Two transformations of inertial reference frames are well-known: the Galileian and the Lorentz transformations. There is a third transformation as well, which will be called the co-Galileian transformation. Below is a derivation of all three transformations, closely following the paper *Getting the Lorentz transformations without requiring an invariant speed* by Andrea Pelissetto and Massimo Testa (*American Journal of Physics* 83 (2015), p.338-340). Their approach is based on the work of von Ignatowsky in the early 20th century.

We wish to characterize the transformations that relate two different inertial frames. Let us consider two inertial observers *K* and *K*′. Let **r** = (*x*, *x*_{2}, *x*_{3}) and **w** = (*t**, * *t*_{2}, *t*_{3}) be space and time coordinates for *K* and **r´** = (*x*´, *x*_{2}´, *x*_{3})´ and **w´** = (*t*´*,* *t*_{2}´, *t*_{3}´) be the corresponding quantities for *K*′.

In order to simplify the argument, we will restrict our considerations to the subgroup of transformations involving *x* and *t* only, setting *x*_{2}´= *x*_{2}, *x*_{3}´ = *x*_{3}, *t*_{2}´ = *t*_{2}, and *t*_{3}´ = *t*_{3}. This is equivalent to choosing coordinates so that *K* and *K*′ are in relative motion along the *x* and *t* directions in *K* and the *x*′ and *t*´ directions in *K*´.

So the displacement magnitude, |Δ**r**| = |Δ*x*|, and the distimement magnitude, |Δ**w**| = |Δ*t*|, is the time along the *t*-axis. Then the speed, Δ**r/**|Δ**w**| = Δ**r**/Δ*t*, and the pace, Δ**w**/|Δ**r**| = Δ**w**/Δ*x*,

We assume the validity of the principle of inertia: in an inertial frame free particles undergo a constant rate of rectilinear motion. Therefore, if the trajectory of a particle is a straight line in the frame of observer *K* and its rate is constant, the trajectory is also a straight line in the frame of observer *K*′ and also the rate in the new frame is constant. This condition implies that the two inertial frames are related by a linear transformation. We can therefore write

*t*´ = *At* + *Bx*,

*x*´ = *Ct* + *Dx*,

where we have defined the time coordinates *t* and *t*´, measured in units of distance, as

*t = qt*_{1},

where *q* is an *arbitrary* constant with units of speed. The inverse transformation follows immediately:

*t* = (1/Δ) (*Dt*´ – *Bx*´),

x = (1/Δ) (–*Ct*´ + *Ax*´),

with

Δ = *AD – BC* ≠ 0.

It is useful to rewrite the transformation in matrix notation, introducing

and its inverse

Then we have

and

The main ingredient in the proof is the requirement that there are no privileged frames: all inertial frames are equivalent. We wish now to express this hypothesis in a more transparent way that allows us to put constraints on the matrix Λ.

Pelissetto and Testa then show that Δ = 1 so that *A* = *D*. These conditions imply that the transformation relating two different inertial frames is of the form

with

*A*² – *BC* = 1.

To further constrain the structure of the matrix Λ, we now add the natural requirement that the transformations connecting two inertial frames constitute a group, i.e., that the combination of two such transformations yields a third transformation of the same form.

By multiplying two transformations, sub-scripted 1 and 2, the diagonal elements must again be equal:

*B*_{1} *C*_{2} = *C*_{1} *B*_{2}.

In order to satisfy this equation for all transformations with equal diagonal elements, we have three different possibilities:

(i) *B = αC*, where α is a nonzero constant; or

(ii) *B* = 0 and *A* = 1; or

(iii) *C* = 0 and *A* = 1.

Case (i) corresponds to the *Lorentz* transformations, as Pelissetto and Testa show. They introduce *C*´ = *C* √(|*α*|) and show that

where *A*² – *C*´² = 1. In this case the observer *K*′ moves with speed *v* with respect to observer *K*, where *v* is determined by

*C*´/*A* = *v*/*c*.

The condition *A*² – *C*´² = 1 implies that

*A* = *γ* = 1/√(1 – *v*²/*c²*), and

*C*´ = *γv*/*c*,

so that Λ is a generic Lorentz transformation, with the speed of light identified with *c*.

Case (ii) corresponds to the *Galileian* transformations:

*t*´ = *t*,

*x*´ = *Ct* + *x*,

the parameter *C* being the relative *velocity* of the two frames in units of *q*.

Case (iii) corresponds to the *co-Galileian* transformations:

*t*´ = *t* + B*x*,

*x*´ = *x*,

the parameter *B* being the relative *legerity* of the two timeframes in units of 1/*q*.

Thus these three cases lead to three transformations of inertial reference frames.