An identity for the Lorentz cross product












0














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.










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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
















0














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.










share|cite|improve this question






















  • 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














0












0








0







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.










share|cite|improve this question













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






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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


















  • 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










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.






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    0














    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.






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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.






      share|cite|improve this answer




























        0














        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.






        share|cite|improve this answer


























          0












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          0






          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.






          share|cite|improve this answer














          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.







          share|cite|improve this answer














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          share|cite|improve this answer








          edited Dec 1 '18 at 11:57

























          answered Dec 1 '18 at 11:51









          S Shemyakov

          162




          162























              0














              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.






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                0














                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.






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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.






                  share|cite|improve this answer












                  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.







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                  answered Dec 1 '18 at 14:17









                  edm

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