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Who came up with the Light Clock Thought Experiment

  1. Jul 26, 2012 #1
    Everyone is familiar with the Light Clock, a thought experiment which forms the entire basis of Special Relativity.

    Through the Light Clock, the Lorentz factor can be derived.

    So who was the genius who first came up with the Light Clock? Was it Lorentz himself? Or someone else?

    When did it make its first appearance?
  2. jcsd
  3. Jul 26, 2012 #2


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    I wouldn't say that it "forms the entire basis of Special Relativity." It's just one way of introducing the topic. It has the advantage of being relatively nonmathematical.

    Interesting question. I suspect it was invented about 70 years after Lorentz's work. The first place I encountered this presentation was in Hewitt's textbook Conceptual Physics, which dates back to 1987. This paper http://arxiv.org/abs/0705.0941 says, "The light clock is a pedagogical device used by many authors for deriving the formula that accounts for the time dilation relativistic effect," and gives two references, the earlier of which is to Space and Time in Special Relativity by Mermin, dating back to 1968. Amazon let me peek at the relevant part of the book with their "look inside" feature. (Mermin's more recent approach to the pedagogy of SR is given in his newer book It's About Time.) This paper http://arxiv.org/abs/physics/0505134 points to an earlier use of the idea, in 1963 in the Feynman Lectures (section 15-4).

    The problem with the light clock as an introduction to SR is that it requires Einstein's 1905 axiomatization of relativity, which, with the benefit of 107 years' hindsight, inappropriately singles out light as having a special role. We have a FAQ about the different possible axiomatizations: https://www.physicsforums.com/showthread.php?t=534862

    A more nitpicky objection to the light clock is that I think most presentations fail to justify a hidden assumption, which is that the length of the light clock is the same in both frames. This assumption would be incorrect if the light clock were oriented longitudinally rather than transeversely.

    There is clearly a close affinity between the Michelson-Morley experiment and the light clock. Feynman pretty much develops it this way (and also doesn't cheat on the issue of longitudinal and transverse length contraction).
    Last edited by a moderator: May 6, 2017
  4. Jul 27, 2012 #3
    It was Gilbert Newton Lewis and Richard Chace Tolman in "The Principle of Relativity, and Non-Newtonian Mechanics"
    Published in the year 1909 in Proceedings of the American Academy of Arts and Sciences, 1909, 44: 709–726


    See Figure 1 and explanation on page 714.
  5. Aug 2, 2012 #4

    Then how did Lorentz derive the Lorentz factor before the Light Clock was thought of?
  6. Aug 2, 2012 #5


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    Lorentz argued entirely from the experimental result that a moving charge does NOT have a differenti electrical field than a moving charge even though Maxwell's equation say it should.
  7. Aug 2, 2012 #6
    where can i find this argument?
  8. Aug 3, 2012 #7
    See this excerpt from the Feynman Lectures, especially the last two pages. It show how Lorentz got the Lorentz transformations from Maxwell's equations.
  9. Aug 3, 2012 #8


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    Well, of course... :wink:
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