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Minkowski's Four−Dimensional Space ("World") (supplementary to section 17)

in place of t, as time−variable. If, in accordance with this, we insert

x₁ = x
x₂ = y
x₃ = z
x₄ =

and similarly for the accented system K¹, then the condition which is identically satisfied by the transformation can be expressed thus :

 

x₁'² + x₂'² + x₃'² + x₄'² = x12 + x22 + x32 + x42 (12).

 

That is, by the afore−mentioned choice of " coordinates," (11a) [see the end of Appendix II] is transformed into this equation.

We see from (12) that the imaginary time co−ordinate x₄, enters into the condition of transformation in exactly the same way as the space co−ordinates x₁, x₂, x₃. It is due to this fact that, according to the theory of relativity, the " time "x₄, enters into natural laws in the same form as the space co ordinates x₁, x₂, x₃.

A four−dimensional continuum described by the "co−ordinates" x₁, x₂, x₃, x₄, was called "world" by Minkowski, who also termed a point−event a " world−point." From a "happening" in three−dimensional space, physics becomes, as it were, an " existence " in the four−dimensional " world."

This four−dimensional " world " bears a close similarity to the three−dimensional " space " of (Euclidean) analytical geometry. If we introduce into the latter a new Cartesian co−ordinate system (x'₁, x'₂, x'₃) with the same origin, then x'₁, x'₂, x'₃, are linear homogeneous functions of x₁, x₂, x₃ which identically satisfy the equation

x'12 + x'22 + x'32 = x12 + x22 + x32

The analogy with (12) is a complete one. We can regard Minkowski's " world " in a formal manner as a four−dimensional Euclidean space (with an imaginary time coordinate) ; the Lorentz transformation corresponds to a " rotation " of the co−ordinate system in the fourdimensional " world."

Next: The Experimental Confirmation of the General Theory of Relativity Relativity: The Special and General Theory Appendix

Appendix III