Translation:The Lorentz-Einstein transformation and the universal time of Ed. Guillaume

Session of 17 March 1921.

D.. – The Lorentz-Einstein transformation and the universal time of Ed. Guillaume.

In a series of communications and articles, Ed. sought to introduce a mono-parametric representation of time in the theory of relativity. He succeeded in giving to this problem an interesting solution, in the case where the number of reference systems is equal to two. This solution involves, as we know, a simple geometric interpretation.

I'd like to propose to give a new interpretation. I'll show that the $$t$$-parameter of only differs by a constant factor of time $$\tau$$ of a particular  system which I call median system. Each pair of reference systems correspond to a median system and a $$t$$-parameter of. Then one realizes better why the procedure of is no longer successful if the number $$n$$ of reference systems is greater than two. Indeed, for $$n > 2$$, the number of reference systems and consequently of $$t$$ parameters is greater than one, and these parameters are generally distinct.

1. Median system. $$S_1$$ and $$S_2$$ are two reference systems, animated to move uniformly with respect to one another along axes $$o_{1}x_{1}, o_{2}x_{2}$$. I assume that the transformation is applicable to these systems and therefore coordinates $$x_{1},x_{2}$$ and times $$x_{1},x_{2}$$ are linked by the relations

where $$\alpha=\tfrac{v}{c}$$, $$\beta^{2}=\tfrac{1}{1-\alpha^{2}}$$, $$v$$ is the velocity of $$S_2$$ with respect to $$S_1$$.

Now a third system $$S$$ parallel to $$S_1$$, $$S_2$$ also conducts a motion of translation along $$ox_1$$. Let $$v_0$$ be the velocity with respect to $$S_1$$. The Lorentz transformation still applies

where $$x$$ $$\tau$$ is the abscissa and the corresponding time in $$S$$, $$\alpha_{0}=\tfrac{v_{0}}{c}$$, etc.

We assume that the velocity of $$S_2$$ relative to $$S$$ is also $$v_0$$. I would say that $$S$$ is the corresponding median system. How can we express $$v_ {0}$$, $$\alpha_ {0}$$, $$\beta_ {0}$$ as functions of $$v$$, $$\alpha$$, $$\beta$$? In order to find it, it is sufficient to express $$x_ {1}$$, $$\tau_ {1}$$ as functions of parameters $$x$$, $$\tau$$ (form. (2)) and the latter as functions of $$x_ {2}$$, $$\tau_ {2}$$ and identify the resulting formulas with (1), which gives

2. Contraction. Consider two points $$P'$$ and $$P''$$. Let $$x'_{1},x'_{2},x'$$; $$x_{1},x_{2},x''$$ be their coordinates in $$S_1$$, $$S_2$$, and $$S$$ at the same moment $$\tau$$ ( time of the median system). By virtue of (2)

$x'_{1}=\beta_{0}\left(x'+\alpha_{0}c\tau\right),\quad x_{1}=\beta_{0}\left(x+\alpha_{0}c\tau\right).$

Therefore

So there is no contraction, provided that $$P'$$ and $$P''$$ are considered at the same moment $$\tau$$.

The converse is true, in other words: If the contraction does not take place by adopting time $$\tau$$ of an system, this system is the median system.

3. Another relation. Let P be a point of the abscissas $$x_1$$ and $$x_2$$ in $$S_1$$ and $$S_2$$. There, by replacing parameter $$\tau_{2}$$ by its expression as function of $$x_2$$ and $$\tau$$ in the first formula (1), it is given

by virtue of (3).

4. The universal time of . Let $$k$$ be an arbitrary function of $$v$$. As $$v$$ is const., $$k$$ is constant. Suppose $$k > 0$$ and put $$t=k\tau$$. If we adopt time $$t$$ instead of time $$\tau$$, simultaneity is not altered. Equality (4) remains true, therefore no contraction, equality (5) is written $$x_{1}=x_{2}+\tfrac{1}{k}\tfrac{\beta}{\beta_{0}}vt$$. In particular we assume that $$k=\tfrac{\beta}{\beta_{0}}$$, where $$t=\tfrac{\beta}{\beta_{0}}\tau$$. Equation (5) is written

Multiplication of the second equation of the second group (2) by $$k=\tfrac{\beta}{\beta_{0}}$$, gives in virtue of (3):

$c\tau_{1}=\frac{c}{\beta}t+\frac{\beta-1}{\alpha\beta}x_{1}$

We come, as seen, to the equation that defines the universal time $$t$$ of. Therefore the time $$t$$ defined by $$t=\tfrac{\beta}{\beta_{0}}\tau$$ is the parameter of. It only differs from time $$\tau$$ of the median system by the constant factor $$k=\tfrac{\beta}{\beta_{0}}$$.

5. Case of three systems. Imagine three systems $$S_{1},S_{2},S_{3}$$ conducting a uniform translatory movement parallel to the axes of $$x$$. Let $$v_{12},v_{13},v_{23}$$ be the relative velocity of $$S_2$$ with respect to $$S_1$$, of $$S_3$$ with respect $$S_1$$, of $$S_3$$ with respect $$S_2$$, and $$t_{12},t_{13},t_{23}$$ the parameters of. Then we have in virtue of (6)

$x_{1}=x_{2}+v_{12}t_{12};\quad x_{1}=x_{3}+v_{13}t_{13};\quad x_{2}=x_{3}+v_{23}t_{23};$

for example, the abscissa $$x_1$$ of $$O_2$$ is given by $$x_{1}=v_{12}t_{12}$$, that of $$O_3$$ by $$x_{1}=v_{13}t_{13}$$. Parameters $$t_{12},t_{13},t_{23}$$ should not be confused with each other.