frequency for approaching trains and lower frequency for receding trains then that is no reason to accept the underlying frequency because the qualitative result may have disagreed with the quantitative result. Have you ever met a person who had surgery? If so then what evidence was provided to the patient which convinced him that they actually needed surgery? Were they doctors themselves and as such they didn't need to learn medicince and understand why they needed the surgery? Or di they merely accept that the doctor was a doctor because they went to school for it and learned what they needed to know in order to do the job right? Do you believe that stars are objects like our Sun? If so then what experimental evidence did you review to come to such a belief? Of course understanding the evidence nearly always requires an educational background in the relavant science as well as a belief that the experiment was not flawed or that the scientist was not lying. Do you know such scientists?

    Regarding "nearly" - There is one case where Einstein himself thought his work was a blunder and that is with regards to the cosmological constant. However, Einstein based his motivation of this constant on the currently accepted idea of a static universe. Onky when he accepted that the universe was expanding did he see his "error." But when he learned this he readily admitted it. And there very few scientists that believe that relativity is wrong. Actually I've never heard of a scientists who didn't believe in it but I'm leaving room for them, just in case.

    Recall the reason why his theory was accepted. Let me ask you this first. Why do you believe that physicits came to accept the theory of relativity? If you think that people accepted it because it came from the genius of Einstein then think again. Einstein's genius wasn't recognized until he wrote his relativity paper. And even then very few people heard of the paper. But the few that did were very intelligent, and well-known, physicists. So there is no reason to believe that people came to believe in this theory for the reason that it came from a genius.

    Last edited: Apr 13, 2008
  13. Apr 14, 2008 #12
    I would say that muons moving-with-near-speed-of-light found to have significantly slower decay rates than whose at rest is a very good argument for relativity.

    I would say the fact that the orbit of mercury slightly rotates about the sun is a very good argument for relativity.

    I would say that the existence of magnetic fields is a very good argument for relativity.

    bottomline is: It matters not in the slightes wether Einsteins logic is flawed - expirements have shown, and still shows, that he was right nonetheless.
  14. Apr 14, 2008 #13
    Those are predictions, not arguments. But the experiments don't validate the theory, merely fail to invalidate it.
  15. Apr 14, 2008 #14
    Good thing then, that it is the coherence of the predictions and the experiments that ultimately judges the validity of a theory, and not the quality of its arguments. Sorry, I am an experimental physicist after all :cool:

    I beg to differ. Of course no theory is ever validated with absolute certianty. But surviving a century's worth of evalutation and critique is coming pretty close to "validated with absolute certainty"

    There are plenty of experiments that does not validate a model, and yet fail to invalidate it, at the same time.
    Last edited: Apr 14, 2008
  16. Apr 14, 2008 #15


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    This is the flaw in your logic. A train is, at best, a very poor analogy to a beam of light. You certainly can't draw any conclusions from the comparison.
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