What's wrong with this FTL hypothetical

Today I was trying to cut a plastic bead with upholstery scissors and rather than cutting the bead it shot out very fast.
<cut to a lot of thinking about this later>
If you had a giant pair of scissors in space, big enough so that the intersection of the closing blades was moving faster than the speed of light, and put a giant bead in the intersection why could’nt you theoretically accelerate the bead past the speed of light?


Re-read the OP. I know it’s not possible, I’m asking why not.

Well, for openers, the energy required to accelerate any object with mass TO c is infinite; you can’t have more than infinite energy. Puts a crimp in the plan right there.

The scissors will break, the bead will be crushed, there isn’t enough energy from any source great enough to close the scissors even withouth the bead, and intersection of two objects is not an object.


Why do you feel the object would bead would accelerate to faster than the speed light? Because the distance of the contact point of the blades from the origin increases at a rate faster than the speed of light? So what? The bead isn’t compelled to shoot off at the same speed as the contact point distance (indeed, that’s silly; even with normal scissors and such, if you had a very heavy bead, it would clearly shoot off more slowly than a very light bead, so the shoot-off velocity is not simply determined by the speed at which you close the scissors)

(Standard science disclaimer: if I’m wrong, people more knowledgeable than me will tell you so)

Also, a relevant link on big scissors and relativity.

Hm… take your pick on whether you want me to say “object would” or “bead would”. :slight_smile:

Also, insert an “of” in “speed of light”.

It won’t work because that can’t happen.

True, but the bead would have to remain in front of the closing angle.

Ah, fair enough. Well, I suppose that aspect of your reasoning is alright, then. But, as all the more knowledgeable people than me said, you have to keep in mind the energy required to close the blades as they propel the bead (scissors with a heavy object in them are harder to close at a given speed than with a light object in them, so the energy to close at a given speed is not independent of the presence or mass of an object within), and at this point, you’re scuttled; no amount of energy will be sufficient to close the scissors fast enough with the bead stuck in there.

The answer, though, is in Indistinguishable’s link. Forget the bead. The point at which the blades intersect cannot move faster than the speed at which force propagates through a solid material, the upper bound of which is the speed of light.

Seems to me it could happen, though, if there was no bead in there. That is, under a suitable interpretation, with the right shape of scissors and flexing of the blades, it can happen. You just have to realize that the contact point of the two blades is not a physical object in itself, and thus not necessarily constrained in its movement the way physical objects are.

Consider, say, taking two long planks and setting them up so that one starts at the origin and points straight up, while the other starts at the origin and points at an angle of thirty degrees off of vertical to the right. Now slowly move the vertical plank to the right; say, at one meter per second. The contact point will move up and to the right along the fixed sloped plank at two meters per second, even though no actual physical object is moving that fast.

Seems to me, the same sort of thing can happen with the scissor blades to get the contact point moving faster than light (as long as we don’t put a mass in the middle, effectively making the contact point into an actual physical object that takes energy to move). Indeed, my link from before backs me up on this, unless I’ve misinterpreted it.

Dude, have you read your own link?

Have you? It explicitly says “The contact point where the two blades meet is not a physical object. So there is no fundamental reason why it could not move faster than the speed of light, provided that you arrange your experiment correctly.” and “When the blades finally come together, if they have the right shape, the contact point can indeed move faster than light.”, and then illustrates this sort of thing (superluminal movement of non-physical “objects”) in its last two paragraphs.

I agree that you cannot move the bead faster than the speed of light. But without it, the contact point itself, yeah, that can change faster than the speed of light. But feel free to correct me if I’m hideously wrong in my understanding or interpretation.

Sorry, I was misunderstanding. As you say, the contact point can travel faster than light. I was looking at it from the point of view of whether this can actually be used to transmit a signal faster than light, which it cannot.

Ah, phew. I was worried I’d made a mess of it for a while there. Glad that’s all cleared up.

One of the better username/questions I’ve seen in a while.

This is at the heart of my question. Suppose you built the hypothetical scissors and operated them such as to cause the intersection to move faster than the speed of light. Then repeated the closing but this time had a (frictionless?) ‘bead’ that would ride in front of the closing intersection. What specifically would prevent the bead from achieving, or surpassing C?

Can you be a bit more specific about what the speed intersection of the scissors means? Are you talking about the relative speed of the two blades as observed by a stationary reference frame?