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Ruler envy

  1. Aug 31, 2006 #1
    Another thing I dont understand about SR. Length contraction. As I move at near the speed of light, a stationary observer observes my length to be contracted. What exactly contracts? Is it the space between my molecules or the molecules themselves? Furthermore, if we switch perspective to my point of view and say the "stationary observer" is contracted in the opposite direction, how does that square with our two rulers. Mine is shorter than his from his perspective, and mine is longer than his from my perspective. How can this be?
  2. jcsd
  3. Aug 31, 2006 #2
    I think to clarify, I should have said, mathmatical points instead of molecules. Using the Euclidian concept of a one dimensional point in space, during length contraction, are points taken away or do the points somehow get smaller?
  4. Aug 31, 2006 #3
    To fix the ideas , say that the ruler has length L in an inertial frame S attached to the ruler. Say that S (and the ruler) move at relative speed v wrt to an observer situated in inertial frame S'. Then, the observer in S' sees the length of the ruler to be L'=L*sqrt(1-(v/c)^2) .
    The contraction is in what S' sees. The reason is that for S' to "see" the ruler in S it needs to see rays of light propagating from the ends of the ruler (and from all the other points in-between). Since light propagates with a finate speed, the visual effect is one of contraction.
    The molecules, atoms, electrons in S do not contract. Lorentz (and FitzGerald before him) believed the opposite but that was proven wrong by a famous experiment ran by Trouton and Rankine and reenacted by others.

    No, absolutely not, neither of the above. Actually, to make things more interesting, the ruler does not appear only contracted but also rotated. Look up the Terrell-Penrose effect.

    Heh, this is a classical. The short answer is that if you have two cars driving away from each others, when the drivers look in the rear mirror both see the other car diminishing. Is this a paradox? Then why should be the symmetric contraction of SR a paradox. Quite the opposite, one would expect symmetry, it is one of the fundamental laws of physics.

    The longer answer is that the whole thing has something to do with the way we measure intervals in relativity. And it has something to do with simultaneity. Look up pages 22-23 in, for example, "Relativity, Thermodinamics and Cosmology" by RC Tolman, a very cheap and good book on relativity
  5. Aug 31, 2006 #4
    What was proven what just that the rest frame cannot measure it, nothing more.

    Whether rods actually contract is a statement of preference for a particular theory, e.g. ether vs relativity, since so far we have not been able to prove that one way or the other.
  6. Aug 31, 2006 #5
    This is the frame that counts, the lab frame, isn't it? The observers are the ones comoving with the rod, rulers,Whetstone bridges, etc. under observation. Do you think otherwise?

    What do we have here? A closet "aetherist"? The many "aether" theories have been discarded in favor of SR. Do you know why? I'll give you a hint: why does the Kennedy Thorndike have difficulties with an "aether" theory explanation?
    A second hint: why does the Ives Stilwell experiment have such difficulties with an "aether" theory explanation.
  7. Aug 31, 2006 #6
    Needless to say that there are many crackpot ether theories that do not make any sense whatsoever.

    But SR and Lorentz ether theory are simply theories, nothing more nothing less. :smile:
    Lorentz ether theory simply presents an alternative with the same results.

    Unlike others I do not get emotional about any favouring or bad-mouthing any one of them for the simple fact that they provide the same numerical results.
    I would wish that more people would demonstrate such an objectivity and open mindedness. :smile:

    SR, in my view, is a more fascinating theory than LET but that does not warrent it to be considered more right than the other.
  8. Aug 31, 2006 #7
    I didn't ask you about crackpot theories, I asked you about legitimate "aether" theories. Do you know which one I was asking you about?

    Really? What are its difficulties in explaining the two experiments I asked you about?

    Then you will have no difficulty in answering my questions, right?

    Then you will have no difficulty....So , could you please back up your point?
  9. Sep 3, 2006 #8
    True, SR and LET are equivalent in predictions, but SR derives some of LET's postulates as consequences from SR's two simple postulates. Therefore, SR is strongly the preferred theory.

    Imagine a new theory that is nothing but a big list of "if you do this, and that, you will measure this". If you make this list infinitely long, you could make a new theory equivalent to SR as well. Yet, I assume you would agree that this theory is not adequate. So maybe that will be a good example to help others understand that agreement with experiment is just one criteria used to evaluate a theory (albeit the most important one).

    Lorentz did not believe the contraction would be measurable in the object's rest frame. In fact he showed the exact opposite, showing that Maxwell's equations had what has become known as Lorentz symmetry, and hence would appear the same in all inertial frames. Once Poincare pointed out and fixed the fact that the charge density was still not described relativistically (in 1905 I believe), LET became equivalent to SR (before the Rankine experiment). Lorentz did continue to grapple with his very "absolutist" view of space/time though (he and Einstein had several discussions on these topics).

    Now you are mixing two notions of "appear". The observer will definitely not measure the ruler to be rotated. He will only measure it to be contracted.

    Only in the sense of "if you could take a photograph" (ie not correct for the time light took to arrive) would it "appear" rotated.

    But that misses the point. Silverdiesel, in the question that started this thread, specifically mentioned a second observer who is doing the measurement (and sees the molecules, atoms, etc moving).

    So, Silverdiesel, in answer to your question: yes, that observer would measure the space between your molecules, your molecules, atoms, etc (all of you) to contract.

    Since MeJennifer specifically mentioned LET, I think your condescending tone implying that she should automatically know you decided you changed the subject without stating so is very inappropriate.
  10. Sep 3, 2006 #9
    You got the wrong LET. You won't find it in any of your books.

    Wrong, he shows exactly the opposite in his 1904 paper. He might have changed his mind after the Rankine results but it took him a looong time. You are right about the fact that he and Einstein had lengthy discussions about this issue.

    You are mixing up Lorentz's theory with LET (hint: LET is a very late arrival, circa late 1990's)

    Did I say anything different than you are saying or are you just being pedantic?

    Yes. Did I say anything different? Terrell - Penrose is quite trivial, no need for you to split more hairs than there are.

    I think that you don't know what LET is. It is not something mentioned in any books and has very little or nothing to do with Lorentz. I'll "let" you research it on your own.
    Last edited: Sep 3, 2006
  11. Sep 4, 2006 #10
    So you are saying that Lorentz's theory of electromagnetism is not mathematically equivalent to SR, but LET is? And that LET is something else?

    These are bold statements. I would appreciate if you backed up such statements if you make them on these physics boards. So please do back them up.

    Lorentz introduced the full Lorentz transformations in 1904 (not just the "first order" ones he showed previously). Lorentz also used Maxwell's equations and the Lorentz force law (both of which have Lorentz symmetry). As already stated, once Poincare pointed out and fixed the fact that the charge density was still not described relativistically (in 1905 I believe), Lorentz's theory became equivalent to SR (before the Rankine experiment).

    The fact that Lorentz continued to grapple with his very "absolutist" view of space/time, and forced himself to work out how "everything cancelled" is his own prerogative. It does not affect the predictions of his theory though. Due to the fact his theory had full Lorentz symmetry (something which strangely Lorentz never fully utilized himself), we can see that it immediately explains Rankine or any other experiment.

    Lorentz even stated in his 1910 book that the theories were equivalent. You comments made me search google for a bit and here is a nice quote from a PHD thesis: To the end of his life, Lorentz maintained, first, that his theory is empirically equivalent to special relativity, and, second, that, in the final analysis, it is a matter of taste whether one prefers the standard relativistic interpretation of the formalism of the theory or his own ether theoretic interpretation (see, e.g., Nersessian 1984, pp. 113–119).

    But for others reading, please remember that while SR and LET are equivalent in predictions, SR derives some of LET's postulates as consequences from SR's two simple postulates. Therefore, SR is strongly the preferred theory. Agreement with experiment is just one criteria used to evaluate a theory (albeit the most important one).

    No, many people abbreviate it that way.
    LET stands for Lorentz Ether Theory.

    Why then does it have Lorentz's name in it? I think you may have misread something.

    Yes, you were combining the ideas of "appear" to mean measuring as well as meaning "according to a photograph".

    Not only should you not combine those ideas, but it is even confusing to introduce the second idea to people just beginning to learn SR because we almost exclusively refer to measurements of observers (in other words, correcting for the finite speed of light). If you really must introduce it, at least explicitly separate the two meanings and ideas.
    Last edited: Sep 4, 2006
  12. Sep 4, 2006 #11
    What I am saying is that you understand neither Lorentz theory (see the issue on length contraction) nor do you know what LET is. Try to find out on your own.
  13. Sep 4, 2006 #12
    According to Justin Levy, he (and many others) regards LET as Lorentz Ether Theory. Given that he has already said what he thinks LET is, saying

    isn't very helpful, is it?
  14. Sep 4, 2006 #13


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    Most of what humans know they know on the basis of what other humans tell them. So the most natural argument in favor of one version of relativity over another is to find out what the experts say. The original developers of these things understood them at great depth (in the context of physics at that time) and could explain the differences between the accepted theories and the rejected theories at a depth that the students of the students of the students of their students cannot. Like how to make a buggy whip, the knowledge is mostly lost.

    When humans want to know something, they look to the opinions of the experts. When the experts disagree, humans look for the most famous of the experts or the majority of the opinion. This tends to suppress the alternatives even if the choice is a matter of preference, not proof. Over the long term, the inevitable tendency is to turn majority opinions into near unanimous opinions that then become accepted as "facts".

    The alternative would be to complicate the education of students by presenting them with a bunch of different equivalent alternatives. Instead, students tend to get taught one way of doing things, and the majority opinion is reinforced. A great example of this is the recent history of string theory.

    The original participants in the debates, who understood both sides of the subject very deeply (in the context of physics as it was then known) typically retain their beliefs whether they agree with the mainstream opinion or not. As time goes on, this makes physicists on the losing side of these arguments more and more out of sync with the rest of the community. Eventually they die and their version of physics mostly dies with them.

    But from a sociological point of view, the important thing to note is that these conflicts were decided not by pure logic, but instead by a sort of majority decision making that crowded out alternative thinking, combined with the mortality of the human race. In addition, as one majority or the other gets control of universities, they will prevent those with the alternative view from obtaining tenure. See Woit's new book, "Not Even Wrong" for the string theory history, or Smolin's new book, "The Trouble with Physics".

    As time goes on, there will always be a few people who rediscover the obsolete alternative. Sometimes things have changed in physics so that the obsolete view looks more attractive. This is the case with the versions of relativity that included an ether. As an example, buy Bohm & Hiley's book, "The Undivided Universe" and read chapter 12.

    If you want to understand the foundations of physics, not just in relativity but also in quantum mechanics, field theory, etc., below the "shut up and calculate" level, you can either go read the original sources (as suggested in the Wikipedia article), or you can hunt around for the very few modern histories of the subject.

    But it should be noted that if you ask the vast majority of physicists their opinions on these things they will tell you that you are wasting your time. The vast majority of physicists treat physics as a set of formulas that one manipulates in order to obtain interesting new results. Most physicsts do not question the formulas any more than most engineers question the formulas they use in their work. Most physicists do not have the intellectual inclination or ability to question the founding principles of physics any more than most engineers can question the founding principles that they use. Shut up and calculate.

  15. Sep 4, 2006 #14
    That's quite a nice tract, but I'm not sure if that was directed at anything I said or if it was a reply of any sort to my post.
  16. Sep 4, 2006 #15
    Look at post 9 , ok?
  17. Sep 5, 2006 #16
    I agree with masudr. You seem like all you want to do is pick a fight for some reason.

    I have taken the time to explain to you my understanding and even cite some sources. You instead just blanketly state that I am wrong and supply no information supporting your statements at all.

    So unless you actually respond with specifics about what you agree with, what you disagree with, and information supporting your statements, I see no reason to continue this "discussion". Have a good day sir.
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