I think this might have an answer to my previous question which I didn’t see at first. I’m not quite sure what is meant by “short enough and wide open to start,” but I imagine scissors in which the handle’s are further away from the fulcrum than the tips are.
Whether that image is the right one or not, in any case, maybe the time it takes for a causal processes (a signal) to get from a guy standing at the fulcrum (my “A” point from my previous poist) to the handles of the blades, (or to any point on the handle-ends where sufficient force can be exerted in order to make the scissors close in a way that “moves” the intersection faster than light,) is necessarily as long or longer than the time it takes to get from A to B (the tip of the blades) at light-speed. (Minus the time it takes for the blades to close.) This would make it impossible to send a signal from A to B at faster-than-light speed, as there would always be the sufficiently lengthy delay between the decision made to close the blade and the actual act of exerting force on the blade to close it.
Alternatively, Frylock, it could be that the signal gets from point A to the handle-closer very quickly, and the point of intersection starts moving almost immediately. The point of intersection is, at first, moving at less than the speed of light. However, later on, as the scissors continue their bending towards complete closure, if you set it up properly, the rate of movement of the point of intersection eventually exceeds the speed of light for the last leg of its journey. This will happen with the average speed during the entire journey remaining less than the speed of light, and thus there will have been no FTL transmission of information from A to the tip of the blades (i.e., the total time for the point of intersection to move from A to the tip of the blades will have been more than light would have taken for the same journey). However, all the same, the point of intersection will have undergone FTL movement over some interval.
The intersection of a carefully arranged (say, curved) pair of scissors can appear to move FTL as they close, but only as a result of force propagating along their length at a speed limited by the material properties (much less than light speed), so no information can be sent FTL by this method - because the ‘data’ is committed to transmission when you initiate the propagation of the force, not when the intersection starts appearing to move.
No it’s simpler than that. You don’t need to consider the fulcrum/handles part. The physical force cannot be imparted from A to B FTL. However, if the blades are shaped right at the time they meet (allowing for flex if necessary), the PPI (perceived point of intersection) may travel all or part of the way from A to B FTL.
Say you start with scissors with short very stiff perfectly straight blades a long way apart. Force is applied at A. It travels to B at less than FTL. Some time later, the force causes B (and indeed the whole blade) to begin to move. There is flexing, but as the blades move towards one another this damps out and by the time each blade reaches the end of travel, the blades are dead straight again. Now the blade angle reduces to zero, and SNIP! the PPI moves from one end to the other near instantaneously (and FTL).
The PPI begins to advance rapidly only well after the physical force starts. Thus the PPI advances from A to B in an instant (FTL) despite the physical force (which starts out on its journey from A to B earlier) doing so slower than FTL. There is no breach of SR because to cause the PPI to seem to move FTL, you had to send a slow signal, and there was no way to modulate the PPI movement (and thereby effect communication) other than by changes to the force applied which can only moderate the PPI at less than FTL speeds.
Do you think if enough people keep chipping in we can get this topic to the sort of "SDMB legend"status held by conveyor belts and planes?
I think your, my, Mangetout’s and Indistinguishable’s most recent posts have been four ways (mine being the least well formulated) of saying the following: The PPI may move from A to B FTL, but this can’t be used to transmit a signal FTL because the relevant signal will have begun being transmitted from A well before the PPI begins moving FTL.
If you were riding the back of a hummingbird, hovering inside a box, suspended by helium balloons inside a plane, which is on an imaginary perfect treadmill that accelerates so as to stop it taking off, would you kill one person in order to harvest his organs and save the lives of five deaf people standing on a railway track in the path of an oncoming train?
Well, a ridable hummingbird in a box suspended by helium balloons in a place on an accelerating treadmill is possible, right? So if it’s possible, it’s necessarily true in our world!
Sorry, before this thread turns completely into SDMB - The Sitcom, we ought to just confirm that everyone is happy the main point of contention is resolved. I propose that it is resolved thus:
-It may be possible to contrive to construct a pair of scissors in such a way that the point of intersection of their blades appears to travel along their length FTL (although no real object is actually moving that fast), but this can occur only after the blades have been set in motion, and that motion has propagated along their length at a speed very significantly less than that of light, and it is this act of setting the apparatus in motion that commits any data to be transmitted, so no violation of fundamental laws is necessary.
OK, wait a second. Frylock’s post got me thinking…
Think of a telegraph key. You let go of one end the other drops and makes a connection sending a signal. If the intersection of the scissors can move FTL doesn’t that mean that the far end of the scissors could transmit data at the same speed? In other words if the ends of the scissors were light years apart, and the intersection traveled that distance FTL couldn’t you use the moving scissor blade as a telegraph key?
This makes me think that the point of intersection could not move FTL.
No, it can only (appear to)do that long after the actual information has been committed to transmission, in the form of initiating the movement of the blades, and allowing that movement to propagate along them (presumably at the speed of sound in that material).