# Einstein Relatively Easy

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Note: the simple derivation of the Lorentz transformation as it will be exposed in this article is done by Einstein in Appendix 1 of his book Relativity. We are giving here a little bit more detailed calculus.

Again with take the hypothesis of two referentials R and R' in standard configuration.
We require to find x' and t' when x and t are given, assuming that R' is moving along the x axis relative to R.

A light-signal, which is proceeding along the positive axis of x of the referential R, is transmitted according to the equation

Likewise, since the same light-signal has to be transmitted relative to R' with the same velocity c, the propagation relative to the system R' will be represented by the analogous formula

The only way that the same event could satisfy the two equations is if

(1)

so that at t = t' = 0, the disappearance of (x - ct) involves the disappearance of (x' - ct')

If now we apply quite similar considerations to light rays which are being transmitted along the negative x-axis, we obtain the equivalent formula

(2)

If we now sum up equations (1) and (2) we get:

Substracting (2 and (1) gives the equation of ct'

By defining a and b as follows

we can rewrite the two equations simply like

Our problem becomes now to find a and b.

If we try to find the the movement of the origin of R', given by x'=0, with the reference to R, we just have to use first equation

We can now express the speed of the referential R' with a,b and c = speed of light.

If now we were to express the length of a solid stick measuring 1m in referential R', from the referential R, we just have to take a photography of this stick at a given time, say at t=0 in R. Using the first equation, we get

(equation 3)

Let's do the same exercise from the R' perspective; we need to take a snapshot of the stick at a given time t' in the R' referential. We guess that we have to choose t'=0

(equation 4)

But we know that the stick length in R' seen from R should be equals to its length in R observed from R', hence equation 3 = equation 4

we can then rewrite our two main equations expressing x' and t', and verify that they conform to the Lorentz expression as given in the precedent article The Lorentz transformations Part I - Presentation

• ### Guest - Huw

E=mc2 from Brian Cox and Jeff Forshaw: I think this contains a nice derivation of the Lorentz factor using moving mirrors. The derivation usually used, the one above, is very 'dry'. Their derivation plays out as a thought experiment which leads to a greater understanding of what is occurring.

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• ### Cyril

Hi Huw,
Thanks for your comment!
You are right, in this book the authors derive the Lorentz factor from both the mirrors experience in Chapter 3 'Special Relativity' and also in Chapter 4 'Spacetime' from the spacetime interval invariance when x=0 => (ct)^2 = (cT)^2 - (vT)^2
But deriving the Lorentz factor is not quite the same as finding the Lorentz transformations themselves (4 equations)
Quite suprisingly, the authors do not mention this quantity as the Lorentz factor, but only as the "stretching factor".
We have ourselves used the moving mirrors to derive the Lorentz factor in the article Introduction to time dilation and Lorentz factor

Comment last edited on about 1 year ago by Cyril
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