On 03/31/2010 11:52 PM, Francis X Roarty wrote: > Am I correct in believing a near luminal basketball could pass through > the eye of a stationary needle? >
No. The basketball is contracted fore-and-aft, but not side-to-side, as viewed by an observer sitting next to the needle. So, it's going to be too wide to fit through the needle's eye, even though it may be *thinner* than the needle's diameter. As Steven Johnson already said, the basketball "pancakes", but the pancake is flying along with one flat side to the front, so it's "splat time" when it hits the needle. You could also say the basketball has been replaced with its silhouette. It's the fact that there's no side to side contraction which leads to all the arguments over whether the contraction is "real". The fore-and-aft contraction is arguably just a "trick of the light". The fore-and-aft contraction seems to be mostly an artifact of the fact that time is skewed between the reference frames, and in order to determine how "long" something is, you need to find the locations of the front and back of the object "simultaneously". The definition of "simultaneous" turns out to be frame dependent, which is how an observer on the basketball and one sitting by the needle end up disagreeing about whether the basketball is contracted or not. Spinning disks, on the other hand, cast a rather different "light" on the matter, as a spinning disk which can't stretch will, in principle, crack as it spins up, due to the contraction of the rim. That makes the contraction seem rather "real". A really long train also cannot accelerate at more than a certain rate without breaking apart, as it turns out the cars at the back of the train must move faster than the ones in the front due to the train's contraction as it speeds up. At some point they'd have to break the lightspeed barrier to avoid being left behind by the shrinking train. That, also, makes the contraction seem rather "real".
