Jaseca

Say you had a scissors and the ends closing were both moving at the speed of light. How fast is the point in between the blades traveling?

Silvy

since both ends are traveling at maximum speed, the points between them must be stationary...

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cliche

if they're moving at equal speed, surely any equidistant point remains stationary. Or are you asking a different question?

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Redlemon

Jaseca is referring to the point where the two blade edges meet; this point moves closer to the tip as the tips approach each other. Towards the point where the scissors are almost closed, that point is moving faster than the tips.

However, that point is not a physical thing.

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Jaseca

That was my thought. Although it is really hard to explain to some people that a point does not move.

Lunchbox7

For every action there is an equal an opposite reaction. SO if something is moveing at light speed in direction A the there must be an equal amount of force going in the opposite direction B. If the two points are both moving a light speed towards each other then the point half way between the two pints should be stationary. Though the speed that the two points travel towards each other should be twice the speed of light thus light speed is not the maximum speed. I hope that makes sense because Im pretty much talking out of my arse.

Sapper

Ouch.

Newton's third law defines to reaction pair forces. Light is not a force as for force = mass x acceleration. Light as we understand it has no mass (which I disagree with).

In this question, to my understanding .. the point can only propogate as fast as the movement of both "blades".

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MuadDib

If the tips of the scissors were travelling at the speed light the base of the blades (right before where the blades meet) would be traveling at some lesser speed that could be determined by knowing the length of the scissor blades.

Speed is distance over time (ie: kilometers per hour, miles per hour, etc.). The base is traveling a vastly shorter distance in the same amount of time so it must be going slower.

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Redlemon

MuadDib, let me try again, because you (and others) aren't getting the point of the original question. Grab a pair of scissors. Open them partway. Put a pencil in between the blades as close to the handles as you can. Now open the scissors further, and see that you can put the pencil in further. That is the "point" Jaseca is talking about.

Now take out the pencil and close the scissors slowly. Notice that the "moving point" moves faster than the tips of the blades. Extrapolate to light speed. Explain.

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Lunchbox7

Light does have mass. Otherwise it would not be able to travel hence the speed of light. Light is consists of vibrating/depolorising (i cant remember which) particals. The particals are still constitued as mass. Just not very big mass. A by product of light is heat. This heat can be considered some of the opposing force of light hence why lasers are so hot. Just a thought.

hundred-peons

This coincides with the theory of relativity.

The scissors closing speed is relative to the pivot point, not to the fact that the scissors are going the speed of light.

The tips of the scissors travel a farther distance in the same amount of time, thus they are moving faster.

This would be hard to calculate as scissors traditionally overlap, and are not an exact "V"

I'd have to search through a calc book to find a fitting equation to calculate the actual speed of the blades closer to the pivot.

Fremen

Why not use a movie clackboard then?

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numist_net

I think that the point would travel at c, and if it exceeded that speed, the universe would end and begin all over again, because we screwed it up.

But really, it seems that as the angle approaches 0, the speed of the point appears infinitismely large (at least on my scissors) so if both blades were EACH moving at c in oppoosite directions, (lets say on opposite ends of the x-axis in a cartesian plane), then the point should be moving at a theoretically impossible number in the positive direction in the x-axis.

So, the answer would exceed c^c, which is supposed to be c, by relativity.

Basically, I couldnt tell you without testing it empirically myself, and I dont have anything that could create enough force to move the blades anywhere near that fast.

CSflim

You are assuming the scissor baldes are rigid.
Rigid bodies don't exist.
Therefore, your pencil will travel only at the speed of light, as that is the maximum speed with which the "signal" of you closing the scisoors can propagate along the baldes.
If you want to visualise it, you must think of the blades as being made of a "rubber" substance.

Think of this situation.
You have a rigid meter stick, and you hold it a milimeter away from a button. you can push the stick into the button in one milisecond.
In other words, you can send a signal from point A to B, a meter apart in one milisecond.
Now what happens when you increase the length of the stick?
Surely you could send a signal from A to B in one milsecond, for an arbitrarily large distance?
Well no...because the signal still has to propagate down the length of the stick.
So if you start to push on the stick at end A, end B will only start to move correspondingly at a time no less than (A-B)/c...i.e. the signal won't travel faster than c.
To visualise it, think of picking up a piece of string and flicking it, so that you create a wave in it, and observe the wave traveling down the piece of string.
The same thing happens with the stick...which you cannot visualise as a uniform rigid body...but instead as a mass of atoms, all bound by elctomagnetic attractions (which cannot exceed c).

Here's a way, where you can actually make "something" move faster tha the speed of light.
Take a very powerful laser pointer.
Point it on the surface of the moon.
Bring the dot over the the far left side of the moons surface.
Now really quickly point your laser pointer a few degrees right.

It will take you only a tiny fraction of a milisecond to move your laser pointer, from right to left, yet the "dot" will have to travel, in that same length of time, a very large distance.
To some one on the moon, they will see a red dot very quickly move from one point on the surafce to another...so fast in fact that it is seen as going faster than the speed of light.

I will leave it up to yourself to see how this does not break relativity.

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CSflim

light has relativistic mass...but no actual mass.
In other words if you took a photon and slowed it down, and stopped it, it would be massless.

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TIO

I would like to politely note that Lunchbox has no idea what he is talking about. First, objects moving at constant velocity, by definition, are under the influence of no (or balanced) forces. So there is no need for any 'reaction' force on the ends of the scissors. You also seem to have a small problem with the idea of action-reaction.
Heat is not a 'byproduct' of light. Light is a method of transmission of energy (as either a wave or photons, which we don't really understand very well), and hence can easily be converted into heat, which is just another form in which energy can exist. Lasers are not hot, per se. They just have the capacity to heat things. Just like microwaves don't produce heat; they just make stuff hot.

numist, you apparently misunderstand the meaning of the word infinitessimal. It means 'really, really small.'
There is nothing preventing that point from moving faster than c. As redlemon correctly states, the point has no mass and is iscapable of carrying information, so it can travel as fast as it wants.

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Xell101

I'd think that point would simply the spot where the two blade converge therefore it is nothing but a human creation since it really doesn't exist therefore has no properties whatsoever...

Lebell

To pick a nit, photons don't have mass, relativistic or otherwise.

What they do have is momentum, which comes from the fact that the momentum acutually has a small speed component in addition to the well known mass component.

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CSflim

That's pretty much what relativistic mass is...

http://www.physlink.com/Education/AskExperts/ae180.cfm

http://www.ibiblio.org/lunar/school/.../massphot.html

You can argue all you like about relativistic mass not being "real", but you won't find me disagreeing with you.
Either way, by virtue of the definition of relativistic mass, you can assign such a value to a photon.

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Lunchbox7

TIO, As I already said I am talking out of my arse but it appears so are you. How can lasers heat things without being hot? Heat can be transfered through several methods (eg transduction, convection etc). It requires some portion of that heat to touch the object being heated. Otherwise you are just creating energy out of nothing which is physically impossible. You can only turn one form of energy into another form of energy. Light is one form of energy. It gives off heat as a byproduct of this energy or mmore accuratley the transferring of another form of energy into light energy. There has to be a reaction to every action. Heat is often a by product of energy transformation but heat in itself is a form of energy.

As for the mass of light. Light is effected by gravitational forces. Gravity bends light so you can still see light if you are behind the dark side of a planet. It might not bend enough to be percieved as bright light but the human eye only needs to have one photon of light to be percieved. How can something wiht no mass be effected by gravitational forces which by definition only effects mass?

lordjeebus

Something doesn't need to be hot to heat something, because thermal energy can arise from another form of energy. For instance, if I drop something, the kinetic energy from the object is converted to heat -- but the object itself could have been at the same initial temperature as the room. A laser carries a lot of energy that can be converted into thermal energy, but temperature is not the right way to describe the nature of its energy.

TIO

So Lunchbox, you're saying that electricity is hot, stretched springs are hot, and a spinning wheel is hot?
They are all capable of generating heat.
You have it right in the second part of that paragraph; light is a form of energy, and can be converted to heat. But light is not, per se, hot.
I also don't understand what you mean by saying that heat is a byproduct of light. Are you saying light produces heat by virtue of its existance? Or that it is impossible to create one without the other?

The human eye cannot percieve one photon of light.
As for the mass of light, I suggest you read up on relativity. Light has no rest mass, but it does have momentum when it is moving (hence the idea of light-sails). You're thinking of Newtonian gravity, which involves two masses pulling against each other. But you need to consider that when something is moving at relativistic speeds, it can have momentum even if it does not have mass, and so can be affected by gravity. It's easier to understand if you think of gravity as a distortion of space-time, rather than a direct interaction between two bodies.

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Lunchbox7

Light has a certain ability for force (laser for example). A laser can manipulate its surrounding through a degree of force. Froce is speed x mass. If something can produce force (even as little as light) then it has to have a component of mass. I have no idea what relativistic speed is so I cant say anything there.

I am currently involved in research on colour vision in the periphery. On my reading I have discovered that "visual receptors can absorb and respond to as little as a single photon of light" (Kalat, 2001, p152).

Isnt what I said all along is that heat is the byproduct of the tranformation of onf one from of energy to another form of energy? It appears that you just argued my original case for me. Thanks.

Isnt somthing hot if it produces heat? whether it comes from energy transformation or not. If you run your hand on a spinning wheel Im willing to bet your hand will get hot. Thus your hand gets hot from the wheel so you could accurately describe the spinning wheel as hot.

DrB

This is an interesting note on the properties of the "point". Is the viewpoint of the "spot" similar to CSflim's transmission of information? (which cannot be faster than c)

Imagine someone standing on the far end of the blade and looking along it towards the pivot point, thus seeing the edge of the other blade closing . (woah .. watch out! )
What do we have in a 2D perspective? Is something (information) getting closer or are discreet pieces of matter (of the other scissor blade) appearing in a sequence that is faster than c?? The observed (relative) motion can definitely not be greater than c. But the matter of the other blade moves with a very small speed (compared to c) in the direction towards the observer.
As a result I would suggest, that for the described perspective the discussed point _can_ move faster than c as it is a completely theoretical (geometric) construct.

Just a thought...

CSflim

Common misunderstanding of physics.

Light is not bent by gravity.

Light always travels in straight lines. It APPEARS to be bent around massive objects...but it is not light which is bent in this case. It is 4-dimensional space-time being distorted by mass, not light being deflected. In three dimsnsions this makes it appear that light is bends.

As an analogy, let us project down from 4 dimensions to 3, as is the usual case when trying to imagine such matters.

Imagine you are on a hilly landscape, and you see a plane flying overhead. You now look at the planes shadow, passing along the hilly ground. Its path will appear to be bent. But at all times, the plane is traveling in a straight line.

Further more, if you are talking out of your arse, why bother to post? There are plenty of questions on this board, which I simply wouldn't have a clue how to answer, so I don't. Yuo have made it quite clear that you don't even understand classical newtonian physics, let alone relativity.

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TIO

Force is mass times acceleration, not speed.
I'll say again: light has no rest mass. It simply gains momentum when it has speed, and it requires force to change momentum, so light is capable of exerting a force. You're trying to apply Newtonian mechanics to relativistic situations. Above a small percentage of the speed of light, Newtonian physics breaks down pretty quickly.

I never disagreed with you that heat can be produced when converting between forms of energy.
It's your next step that bothers me. Like I said before, would you consider electricity hot? Or propane gas at 0 deg celcius? They can both produce heat when properly manipulated. Likewise, the spinning wheel probably isn't hot, at least not by virtue of what it is. If you put a thermometer on the wheel, it would register that the material of the wheel is at room temperature. When you grab the wheel, friction would convert the wheel's kinetic energy to heat, and then the wheel would become hot. It's not the same thing as the wheel being hot in the first place.

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CSflim

"Froce is speed x mass" - Oh dear god!

Newton's law do not hold for things with very large mass, or for things at very high speed. Light travels at very high speed. As such, you cannot use Newton's laws to try to explain it (even if you manage to state said laws correctly).
Light is massless, but it still has a momenteum, or impart a force. If light had a mass, it would not be able to travel at light speed!
This can only be understood by modern physics. This is why newtonian physics is reffered to as "classical" physics.


And you are now changing the definition of heat.
Running your hand along a moving wheel will cause friction, which will generate heat. But by no means what-so-ever could you describe the spinning wheel to be hot.
Tape a thermometer to the spinning wheel, and you will find out how hot it is. It will be at room temperature. Rub your hand against it, and some of the kinetic energy will be converted by friction into heat energy, and the temperature will rise. The wheel is now hot, but it wasn't before you converted its kinetic energy into heat.
Without your intervention by no means could you claim the wheel was hot.

Now it appears, that you are not only wrong, but you are being arrogant about it.

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Lunchbox7

Im being arrogant? Ive allready said twice Im talking out of my arse. That would imply that there is a good probability that Im wrong but I dont care. I like to ponder and throw a spanner in the works at times just to keep people thinking. A theory is never fully validated if it isnt contested. I find this fun hence why I have posted about stuff i dont know much about. Whilst I said a lot that I didnt know much about I also said some stuff that I know to be true. It is up to you to critique it to find out which is which. So on the aspects Im wrong then Im wrong. I really dont care. I dont see how Im being arrogant if i dont care if Im wrong or not. My first post was just ponderings and I said so (befoer mentioned arse comment). Some people decided to work their mind and say so. I felt like keeping it going to keep those minds running.

Oh yeah one more thing theories on physics are just that, theories. There are many physics theories ( hard science theories) that contradict each other. They are still adopted for absence of a better theory. Just keep that in mind when talking about the absolute truth of a physics theory. Theories have to be able to be proved or disproved. Somestuff is proved and it becomes fact. No longer a theory. Just because a physisist says something it doesnt necessarily mean its true. We are in an age where science supposidley 'proves' the nature of the universe. Whilst it definately has its strengths it does have its weaknesses too. Science is just a way to explain our universe. An interpretation. At the moment science has very good explainations. But its explainations are not absolute. Challenge what your told as some of you have done. Scientific theorys change as new information is aquired. Is there any reason to believe physics wont?

TIO

Lunchbox, it's not really good form to knowingly talk out of your arse, especially when you've been corrected repeatedly. There is also no room for a devil's advocate in matters of fact, and if you don't care if you're right or wrong, you should probably go to one of the other forums.
It should not be up to us to figure out the truth in your writing. You should comment on what you know about, and not spout off about things you don't have the slightest clue about.

We are all aware that theories are just that. However, it pays to keep up with which theories have been refuted: namely, Newtonian mechanics. As far as we know, relitivity works, and we know for certian that light is massless and that force is not equal to mass*speed.

I don't mind differing viewpoints, but I don't have time for someone who won't back down when he doesn't know what he's on about. Your first post was fine, but you were corrected, and should have read what we said and left, happy at having improved your knowledge. To maintain an uneducated viewpoint in the face of several people who know what they are talking about is not stimulating discussion. It is arrogance.

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KnifeMissile

First of all, I understand the question and I think I have a reasonable explanation for it all. But I will save that for later. For now, let me address a few things...

There are many examples of things that travel faster than light. The scissor example is but one. They're all kind of contrived, especially the quantum examples, but here's another for you guys. You have a flashlight that shoots a straight beam (it could very well be a laser but I wanted to make the example simple). Now, put it in a circular room and spin it. You will see a dot on the wall and it will be moving. How fast will it be moving? Well, that depends on the radius of the room, doesn't it? Well, make the room arbitrarily big. What happens? You can make the dot move arbitrarily fast - faster than the speed of light, even. How about that? Can anyone explain that?

Like I said, I can explain it but I will save that til I see some more conjectures...

Oh, and while I'm at it, let me address something someone said:First, I can't believe you didn't state the obvious. Force is not the product of speed and mass, that's momentum...

Secondly, light has no rest mass but, since light is never at rest, this is not a problem. However, I would tend to agree that light has no mass only because of the definition of mass: the measure of inertia. It's hard to talk about the inertia of something that moves at a constant speed regardless of your frame of reference!

CSflim

Thats exactly like the example of shining a laser on the surface of the moon.

Quote:

Oh, and while I'm at it, let me address something someone said: First, I can't believe you didn't state the obvious. Force is not the product of speed and mass, that's momentum...

I didn't say that. I was quoting it from Lunchbox.

When most people use the word "mass", they are reffering to invariant mass (rest mass) not relativistic mass.
As such, it is correct to say that light is massless.

If you wish to communicate the idea of relativistic mass, you must explicitly say it, i.e. light has a relativistic mass.

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