1. The problem statement, all variables and given/known data How fast must a meter stick be moving if its length is observed to shrink to 0.5 m? 3. The attempt at a solution A meter stick moving at speed V to a stationary observer is the same thing as a stationary meter stick and an observer moving at speed V. Lproper = Lp = 1 meter Lobserved = L' = .5 meter y = 1/sqrt(1- v^2/c^2) Time it takes for observer to cover the distance of 1 meter, /\t: /\t = Lp/V Length moving observer sees, L': L' = V*/\t' where /\t' is the time it takes for the meter stick to fly by in observer's reference frame Choosing Proper Time: If I choose proper time to be /\t then: /\t*y = /\t' Through substitution this leads to: L' = Lp*y (WRONG) If i choose proper time to be /\t' then: /\t = /\t'*y Through substitution this leads to: L' = Lp/y (RIGHT) Why can't I choose the wrong one? What makes the other one right?