Einstein Relatively Easy

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To make the meaning of the equations of covariant differentiation seen in last article Introduction to Covariant Differentiation more explicit, we will consider the covariant derivative of vector V with respect  to θ in cylindrical coordinates (so x1=r, x2=θ, and x3=z).

Setting β=2 in the following equation, since we are interested in the covariant derivative with respect to θ:

we get

We know the values of the first two Christoffel symbol as we have already calcuted them in the previous article Christoffel symbol exercise: calculation in polar coordinates part I

so that we already  know that

We know also that since

 

all the symbols from the following form vanish

thus we end up with this equality

 

which says that a change in the r-component of vector V caused by a change in θ is caused both by a change in Vr with respect to θ and by a change in the basis vectors which causes a portion of V that was originally in the θ-direction to now point in the r-direction.

Likewise, for the change in Vθ as the value of θ is changed, we have

which says that a change in the θ-component of vector V caused by a change in θ is caused both by a change in Vθ with respect to θ and by a change in the basis vectors which causes a portion of V that was originally in the r-direction to now point in the  θ-direction.

 Thus finally the covariant derivative of vector V with respect to θ in cylindrical coordinates is

 

 

 

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"Pas plus de cinq ou six semaines s'écoulèrent entre la conception de l'idée de la relativité restreinte et la rédaction de l'article correspondant."
Einstein à Carl Seelig, 11 Mars 1952

"N'importe quel étudiant dans les rues de Göttigen en connaît plus qu'Einstein sur les géométries à 4 dimensions. Et pourtant ce fut Einstein qui accomplit le travail, et non les mathématiciens."
David Hilbert

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