Friday, June 23, 2017

electromagnetism - Can I transform electromagnetic tensors by matrix multiplication?


I know that the eletromagnetic field tensor Fμν, can be transfomed to another reference frame by Fαβ=ΛαμΛβνFμν


Since these tensors can be represented by matrices so I thought that I could represent the electromagnetic field tensor in another inertial reference frame by doing matrix multiplications, but I ended up with:


Fαβ=[γγβ00γβγ0000100001][γγβ00γβγ0000100001][01cEx1cEy1cEz1cEx0BzBy1cEyBz0Bx1cEzByBx0]= =[2γ2βcExγ2cEx(1+β2)γ2cEy(1+β2)+2γ2β2Bzγ2cEz(1+β2)2γ2βBy1cEx2γ2βcEx2γ2βcEyγ2Bz(β2+1)2γ2βcEz+γ2By(β2+1)1cEyBz0Bx1cEzByBx0] But as it can easily be seen, this matix is not anti-symmetric as an eletromagnetic field tensor should be, by definition, so does this mean that I cannot use matrix multiplication on tensors or does it mean that I made a mistake somewhere in the calculations of the matrix products?



Answer



Fαβ=[γγβ0012γβγ0012001012000112][0ExEyEz12Ex0cBzcBy12EycBz0cBx12EzcBycBx012][γγβ0012γβγ0012001012000112]=[γβExγExγ(EyβcBz)γ(Ez+βcBy)12γExγβExγ(βEycBz)γ(βEz+cBy)12γβEycBz0cBx12γβEzcBycBx012][γγβ0012γβγ0012001012000112]=[0Exγ(EyβcBz)γ(Ez+βcBy)12Ex0γ(βEycBz)γ(βEz+cBy)12γ(EyβcBz)γ(βEycBz)0cBx12γ(Ez+βcBy)γ(βEz+cBy)cBx012]=[0ExEyEz12Ex0cBzcBy12EycBz0cBx12EzcBycBx012] Since β=υ/c Ex=Ex12Ey=γ(EyυBz)12Ez=γ(Ez+υBy)12Bx=Bx12By=γ(By+υc2Ez)Bz=γ(Bzυc2Ey)





For Your Information :


The equations of a more general Lorentz Transformation between two systems S(x,t) and S(x,t), the latter translating with constant velocity v=υn,n=1,υ(c,+c), with respect to the former, are : x=x+(γ1)(nx)nγvtt=γ(tvxc2)γ=(1υ2c2)1/2 see Figure.(1)


Under (ft-01) the vectors E,B of the electromagnetic field in empty space are transformed as follows :
E=γE(γ1)(nE)n+γ(v×B)B=γB(γ1)(nB)nγc2(v×E) Equations (02),(03) are a special case of (ft-02) for n=(1,0,0).


enter image description here




(1) See a 3D version of this Figure here : Figure 3D version


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