Einstein Relatively Easy

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In this article, our goal is to show that the geodesics for a two-dimensional Euclidean space are straight lines.

In Cartesian coordinates and in two dimensional space, as there is no z coordinates, the Euclidean line element there becomes:

dl2 = dx2 + dy2

Therefore, the corresponding metric is - we are using Latin indices as we are not working in spacetime:

We also know from our previous article Geodesic equation and Christoffel symbols that the geodesic equation can be written as

But the Proper time is clearly not a convenient parameter in the case of the propagation of photons (the proper time is not defined for massless particles)

We should better use a so called affine parameter λ, as per below:

In order to calculate the eight Christoffel symbols (2*2*2 in 2D space), we need to use the equation given in Christoffel symbols in terms of the metric tensor

 But as the values of the metric are constant (equal to 0 or 1 as pointed out above), the partial derivatives gij,k = 0 for all values of i, j and k. Therefore Γjki = 0 for all values i, j, k and the geodesic equation simply becomes :

The function xi = aλ + b where a and b are constants is obviously a solution to this equation as when derived twice it gives 0.

As we are using Cartesian coordinates where xi equals x and y, the above equation becomes then

 Solving for λ gives:

which is the equation of a straight line with gradient c/a and constant (ad-bc)/a.

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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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