*The Elegant Universe*a while back, and this issue is still bugging me. he kinda left it hanging.

Here's the issue: Let's say you (Bob) and a friend (Mary) are on a merry-go-round. A third friend (Dave) watches from off the merry-go-round. assume the merry-go-round is perfectly rigid.

Initially, everyone takes measurements. Bob measures the cicumfrence from on the merry-go-round by laying a ruler down head to tail several times. He measures the total length as X. Mary measures the radius. she measures it as she measures is as X/(2*pi*r). Dave measures the cicumfrence from off the merry-go-round ("disk" hereafter) using the same head-to-tail method Bob used. he gets X as the cicumfrence.

Spin the disk at a speed such that special relativity comes into play... let's say .3c (assume they don't die for the sake of the problem ). Everyone takes measurements again. here's where it gets funky.

a) Mary measures the radius. Because she is not measuring along the direction of travel, her ruler is not contracted. she measures X/(2*pi*r) as she did when the disk was at rest.

b) Dave measures the disk from a stationary position. he goes head-to-tail with the ruler very close to the spinning disk. he measures the same cicumfrence, X when he comes back to his starting point.

c) Bob measures the disk from onboard. what happens here? the circumfrence of the disk should be shorter because of length contraction, as should Bob's ruler. so one would assume Bob would measure X as the circumfrence since both his ruler and the disk would be contracted by the same amount due to SR. But Dave measured the circumfrence as X with his ruler which is NOT contracted. and the circumfrence is not effected. the disk would still appear from the air to have a circumfrence of X and a radius proportional to X. so what's up?

-IggDawg