What's wrong with this FTL hypothetical

Factual Questions
121

I mean, if you want a simpler example of something appearing to move FTL, consider this:

-A laser beam is directed at a hypothetical very large wall, one light year away - it’s a super-bright laser, so after the beam has reached the wall and spent a year returning, we can see the dot from our viewpoint aboard the spaceship.
Now, in the space of one minute, we turn the laser so that it ends up pointing at a place on the wall that is one light year distant (I told you it was a big wall) from the original spot. Nothing appears to happen, because the beam takes a year to arrive at the wall, but when it does, the spot appears to move one whole light year distance in one minute. The reason this is possible is that the spot isn’t a thing - it’s a succession of unconnected states. The spot can appear to move as fast as you like, but it can’t convey any information from one point to another on the wall, because it’s controlled by the guy in the space station.

Similarly with the scissors, it’s the act of initiating the movement that commits the information - once the movement of the blades is underway, the apparently-FTL movement of the intersection is inevitable, so you can’t use it to send any information that wasn’t consistent with the initial decision to close the blades.

122

If the intersection can travel FTL there should be no upper limit to that speed. Suppose you built even bigger scissors such that the intersection moved 100xC. Are you still saying that info couldn’t be transmitted at the leisurely speed of light?

I admit I’m confused, but :dubious: .

123

No, because the apparent movement of the intersection is just an interesting optical illusion - it doesn’t matter how fast it can appear to move, because it can only occur when the whole apparatus has already been set in motion, a process which cannot roll out very fast at all, and once rolled out, cannot be stopped very fast at all - so you can’t modulate the movement of the intersection.

124

ARRGH!

I hate to start this thing up again, but the movement of the intersection in NOT an optical illusion, and for the light-year long scissor case, that intersection CAN NOT move FTL. See the second paragraph of this (Indistinguishable’s)link: http://atschool.eduweb.co.uk/rmext04/92andwed/pf_quant.html#Q24c

125

I’ve been thinking about this and I think I have an explanation that will work. It would help to have diagrams, but let’s try anyway.

Start with an ordinary pair of scissors. The blades are each six inches long and the tips can open up to a width of six inches. They are idealized blades so we won’t worry about their thickness or any other physical property. They are on a perfect hinge so the intersection point always starts at the hinge and moves a full six inches when the tips close no matter how far the tips are apart when they start. (You can imagine a compass, the two-pronged thing you use to draw a circle, instead if that helps.)

Now we label points on each blade at one inch intervals, so that each blade looks like a six inch ruler.

Open the blades as far apart as they will go, which is six inches. Now close the blades. Each tip moves three inches to meet in the middle. The intersection moves - or appears to move - six inches. The intersection moves twice as fast as the blades.

Already you should see a problem with this. If the intersection is real, how could it be moving so much faster? But it gets worse.

Open the blades four inches apart and then close them. Now each tip moves two inches but the intersection still moves six. It is now moving three times as fast.

Open the blades two inches apart and close them. Each tip moves one inch but the intersection still moves six in the same time. So the intersection is moving six times as fast.

Continue doing this as long as you want. (Remember that these are idealized scissors.) As the tip movement goes to zero, the speed of the intersection goes to infinitely times the speed of the tips. At some point this intersection speed will pass the speed of light.

Obviously this is impossible for a real object. It is possible for a virtual object, though.

So let’s look at what is “moving” at the intersection.

Whenever the blades close you can watch first the two one inch markers meet, then the two inch markers meet, then three, four, five and six at the tips. The markers are real. You can trace out a continuous path for them at all times. But - here’s the important part - they move only laterally and slower than the tips. The one-inch marker never moves down the blade. It becomes the apparent intersection point for an instant and that’s all.

The intersection point is not a real movement. It’s a series of points, one handing the honor of being the intersection off to the next. It’s an imaginary, a virtual set of points, because it’s just an apparent matchup of real points. The intersection is no more passing information than your eye does when it moves along a ruler from the start to the six inch marker. It goes through all the points - an infinite number of them since this is the real number line we’re talking about - but the points themselves never move.

That’s how the intersection point of a scissors can appear to move faster than light. It’s the same way your eye can trace out a pathway of gazillions of light years in the sky just by turning your head from one side to the other. You were apparently at one point in the sky and now you’re apparently at another point in the sky and you got there faster than light ever could. But obviously you haven’t transmitted any information between the two points.

The same principle is at work in the example of the laser tracing out a series of individual points on the moon. Each point is there for an instant. The points don’t move; they just hand off the apparent landing point to the next point on the line. It’s just the extension of shining a light along a six-inch ruler. The light doesn’t transmit information; it just illuminates the path. If you make the path long enough, the apparent speed of moving along the path will eventually exceed the speed of light. But it’s an illusion. All you’ve done is look.

I think this gets to the heart of the issue. If I’m unclear at any point, let me know.

126

Good lord, beowulff, stop ignoring the large swaths of my link which very explicitly mention that the point of intersection can change FTL.

But I agree that it is misleading to speak of the movement of the intersection as though it were a mere optical illusion. What I would say is that the point of intersection does indeed change very quickly. But this doesn’t mean any physical object is moving at that rate, because the point of intersection doesn’t track a fixed particular physical object, but, rather, a successive series of distinct points along one.

127

OK, look:

There are TWO cases!

Case 1:
A very long scissors, where the motive force is applied at the hinge end. If the intersection of the scissor blades could move FTL, then information could be transmitted that way. As the paragraph mentioned above notes, this is impossible.

Case 2:
Let’s call this the “I-Beam” case.

You have two light-year long I-beams. You accelerate one to near light-speed, and arrange it to pass by I-Beam two at a very slight angle from parallel. The occultation that occurs as the I-beams pass each other may very well move FTL, BUT:
This is not the same as case 1! (Which is what the OP asked about).

IS THIS CLEAR?

128

Did you read my post at all?

129

I just did.
It’s wrong.

Please read the first case in Indistinguishable’s post.

Why are you bringing in “Virtual objects”?

130

I don’t believe this is true, and it may be the central point that is tripping you up. Explain how this will transmit information FTL.

131

One more time.

If you had a light-year long, mass-less, infinitely stiff pair of scissors you could arrange a photodetector at the end (or anywhere in between) and send Morse code from the handle end to the detector at any speed you want. However, if you want to stick to this universe, you can’t move the atoms in any physical pair of scissors that fast because:

  1. They are not infinitely stiff.
  2. They are not mass-less.

Clear?

132

The only way the intersection of a “scissor” (an not just two independent objects) can move FTL is if the force applied is applied simultaneously long all points. This cannot be realized.

I agree with beowulff that we are confusing two separate problems:
A scissor scenario, where the two objects are NOT independent of each other

AND

Two independent objects passing by.

133

This is not a debate. There is no other side here. The intersection can move faster than light because it is a virtual object. That is what you aren’t understanding.

Even though faster than light speeds are being talked about, this has nothing to do with relativity. It’s not even Newtonian. The Greeks could easily have understood the principles involved.

If you were to tell a group of physicists what you’ve posted here, some would laugh, some would be annoyed, some might try to explain to get you to understand. None would say, oh my god, everything we’ve ever learned about physics is wrong, thank you for showing us the light.

The only question to be answered is what is keeping you from understanding this fairly straightforward point and why you keep repeating the same wrong notions over and over without apparently even grappling with the explanations that have been provided. This is a textbook. The answer is in the back. Your job is to understand the answer, not to debate it or declare that it is wrong.

Admittedly, physics doesn’t care if you understand it or not. The burden is entirely on you.

134

The thing is that beowulff still believes the scissor intersection movement can be used to transmit information FTL. What’s tripping him up is that the movement doesn’t necessarily start or hit FTL speeds at the same time that it is triggered. What we’re saying is that you could close the scissor handles, and then, a long long time later, the intersection will move FTL. Because there’s such a long delay between deciding to move the intersection and the intersection actually moving FTL, you can’t use this to transmit information FTL.

Is that clear enough?

135

Well, it’s clear, but irrelevant.

Let’s talk about the conditions of this thought experiment.

  1. The apparatus is a very long scissors (let’s say one light year).
  2. The cutting edges are straight.
  3. The scissors are made out of - choose one:
    a) Unobtainium - infinitely strong and massless.
    b) Steel.

Then we’ll proceed.

136

Why is it irrelevant? Take those conditions, start the scissors open, and close the handle. A wave goes down the blades at the speed of sound within them (< c, naturally), and causes them to flex. This flexing causes the point of intersection to move. Eventually, the point of intersection moves from the vertex of the scissors to the tip of the blade. The amount of time it takes to do this will be more than a year (i.e., the trip as a whole has an average speed < c). It [the POI] must initially move at a speed < c; over every time segment of its journey starting with the time the handles were closed, indeed, it must have an average speed < c. However, there’s nothing preventing it from eventually obtaining a speed of movement > c at a later time; for example, it might move at less than half the speed of light for the first half of its journey down the scissors, and then move almost instantaneously from the halfway point to the very tip of the blades. What would be wrong with that?

137

Well nothing is “wrong” with that, except you haven’t explained under what circumstances the motion might have such a profile. What is this Space:1999 with the “Mysterious Unexplained Force”?

138

Does infinitely strong = infinitely rigid? Because if it did, it would have to have some mechanism inherent to unobtainium which could transmit information FTL. In which case we could transmit information FTL by simply tapping on the unobtainium rod rather than setting up a complicated scissors mechanism.

But yes, if the scissors were infinitely rigid, then you could transmit information FTL in many ways, including via the scissors method (I wonder, though, if Chernokov radiation, especially on the edges of the rod, wouldn’t mean you wouldn’t wanna be even close to the rod when it’s in operation!)

139

If there’s nothing wrong with it, beowulff, I don’t see why you argue so adamantly that such a thing is not possible. Is it your contention that there are no circumstances under which motion might have such a profile? It’s easy enough to describe or give an equation defining a flexing and curving of the scissor blades over time which satisfies all the properties I stated, in terms of its geometrical properties at various times. Is that what you want? Is that mathematical object what you don’t think can exist?

Incidentally, why do you have as a condition in your list that the scissor edges must be initially straight? Is it your contention that their straightness is essential to using the moving POI to transmit information FTL?

140

No, the straightness requirement was just to simplify things.

So, if you think you can come up with a real-world example of the intersection profile that exhibits the velocity curve you have stated above, do it.