An identity for the Lorentz cross product
I need some help finding the error in my proof for an identity for the Lorentz cross product.
For $x=(x_1,x_2,x_3)^T, y=(y_1,y_2,y_3)^T in R^{2,1}$ the Lorentz cross product is defined as
det[x,y,z]=$langle,x times y,zrangle$
for all $z in R^{2,1}$ where $langle,cdot, cdotrangle$ is the Lorentz scalar product of $R^{2,1}$.
In coordinates we have
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\x_1y_2-y_1x_2end{pmatrix}$.
Now let
$a=(a_1,a_2,a_3)^T, b=(b_1,b_2,b_3)^T, c=(c_1,c_2,c_3)^T in R^{2,1}$. I want to prove the identity
$a times (b times c)=clangle,a, brangle-blangle,a,crangle$
I did it in coordinates. We have
$b times c=begin{pmatrix}b_2c_3-c_2b_3\c_1b_3-b_1c_3\b_1c_2-c_1b_2end{pmatrix}$
and
$a times (b times c)=begin{pmatrix}a_2(b_1c_2-c_1b_2)-(c_1b_3-b_1c_3)a_3\(b_2c_3-c_2b_3)a_3-a_1(b_1c_2-c_1b_2)\a_1(c_1b_3-b_1c_3)-(b_2c_3-c_2b_3)a_2end{pmatrix}$
$=begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1b_3-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$
We also have
$clangle,a, brangle-blangle,a, crangle$=
begin{pmatrix}c_1a_2b_2-c_1a_3b_3-b_1a_2c_2+b_1a_3c_3\c_2a_1b_1-c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}
So far the equations do not coincide. It seems that I have made a mistake but I don't see where. Any help would be appreciated.
geometry hyperbolic-geometry
add a comment |
I need some help finding the error in my proof for an identity for the Lorentz cross product.
For $x=(x_1,x_2,x_3)^T, y=(y_1,y_2,y_3)^T in R^{2,1}$ the Lorentz cross product is defined as
det[x,y,z]=$langle,x times y,zrangle$
for all $z in R^{2,1}$ where $langle,cdot, cdotrangle$ is the Lorentz scalar product of $R^{2,1}$.
In coordinates we have
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\x_1y_2-y_1x_2end{pmatrix}$.
Now let
$a=(a_1,a_2,a_3)^T, b=(b_1,b_2,b_3)^T, c=(c_1,c_2,c_3)^T in R^{2,1}$. I want to prove the identity
$a times (b times c)=clangle,a, brangle-blangle,a,crangle$
I did it in coordinates. We have
$b times c=begin{pmatrix}b_2c_3-c_2b_3\c_1b_3-b_1c_3\b_1c_2-c_1b_2end{pmatrix}$
and
$a times (b times c)=begin{pmatrix}a_2(b_1c_2-c_1b_2)-(c_1b_3-b_1c_3)a_3\(b_2c_3-c_2b_3)a_3-a_1(b_1c_2-c_1b_2)\a_1(c_1b_3-b_1c_3)-(b_2c_3-c_2b_3)a_2end{pmatrix}$
$=begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1b_3-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$
We also have
$clangle,a, brangle-blangle,a, crangle$=
begin{pmatrix}c_1a_2b_2-c_1a_3b_3-b_1a_2c_2+b_1a_3c_3\c_2a_1b_1-c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}
So far the equations do not coincide. It seems that I have made a mistake but I don't see where. Any help would be appreciated.
geometry hyperbolic-geometry
You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59
add a comment |
I need some help finding the error in my proof for an identity for the Lorentz cross product.
For $x=(x_1,x_2,x_3)^T, y=(y_1,y_2,y_3)^T in R^{2,1}$ the Lorentz cross product is defined as
det[x,y,z]=$langle,x times y,zrangle$
for all $z in R^{2,1}$ where $langle,cdot, cdotrangle$ is the Lorentz scalar product of $R^{2,1}$.
In coordinates we have
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\x_1y_2-y_1x_2end{pmatrix}$.
Now let
$a=(a_1,a_2,a_3)^T, b=(b_1,b_2,b_3)^T, c=(c_1,c_2,c_3)^T in R^{2,1}$. I want to prove the identity
$a times (b times c)=clangle,a, brangle-blangle,a,crangle$
I did it in coordinates. We have
$b times c=begin{pmatrix}b_2c_3-c_2b_3\c_1b_3-b_1c_3\b_1c_2-c_1b_2end{pmatrix}$
and
$a times (b times c)=begin{pmatrix}a_2(b_1c_2-c_1b_2)-(c_1b_3-b_1c_3)a_3\(b_2c_3-c_2b_3)a_3-a_1(b_1c_2-c_1b_2)\a_1(c_1b_3-b_1c_3)-(b_2c_3-c_2b_3)a_2end{pmatrix}$
$=begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1b_3-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$
We also have
$clangle,a, brangle-blangle,a, crangle$=
begin{pmatrix}c_1a_2b_2-c_1a_3b_3-b_1a_2c_2+b_1a_3c_3\c_2a_1b_1-c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}
So far the equations do not coincide. It seems that I have made a mistake but I don't see where. Any help would be appreciated.
geometry hyperbolic-geometry
I need some help finding the error in my proof for an identity for the Lorentz cross product.
For $x=(x_1,x_2,x_3)^T, y=(y_1,y_2,y_3)^T in R^{2,1}$ the Lorentz cross product is defined as
det[x,y,z]=$langle,x times y,zrangle$
for all $z in R^{2,1}$ where $langle,cdot, cdotrangle$ is the Lorentz scalar product of $R^{2,1}$.
In coordinates we have
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\x_1y_2-y_1x_2end{pmatrix}$.
Now let
$a=(a_1,a_2,a_3)^T, b=(b_1,b_2,b_3)^T, c=(c_1,c_2,c_3)^T in R^{2,1}$. I want to prove the identity
$a times (b times c)=clangle,a, brangle-blangle,a,crangle$
I did it in coordinates. We have
$b times c=begin{pmatrix}b_2c_3-c_2b_3\c_1b_3-b_1c_3\b_1c_2-c_1b_2end{pmatrix}$
and
$a times (b times c)=begin{pmatrix}a_2(b_1c_2-c_1b_2)-(c_1b_3-b_1c_3)a_3\(b_2c_3-c_2b_3)a_3-a_1(b_1c_2-c_1b_2)\a_1(c_1b_3-b_1c_3)-(b_2c_3-c_2b_3)a_2end{pmatrix}$
$=begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1b_3-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$
We also have
$clangle,a, brangle-blangle,a, crangle$=
begin{pmatrix}c_1a_2b_2-c_1a_3b_3-b_1a_2c_2+b_1a_3c_3\c_2a_1b_1-c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}
So far the equations do not coincide. It seems that I have made a mistake but I don't see where. Any help would be appreciated.
geometry hyperbolic-geometry
geometry hyperbolic-geometry
asked Nov 28 '18 at 21:20
Polymorph
1236
1236
You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59
add a comment |
You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59
You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59
You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59
add a comment |
2 Answers
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The problem is with your formula for Lorentz cross product. It should be:
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\y_1x_2- x_1y_2end{pmatrix}$.
With this formula everything fits.
add a comment |
The correct identity is $$atimes (btimes c)=blangle,a,crangle-clangle,a, brangle.$$The right-hand-side is negative of $clangle,a, brangle-blangle,a,crangle$.
You calculated $atimes (btimes c)$ wrongly. I calculated it as $$begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1color{red}{b_1}-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$$ The highlighted symbol is where you calculated wrong. Double check that part.
You also calculated $clangle,a, brangle-blangle,a,crangle$ wrongly. I calculated it as $$begin{pmatrix}c_1a_2b_2color{red}+c_1a_3b_3-b_1a_2c_2color{red}-b_1a_3c_3\c_2a_1b_1color{red}+c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}$$
Again, the highlighted part is where you were wrong. Double check those.
add a comment |
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2 Answers
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2 Answers
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The problem is with your formula for Lorentz cross product. It should be:
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\y_1x_2- x_1y_2end{pmatrix}$.
With this formula everything fits.
add a comment |
The problem is with your formula for Lorentz cross product. It should be:
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\y_1x_2- x_1y_2end{pmatrix}$.
With this formula everything fits.
add a comment |
The problem is with your formula for Lorentz cross product. It should be:
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\y_1x_2- x_1y_2end{pmatrix}$.
With this formula everything fits.
The problem is with your formula for Lorentz cross product. It should be:
$x times y=begin{pmatrix}x_2y_3-y_2x_3\y_1x_3-x_1y_3\y_1x_2- x_1y_2end{pmatrix}$.
With this formula everything fits.
edited Dec 1 '18 at 11:57
answered Dec 1 '18 at 11:51
S Shemyakov
162
162
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The correct identity is $$atimes (btimes c)=blangle,a,crangle-clangle,a, brangle.$$The right-hand-side is negative of $clangle,a, brangle-blangle,a,crangle$.
You calculated $atimes (btimes c)$ wrongly. I calculated it as $$begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1color{red}{b_1}-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$$ The highlighted symbol is where you calculated wrong. Double check that part.
You also calculated $clangle,a, brangle-blangle,a,crangle$ wrongly. I calculated it as $$begin{pmatrix}c_1a_2b_2color{red}+c_1a_3b_3-b_1a_2c_2color{red}-b_1a_3c_3\c_2a_1b_1color{red}+c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}$$
Again, the highlighted part is where you were wrong. Double check those.
add a comment |
The correct identity is $$atimes (btimes c)=blangle,a,crangle-clangle,a, brangle.$$The right-hand-side is negative of $clangle,a, brangle-blangle,a,crangle$.
You calculated $atimes (btimes c)$ wrongly. I calculated it as $$begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1color{red}{b_1}-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$$ The highlighted symbol is where you calculated wrong. Double check that part.
You also calculated $clangle,a, brangle-blangle,a,crangle$ wrongly. I calculated it as $$begin{pmatrix}c_1a_2b_2color{red}+c_1a_3b_3-b_1a_2c_2color{red}-b_1a_3c_3\c_2a_1b_1color{red}+c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}$$
Again, the highlighted part is where you were wrong. Double check those.
add a comment |
The correct identity is $$atimes (btimes c)=blangle,a,crangle-clangle,a, brangle.$$The right-hand-side is negative of $clangle,a, brangle-blangle,a,crangle$.
You calculated $atimes (btimes c)$ wrongly. I calculated it as $$begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1color{red}{b_1}-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$$ The highlighted symbol is where you calculated wrong. Double check that part.
You also calculated $clangle,a, brangle-blangle,a,crangle$ wrongly. I calculated it as $$begin{pmatrix}c_1a_2b_2color{red}+c_1a_3b_3-b_1a_2c_2color{red}-b_1a_3c_3\c_2a_1b_1color{red}+c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}$$
Again, the highlighted part is where you were wrong. Double check those.
The correct identity is $$atimes (btimes c)=blangle,a,crangle-clangle,a, brangle.$$The right-hand-side is negative of $clangle,a, brangle-blangle,a,crangle$.
You calculated $atimes (btimes c)$ wrongly. I calculated it as $$begin{pmatrix}-c_1b_2a_2-c_1b_3a_3+b_1a_2c_2+b_1a_3c_3\-c_2a_1b_1-c_2a_3b_3+b_2a_1c_1+b_2a_3c_3\-c_3a_1color{red}{b_1}-c_3a_2b_2+b_3a_1c_1+b_3a_2c_2end{pmatrix}$$ The highlighted symbol is where you calculated wrong. Double check that part.
You also calculated $clangle,a, brangle-blangle,a,crangle$ wrongly. I calculated it as $$begin{pmatrix}c_1a_2b_2color{red}+c_1a_3b_3-b_1a_2c_2color{red}-b_1a_3c_3\c_2a_1b_1color{red}+c_2a_3b_3-b_2a_1c_1-b_2a_3c_3\c_3a_1b_1+c_3a_2b_2-b_3a_1c_1-b_3a_2c_2end{pmatrix}$$
Again, the highlighted part is where you were wrong. Double check those.
answered Dec 1 '18 at 14:17
edm
3,6131425
3,6131425
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You'll find proofs here: math.stackexchange.com/questions/1905883/… Some, including my own, don't even require you to chug through the algebra as much as your approach does.
– J.G.
Dec 1 '18 at 11:59