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Really moving. What does Bridgman mean.

  1. Oct 23, 2009 #1
    This extract is from Bridgman, A Sophisticate's Primer of Relativity. 1962 Page 28. Only the last couple of sentences are relevant but the rest is included to put it in context.

    He starts---

    Velocity IS a relative concept, and whenever the physicist allows himself to speak of velocity with an implication of absoluteness, he is either forgetting something or is tacitly implying something he has not taken the trouble to make explicit. Sometimes, for example, the Michelson-Morley experiment is described as showing that "absolute" velocity does not "exist." Of course it does not exist, because it is not that sort of thing BY DEFINITION. What the physicist is actually saying here is that there is no evidence for the existence of the old-fashioned ether, which if it existed could be taken as a universal frame with respect to which velocities could be measured.------

    So far so good. Then he continues-----

    One of the most insidious, and because it is so insidious, one of the most vicious formulations of this point of view is; "Relativity theory says that if two frames of reference are moving with respect to each other, it is impossible to say which frame is 'really' moving". The usual implication here is that nature is so constructed that it is impossible to make the decision. The impossibility is entirely man-made. This point of view is behind some of the intuitive difficulties exploited in some recent discussions of the paradox of the "space traveler".-------

    Does anyone know What he means by that. I can think of one or two interpretations, none of which fit in with SR.

  2. jcsd
  3. Oct 23, 2009 #2


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    when he says "if two frames of reference are moving with respect to each other, it is impossible to say which frame is 'really' moving"." he is repeating what he had just said. All motion is relative and there is no such thing as "really moving". He makes that clear when he says "The usual implication here is that nature is so constructed that it is impossible to make the decision." and then denies that and says "The impossibility is entirely man-made." That is, that there is no physical law that says we cannot decide which frame is "really moving". It is, rather, that the whole notion of "really moving" is a figment of our imagination. So, basically, when he speaks of "really moving" he is refering to "absolute motion"- motion that is NOT relative to some other body- and his point is that such a thing does not exist.
  4. Oct 23, 2009 #3
    I know that to ask such a question as who is "really" moving is meaningless. It must be my reading of Bridgman which is at fault because it seems to me that he is saying otherwise. And of course that is something that a relativist would not say. That's what made me query it. I must read your interprtation more closely. thanks.

  5. Oct 23, 2009 #4
    Hello HallsofIvy,

    I have re-read your interprtation and it is of course correct and I agree with you that this is what he meant to convey. I think perhaps the wording is open to misinterpretation, by some people, or perhaps by only one, me. It would be interesting to know if anyone else is confused by it.

  6. Oct 23, 2009 #5


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    I think Bridgman is wrong. The impossibility is not entirely man-made, but a law of nature, which we usually call the "Principle of Relativity", and is subject to experimental refutation.
  7. Oct 23, 2009 #6
    It is confusing, but HallsofIvy is correct in his explanation. The formulation is "insidious" precisely because it implies the existence of "really moving" and of a preferred reference frame, and, in idealized special relativity, there's neither.

    Of course, once you dig deeper, you discover that there IS a special reference frame in our universe (the Earth as a whole is moving at the speed of ~600 km/s with respect to that reference frame) ... but it's not special because it's somehow chosen by laws of physics, it's just a historical accident. So the argument is more philosophical that scientific.
  8. Oct 24, 2009 #7
    Hello all.

    As I have already said, I am aware of the meaninglessness of the question who is "really" moving. I see now what is going on. I thought that Bridgman was criticising the statement "it is impossible to say which frame is 'really' moving" and, by doing so, contradicting it. Whereas in fact he was merely objecting to the fact that the phrase might give the impression that such a frame exists but it is impossible to decide upon it. Obviously it could mean nothing else.

  9. Oct 24, 2009 #8

    George Jones

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    I think this might have to do with bridgman's philosophy of science, operationalism. According to this, physical quantities are defined by how we choose to measure them, and thus are "man-made."
  10. Oct 24, 2009 #9


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    I think Bridgman is right in the sense that all motion is relative by definition - so that is not any law of nature. Where I think Bridgman is wrong is that it is not a matter of definition that the laws of nature are the same for a class of motions called "inertial".
  11. Oct 24, 2009 #10


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    Most likely Bridgman refers here to the SR interpretation (sometimes referred to as 'Lorentzian interpretation) that holds that effectively the laws of physics conspire to hide the effects of the actual velocity relative to the background.

    Using the expression 'Lorentzian interpretation': the underlying assumption of a Lorentzian interpretation is that velocity with respect to the background structure does exist. But it remains hidden.

    As you quoted, Bridgman criticises that with extraordinary fierce language ('insidious', 'vicious').
    I surmise that Bridgman asserts that there is no "conspiracy" in the first place; it's not hidden: it doesn't exist

    On a more general note (not referring to Bridgman):
    When textbook authors try to write vividly, their imagary tends to have a lorentzian flavor. For instance, a textbook author may write: "As an object moves faster, it becomes heavier." Although not so intended, by summerizing it in that way you get that Lorentzian flavor. Strictly speaking the textbook author should write: "If an object has a velocity relative to us then in our frame of reference it has a larger mass than its rest mass."
    Technically that is better, but already it start to sound like legalese.

    Possibly Bridgman is so fierce because he hasn't given the benefit of the doubt. Possibly he has read all kinds of stuff that was written to be vivid imagery, with Bridgman thinking each time there was Lorentzian intepretation behind it.

    Last edited: Oct 24, 2009
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