Any real-world scissors made of ordinary matter still have to obey the laws of physics.
So if you try to move the handle of real-world giant scissors, what happens is that the giant scissors bend or break long before any part of those scissors reach an appreciable fraction of the speed of light. Even metal beams a few hundred FEET long will noticeably bend and sway when large forces are applied, rather than move solidly.
Now, suppose we change the conditions of the experiment so this doesn’t apply. And there’s a simple way to do this. You just set up two lasers on pivots and intersect the beams, and change the angle of the lasers such that the point of intersection moves back and forth. And the point of intersection could definately move along the laser beams faster than the speed of light…except for the trouble that the photons shot out of the laser keep going in the direction the started, they don’t turn because you pivoted the laser.
Forgetting lasers and everything, it’s easy to make a point of intersection move at the speed of light or faster. Take two yardsticks. Align them so that they intersect at a thirty degree angle to each other. Then move one of the sticks at half the speed of light or faster in a direction perpendicular to its slope, without bending it. Nothing in relativity says you can’t do this. And when you do it, the location in space of the intersection will move at a speed greater than or equal to that of light. There’s no problem with this, since this doesn’t correspond to an actual physical object moving at such a speed, or any information/causality being transmitted at such a speed.
Or like the link said, take two perfectly rectangular yardsticks and lay them side by side in a scissors-like formation but without an overlap. The gap between the two will move instantaneously at the exact moment the two sides connect, which is much much faster than the speed of light
Note that the further down the blades (or yardsticks) that you go, the more force is directed towards crushing the bead than propelling it forward. At some point, it will be nearly indistinguishable from being squeezed between two parallel plates, right?
If you replace the scissor action with a pair of beams crossing, but not closing like scissors (angle one beam and bring it down across the other), then no.
The answer, as I learned from that thread, is that when you push on an object you push the atoms in your hand, which push atoms next to them, which push atoms next to them, and so on. This propagates through the object at the speed of sound.
I’d guess that the same thing applies to those big scissors in the OP. If you pull on the handles, it takes many centuries to reach the point of intersection. And the point of intersection itself will move at the speed of sound, or slower.
So the bead just accelerates to FTL instantaneously? How do you keep the bead going in the plane that you want it to go? I think that necessarily the bead will push back on the beams sufficiently to stall their motion, or be crushed itself, right?
Not quite - the point of intersection isn’t a physical object, so it can move faster than any of the objects that contrive to create it - and can, in principle, move faster than the speed of light.
No - there’s no limit to the speed at which the intersection can move - it’s not a real object - merely an idea.
Here is a quick and dirty diagram - the scissors close and the point of intersection moves rapidly along the length of the blade - and gets faster as it goes, because the angle of the blades is closing.
With the unconnected bars, the top bar is brought down, all at once, across the bottom horizontal one - the point of intersection again moves very rapidly along the setup.
It’s possible to create quite a small version of this where the intersection’s movement would exceed the speed of light - all that is required is to make the angle of the top bar shallower - but when we talk about the intersection moving at high speed, in fact, nothing is moving like that at all - because the intersection isn’t a real, physical object.
Yes, I understand that. But the blades themselves are real, and moving in a particular way. The point of intersection is determined by the movement of the blades.
Hang on, doesn’t that assume that the blades are rigid and move instantaneously? Which is wrong, isn’t it?
Here’s the way I see it.
You start by pulling on the handles. The atoms in your hand move immediately.
Those atoms pull on the next atoms in line, which also move.
Those atoms pull on other atoms, which also move. This ripples down the line at the speed of sound.
Eventually, it reaches the point of intersection. The atoms at that point get pulled aside. Thus the point of intersection moves forward. By the width of an atom.
But those atoms also pull the next in line, so the next atom also gets pulled aside. And the point of intersection moves forward again. By the width of an atom.
And so on. Every time an atom gets pulled aside, the point of intersection moves forward. And if atoms are being pulled aside at the speed of sound, then the point of intersection also moves at the speed of sound.
They needn’t be - force could be applied instantaneously at various points all along their length - but even that’s not necessary - we could design the apparatus so that even taking into account the elasticity of the materials, the intersection of the two blades still appears to move at more than the speed of light - just by flattening the angle of the intersecting top blade.
Think about it this way - in the second diagram, if the top blade is brought down completely parallel with the bottom one, the intersection event happens everywhere, all at once - that’s faster than light. if we now tilt the top blade only by the tiniest amount, then the intersection appears to move almost instantaneously from one end to the other.
But it’s all moot - you can make the intersection appear to move any speed you like - you can’t use that to move a physical object at any speed you like, or transmit information that way.
If it’s true that the intersection can move at any speed (which I doubt), then just hook up a photo-interrrupter, and voilà! - you have a faster-than-light communications system.
No, because as people keep saying, the intersection is a virtual object rather than a real object. In other words, the intersection is a succession of non-connected points unlike the actual blades of the scissors, which are real connected points.
A pattern can move across a series of non-connected points at any speed.
I think it’s easier to visualize if you use the example of a laser moving across the moon. The series of points the laser marks out on the moon are not connected to one another. We see the end point of the laser at time a, then the end point of the laser at time b, then the end point of the laser at time c, etc. In that way the laser’s beam can appear to sweep across the moon at faster than the speed of light. But it’s only a virtual pattern that you’re seeing.
Another analogy is to think of your shadow as you move from light to light. Your shadow is behind you at first, then catches up, then sweeps out in front of you. It moves much faster than you do. But nothing is really moving to make up the shadow. It’s a succession of individual points.
So the intersection of the scissors is a succession of points that aren’t connected to the actuality of the blades. It’s entirely virtual, a mere pattern. You cannot interrupt it and get something physical as a result any more than you can get work out of your moving shadow by adding another light or taking one away.
You’re not sending any information from the first location of the intersection to the second location of the intersection. You are at best sending information from some other point or points to the various locations of the intersection, successively, at a speed in each case less than that of light (i.e., the time the information takes to get from its origin to any particular location of the intersection is less than that light would take).
If the Intersection can move FTL, then I can send Morse code from the “handle” end, and decode it at the other end in less time than a beam of light can travel, since the intersection moves away from the handle end.
With scissors, in the setup in my mind, where the information transmitted by seeing an intersection is “Oh, I guess the handles flexed” and thus originates at basically the original location of the intersection, the point of intersection doesn’t move faster than light for the entirety of its journey; it just may do so for some interval of its journey after the beginning. The total time for the point of intersection to get from the origin to any point will be less than light would take; however, even though this means its average speed over the entire journey will be less than that of light, it may have had higher speeds at some point along the way. This will not give us any ability to transmit information “picked up” along the way, though.
Another illustrative example of faster-than-light motion of an intersection: Let’s say we have a long stick with tabs on both ends. Hover it above another long stick, with its left tab touching and its right tab just hovering above. Now lift the left tab just a bit while at the same time depressing the right tab just a bit. Voila, the point of intersection just shifted a long amount in a very small amount of time. But nothing in this involved information being transmitted from the left tab of the stick to the right stick in a very small amount of time.
(This is basically the two flat rulers, instantaneous movement example, but without any possible complaints about “No fair; there’s more than one point of intersection when they touch”)
We have discussed the way if you actually built a light-year long set of blades, with the tips one inch apart, then closed them from one end, you wouldn’t actually get the intersection to move faster than light, because the closing force you apply at one end wouldn’t be transmitted instantaneously down the blades: they would actually flex and the tip would only close well over a year later.
However, imagine you build a set of seriously clever blades that are not straight but curved very slightly, like pincers ie bowed outward. The tips are still set one inch apart, and the distance between pivot and tip is still a light-year. You have carefully designed the blades so that when you squeeze on one end to close them, the flex you introduce will tend to straighten the blades out such that as the flex reaches the far end, the blades will be have flexed dead straight. They will now meet with the universe’s mightiest SNIIIIIIIP!, near instantaneously along the blades’ length.
The point of intersection will have moved FTL, but that doesn’t breach Special Relativity because nothing physically moved FTL, only an abstraction.
No signal will have moved FTL, because the blade tips didn’t move (causing the FTL SNIP) until well over a year after you first attempted to close the blades.
Again, no. The only way you can send information is along the blades, but we’ve already established that they move move slower than light. The intersection isn’t real. You can’t send information via the intersection any more than you can send information via your shadow.
You don’t seem to get analogies, but I’ll try one more. Film moves at 24 frames per second at all times. But the picture that results can depict objects moving at any speed. That’s an illusion caused by the rapid sighting of a succession of individual frames. Same with the intersection. The blades are real like the frames of film. The intersection is an illusion like the picture on the screen. No matter how you manipulate the picture on the screen the film moves at a fixed speed and is the limiting factor.
The intersection can only move FTL after the blades at the far end have moved. You can’t transmit movement to the blades at the far end FTL.
To put it another way, the abstraction we are referring to as “the intersection” (ie the point at which the blades meet) can be, in a very short space of time, at points separated by more distance than light could travel in the same period of time. However, to set that up, you first need to arrange by some means for the intersection to occur over that distance. And you can’t do that FTL.
An example is given in Indistinguishable’s first link which helps. Imagine a light-year long row of lights . You could (by prearrangement) make them flash in series such that the flash would seem to pass from one end to the other in less than a year. ie the “flash point” would appear to travel at faster than the speed of light. However, in order to do this you could not use a signal setting off each light starting at one end and moving to the other, because the signal couldn’t go FTL. What you could do is set each one up so that (using pre-arranged timing) the first one went off on 1 January, and the last one on 30 June, and all the ones along the way at suitable intervals. But moving along the lights to set this up would take longer than a year.
