Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I What Actually is Length Contraction?

  1. Mar 19, 2016 #1
    When I first learned special relativity it was on an elementary level. I was told that "space itself" contracts in a moving reference frame. Now I am studying electrodynamics out of Griffiths. I just read the derivation of the Lienard-Wiechert potential and the fields for a moving charge. Griffiths on the field of a moving charge: "In the forward and backward directions E is reduced by a factor of (1-v^2/c^2)..." So now putting special relativity aside, doesn't this reduction in the field strength imply that the intermolecular forces would be weakened and thus that the intermolecular separations would be contracted along the direction of motion? Does special relativity account for this within the framework of the Lorentz transformation, or is this "Lienard-Wiechert contraction" a separate effect that has to be calculated independently to find an accurate result?
     
  2. jcsd
  3. Mar 19, 2016 #2

    strangerep

    User Avatar
    Science Advisor

    You can't "put special relativity aside" in this case. The Lienard-Wiechert potential is derived within the framework of SR, i.e., as a relativistically correct version of the EM potential produced by a moving point source.
     
  4. Mar 19, 2016 #3
    Except it was derived in 1898 and SR in 1905...? Griffiths doesn't invoke SR at all in deriving the Lienard-Wiechert potential.
     
  5. Mar 19, 2016 #4

    strangerep

    User Avatar
    Science Advisor

    The Lorentz transformations, and various puzzles surrounding it, were being investigated earlier than 1898. See, e.g., the history section in Wikipedia's article on LTs.

    I don't have a copy of Griffiths, but if it's similar to the derivation in the Wikipedia article on Lienard-Wiechert I'm guessing it relies on Maxwell's equations, which are SR-compatible. (I.e., SR-like features are already built-in to Maxwell's equations.)
     
  6. Mar 19, 2016 #5

    Dale

    Staff: Mentor

    Intermolecular separations (at equilibrium) are definitely contracted along the direction.

    The question of forces is a little more complicated.

    The LW potentials are fully relativitistic. You don't need to calculate anything else independently.
     
  7. Mar 19, 2016 #6
    Of course it relies on Maxwell's equations. I don't think that means it has SR "built in."

    Thanks for your reply. I guess my question is the inverse of the one you answered. Let there be a measuring rod traveling in some direction at constant velocity. SR tells you how much the length of the rod contracts (measured in stationary frame) due to the changing geometry of spacetime, but as I learned it SR doesn't mention anything about contraction due to intermolecular spacing. Since there definitely is contraction due to changes in intermolecular spacing, why doesn't SR mention that? Is it just sort of built into the math and nobody talks about it? You say that LW potentials are fully relativistic, so I'm assuming that means you don't have to add any calculations to SR to account for intermolecular effects. But I still don't understand why SR doesn't mention intermolecular contraction explicitly. Maybe I just learned a watered down version...
     
  8. Mar 19, 2016 #7

    strangerep

    User Avatar
    Science Advisor

    Then you need to learn more about SR, and the subject of symmetry groups of dynamical equations.

    Anyway, it sounds like I'm not the right person to help you. Bye.
     
  9. Mar 19, 2016 #8

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    No, it doesn't. The geometry of spacetime in SR is the same everywhere--flat Minkowski spacetime. SR tells you how much the length of the rod contracts due to the different "angle in spacetime" at which the length is being measured (the "angle" is due to relative velocity and is a hyperbolic angle).

    It may not have been mentioned, but SR length contraction is contraction due to a change in intermolecular spacing. When measured at a different angle in spacetime, the spaces between the molecules along the direction of motion of the object are shorter, just like the spaces between any other objects.
     
  10. Mar 19, 2016 #9
    Sorry, I always get my words mixed up on this.

    But why don't you have to calculate the contraction due to changes in the strengths of the fields of individual molecules (a la LW potential) and sum them over the entire object? Is this effect accounted for implicitly by the spacetime rotation?
     
  11. Mar 19, 2016 #10

    Dale

    Staff: Mentor

    It is the same thing.

    It doesn't matter if the length is the distance between molecules, the distance between ends of a rod, or the distance between stars. All lengths of any type exhibit length contraction.
     
    Last edited: Mar 19, 2016
  12. Mar 19, 2016 #11
    So the coordinate transformation of SR is actually calculating all the LW contracted fields between the molecules and summing them up and telling us the net effect this has on the length of the object? Could you get the same answer by explicitly calculating the length contraction using the retarded potential between molecules, assuming this were computationally feasible?
     
  13. Mar 19, 2016 #12

    Dale

    Staff: Mentor

    Maxwell's equations have SR built in, so anything based on Maxwell's equations also has SR built in.
     
  14. Mar 19, 2016 #13

    Dale

    Staff: Mentor

    Assuming that there are no other forces involved besides EM forces on classical point charges, yes.

    Maxwell's equations are more general than the LW potentials since they apply to other charge configurations. And QED is more general than Maxwell's equations since it applies to quantum scales. And SR is more general than QED since it applies to the weak and strong forces too. But wherever LW applies, yes, you could get the same answer either way.
     
  15. Mar 19, 2016 #14
    Electrodynamics says that a moving system of a positive and a negative charge is time dilated and length-contracted? That's quite interesting.

    Let's consider a relativistic water droplet. If the longitudinal surface tension is weaker than the transverse surface tension, then the droplet tends to become longer. That does not sound right, so we conclude that it is not so that longitudinal intermolecular forces inside a moving droplet become weaker.

    Now we just have to find out what "In the forward and backward directions E is reduced" really means.

    Special relativity says that a moving field is contracted, so if a field changes by 10 % per one meter at rest, then at speed 0.87 c the "same" field changes 10 % per half meter, and maybe about 20% per one meter.

    SR says that in different frames the "same" point of an E-field is at different distance from the charge, while electrodynamics says that in different frames the "same"point of an E-field is not at different distance from the charge, but the EM-field strength is different in different frames?
     
  16. Mar 19, 2016 #15

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    No, it doesn't. The way EM fields transform under a Lorentz transformation is not a simple "length contraction" of a static field.
     
  17. Mar 20, 2016 #16

    Dale

    Staff: Mentor

    The surface tension forces and the forces inside the droplet all transform the same way. It isn't about "sounding right", it is about working through the math.

    SR and electrodynamics say the same thing. SR is built in to electrodynamics. The difference is only that SR is more general and applies to situations where electrodynamics does not.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted