Hello All,
I read somewhere that the point directly in front of the vertex where the two blades of a pair of scissors meets travels at or near the speed of light…???
Can any of you confirm or quell this theory?
– Matt
Think about this: the speed of sound is much, much less than the speed of light.
So if any part of this physical system were traveling near the speed of light, it would have easily exceeded the speed of sound.
Heard any sonic booms while clipping coupons with your favorite scissors? 
If you had perfectly rigid blades on your scissors, and they were very long, then this could be true. For instance, if you had scissors a lightyear long, and it took you a second to close them, then the point of contact would travel half a lightyear in a second.
However, in actuality, there’s no such thing as a perfectly rigid material. In fact, one of the limitations on how rigid a material can be is the speed of light. When you move the handles of the scissors, the atoms of the handle move. These moving atoms will pull the next atoms along, and those next ones will pull the ones after, etc., by electromagnetic forces. Electromagnetic forces only travel at the speed of light, though, so the atoms way at the end of the scissors won’t move until at least a year later, and they’ll end up bending a bit in between.
Let’s look at this from another angle. (Pun intended.)
What is the vertex of the scissors? The point where the two blades meet. In order for the vertex to move, the blades have to move. And, of course, in order for the blades to move, your hand - or rather your fingers - have to move.
Thus, in order to move the vertex at the speed of light, your fingers must move at the speed of light. And since your muscular structure in your hands probably wouldn’t be able to handle the increase in the mass of your fingers should they approach the speed of light, it’s likely that your fingers won’t be able to move that fast, and therefore neither will the vertex of the scissors. QED.
Actually, Chronos, since a lightyear is the amount of distance light travels in a year, the vertex would exceed the speed of light by a very impressive amount if it traveled that distance in a second.
How do you get half a lightyear out of that, by the way? If the scissors is a lightyear in length, that means each blade is a lightyear in length, and closing it causes the vertex to travel a full lightyear, not half.
Arrgh. It isn’t an actual physical thing that’s moving at the speed of light, it’s our perception of something that moves that fast.
Suppose you go out to the ends of those scissors and look straight down the blades. The ‘vertex’ doesn’t seem to move at all. In fact, it vanishes once the blades are closed.
Eppur si muove!
If you’re looking down the barrel of a gun, the bullet that’s shot out of it doesn’t appear to move either, but you end up dead anyway.
The vertex is indeed a physical thing, because it is the juncture of two very physical things.
Okay, Olentzero, then where does you lap go when you stand up? The question in the OP wasn’t, “if we balance, say, a marble at the vertex of a pair of scissor blades…” In that case, having the marble move that distance in that amount of time would indeed violate way lots of laws. This vertex thingie is different. Asking how that vertex moves at the speed of light makes as much sense as, say, if you hold your thumb and forefinger out and ‘crush peoples’ heads’ (“crush! crush! I’m crushing your heads!”), why don’t their heads actually get crushed?
When I read the thread title, I thought this would be a discussion about how when you buy a pair of scissors, and happen to have pre-teen/teen offspring (especially the female variety), the scissors mystically disappear at the speed of light.
Carry on…
Nothing to add about the OP. I just wanted to say thanks for the KiTH reference. (My favorite “I’m crushing you head” guy skit was the big battle with the “I’m pinching your face” guy.)
So “seeing” (pun intended) as we may be speaking in a hypothetical sense about the “invisible” point just ahead of where the two blades meet, Is it possible for this non-mass thing to be travelling at all? if so why couldn’t it be going at the speed of light? Ecspecially seeing as if we don’t know if a non-mass object would make a sonic boom. Am I just being silly here?
hmm… I think he is wondering about the fact that, when scissors are closed, their vertex moves significantly faster than the speeds of the blades even when added together.
so the question might be, “would it be possible for the vertex’s apparent motion to reach the speed of light, using a pair of scissors that might actually exist (excluding alternate realities with funky physics)?”
hmm… I think he is wondering about the fact that, when scissors are closed, their vertex moves significantly faster than the speeds of the blades even when added together.
so the question might be, “would it be possible for the vertex’s apparent motion to reach the speed of light, using a pair of scissors that might actually exist (excluding alternate realities with funky physics)?”
It’s possible for certain abstractions, such as geometric points, to move faster than the speed of light.
Simple example: In New York City, the traffic lights on many avenues go off in succession. If traffic is light, it is possible to “surf” the green for miles.
This “green wave” travels at about 30 mph, but in principal, the lights could be reprogrammed so that the “green wave” travels at any speed, including and exceeding the speed of light. In fact, all the lights could be programmed to change (basically) at the same moment, creating a “green wave” that travels infinitely fast.
Note that there’s no way to use this technique to transmit information faster than the speed of light.
Similarly, I believe that a pair of scissors could be constructed (out of ordinary steel) where the point of intersection of the blades travels faster than the speed of light. In fact, if you take two blades of steel (that aren’t connected) and bring them together so that they remain parallel at all times, the point of intersection (arguably) moves infinitely fast.
Again, there’s no way to use this technique to transmit information faster than the speed of light.
Wouldn’t it transmit the information that the scissors had closed? 
If the two blades are not connected and are parallel, where do they intersect? Wouldn’t they by definition never cross (in Euclidean space)?
The question is - transmit information from where to where?
Going back to the Manhattan analogy, let’s suppose you’re standing on the corner of 125th and Park, and all the north-south lights go red at about the same moment. Well, you (basically) KNOW the light is red at 42nd and Park, but how can you use that knowledge to transmit, say, a message between the two points?
Once you bring them together, they intersect at every point. Before that, they don’t intersect at any point.
I think this comes from an article in Scientific American.
It’s just that ever since I was a wee lad, I wondered about this very question. I wondered if the vertex could actually be made to travel faster than the speed of light by closing some very long scissors. To find it here amongst the SDMB ruminations is a treat. To be edified by the conversation is probably more than the average bear could ask for. Thanks, boys and girls.