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

Lorentz transformations

  1. Feb 1, 2005 #1

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In all the textbooks I read on SR, they always list the LT assuming y'=y and z'=z. But how does the time coordinate transform if the speed has a y and a z component?

    I'm guessing

    [tex]t' = \frac{t-(v_x x + v_y y +v_z z )/c^2}{\sqrt{1-v^2/c^2}}[/tex]
     
  2. jcsd
  3. Feb 1, 2005 #2
    I think so too.
     
  4. Feb 1, 2005 #3
    Yes. Absolutely.

    Pete
     
  5. Feb 1, 2005 #4

    JesseM

    User Avatar
    Science Advisor

    Did you actually verify this using the Poincare transformation? I tried starting to figure this out by imagining a ruler moving diagonally with clocks mounted on it that were synchronized by light-signals in the ruler's own frame, and things were coming out pretty complicated, although I haven't finished the calculation yet.
     
  6. Feb 1, 2005 #5
    This is the Lorentz transformation we're talking about. Not the Poincare transformation.
    Its a standard equation which can be found it many relativity texts. However I did doublecheck it in Classical Electrodynamics - 2nd Ed., J.D. Jackson. I also recall verifying it myself in the past.

    What do you mean by a ruler moving diagonally? Remember that this equation still relates to axes between two systems for which the axes are parallel. Only thing that's changed is the direction of motion of the origin.

    Note that [itex]v_x x + v_y y +v_z z[/itex] can can be replaced by [itex]\beta r[/tex] where [itex]\beta r[/itex] is the dot product of [itex]\beta[/itex] = v/c and r = (x, y, z). Then we simply replace

    [tex]t' = \frac{t - (v_x x + v_y y +v_z z )/c^2}{\sqrt{1-v^2/c^2}}[/tex]

    with

    [tex]t' = \frac{t - \beta r/c^2}{\sqrt{1-v^2/c^2}}[/tex]

    I.e. all you do is rotate the axes. TGive it a try. Rotations are part of the Lorentz group. Use matrices for this transformation. It should make it simple. I think that will give the correct result. Give it a whirl and let me know how it works out.

    Pete
     
    Last edited: Feb 1, 2005
  7. Feb 1, 2005 #6

    JesseM

    User Avatar
    Science Advisor

    Ah, that's what I got confused about. If you actually rotate the axes, would you have to use the Poincare transformation then? I was under the impression that the Lorentz transform could only be used when the axes are parallel and the origins of the two systems coincide, and the Poincare transformation is more general and can deal with systems not covered by the Lorentz transform, but maybe I've got it wrong, I don't think I ever actually studied the Poincare transformation in my college SR class.
     
  8. Feb 1, 2005 #7

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Nope.

    Rotations = Those Lorentz transformations that leave the t coordinate unchanged

    Lorentz transformations = Poincaré transformations with zero translation
     
  9. Feb 1, 2005 #8
    Lorentz transformations are those transformations of the form x' = Lx where x',x are column vectors and L is a matrix. Poincare transformations are those transformations of the form x' = Lx + a where a is a point in spacetime.

    No. Lorentz transformations can represent transformations such as rotating axes. give it a try. Employ the usual coordinate transformation for a rotation of axes. You'll see what I mean.

    Pete
     
  10. Feb 1, 2005 #9

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    Within each spatial slice the chocie of the x coordiante is arbiatry and is usually choosen for convenience, so the product of a spatial rotation and a Lorentz boost (a Lorentz boost is a change in velocity) is also a Lorentz transformation (as has really already been pointed out). The Lorentz transformation is best viewed as a generalized 'rotation' in spacetime (or at least that's how I like to view it at an intuitve level YMMV)
     
  11. Feb 11, 2005 #10

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Mmmh, and also in this case, the transformations of position have different values of gamma, right? For exemple

    [tex]y' = \gamma_y (y - v_y t)[/tex]

    where

    [tex]\gamma_y = \frac{1}{\sqrt{1-v_y^2/c^2}}[/tex]
     
  12. Feb 11, 2005 #11

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Nope,IIRC,for all possible cases of LT,there's only one "gamma",namely the one with the "beta" squared in the denominator.

    Daniel.
     
  13. Feb 11, 2005 #12

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Are you certain? It seem very weird that if we have a square, moving in the xy plane at speed near c with all of its speed in the x direction, its height suffers no contraction at all. But if most of the component of speed is in the x direction and very little is in the y direction. Then the height is contracted just as much as the width, and just as much as if most of the speed were in the y direction.
     
  14. Feb 11, 2005 #13

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    I'm absolutely sure.I've checked my EM course and,for arbitrary LT,it's still the "old" gamma involved.
    You may wanna check Jackson,if u don't believe me...

    Daniel.
     
  15. Feb 11, 2005 #14

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Ok, I believe you.

    Another weird effect of this is that if we look at the square in a way such that its speed only has an x component, it is a rectangle. But if we tilt our head/coordinate system just a tiny winy bit (or by any amount in [itex]]0,\pi[ [/itex] for that matter), it instantaneously becomes a tiny square.

    There is a discontinuity in the process of transformation of the figure from rectangle to tiny square.
     
  16. Feb 11, 2005 #15

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    It is contracted in the direction of motion, so no height and width are not contracted equally as much as that suggests that the square shrinks, infact in the case where some of it's motion is not parallel to one of the square's sides it will no longer be a square in that frame.
     
  17. Feb 11, 2005 #16

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    But aren't the contraction governed by

    [tex]\Delta x = \frac{\Delta x'}{\gamma}[/tex]

    [tex]\Delta y = \frac{\Delta y'}{\gamma}[/tex]?

    For equal gammas and equal sides of the square in its rest frame (S'), [itex]\Delta x = \Delta y[/itex]
     
    Last edited: Feb 11, 2005
  18. Feb 12, 2005 #17

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    This would suggest that Lorentz-FitzGerald contraction is not just depednent on the velocity, but on spatial orienation also which is not the case. If the square shrinks then there MUST be contraction in the direction perpendicular to the motion of the object (which is actually can be shown to be in violation the postulates of relativity). What happens when the motion is not parallel to a set of the square side is thay the square appears to be a parallelogram.

    see the attachment for illustration of this (it's just a quick sketch done in paint so obviously it's not 100% accuarte).
     

    Attached Files:

    Last edited: Feb 12, 2005
  19. Feb 12, 2005 #18

    quasar987

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I see, thanks for the sketch it helps a lot. But are the equations I wrote in the last post wrong?
     
  20. Feb 12, 2005 #19

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Since the motion you describe is along a diagonal you should put in a factor of [tex]\sqrt{2}/2[/tex] in each one
     
  21. Feb 12, 2005 #20

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    You just need to use a bit of trig.

    [tex]\Delta x'=\sqrt{\frac{1}{\gamma^2}\Delta x^2 \cos^2 \theta + \Delta x^2 \sin^2\theta}[/tex]

    Where theta is the angle that the length Delta x makes with the direction of motion (I've not simplifed the formula in odrer that you can see where all the bits come from).

    edited to correct equation.
     
    Last edited: Feb 12, 2005
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Lorentz transformations
  1. Lorentz transformation (Replies: 3)

  2. Lorentz Transformation (Replies: 2)

  3. Lorentz transformation (Replies: 3)

  4. Lorentz Transformation (Replies: 7)

  5. Lorentz transformation (Replies: 1)

Loading...