# Invariance of the y coordinate for a boost along the x axis

1. Jan 26, 2013

### homerT

1. The problem statement, all variables and given/known data
I've been reading through Spacetime Physics by Taylor & Wheeler, but this argument about the invariance of the $y$ coordinate for inertial frames, one moving relative to the other on the $x$ axis, is tripping me up. I'll just write the text word for word:

I'm just not understanding this argument at all. There is a way to distinguish between whether the lab clocks were painted by the rocket or not if the lines painted at $y=1$ (rocket frame) weren't at $y=1$ (lab frame)??? Is that the experiment to distinguish the two frames?

2. Relevant equations
The laws of physics are the same in any two inertial frames

3. The attempt at a solution
Just lost trying to understand how non-invariance of the $y$ coordinate makes the laws of physics different in between the inertial lab and rocket frames.

Last edited: Jan 26, 2013
2. Jan 27, 2013

### tiny-tim

welcome to pf!

hi homerT! welcome to pf!

it's a little confusing, but i think what they're saying is this …

all we've stipulated is that the y = 0 line (the x-axis) is the same in both systems

in other words, if one clock on y = 0 was using paint, then we've stipulated that the paint must fall on the clocks on y = 0 in the other frame

we haven't stipulated anything about clocks on y = 1

but if the paint from one clock on y = 1 fell inside the clocks on the other frame on y = 1, then that would be a way to distinguish between the frames, which can't happen, so the paint must fall exactly on the other frame's y = 1 clocks