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I Is it time to "retire" time dilation and length contraction?

  1. Nov 14, 2015 #1
    Considering the enormous number of questions posed on this forum and other places, the concepts seem fundamentally flawed (because both are formally and practically unobservable). The calculations themselves (together with the Lorentz Transform) are highly error-prone and the results misleading (the "Mr Tomkinson" phenomenon) and unsatisfying (you can't directly see either except in very specific circumstances).

    I realize this might come across as flamebait, but as an amateur learner I have not found either concept particularly useful to my studies. They seem most common in pop science "wow look how weird relativity is" presentations, and the fallout is seen in all of the forums that I visit (here, Physics Stack Exchange, reddit/AskScience etc)

    As an alternative, I would propose approaches that I have seen in a few places, but which are not very common.

    1) Formal derivation of the Lorentz Transform, leading swiftly to defnition of the space-time interval and space-time diagrams.

    2) Introduction of four-vectors, including four-momentum and four-frequency

    3) Extension of the 1+1 spacetime to 2+1 (which is necessary and sufficient for first-person calculations of what we can see/observe/measure)

    4) Develop formulas for aberration/Doppler/headlight effects, so students will be able to calculate eg. the shift in apparent positions etc. of stars/galaxies

    5) maybe introduce time dilation etc as a historical formality for the interested (or bored!) student

    I'll leave it here for now, rest assure that I have other practical arguments in reserve if anyone deems this post worthy of comment!
  2. jcsd
  3. Nov 14, 2015 #2


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    I think that is a plausible approach, consistent with de-emphasizing coordinate dependent features (which time dilation and length contraction are).
  4. Nov 14, 2015 #3
    I agree that this should be the approach for all students who wish to pursue GR into the future. It makes the algebraic to topological transition much easier later on.
  5. Nov 14, 2015 #4


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    I expect that no matter what approach is adopted, the essential challenge will be the same: The student has to let go of the notions of absolute time and absolute simultaneity first. Although I agree that length contraction and time dilation are widely misunderstood, I also find that these misunderstandings are almost always the result of the deeper misunderstanding of absolute time.
  6. Nov 14, 2015 #5
    Agreed. In my case I did just that, but found that after letting go of absolute time and simultaneity I was left floundering, wondering what was left to trust ;) The thing the brought me back into the "reconstructive" phase was precisely the concept of proper time, which I felt had been hidden behind layers of deconstructive and tedious calculations (crushed spaceships and bloody trains!) in the majority of presentations.

    Based on my own experience I am convinced that if a student has to let go of such fundamentals (and they must), they need to be given something else to hang on to!

    I think that learning about proper time, together with calculating what we can actually see, were my salvation. Once learned, I could look back on all my failed attempts to understand SR and see that much of the teaching material itself had been the major obstruction in my case.
  7. Nov 14, 2015 #6
    Even for SR, in my opinion (regarding 1); If proof be needed, then surely the twin paradox is the golden example of where the space-time interval and diagrams are the right way to go.

    Just reading people's explanations of what happens with simultaneity at the turnaround makes me cringe, and don't even get me started on explanations based on acceleration ;)
  8. Nov 14, 2015 #7


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    @m4r35n357, while I agree w/ your annoyance at all the time and energy that is spent on correcting unfortunate misconceptions (for which, I think, Nugatory has identified a deeper root cause), and I do not argue at all with your proposal, I would point out that is it not going to have any effect. Beginners, and even more to the point amateurs, are going to keep asking those same questions over and over whether you or I like it or not and I think part of the charter of PF is to help them clarify the issues by having those same damned long discussions every time a noobie asks the questions. For a lot of those folks the very phrase "Formal derivation of the Lorentz Transform" will make their eyes glaze over. I think you have to guide them to that, not hit them over the head with it up front even though doing so would be better for the more math-oriented.
  9. Nov 14, 2015 #8
    My "annoyance, list of demands, whatever" was more aimed at courses, books, etc. that keep peddling the same old line than at what happens on this forum (the OP was tagged "teaching", but it's not really obvious). I only mentioned this forum (amongst others) in the sense that this is one of the places that the casualties turn up!

    I'm not trying to tell anyone here what to do, just giving some idea of what I feel is likely to be effective. Although if I was starting by asking questions here I would want to learn about proper time first, purely because the concept is simpler (as are the calculations).

    BTW I'm slightly amused that you seem to think that "Formal derivation of the Lorentz Transform" was aimed at "noobies" here ;)
  10. Nov 14, 2015 #9


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    fair enough.

    One of the challenges faced here is that we get SO many people who don't really want to study anything, they just want simple answers to, for example, what they heard Michio Kaku spouting on a TV show.

    I took your whole post as being addressed at noobies, since they are the ones that create the issue that I perceived you to be addressing. Certainly a noobie with a PhD in physics isn't likely to have the same level of difficulty as a much less knowledgeable noobie, but the latter are WAY more common here.
    Last edited: Nov 14, 2015
  11. Nov 14, 2015 #10
    I trust we now all understand that my ire was aimed at the teaching materials and not the students . . .
  12. Nov 14, 2015 #11


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    fair enough, but I don't think that was clear in the original post
  13. Nov 14, 2015 #12
    Hmm, I thought I was quite explicit that I was talking about my personal learning experience and how TD and LC (obvious abbreviations) hindered rather than helped.

    Here is a more pointed attempt at the title of my original post (too long for a real title without the abbreviations):

    "Should the concepts of TD and LC be regarded in same way as relativistic mass?"

    Neither set of ideas is actually wrong, but they all serve to make the subject of SR harder to learn than it should be (in my experience of course).
  14. Nov 14, 2015 #13
    From my experience (when I struggled to understand SR, or trying to explain it to others), the relativity of simultaneity (I call it "time's slope" :oops:) is the first thing that needs to be explained and understood. Without it, the mutual length contraction and mutual time dilation don't make any sense, which is the main obstacle in accepting those 2 concepts.

    Even noobies have heard about "time is relative", but if I say that my clock runs twice as fast as yours, and yours are running twice as fast as mine, they are left scratching their heads.

    Sooner or later you'll need to introduce the Lorentz transformation, but starting with it isn't likely to be effective. IMHO.
  15. Nov 14, 2015 #14
    I've never computed either, and don't see why I would need to (nor have I ever computed relativistic mass).

    My point is they don't really help much so why bother going to the extra effort (at least to start with)? All they tell you is some unobservable weirdness associated with two moving objects (in two frames of reference). Using the spacetime interval, a piece of paper and a ruler one can explain the twin paradox in words of one syllable (almost). That is a fairly complex example using three frames of reference, and the result is observable. Maybe it's a personal thing but I find the latter far more satisfying, as well as easier to learn.
    Last edited: Nov 14, 2015
  16. Nov 14, 2015 #15
    In the twin paradox, once the twins are re-united, there can be age differences that are not coordinate dependent.

    So is it really correct to say that time dilation is a purely coordinate dependent feature?
  17. Nov 14, 2015 #16
    The age difference, like the aging itself, is coordinate-independent (invariant). Time dilation does not enter into the discussion or the calculation.
  18. Nov 14, 2015 #17


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    We can't even seem to "retire" relativistic mass.
  19. Nov 14, 2015 #18
    And what term other than "time dilation" would you propose to describe the fact that one person has aged at a different rate than the other or some physical process has progressed at a different rate?
  20. Nov 14, 2015 #19
    Difference in "length" of world lines.
  21. Nov 14, 2015 #20
    Hmm so when I fly to Alpha Centauri at 0.9c, I only age 1.83 years.
    How do you explain it without length contraction?
    How can you even use Lorentz transformation without referring to length contraction?
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