Therefore, physicists DON'T drop the first order effect in either distance (x) or time (t). They only drop effects that are second order or higher when they take the Galilean limit. However, you apparently haven't thought out which effect is which. With what expansion are the effects in any order?
The expansions are usually made in terms of 'v/c'. In your units where 'c=1', 'v'. Okay, let 'c=1'. The Galilean limit neglects terms like v^2 and (vx). We do not ignore first order terms like 'v' or 'vx'.
The expansions that you are obviously thinking of are Taylor expansions in terms of 'v'. The other quantities in the Lorentz transformation of independent of 'v'. So both 'v' and 'vx' are first order, not second order.
What disappears is terms like 'v^2'. So the Galilean approximation is really,
sqrt(1-v^2)=1.
The reason one that the 'vx/c^2' term disappears is because for the observer on earth looking at the rocket, 'x=vt'. This is subjective. So in this case:
xv=(vt)t
(vt)t=t v^2
xv =t v^2
Thus, 'xv' is second order in 'v' for this observation only.
Let me elaborate a little bit farther.
You chose units where c=1. Fine. Therefore, v<1 because a signal can't travel faster than the speed of light. So:
v<1
v^2<v
t v^2 < t v
Now, if the observer of earth (x=0) is looking at the rocket after a time t,
X=vt where X is the turn around distance. Substitution into the last inequality expression shows that
vx <vt.
The earth observer is at x=0
The complete Lorentz transform with c=1 is
x'=(-vt)/sqrt(1-v^2)
t'=(t-vx)/sqrt(1-v^2)
Suppose the speed of the rocket is MUCH less than the speed of light, so v<<<1. Because v is much less than 1, we can use the approximation v^2<<v.
x'=-vt.
t'=t.
So, at turn around, the earth twin sees.
x'=X
t'=t
These are the Galilean equations. So if v<<1, the Lorentz transformation becomes the Galilean equations. There is no inconsistency in the Galilean limit.
If you want to know how the 'a' term survives, then we have start with the calculus. We have to subtract quantities very close together in time and space. This has nothing to do with the Galilean limit of the Lorentz transformation.
Your concrete error is still the same. You are hypothesizing that the proper acceleration of the rocket and earth is negligible. The proper acceleration is not negligible even if it is intense for only a very short time.
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