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Lorentz Transformation

  1. Apr 8, 2013 #1
    Can Lorentz Transformation be applied directly to a four velocity vector?

    I mean let v[itex]_{α}[/itex] be a four velocity vector.

    Is there a form of Lorentz tfm matrix such that:

    v[itex]^{'}[/itex][itex]_{α}[/itex] = [itex]\Lambda^{β}[/itex][itex]_{α}[/itex]v[itex]_{β}[/itex] ?
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  3. Apr 8, 2013 #2


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    Yes, why not ? As long as the velocity in the transformation is the relative velocity between frames, thus independent of the spatial components of the v 4-vector. And we usually write

    ##v'_{\alpha} = \Lambda_{\alpha}^{~\beta} v_{\beta} ##
  4. Apr 8, 2013 #3


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    yeah man, as wikipedia says: "The transformation matrix is universal for all four-vectors, not just 4-dimensional spacetime coordinates" on this page https://en.wikipedia.org/wiki/Lorentz_transformation about halfway down, under the subsection heading "transformation of other physical quantities"
  5. Apr 8, 2013 #4
    I didn't encounter the matrix anywhere, they always use the addition of the velocities, so I wasn't sure. I think I should construct on my own that matrix. Thank you
  6. Apr 10, 2013 #5
    I was trying to ask is there a matrix which includes all the "four vector information" in itself and we can act it on salt
    [itex]\vec{v}[/itex] 's as (c, [itex]\vec{v}[/itex] ) (not a four velocity but still)

    Maybe it seems meaningless but I was confused with all the γ factors because I couldn't find out which of the velocities should be used in them.

    I tried to find L in the form of a matrix equation:

    V' = L(not the Lorentz tfm matrix) V

    and V vectors have 4 components.

    Thanks again anyway.
  7. Apr 10, 2013 #6


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    In units such that c=1,
    $$\Lambda=\frac{1}{\sqrt{1-v^2}}\begin{pmatrix}1 & -v\\ -v & 1\end{pmatrix},$$ where ##v\in(-1,1)## is the velocity of the new coordinate system in the old. This is of course just a Lorentz transformation. I don't understand what you want when you say "not the Lorentz tfm matrix".
  8. Apr 10, 2013 #7


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    Of course, you cannot simply transform a non-covariant object. The right way to "add" velocities in the sense of three-vector velocities is to first go over to the proper velocity
    [tex]u^{\mu}=\gamma \begin{bmatrix}{1 \\ \vec{v}}\end{bmatrix}[/tex]
    with [itex]\gamma=1/\sqrt{1-v^2}[/itex].

    Of course you get the three-velocity as
    Then, by definition you define the three-velocity in the new frame as
    [tex]u'^{\mu}={\Lambda^{\mu}}_{\nu} u^{\nu},[/tex]
    where [itex]{\Lambda^{\mu}}_{\nu}[/itex] is an arbitrary O(1,3) matrix.
  9. Apr 10, 2013 #8


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    I think vanhees has explained it pretty well. From the equations, we get:
    [tex]u'^{\mu}=\gamma' \begin{bmatrix}{1 \\ \vec{v}'}\end{bmatrix} [/tex]
    So the velocity of the object (or whatever), with respect to the old frame is unprimed. And the velocity of the object according to the new frame is primed. (when I say primed, I mean ' this thing).
    And also, the unprimed gamma corresponds to the unprimed velocity and the primed gamma corresponds to the primed velocity.

    The reason that this form of the 4-velocity is used, is because it is a four-vector. i.e. it has an invariant scalar product, and you can use a Lorentz matrix on it to boost it. And it has a lot of nice properties. On the other hand, the column matrix:
    [tex]\begin{bmatrix}{1 \\ \vec{v}}\end{bmatrix}[/tex]
    Does not have all the nice properties of a 4-vector. If you wanted an equation for this thing, then you could rearrange the equation for the four-vector as:
    [tex]\begin{bmatrix}{1 \\ \vec{v}'}\end{bmatrix} = \frac{\gamma}{\gamma'} {\Lambda^{\mu}}_{\nu} \begin{bmatrix}{1 \\ \vec{v}}\end{bmatrix}[/tex]
    But people don't usually write stuff like this, because the physics is really contained in the equation for the four-vector. So the 'proper' four-vector is the thing that you should try to get used to thinking about.

    Also, as vanhees says, if you want to then talk about the new 3-velocity, then you can do this:
    Which is a much nicer way, because here we can clearly see how it relates to the four-vector, which is the physically important quantity.
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