Well, now we may be getting somewhere.
So the whole “scissors” thing was incorrect, or at least a really bad example.
I believe you might be able to use parallax to cause an imaginary point to move FTL, but that’s not really what was posited way back at the start of the thread…
I think we’re in agreement. You could make the intersection of the scissors move at any speed you like if you could contrive to apply force to the entire length of the blade simultaneously (although this would then be the same as the unconnected bars example), but there isn’t -and can’t be - any material rigid enough to permit the closure of scissors to propagate along their length faster than the speed at which force propagates along them.
beowulff, answer my questions at #63. It’s critical.
Well, I don’t believe (for the scissor case) that the intersection can move FTL.
Back at the start of the thread I was positing that an actual object (the bead) be made to move ahead of a virtual object (the intersection) by means as demonstrated by my actual experience.
Maybe, beowulff, you could try to demonstrate how morse code, for example, could be sent down the scissor’s intersecting blades and I think you will see why information cannot be transfered that way.
I know the intersection can move FTL, and I know that the bead cannot. Most explanations go along the path as offered by Chronos:
“While I’m here, the interpretation of “mass increases as you near the speed of light” isn’t considered very useful by most physicists. It’s just a dodge to make the formula for momentum look the same as what we’re familiar with. But it makes more sense to change the formula for momentum, or better yet, to change what one means by speed. In relativity, the momentum of an object of rest mass m travelling at speed v is given by p = mgammav, where gamma depends on v and is 1 at small speeds, but approaches infinity as v approaches the speed of light. Now, you can interpret that formula as "relativistic mass is mgamma”, so p = mRv. But it’s more natural to say that there’s some sort of “relativistic speed” (actually called “proper speed”, or u), with u = gammav, and p = mu. Working with proper speed instead of the familiar sort makes a lot of things in relativity easier, since proper speed can get arbitrarily large, and it adds according to the familiar rules. Then, you just have to convert proper speed back to regular speed at the end of the problem, after you’ve done all of the calculations."
But what I was really asking was not what is the math that proves it can’t happen, but; as an actual object like the one in the OP reaches a substantial percentage of C, what physically happens to it that prevents it from being pushed all the way to C.
In other words, taking our hypothetical floating, stationary viewpoint what would the bead look like as it passed. I was making the mistake Chronos pointed out and was assuming it would have nearly infinite mass (taking up the whole universe?).
I don’t think anyone has adequately shown that for the “scissor case”, the intersection CAN move FTL.
There may be other tortured cases involving parallax or imaginary intersections, but those are different.
The intersection can move FTL in Princhester’s curved blade variant in post #58, but the point there is that once you’ve started the process, you can’t stop it happening (due to the propagation delay of any force applied at the handles), so you still can’t modulate the effect to transmit information.
I don’t agree that the answer to these questions is yes, yes, yes and yes.
The answers are
- “yes”,
- "you can detect the position of an intersection. But there isn’t just one intersection. There is a series of intersections. The position of the place where there is an intersection at any given point in time can be measured.
- “depends what you mean by move” and
- “see my answer to Q3”.
While there is an illusion of movement involved, there is no movement in the physics sense. Nothing moves.
In the loose sense you can say that the intersection moves, just as you could say that if you turn off a light in one place in a room and turn on a light in another place in the room “the light” in the room has moved. But really, nothing moved.
Well, if you are talking about something like two serrated edges passing each other, than I agree that two widely separated points may intersect faster that the speed of light could travel between those points, but that’s not the “scissors” case, now is it?
So, I think (or at least hope) that this is done…
Yes it is the scissors case. There is no difference between the scissors case and the case of two unconnected edges passing each other. The apparent point of the intersection of the blades is exactly as imaginary as the apparent point of the intersection of the two unconnected edges.
Are you still trying to say that in the scissors case the physical intersection between the blades can move away from the handle end at FTL? :smack:
I’m not sure that I understand what you are saying here. But I think the answer is: there is no relevant difference. I don’t know whether we are done. I’m not sure the matter can be explained to you any more clearly.
Really, Mangetout’s post #50 explains it as well as it can be explained.
However, if it helps, consider this: I think you would accept that the perceived point of intersection in an ordinary pair of scissors moves faster than the blades move. The point of intersection appears to travel from one end of the blades to the other while the blades move from being just slightly apart to being together, so the fomer must seem to move faster.
I think you must also accept that if the blades are thinner, the angle between the blades just before they meet becomes more and more acute. Consequently, when they do meet the perceived point of intersection will all the more suddenly go from one end to the other ie appear to move faster.
The question for you is: what do you say sets the upper bound on the perceived speed of the intersection?
Well, that’s exactly the point, now isn’t it?
The perceived intersection of the scissor blades is a physically measurable point, and as such** if it could move FTL ** then you could transmit information with it. Since that would violate one or more of the tenants of Relativity, and get you thrown in Relativistic jail, I manitain that the intersection cannot move FTL.
Sure, you seem somewhere to have gained the impression this has been conceded. On the contrary, it is clearly the case that the perceived movement of the intersection can be FTL. Read Indistinguishable’s first link.
That’s not at all what his link says:
"The point at which the blades bend propagates down the blade at some speed less than the speed of light. On the near side of this point, the scissors are closed. On the far side of this point, the scissors remain open. You have, in fact, sent a kind of wave down the scissors, carrying the information that the scissors have been closed. But this wave does not travel faster than the speed of light. It will take at least one year for the tips of the blades, at the far end of the scissors, to feel any force whatsoever, and, ultimately, to come together to completely close the scissors."
Which is what I’ve been saying all along…
You’re going round in circles. You say that the perceived point of intersection (“PPI”) can’t move FTL because that would violate SR. I pointed out to you that this is incorrect because the PPI can only seem to move FTL after transmission of a physical impulse at slower than light speed. I tried to highlight this by my question at #68, and you ducked the question by saying you didn’t believe that the PPI could move FTL. When I ask you why it can’t move FTL, you say it can’t because that would violate SR. And we’re back at the beginning.
There’s no point in quoting selectively. Read the last paragraphs. Trying to pretend that the paragraph you have quoted is the sum total of what the author of that page considers correct when his further paragraphs clearly contradict your point is not going to take this debate anywhere.
Sorry, that should be my question at #63, not 68
Well, the author says two contradictory things.
But, the only one that applies to this discussion is the first one…“If you had a giant pair of scissors in space…”
Now, I still think that the second case is suspect, but on a microscope scale, I guess it’s possible for the point of contact to move FTL and still not be able to convey information.
But, as the author points out, on a megascopic scale, the point of intersection can’t move FTL. Which is what this question devolved to.
I fear to tread here.
When the scissors are open, call the point at their intersection A. When the scissors are closed, call point at the tips of the two blades B.
Assume A to be motionless.
I think if the following were true:
FTL: It is possible, as the scissors close, for the point of intersection between the two blades to “move”* from A to B at a speed faster than light
then you could send a signal faster than light. For once we have the scissors set up, I can stand at the “A” end, and at will, close the scissors, thus sending a signal to someone standing at “B” faster than light.
Since you can not send a signal faster than light, the sentence I named “FTL” must be wrong. The question is why is it wrong?
Some people here are saying the point of intersection can “move” faster than light because its “movement” is like the “movement” of a shadow against a wall. The shadow can be made to “move” faster than light, though no signal is being moved faster than light in that process. This makes sense to me. But I don’t see how the intersection between the blades of the scissors could “move” faster than light without the possibility of thereby sending a signal faster than light–for, as I said, it seems one person could, at will, shut the scissors at A, thereby exerting a causal influence on someone at B at a time quicker than would be possible below light speed.
This makes me think that the point of intersection can’t be made to “move” (or of course simply move for that matter) faster than light, and that this impossiblity can be explained by reference to the physical properties the blades must have and the limits that can be placed on the propagation of motion through them and so on. Yet people here are saying that’s not right either.
So I’m confused. If FTL above is false, then why is it false? Is it false because of the physical properties of the scissors? Or is it false because the point of intersection turns out not to be relevantly like the movement of a shadow? Or neither or both? Or what?
-FrL-