Here is a simpler approach (forget about setting c = 1), which is clearly distracting you. In fact I see an error in my earlier post where I set c = 1 but then proceeded to use c = 3*10^8 anyway, which exaggerated the resulting orders of magnitude (dumb mistake). So:
We have: x' = γ(x - vt)
and t' = γ(t - vx/c^2
...
I think that I know a way to get the 'asymmetry' that Wogaga wants. We require the twin on earth to determine the time at the position of the rocket in the 'stationary' frame. Therefore, we set x' = 0.
We have: x' = γ(x - vt)
and t' = γ(t - vx/c^2)
Let us set the proper acceleration of earth and rocket to zero. So (v/c) is constant in time.
Now, the earth twin wants to know how fast the clock is moving at a particular point on the rocket. Say he wants to know what the time, t', is at a particular place, x'=0.
x'=0
so
x = vt.
Now the earth twin wants to calculate t' under the conditions set above. So he substitutes for x in t'. So,
t' = γ(t - v[vt]/c^2)
So
t'= γ(t - tv^2/c^2)
t'= γ(t - t|v/c|^2)
t' = γt (1 - |v/c|^2)
Now
1/γ = sqrt(1 - |v/c|^2)
So:
t'=t/γ when x'=0.
So not this is consistent with
t = γ t' when x'=0.
The problem here is justifying the condition x' = 0. The dynamic asymmetry in the problem is hidden in this condition. Because the clock in the
rocket is not reset during the proper acceleration, we can't set x = 0.
That is, we made the problem asymmetric when we set x'=0 rather than x=0. If we set x=0, we would have the 'opposite' result.
The physical justification of x'=0 would require a little knowledge of how the clocks communicate. In other words, we would have to know what forces are involved in synchronization. In other words, I haven't really avoided proper acceleration. I just delayed its introduction a bit.
The x'=0 is only a special case in the sense that Wogoga is claiming. However, that is not important for the issue of whether the Galilean transformation is the low order approximation of the Lorentz transformation.
We note that v<c in all systems of units. Therefore, |v/c|<1.
We expand γ by a Taylor series. Or, if you wish, by binomial seres.
γ= 1-0.5|v/c|^2 + 0.75|v/c|^4 -...
The sign of the coefficient alternates between + and - for all orders. Therefore, we note that the infinite series above converges for all values of |v/c|<1. The proof of convergence requires calculus just a little above introductory. However, all of us here at least understand the concept of limits, right? So we can look up the conditions of convergence when we can't derive them.
We can drop all terms higher than first order, leaving only terms first order in |v/c|. However, we note that there are not high order terms in the Taylor expansion above.
So to first order in |v/c|, γ= 1.
Substituting this value into the formula for t:
t=t'.
Further, γ= 1 so,
x'=x-vt.
These are the
The important assumption here is not c=1. That doesn't matter because v and c are in the same units. The important assumption here is that |v|<c. Then, the series converges because the signs alternate.
This is about as mathematically rigorous as I know how. I have used no mathematics that Newton didn't know. There are important differences between relativity and Newtonian physics, but are not with the arithmetic.
Also note that the sign of v doesn't matter. One can substitute +v for -v and get similar results. Thus, the results are symmetric with respect to the sign of v.
I suggest that anyone who disagrees try the special case of v=0.1c. Choose t to be any value you like just as long as the units of x, t, v and c are consistent. Use the rules for round off significant figures.
Anyway, I thank Wogoga for asking the question. I love these complicated questions that masquerade as simple ones.