tino@hou2f.UUCP (A.TINO) (06/05/85)
> A way to look at the long scissors experiment to demonstrate >to yourself that you cannot use them to transmit information at faster than >the speed of light (in a vacuum) is that one bar of the scissor can be >held stationary, while the other moves. The moving bar must be waved back >and forth in a way that represents the information being sent. The stresses >in the bar which cause each succeeding layer of atom to sway one way >or the other cannot move faster than the speed of light. > Of course if you are only looking for one sweep of the blades, >you can get the cross-over moving at faster than the speed of light, >but you cannot send information faster than light. Let person A squeeze the handle and person B, at the far end, rest his neck on the bottom blade. If the scissors were rigid I guarantee that some serious information could be transmitted at faster-than-light speed even with a single pass of the blade. You don't need any wave-like motion to send information. The intersection point of the blades never travels at faster-than-light speed because, in the relativistic realm, the blades aren't rigid. (As pointed out by Hummel. Hi Ed!) Al Tino, Bell Labs at Holmdel
tino@hou2f.UUCP (A.TINO) (06/21/85)
In response to my comments on the Faster-than-light scissors: >I am missing something. > >Why can't the crossing points at different locations along the scissors >be treated as independent events that occur in the absolute elsewhere? >The speed at which the blades of the scissors are moving does not have >to be fast at all for the events to appear to be separated by times small >enough that the apparent "rate of propagation" of the intersection exceeds >the speed of light. > >Just drop a board or rotate a ruler. There is no reason that a locus of >points on the ruler cannot all cross a plane at the same time (or a series >of times separated by a time difference as small as you like). > >With respect to the neck--the time it takes to slice the neck is only the >time it takes for the intersection to propagate through it if the neck is >infinitely thin, right? So that argument is not valid. > > --Bruce Failor > MFE @ LLNL ------- Bruce: Maybe my original discussion of the "faster-than-light-scissors" was muddled. I'll try again. When the person at the handle begins to squeeze the scissors, she starts an impulse (shear force, whatever) that requires a finite time to reach the tip of the blade. In fact, the impulse travels at the speed of sound in the blade material. And you'll admit, I hope, that there is no way that the blade tip can move, much less connect with the second blade, until that impulse ( the "causal influence") reaches the the tip. So I don't see any hyper-light velocities here. If there were hyper-light speeds something would be very wrong because relativity prohibits any "causal influence" from traveling faster that light. And someone squeezing the handle is very much the "cause" of the subsequent motion of the blade. There is a very different situation that DOES lead to hyper-light speeds that may be confusing the discussion. If the top blade wasn't attached to the bottom blade but, instead, traveled past the bottom blade at a uniform velocity while making a small angle with it, THEN the intersection point could very well reach hyper-light speed. But so what? No "causal influence" is transmitted along the blade -- everything's already moving, no shear forces needed -- so all's well. (I think this is what you are pointing out when you talk of "dropping a board".) (P.S.: I don't get your point about the time it takes to slice the neck. I was refering only to the time it takes for the blades to reach the neck from the time that the handle-sqeezing began.) Al Tino Bell Labs at Holmdel
levy@ttrdc.UUCP (Daniel R. Levy) (06/22/85)
I've been waiting a couple of weeks for someone to say something about this, but I've heard nothing yet, so I'm speaking my 2 centavos worth. It could well be possible for the "point of intersection" of the blades to move fas- ter than light, but sure as heck one couldn't cause any motion to occur in the far end of the scissors any sooner after BEGINNING to squeeze the han- dles than it would take light to go from the handles to the far end of the scissors. Any attempt to do so would result in the relativistic effective mass of the blades tending to infinity, so that no finite force could pos- sibly move the blades that fast. here's hacking, dan levy at&t teletype corp. skokie, ill.