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Rod shortening - physical reality?

  1. Apr 19, 2010 #1
    Hi Guys,
    I seem to have gone in circles YET AGAIN!!!!!!! When discussing the expansion of the Universe I was told by friends that masses and objects do not expand with space because the forces within the mass are so much stronger than the inflationary force. When discussing General relativity we decided that the rod shortening WAS a physical reality. Now the debate has turned to Special Relativity and the question is: Is the rod shortening in Special Relativity physically real or only "observed"
  2. jcsd
  3. Apr 19, 2010 #2
    With the risk of confusing you, the fact that shortening is "observed" (from a moving frame) means that it (the shortening) is "physically real". To further confuse you, the shortening is:

    -dependent on the relative speed between the frame comoving with the object and the observer frame (so, it "varies")

    -strain-free (it does not induce any strain in the rod)
  4. Apr 19, 2010 #3


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    It depends on your definition of "physical reality."

    The energy and momentum of an object also depend on the reference frame. Do you consider this to be "physically real" or "only observed?" Whichever answer you choose, you should apply it to the length of an object also, for consistency.
  5. Apr 19, 2010 #4

    Very good point.
  6. Apr 19, 2010 #5
    Consider a macroscopic version of the Bohr atom, with a central, fixed (massive), positively charged body and a circularly orbiting, very light, negative satellite. Adjust the satellite orbital parameters so that it goes in a circle, under the influence of the central body's electrostatic field. Now view the system from a frame in which the central body translates to the left at speed v equal to the electron's speed in the first frame. (In this second frame the central body of course engenders a magnetic field, as well as an electric field.) Compute the electron's trajectory in this second frame. You'll find out that it isn't a true cycloid; the electron cuts "in front of" and "behind" the central body at a length-contracted distance. Also, viewed from the second frame, the time for one complete cycle of the electron is time dilated, quite as SRT predicts. To the extent such results pertain to actual atoms, I would say that length contraction and time dilation are "real".
  7. Apr 19, 2010 #6


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    I agree completely.

    To the OP, how would you experimentally distinguish between something that was "physically real" and something that was not?
  8. Apr 24, 2010 #7
    Thanks for the input Guys. "physically real" would be the aspect of something that does not change with or depend on the observer's relative movement.You have managed to confuse me further, but I still maintain that the shortening caused by GR is depends primarily on the gravitational potential.Yes, the observers relative speed would affect the shortening further but surely the GR shortening aspect cannot be viewed as not "physically real"
  9. Apr 24, 2010 #8


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    There are no forces involved in either case. Here is a discussion of the issue of whether space actually expands cosmologically: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.2 [Broken] (subsection 8.2.5)

    Putting these two statements together results in something incorrect. When you say "rod shortening," it sounds like you're talking about special-relativistic length contraction. The description of SR length contraction doesn't change when you discuss it in the context of GR, and in GR it's still frame-dependent.

    On the other hand, if you have cosmological expansion in mind, then there are two problems with this: (1) it's an expansion, not a contraction; (2) it doesn't affect a measuring rod by any realistically detectable amount.

    If you want to make people understand you, I'd suggest you change the terms you're using. Since you say you're defining "physically real" as "frame-independent," please use the latter term, and people here will know what you mean. Also, I'd suggest that you not use the term "rod shortening" to refer to cosmological effects in GR, for the reasons given above.
    Last edited by a moderator: May 4, 2017
  10. Apr 24, 2010 #9
    Hi bcrowell,
    Thanks for your assistance. I suppose "frame independent" explains more accurately my question. Can I rephrase the question because no one has actually attempted to answer it. Is the distance contraction (rod shortening) of GR frame independent?
  11. Apr 24, 2010 #10
    By your definition of "physically real", the length contraction of a rod observed in both SR and GR is not real because the measurement is observer dependent. If you define "physically real" as "that which is measured by an observer" then the length contraction in SR due to relative motion and the length contraction in GR due to relative gravitational potential are both physically real.

    As jtbell said, "It depends on your definition of 'physical reality.'"

    Either way, the physical reality of length contraction in either SR or GR stand or fall by the same criteria.
  12. Apr 24, 2010 #11

    Vanadium 50

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    As mentioned before "frame independence" is a better name for this.

    Note that a lot of things fail your criteria for being physically real (even in classical physics): energy, momentum, frequency, velocity...
  13. Apr 24, 2010 #12


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    A whole bunch of people have tried to answer your question, but you haven't been making it easy, because you've been ignoring our attempts to get you to phrase the question in such a way that we can make sure we're answering it. Please re-read #8, and see if you can state the question in such a way that we can understand what you mean by "rod shortening."
  14. Apr 24, 2010 #13
    Very good point Vanadium.
  15. Apr 25, 2010 #14
    OK, so the guy is having trouble anticipating what specific terminology is being asked for, but I still think that the question is worth an answer.

    The original question was:
    Is the rod shortening in Special Relativity physically real or only "observed"

    To help the experts let's clarify that as
    Is the rod shortening in Special Relativity frame independent or only "observed" (e.g. has a specific length measured by an observer who is peculiar to only one specific uniformly moving, inertial, non-rotating frame)

    It's a great question !
    It also leads to other questions like:

    What length is the rod when it is not being "observed" ?
    What length is it if there are multiple observers ?
    What happens to the rod if the observer blinks rapidly ?
  16. Apr 25, 2010 #15
    If two observers from two reference frames can see the same rod having at the same time two diffrent lengths, then can it be real:rolleyes:?
  17. Apr 25, 2010 #16

    Doc Al

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    That's already been answered: Length of a rod is a frame-dependent quantity. (This, of course, is one of the surprises of special relativity. Who knew?)

    Don't get hung up on the idea of "observation". There is a prescription for defining the length of an object. Each frame gets its own answer, which is independent of the existence of an actual "observer". The length of a rod might be frame dependent, but it's not dependent on someone actually "looking" at it! (Relativity is not that crazy. :smile:)

    This thread seems to be going around in circles.
  18. Apr 25, 2010 #17


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    When you say "anticipating," you make it sound as if we expected him to read our minds. There is a standard terminology used in the field. It's widely known and accepted. It's described in all the standard textbooks. It's not a big deal if a newbie makes mistakes in terminology, but when that's pointed out to him, he needs to pay attention to fixing it. We can't have a discussion unless we can use words that have a mutually understood meaning.

    The answer is yes.

    These are all meaningless questions. If two different people propose two different answers to one of these questions, there is no way of settling the disagreement by performing experiments.
  19. Apr 25, 2010 #18
    These are all meaningless questions. If two different people propose two different answers to one of these questions, there is no way of settling the disagreement by performing experiments.

    So if you propose it is shortened, and I propose it just looks like it is shorter, then there is no experiment that can prove/dis-prove either of us ?

    I am going to propose instead, that from an investigate position there are no meaningless questions - just meaningless answers :smile:
  20. Apr 25, 2010 #19
    Here is a proposed demonstration by Rindler and others. Unfortunately I cannot draw the diagram. This would save a lot of words. I hope I have the wording correct because I am doing it from memory.

    Take three identical rods. Let one, for the present purposes, be the stationary rod. Let the two other rods be moving at equal speeds but opposite directions towards and along the length of the stationary rod. Arrange for the left hand end of the rod moving to the right to coincide with the left hand end of the stationary rod, and the opposite for the rod moving to the left, at the same time in the rest frame of the stationary rod. Each of the two moving rods will appear shorter than the staionary rod to an observer at rest relative to the stationary rod.

    But once again, do we describe this as "real" or not.

  21. Apr 26, 2010 #20


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    I think it's necessary to get one step more abstract, towards the spacetime view of SR:
    A rod is not something three dimeansional, it is something four dimensional. It is extended in time also.
    If it were three dimensional, it would make sense to discuss whether it's really changing or not.
    But it is fourdimensional.
    Different observers will call a different 3-D slice of it "the rod", so yes, the 3-D rods all really are different.
    But the real rod, the 4-D entity, exists only once, unimpressed by observers. It doesn't change upon observation.
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