Albert Einstein (1879–1955). Relativity: The Special and General Theory. 1920.

**XXVI**

**Part II: The General Theory of Relativity**

# XXVI. The Space-Time Continuum of the Special Theory of Relativity Considered as a Euclidean Continuum

W*x, y, z, t,* which determine an event or—in other words—a point of the four-dimensional continuum, are defined physically in a simple manner, as set forth in detail in the first part of this book. For the transition from one Galileian system to another, which is moving uniformly with reference to the first, the equations of the Lorentz transformation are valid. These last form the basis for the derivation of deductions from the special theory of relativity, and in themselves they are nothing more than the expression of the universal validity of the law of transmission of light for all Galileian systems of reference.

Minkowski found that the Lorentz transformations satisfy the following simple conditions. Let us consider two neighbouring events, the relative position of which in the four-dimensional continuum is given with respect to a Galileian reference-body *K* by the space co-ordinate differences *dx, dy, dz* and the time-difference *dt.* With reference to a second Galileian system we shall suppose that the corresponding differences for these two events are *dx’, dy’, dz’, dt’.* Then these magnitudes always fulfil the condition.

**ds**

^{2}=dx^{2}+dy^{2}+dz^{2}-c^{2}dt^{2}*x, y, z,*

*ct,*by

*x*

_{1},

*x*

_{2},

*x*

_{3},

*x*

_{4}, we also obtain the result that

is independent of the choice of the body of reference. We call the magnitude

*ds*the “distance” apart of the two events or four-dimensional points.

Thus, if we choose as time-variable the imaginary variable

*ct*instead of the real quantity

*t,*we can regard the space-time continuum—in accordance with the special theory of relativity—as a “Euclidean” four-dimensional continuum, a result which follows from the considerations of the preceding section.