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kyp4
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Hello, I am new to the forums and I hope this fundamental topic has not been previously treated, as these forums don't seem to have a search function. I am studying general relativity using S. Carroll's book (Geometry and Spacetime) and I am having a fundamental problem with basis vectors under a Lorentz transformation.
Consider a four-vector [tex]V[/tex] having the components [tex]V^{\mu}[/tex] with respect to some basis vectors [tex]\hat{e}_{(\mu)}[/tex] associated with a coordinate system [tex]x^\mu[/tex]. According to the book (and using his notation) the coordinate system transforms to the primed coordinate system according to
[tex]x^{\mu'}=\Lambda^{\mu'}_{\nu}x^\nu[/tex]
for some Lorentz transform [tex]\Lambda^{\mu'}_{\nu}[/tex]. Similarly the vector components transform in the same way according to
[tex]V^{\mu'}=\Lambda^{\mu'}_{\nu}V^\nu[/tex]. (1)
As I understand it the vector components are with respect to some basis and the primed components with respect to a corresponding primed basis. The (invarient) vector can thus be expressed with respect to either basis as
[tex]V=V^\mu\hat{e}_{(\mu)}=V^{\mu'}\hat{e}_{(\mu')} [/tex] (2)
where, in going from the unprimed to the primed coordinate system, the bases are related by the inverse transform:
[tex]\hat{e}_{(\nu')}=\Lambda^{\mu}_{\nu'}\hat{e}_{(\mu)} [/tex]
where [tex]\Lambda^{\mu}_{\nu'}[/tex] (subtly) denotes the inverse transform (because the prime is on the second index).
This is all well and good but when I try to apply it to a concrete example it seems as though the basis vectors should transform (from unprimed to primed) via the transform, not its inverse. To construct an example, consider a rotation in the [tex]x^1x^2 [/tex] ([tex]xy [/tex]) plane, which I believe is Lorentzian and is accomplished via the transformation matrix
[tex]\Lambda=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \cos{\theta} & \sin{\theta} & 0\\
0 & -\sin{\theta} & \cos{\theta} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex].
To make this even more concrete let us set the angle at 45 degrees, i.e. [tex]\theta=\pi/4[/tex], in which case the transform becomes
[tex]\Lambda=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & -\frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex],
which has an inverse transform of
[tex]\Lambda^{-1}=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \frac{\sqrt{2}}{2} & -\frac{\sqrt{2}}{2} & 0\\
0 & \frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex].
If we then let the unprimed basis be orthonormal and lie on each axis with unit length then we can express a vector lying at a 45 degree angle from the x-axis and in the xy plane can be expressed as [tex]V=\hat{e}_{(1)}+\hat{e}_{(2)}[/tex] and has components
[tex]V^0 = 0[/tex]
[tex]V^1 = 1[/tex]
[tex]V^2 = 1[/tex]
[tex]V^3 = 0[/tex].
A diagram would be useful here but it should be easy enough to visualize. Anyway, the components with respect to the primed coordinate system (and the yet-to-be-determined primed basis?) are, by (1),
[tex]V^{0'} = 0[/tex]
[tex]V^{1'} = \sqrt{2}[/tex] (3)
[tex]V^{2'} = 0[/tex]
[tex]V^{3'} = 0[/tex],
which makes pefect sense if one visualizes the vector and the primed and unprimed coordinate axes. According to the book the basis vectors change by the inverse transform and are found to be
[tex]\hat{e}_{(0')} = \hat{e}_{(0)}[/tex]
[tex]\hat{e}_{(1')} = \frac{\sqrt{2}}{2}(\hat{e}_{(1)}-\hat{e}_{(2)})[/tex] (4)
[tex]\hat{e}_{(2')} = \frac{\sqrt{2}}{2}(\hat{e}_{(1)}+\hat{e}_{(2)})[/tex]
[tex]\hat{e}_{(3')} = \hat{e}_{(3)}[/tex],
Which, already at this point, do not correspond to the primed coordinate axes. So the vector in terms of its primed components and primed basis vectors is
[tex]V=\sqrt{2}\hat{e}_{(1')}=\sqrt{2}\frac{\sqrt{2}}{2}(\hat{e}_{(1)}-\hat{e}_{(2)})=\hat{e}_{(1)}-\hat{e}_{(2)}[/tex],
which is clearly different from the original vector, and is rather its rotation in the wrong direction.
I am convinced that I am doing something wrong here or misinterpreting things but I cannot find the source of error and am hoping that another pair of eyes might see it clearly. Any help with this is much appreciated.
Consider a four-vector [tex]V[/tex] having the components [tex]V^{\mu}[/tex] with respect to some basis vectors [tex]\hat{e}_{(\mu)}[/tex] associated with a coordinate system [tex]x^\mu[/tex]. According to the book (and using his notation) the coordinate system transforms to the primed coordinate system according to
[tex]x^{\mu'}=\Lambda^{\mu'}_{\nu}x^\nu[/tex]
for some Lorentz transform [tex]\Lambda^{\mu'}_{\nu}[/tex]. Similarly the vector components transform in the same way according to
[tex]V^{\mu'}=\Lambda^{\mu'}_{\nu}V^\nu[/tex]. (1)
As I understand it the vector components are with respect to some basis and the primed components with respect to a corresponding primed basis. The (invarient) vector can thus be expressed with respect to either basis as
[tex]V=V^\mu\hat{e}_{(\mu)}=V^{\mu'}\hat{e}_{(\mu')} [/tex] (2)
where, in going from the unprimed to the primed coordinate system, the bases are related by the inverse transform:
[tex]\hat{e}_{(\nu')}=\Lambda^{\mu}_{\nu'}\hat{e}_{(\mu)} [/tex]
where [tex]\Lambda^{\mu}_{\nu'}[/tex] (subtly) denotes the inverse transform (because the prime is on the second index).
This is all well and good but when I try to apply it to a concrete example it seems as though the basis vectors should transform (from unprimed to primed) via the transform, not its inverse. To construct an example, consider a rotation in the [tex]x^1x^2 [/tex] ([tex]xy [/tex]) plane, which I believe is Lorentzian and is accomplished via the transformation matrix
[tex]\Lambda=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \cos{\theta} & \sin{\theta} & 0\\
0 & -\sin{\theta} & \cos{\theta} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex].
To make this even more concrete let us set the angle at 45 degrees, i.e. [tex]\theta=\pi/4[/tex], in which case the transform becomes
[tex]\Lambda=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & -\frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex],
which has an inverse transform of
[tex]\Lambda^{-1}=\left[\begin{array}{cccc}
1 & 0 & 0 & 0\\
0 & \frac{\sqrt{2}}{2} & -\frac{\sqrt{2}}{2} & 0\\
0 & \frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2} & 0\\
0 & 0 & 0 & 1
\end{array}\right]
[/tex].
If we then let the unprimed basis be orthonormal and lie on each axis with unit length then we can express a vector lying at a 45 degree angle from the x-axis and in the xy plane can be expressed as [tex]V=\hat{e}_{(1)}+\hat{e}_{(2)}[/tex] and has components
[tex]V^0 = 0[/tex]
[tex]V^1 = 1[/tex]
[tex]V^2 = 1[/tex]
[tex]V^3 = 0[/tex].
A diagram would be useful here but it should be easy enough to visualize. Anyway, the components with respect to the primed coordinate system (and the yet-to-be-determined primed basis?) are, by (1),
[tex]V^{0'} = 0[/tex]
[tex]V^{1'} = \sqrt{2}[/tex] (3)
[tex]V^{2'} = 0[/tex]
[tex]V^{3'} = 0[/tex],
which makes pefect sense if one visualizes the vector and the primed and unprimed coordinate axes. According to the book the basis vectors change by the inverse transform and are found to be
[tex]\hat{e}_{(0')} = \hat{e}_{(0)}[/tex]
[tex]\hat{e}_{(1')} = \frac{\sqrt{2}}{2}(\hat{e}_{(1)}-\hat{e}_{(2)})[/tex] (4)
[tex]\hat{e}_{(2')} = \frac{\sqrt{2}}{2}(\hat{e}_{(1)}+\hat{e}_{(2)})[/tex]
[tex]\hat{e}_{(3')} = \hat{e}_{(3)}[/tex],
Which, already at this point, do not correspond to the primed coordinate axes. So the vector in terms of its primed components and primed basis vectors is
[tex]V=\sqrt{2}\hat{e}_{(1')}=\sqrt{2}\frac{\sqrt{2}}{2}(\hat{e}_{(1)}-\hat{e}_{(2)})=\hat{e}_{(1)}-\hat{e}_{(2)}[/tex],
which is clearly different from the original vector, and is rather its rotation in the wrong direction.
I am convinced that I am doing something wrong here or misinterpreting things but I cannot find the source of error and am hoping that another pair of eyes might see it clearly. Any help with this is much appreciated.
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