GRED Answer: Scissor vertex speed

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GRED Answer: Scissor vertex speed

Post by RJN » Wed Jun 23, 2010 2:41 pm

GRED: Guess the Result of the Experiment of the Day: Scissors Vertex Speed

Can the vertex of a scissors move faster than the speed of light?

Please post answers, comments, and discussion below. I will post what I believe to be the correct answer in a few days.
OK, I have now posted what I believe to be the answer later in this thread here: http://asterisk.apod.com/vie ... 17#p125917


The initial poll, where spoilers were not allowed, can be found here: http://asterisk.apod.com/vie ... 30&t=19963 . If you are new to this GRED and want to ponder this question without seeing spoilers, please go there now instead of scrolling down.

- RJN

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Re: GRED Answer: Scissor vertex speed

Post by bystander » Wed Jun 23, 2010 3:04 pm

We need to clarify to what we are referring as the vertex. If we are talking about the point about which the blades pivot, the the answer is obviously no. That point is fixed with respect to the blades and does not move at all (unless the scissors move). However, if we are referring to the point where the cutting edges meet, this point does move with respect to the blades at an apparent speed that is faster than the speed of the blades. As this point is not a physical entity, I think it is possible for its apparent motion to exceed the speed of light, if the blades are moving at a sufficiently high rate of speed.

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Re: GRED Answer: Scissor vertex speed

Post by RJN » Wed Jun 23, 2010 3:17 pm

bystander wrote:We need to clarify to what we are referring as the vertex. If we are talking about the point about which the blades pivot, the the answer is obviously no. That point is fixed with respect to the blades and does not move at all (unless the scissors move). However, if we are referring to the point where the cutting edges meet, this point does move with respect to the blades at an apparent speed that is faster than the speed of the blades. As this point is not a physical entity, I think it is possible for its apparent motion to exceed the speed of light, if the blades are moving at a sufficiently high rate of speed.
By vertex, I am indeed referring to the point at which the blades meet. As usual, for simplicity, I was trying to use as few words as possible, and chose the word "vertex" hoping that it was clear. Thanks for helping to make this clearer.

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Re: GRED Answer: Scissor vertex speed

Post by alter-ego » Wed Jun 23, 2010 3:25 pm

bystander wrote:We need to clarify to what we are referring as the vertex. If we are talking about the point about which the blades pivot, the the answer is obviously no. That point is fixed with respect to the blades and does not move at all (unless the scissors move). However, if we are referring to the point where the cutting edges meet, this point does move with respect to the blades at an apparent speed that is faster than the speed of the blades. As this point is not a physical entity, I think it is possible for its apparent motion to exceed the speed of light, if the blades are moving at a sufficiently high rate of speed.
Ditto.
The intersection point is an abstraction - it is not constrained to a particular set of atoms or location wrt the scissor blades, and that point cannot be used to transmit information. Just don't get caught at that point or the blades will they have their say!
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ScissorS vertex speed

Post by neufer » Wed Jun 23, 2010 4:02 pm

Scissor, v. t. To cut with scissors or shears; to prepare with the aid of scissors.

ScissorS, n. pl. [OE. sisoures, OF. cisoires (cf. F. ciseaux), probably fr. LL. cisorium a cutting instrument. The modern spelling is due to a mistaken derivation from L. scissor one who cleaves or divides.] A cutting instrument resembling shears, but smaller, consisting of two cutting blades with handles, movable on a pin in the center, by which they are held together.

http://www.youtube.com/watch?v=c6N5gXr784Y

"I believe, said Pantagruel, that all intellectual souls are exempted from Atropos's scissors."
- François Rabelais — Gargantua and Pantagruel (1518)
http://en.wikipedia.org/wiki/Scissors_crossover wrote:
<<A crossover is a pair of switches that connects two parallel rail tracks, allowing a train on one track to cross over to the other. Like the switches themselves, crossovers can be described as either facing or trailing. When two crossovers are present in opposite directions, one after the other, the four-switch configuration is called a double crossover. If the crossovers overlap it is dubbed a scissors crossover, scissors crossing, or just scissors; or, due to the diamond in the center, a diamond crossover. This makes for a very compact track layout at the expense of using a level junction.

Image
A scissors crossover: two pairs of switches linking two tracks to each other in both directions>>
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Re: GRED Answer: Scissor vertex speed

Post by makc » Wed Jun 23, 2010 5:31 pm

yes, scissors' vertex can move as fast as you want, as long as there is no friction. this is because vertex speed can be any number of times faster than blades speed. finally, in case oh perfectly parallel guillotine the speed in question is ∞.

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Re: GRED Answer: Scissor vertex speed

Post by neufer » Wed Jun 23, 2010 6:04 pm

makc wrote:
yes, scissors' vertex can move as fast as you want, as long as there is no friction. this is because vertex speed can be any number of times faster than blades speed. finally, in case oh perfectly parallel guillotine the speed in question is ∞.
  • Or -Image
http://en.wikipedia.org/wiki/Guillotine wrote:
<<In 1791, as the French Revolution progressed, the National Assembly researched a new method to be used on all condemned people regardless of class. Their concerns contributed to the idea that capital punishment's purpose was the ending of life instead of the infliction of pain.

A committee was formed under Antoine Louis, physician to the King and Secretary to the Academy of Surgery. Joseph-Ignace Guillotin, a professor of anatomy at the faculty of medicine in Paris, was also on the committee. Laquiante, an officer of the Strasbourg criminal court, made a design for a beheading machine and employed Tobias Schmidt, a German engineer and harpsichord maker, to construct a prototype. An apocryphal story claims that King Louis XVI (an amateur locksmith) recommended a triangular blade with a beveled edge be used instead of a crescent blade, but it was Schmidt who suggested placing the blade at an oblique 45-degree angle and changing it from the curved blade.

Image

From its first use, there has been debate as to whether the guillotine always provided a swift death as Guillotin hoped. There is the possibility that the very swiftness of the guillotine only prolonged the victim's suffering. The blade cuts quickly enough so that there is relatively little impact on the brain case, and perhaps less likelihood of immediate unconsciousness than with a more violent decapitation, or long-drop hanging.

Audiences to guillotinings told numerous stories of blinking eyelids, speaking, moving eyes, movement of the mouth, even an expression of "unequivocal indignation" on the face of the decapitated Charlotte Corday when her cheek was slapped. Anatomists and other scientists in several countries have tried to perform more definitive experiments on severed human heads as recently as 1956. Inevitably, the evidence is only anecdotal. What appears to be a head responding to the sound of its name, or to the pain of a pinprick, may be only random muscle twitching or automatic reflex action, with no awareness involved. At worst, it seems that the massive drop in cerebral blood pressure would cause a victim to lose consciousness in several seconds.

The guillotine was the only legal execution method in France until the abolition of the death penalty in 1981, apart from certain crimes against the security of the state, which entailed execution by firing squad.>>
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Re: GRED Answer: Scissor vertex speed

Post by makc » Wed Jun 23, 2010 7:00 pm

they should have made multiple blades that slice the head in thin layers. then, brain neurons would be massively disconnected, and victim would not be able to experience any suffering. too bad they couldn't know this back in 1791 :(

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Re: GRED Answer: Scissor vertex speed

Post by swainy » Wed Jun 23, 2010 8:51 pm

makc wrote:they should have made multiple blades that slice the head in thin layers. then, brain neurons would be massively disconnected, and victim would not be able to experience any suffering. too bad they couldn't know this back in 1791 :(
Have a real good think about this makc. let me tell you, if Osama bin Laden, hurts any member of my family, I am going to have to hold my red mist back huh? 1791 was a very different time?

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Re: GRED Answer: Scissor vertex speed

Post by makc » Wed Jun 23, 2010 9:45 pm

I was referring to
the idea that capital punishment's purpose was the ending of life instead of the infliction of pain
If you disagree (and are in position to do anything about it), feel free to dig him into anthill or something.

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Re: GRED Answer: Scissor vertex speed

Post by Beyond » Thu Jun 24, 2010 3:15 am

Anything is possible -- but if the vertex did move faster than the speed of light - photons could not capture the event and you would not see it. You would only see opened and then closed with nothing in between. kinda takes all the excitement out of it.
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Re: GRED Answer: Scissor vertex speed

Post by Chris Peterson » Thu Jun 24, 2010 4:33 am

beyond wrote:Anything is possible -- but if the vertex did move faster than the speed of light - photons could not capture the event and you would not see it. You would only see opened and then closed with nothing in between. kinda takes all the excitement out of it.
Nope, you can see the apparent motion exceed the speed of light. That doesn't violate any physical laws. The photons coming from one position of the vertex are not causally connected with the photons coming from a different portion, that's all.
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Re: GRED Answer: Scissor vertex speed

Post by Amir » Thu Jun 24, 2010 9:44 am

i still think it cannot. because of this:
information travels at the speed of light.
blades should know when to meet each other, and that's information, isn't it?
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Re: GRED Answer: Scissor vertex speed

Post by neufer » Thu Jun 24, 2010 10:51 am

Amir wrote:i still think it cannot. because of this:
information travels at the speed of light.
blades should know when to meet each other, and that's information, isn't it?
http://asterisk.apod.com/vie ... 71#p124271
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Re: GRED Answer: Scissor vertex speed

Post by makc » Thu Jun 24, 2010 11:17 am

Amir wrote:blades should know when to meet each other
aren't blades just two rigid pieces of metal?

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Re: GRED Answer: Scissor vertex speed

Post by wonderboy » Thu Jun 24, 2010 11:30 am

I've often thought about this. if you shake your hand really fast it appears that it is in 2 places at the one time. Now I have thought that if you were to take the speed that your hand or even your finger takes to get from one point to the other and put the exact same speed into a space ship, would it move at the speed of light.

I'd like to say yes, but I think that for some strange reason it will not.

Its the same as a previous question on here. If you stood in the centre of the universe holding a pole which reached the outer edge of the universe and turned round, would the end of the pole be moving faster than the speed of light? the answer to that question was no. So i'm running with that.

Hopefully this isnt a spoiler though.
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Re: GRED Answer: Scissor vertex speed

Post by Amir » Thu Jun 24, 2010 12:18 pm

makc wrote:
Amir wrote:blades should know when to meet each other
aren't blades just two rigid pieces of metal?
let me explain. assume you're are few light years away and i wanna talk to you using a gigantic scissors. for example i use it to send you a signal. thus we are communicating with a speed faster than light because vertex which carries the signal is traveling faster than light. so we are transmitting information faster than light. is that possible?
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Re: GRED Answer: Scissor vertex speed

Post by wonderboy » Thu Jun 24, 2010 12:25 pm

You arent travelling at the speed of light at all. because one end of the scissors is at your end and the other is at makc's so the communication would be instantaneous i presume! I get your point tho Amir.

The blades themselves would be moving at just the same speed as a normal pair of scissors because you have increased the relative size by a few light years. the speed depends on how fast you work the scissors at your end.


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Re: GRED Answer: Scissor vertex speed

Post by Henning Makholm » Thu Jun 24, 2010 1:27 pm

makc wrote:
Amir wrote:blades should know when to meet each other
aren't blades just two rigid pieces of metal?
No. A rigid body is a theoretical impossibility in SR, because it would transmit information faster than light. In practice, if you assume rigidity and end up with an absurd result, it tells you that you need to do your analysis over without ignoring the elasticity/pasticity of the materials involved.

I think the confusion over this question might be because the question does not really describe an experiment. To be an experiment it would need to describe an apparatus for setting the scissors in movement and ask whether that particular apparatus would make the vertex move faster than light. As it is, there are different correct answers depending on which assumptions we make.

Clearly, if we work the scissors exclusively from one end, we cannot make the vertex travel faster than light. (At least not unless the blades have some very weird elastic properties such that they won't meet close to us until our impulse has had time to travel down to points far from us, in which case it would be a stretch to describe the thing as a scissors).

On the other hand, if we plan out the whole closing of the scissors in advance and position a lot of helpers along the blades at regular intervals, all with synchronized watches, and all push on the blades at just the right moments, then there is no reason why we could not succeed in making the meeting point "move" at above the speed of light. Each of us would see the part of the blades at our own position close before any other point along the blades (remember that they are necessarily bendy), but when we meet and compare notes after the operation, we can deduce the actual movement pattern.

This kind of superluminosity is kind of bogus, but mostly because the concept of the vertex is bogus. If the scissors are so long that we can make "the" vertex "move" faster than c, it must also be bendy enough that we can make the blades meet and unmeet at several vertices at the same time.
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Re: GRED Answer: Scissor vertex speed

Post by wonderboy » Thu Jun 24, 2010 2:03 pm

Henning Makholm wrote:
makc wrote:
Amir wrote:blades should know when to meet each other
aren't blades just two rigid pieces of metal?
No. A rigid body is a theoretical impossibility in SR, because it would transmit information faster than light. In practice, if you assume rigidity and end up with an absurd result, it tells you that you need to do your analysis over without ignoring the elasticity/pasticity of the materials involved.

I think the confusion over this question might be because the question does not really describe an experiment. To be an experiment it would need to describe an apparatus for setting the scissors in movement and ask whether that particular apparatus would make the vertex move faster than light. As it is, there are different correct answers depending on which assumptions we make.

Clearly, if we work the scissors exclusively from one end, we cannot make the vertex travel faster than light. (At least not unless the blades have some very weird elastic properties such that they won't meet close to us until our impulse has had time to travel down to points far from us, in which case it would be a stretch to describe the thing as a scissors).

On the other hand, if we plan out the whole closing of the scissors in advance and position a lot of helpers along the blades at regular intervals, all with synchronized watches, and all push on the blades at just the right moments, then there is no reason why we could not succeed in making the meeting point "move" at above the speed of light. Each of us would see the part of the blades at our own position close before any other point along the blades (remember that they are necessarily bendy), but when we meet and compare notes after the operation, we can deduce the actual movement pattern.

This kind of superluminosity is kind of bogus, but mostly because the concept of the vertex is bogus. If the scissors are so long that we can make "the" vertex "move" faster than c, it must also be bendy enough that we can make the blades meet and unmeet at several vertices at the same time.

It took me a while to get this but I think I get it now. Obviously with something so big if you were to work it, it would be like grabing a piece of rubber and moving it back and forwards creating undulating waves.

However, my point is that if we had scissors which stretched over say a light year, then if the scissors were in proportion they would be as rigid as the normal sized scissors we see on earth. Im not talking about normal scissors with light year long blades.

Paul.
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Re: GRED Answer: Scissor vertex speed

Post by Henning Makholm » Thu Jun 24, 2010 2:47 pm

wonderboy wrote:However, my point is that if we had scissors which stretched over say a light year, then if the scissors were in proportion they would be as rigid as the normal sized scissors we see on earth.
The blades would be so thick that before we even get to longitudinal stability, they would collapse around the length axis under their own gravity. (In fact, the entire scissors would weigh around a million galaxies, and form a black hole several kiloparsecs in radius). But that is, of course, neither here nor there.
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Re: GRED Answer: Scissor vertex speed

Post by wonderboy » Thu Jun 24, 2010 3:17 pm

i struggle to see how scissors a light year in proportion would way the same as the matter of a million galaxies combined! Especially when I haven't even told you what they are made out of yet lol.

This is why its a theory. I don't know much, but I like to think outside the box and im not afraid to say what I think..........

It can be problematic at times :P.

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Re: GRED Answer: Scissor vertex speed

Post by neufer » Thu Jun 24, 2010 3:37 pm

Henning Makholm wrote:
wonderboy wrote:However, my point is that if we had scissors which stretched over say a light year, then if the scissors were in proportion they would be as rigid as the normal sized scissors we see on earth.
The blades would be so thick that before we even get to longitudinal stability, they would collapse around the length axis under their own gravity. (In fact, the entire scissors would weigh around a million galaxies, and form a black hole several kiloparsecs in radius). But that is, of course, neither here nor there.
Forget the scissors for the moment.

Go to "Echo Point" and clap your hands.

There will be points quite distant from each other along the reflecting "echo wall" that receive your clapping sound simultaneously.

Just because such distant points are affected simultaneously does not mean that they are actually communicating with each other instantaneously.
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GRED: Scissor vertex speed

Post by DaveBone » Thu Jun 24, 2010 5:48 pm

Since the vertex is not a physical object, but rather is a point in space without substance, it is not bound by the laws of physics not to exceed the speed of light. Imagine two wavefronts moving toward each other at the speed of light. As long as they are meeting at some angle, the point of their meeting, again not a physical object, must move at greater than the speed of light.

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Re: GRED: Scissor vertex speed

Post by dirtch » Thu Jun 24, 2010 5:50 pm

Long metal scissors will bend from inertia before moving at much speed.