I am reading Taylor and Wheeler's Spacetime Physics. I am enjoying it a lot and find it extremely readable but I have a question regarding something. I am not sure how to determine the dimensions of a frame necessary for it to be called a free float frame. This is a general question I know. To be more specific, one problem I am working on discusses an earthbound laboratory in which an elementary particle passes from side to side, traveling at .96c through a cubical spark chamber one meter wide. The first question asks for what length of laboratory time is this particle in transit through the spark chamber. This is 1.04 meters or 3.4 X10^-9 s of time. No problem. Then they ask how far a separate test particle released from rest will fall in this time? This is 6 x 10^-17 m. Once again no problem. Now I understand that in order for the frame to be considered a free float or inertial frame, there must be no observable relative acceleration between the two particles. They ask how wide the spark chamber can be and still be considered a float frame? I am not sure. Then they say to assume that the optical equipment being used can detect a test particle change of position as small as 5 * 10^-7 m. Now I know that it takes the test particle being dropped from rest 3 X 10^-4 s to cover this distance. And the particle moving at .96 c, moves a distance of about 8.64 X 10^4 m in that time. How now can I determine how long an earthbound spark chamber must be to be considered free-float for this sensitivity of detection? Do I just find a time interval short enough that difference in position between the fast moving particle and particle being dropped from rest is less than 5 * 10^-7m?