As you note, it’s slightly irrelevant to the OP’s question, because once we put the bead in there, it becomes progressively harder and harder for the scissors to close as the bead speeds up and acts more massively, and this increasing difficulty of pushing the bead ends up making it impossible for the POI to move FTL. (In the sense that the amount of force required to close the scissors at the desired rate will be infinitely large). Without the increasingly-challenging bead in your way, though, it’s easy to do.
If you want to do it with actual scissors, then yes, there are serious practical engineering obstacles to be overcome, but any of the other equivalent models discussed above works in exactly the same way (for example the flagpole/monument one - which doesn’t even require the apparatus to move - only the observer) - so it’s possible to realise the effect with physical materials, just perhaps difficult to realise the effect with a pair of scissors of ordinary construction.
By the way, the OP suggests the bead (space capsule) being driven forward by the closing scissors. Is this likely to happen? In my experience closing scissor blades tend to trap objects beween them, and then break them apart. It’s called cutting, and its what scissors are designed to do. Is there an easy way to convert closing scissors action into forward momentum?
it’s true though that as the closure progresses, the blades become nearer and nearer to parallel, so they’re exerting less and less force in a forward direction and more and more in such a way as to crush or cut the bead, so yes, apart from all the other engineering problems, the bead would just eventually stall and be cut.
I don’t think that’s fair, as a shortcut solution. I think it’s pretty implicit in the OP that the bead is too strong to be cut. I mean, in a non-relativistic universe, say a Newtonian one, a greased up unobtainium bead placed between unobtainium scissors in the manner of the OP could be accelerated to arbitrary speeds. One does need some distinctly relativistic reasoning to make this fail.
It doesn’t matter - as the cut progresses, the closure of the blades means that the force exerted upon the bead approaches closer and closer to perpendicular to the desired direction of travel - if you look again at this diagram, you can see it - when the blade is in the position coloured palest in the diagram, it’s closing, pushing the bead significantly in a forward direction, but when the blade is in the position coloured darkest, it’s pushing the bead forward less, and down more.
And yes, pleading for unobtainium is one of the reasons why relativistic reasoning makes it fail - relativity doesn’t permit infinitely rigid matter to exist.
I would disagree, the edge of the cut is a physical thing, not an optical object or series of points. This case is distinctly different than the case of placing your thumb in front of the moon. You have now made an “irreversible” change. Whereas before and after the occlusion of two objects you have no idea whether they actually passed, after a cut definite “information” has been transmitted. after the cut has been made an irreversible change has occurred.
Yeah, but it never stops pushing forward. The bead will always stay ahead of the point of intersection, as the OP noted. Clearly, it will be possible to accelerate the bead to some speeds, and, in Newtonian mechanics, everything scales up without problem, so it should be classically possible to accelerate the bead to arbitrary speeds.
Sure. My unobtainium wasn’t meant to be infinitely rigid; it was just meant to not get destroyed by squeezing of the blades. It can get compressed all you want, though.
The bead can’t relativistically be accelerated to FTL, of course, but I think that the fact that the scissors are eventually pushing much less forward than up-down is not the essence of this. I think it’s vital to mention the fact that as the bead gets faster, it takes more force to accelerate it, in such a way as that no amount of force could ever get it up to lightspeed.
You can get an idea of whether the POI got to a certain distance… just sit there and look for it. This isn’t a problem for reasons explained in many, many posts in this thread.
The same amount of information is transmitted by a cut as by the moving POI. (Think of the scissor being fitted to drop a blot of paint wherever its POI is; this leaves an irreversible change, but you could still get the POI to move FTL for all the reasons explained in many, many posts in this thread. It’d have to start off slower than light and have a total average speed slower than light, but it could zoom to FTL for later legs of its journey; it has to stay behind a beam of light emitted at the same time and point it was set into motion, but it can lag behind a lot and then do some catching up, the catching up being, of course, FTL)
Seriously, although the incident that started all of this worked really well at accelerating the object (bead) I realize for the OP to have a shot a working you would have to build something that worked with bearings so as to reduce friction.
The POI for a scissor is different depending on what angle you are looking at the thing from. If you are looking at it head on then there is only on POI and it happens once. The cut of the paper is independent of viewing angle, but perhaps there lies the rub, if you look at the scissors head on you see no cut. ( I am thinking aloud) In any case we both agree that the “cut” will NOT get the end before a beam of light traveling at the same time would. I would love to see the mathematics worked out for this. I might just try to do that this weekend. I am still dubious because I feel as if somewhere there is an assumption of simultaneity that is not realizable in SR.
btw a group of would not laugh at beowulff’s previous posts, I know. This problem is not a simple as it appears to be at first glance. The linked web page is at best an appeal to authority and at worst incorrect.
This may help someone - the point at which the scissors’ vertex appears, and where it moves to, is really simply an optical illusion. If you look down on the scissors, you do not see the intersection. It is merely the result of the motion of one object, compared in one plane with the motion of another object. As such (as has been mentioned many times) it does not actually exist in the way that an object exists. That being the case, you might restate the OP this way: Suppose I point my finger at a star. Then, suppose I move my finger in a consistent way so that it points to a different star in one second. Given the fact that those stars may be millions of light years away from each other, did the point in space that my finger pointed at move many times the speed of light? Well, yes, in a manner of speaking. Could I attach a little plastic ball to that line in space between those two stars and could I accelerate it to that speed? Nope. Why not? Because mass can’t accelerate faster than the speed of light.
