Let us just start with the following line of work, which starts off by assuming that the Lorentz transformations are correct:
Equation 1: v = \frac{x^\prime}{t^\prime} = \frac{x-vt}{t-vx/c^2}
Lorentz Transformations
x^\prime = { { x - vt } \over \sqrt { 1 - { v^2 \over c^2 } } }
t^\prime = { { t - { v \over c^2 } x } \over \sqrt { 1 - { v^2 \over c^2 } } }
In this problem, the relative speed v happens to equal c. Look at the denominator of the RHS of equation 1.
t - vx/c^2
In the case where v=c, the denominator of equation 1 reduces to:
t - x/c
And we know that x/t=c, from which it follows that x/c=t, from which it follows that t-x/c=0. Thus, the denominator of the RHS of equation one contains a zero. This does not necesarily mean that we have the division by zero error of algebra, we need to consider the numerator. Let us now look at the numerator. The numerator of equation 1 is:
x-vt
And in this case, v=c, hence the numerator of equation 1 is x-ct. Since x/t=c, it follows that x=ct, from which it follows that x-ct=0. Thus, in the case were v=c, it follows that x`/t`=0/0, which is an indeterminate form.
Now, we can write the relative speed v (which happens to equal c) as follows:
v = c = \frac{x-ct}{t-x/c}
Multiplying both sides by (t-x/c) gives:
c(t-x/c) = x-ct
From which it follows that
ct - x = x-ct
From which it follows that:
x-ct+x-ct = 0
From which it follows that
2x-2ct=0
From which it follows again that
x/t=c
Again, from this we can only conclude that
x/t=x`/t`
And we start off knowing this.
The customary way to evaluate an indeterminate form, is to use L'Hopital's rule. However, let us pick up the argument here:
x`/t` = 0/0
We start off with the following equation:
v = x/t=x`/t`
So let us take the limit of both sides of the above equation, as v approaches c. We reach the following conclusion:
lim v--> c [x`/t`] = c
This saves us the effort of using L'Hopital's rule. Does anyone know what this means?
For those who wish to use L'Hopital's rule... in the middle of the previous work we arrived here:
v = \frac{x-ct}{t-x/c}
Since c>0, we can divide both sides of the above equation by c, and obtain:
v/c = \frac{x-ct}{ct-x}
Now, if we take the limit as v approaches c of the LHS, we get 1.
The only thing left to do, is take the limit as v approaches c of the RHS.
Since v=x/t, therefore x = vt. Thus, we can write the following:
v/c = \frac{vt-ct}{ct-vt}
Thus, we can write:
v/c = \frac{t(v-c)}{t(c-v)}
The t's cancel out, and we are left with:
v/c = \frac{v-c}{c-v}
The limit as v approaches c of the LHS is equal to 1. The limit as v approaches c of the RHS needs to be broken up into two cases:
Case 1: v>c
Case 2: v<c
Is there anyone who wants to field these cases?