Another illustration of the basic issue:
You and I are going to be a light year apart, standing on a perfectly flat plane. We need to communicate FTL. I have a plan…
Well, I’m still not following this argument, and it’s getting late here, but let me just say this:
- The intersection of the “scissors” is NOT just an abstraction - it can be easily measured optically. So, if this point can “travel” FTL, I should be able to send information FTL.
- The point mentioned above about not being able to send information with my shadow is not true - I can easily wave my hands in front of a light source and turn on or off a photocell
Maybe I need a picture…
Ask yourself this question: what is required to get the intersection to travel FTL? What has to occur? Can it occur on its own, or need there be precursor steps?
It is at least partially my fault (since I was loose with my language) that you are confusing the intersection with the movement of the intersection. The intersection is not an abstraction. What is an abstraction is the idea that the intersection “moves”. In fact, a place on the scissors intersects, then later another place on the scissors intersects. Nothing actually moves between those places.
To tie this in with my last post, ask yoursel what has to occur before the intersection can be perceived at the “receiving” end of your faster than light system?
Suppose you are at point C and you do something which causes your shadow to eventually swing wildly from point A to point B in less time than it would take light to go from A to B. Can this eventual wild swing transport information from point A at the start of the swing to point B at the end of the swing? No, it cannot. In that sense, you can have FTL travel of a “thing” without there being any FTL information transmission.
However, what goes on above does transmit information from point C at the time of the initial action to point A at the start of the swing and it does transmit information from point C at the time of the initial action to point B at the end of the swing. So, while it’s possible to set up a situation where the shadow does swing from A to B in less time than light would move between them, the precipitating event at C doesn’t kick off the swing at A any sooner than light could move from C to A, nor does the initial event at C cause the swing to arrive at B any sooner than light could move from C to B.
That’s the sort of thing that happens with the scissors.
I got caught up in editing my post and totally forgot it was a post edit and not a new post, missing the deadline. Well, here’s what I edited it to:
Suppose you are at point X and you do something which causes your shadow to eventually swing wildly from point A to point B in less time than it would take light to go from A to B. Can this eventual wild swing transport information from point A at the start of the swing to point B at the end of the swing? No, it cannot. (How would you try to do so? You can’t attach information to a shadow and let it go along for the ride) In that sense, you can have FTL travel of a “thing” without there being any FTL information transmission.
However, what goes on above does transmit information from point X at the time of the initial action to point A at the start of the swing and it does transmit information from point X at the time of the initial action to point B at the end of the swing. So, while it’s possible to set up a situation where the shadow does swing from A to B in less time than light would move between them, the precipitating event at X doesn’t kick off the swing at A any sooner than light could move from X to A, nor does the initial event at X cause the swing to arrive at B any sooner than light could move from X to B.
That’s the sort of thing that can happen with the scissors. The point where you initially close the scissor-handle is point X. Somewhere further down the scissors will be points A and B. The average speed at which the point of intersection moves from X to A will be less than that of light. Similarly, the average speed at which the point of intersection moves from X to B will be less than that of light.
However, the average speed at which the point of intersection moves from A to B can be greater than the speed of light. That’s ok, though, because someone standing at A when the point of intersection reaches them can’t do anything to take advantage of this to piggyback a message of his for a ride along with the moving point of intersection. When the point of intersection reaches B, an observer there will gain some knowledge “Oh, someone closed the scissors at X so-and-so long ago”, but this is knowledge that came from X to B at less than the speed of light. He won’t gain any knowledge about what things were like at A when the point of intersection reached A, though. There will have been no FTL transmission of information from A to B.
Does that clear anything up?
OK, but what if the scissors are on a treadmill?
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has been taken care of.
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Look at what you’re saying. Nothing whatsoever about your scenario includes your shadow at all. All the interaction has been photons between your hand and the photocell. And that is restricted to the speed of the photons. That’s because the photons are real.
Remember that a shadow isn’t a thing. It’s merely the lack of photons. You can use the photons to send a signal. You can’t use a shadow to do so. It doesn’t exist in the same way.
I still think this is wrong.
If this diagram: http://www.gurman.co.uk/stuff/ftl.png (referenced above) is what you guys are talking about, and you insist that the intersection can travel from right to left at FTL, then it’s patently obvious that you can send information that way.
If this diagram is not correct, than maybe someone ought to come up with a better one…
(or maybe I’m just dense).
No, you can’t because the intersection isn’t a thing. It’s just a series of unconnected states, that happen in a sequence that resembles FTL movement, but isn’t actually movement at all.
Just wait a second…
Can you detect this “intersection”?
Can you measure it’s position?
Does it’s position move?
Does this movement have a velocity?
You can use a shadow to send a signal, but only from the object casting the shadow to the point where it lands, and only at the speed of dark, which happens to be the same as the speed of light (the speed at which the last photons from the light source arrive, before the light is occulted.
Then you have to take it up with every physicist in the world, because this is a completely understood and defined problem.
I’m not sure why you keep insisting that the intersection is physical and not virtual, a series of unconnected points. That seems to be the stumbling point for you. I’d keep away from the scissors example and look at some of the other analogies until you understand them and only then return to the scissors. Or maybe just look at the two unlinked bars in that illustration and note that the same progression of the intersection can be viewed, even though nothing is actually touching and therefore no physical point is actually moving.
Answer my questions in post 71, please…
Yes, yes, yes, yes, and so what? Those answers are only meaningful if you understand the context of the question. The context is that you are asking those questions about a virtual object and not a real one, and that’s the exact piece that you aren’t getting.
OK, try another example, similar to the scissors one.
You live near the Washington Monument, and you have an excellent view of it from your front garden. Being a patriotic type, there’s a flagpole out front too, and because you’re a perfectionist, your flagpole is dead plumb.
You’ve noticed that when you stand a little distance behind the flagpole , it looks the same height as the monument, and when you shift your weight onto your left foot, you can make the base of the flagpole just (appear to) touch the base of the monument, but it doesn’t (appear to)touch the top of the monument because the monument has slightly sloping sides.
When you shift your weight onto your right foot, the flagpole appears to move in front of the monument, but because of the sloping sides of the monument, the (apparent) intersection of the flagpole and monument appears to progress upward along its length.
Nothing moved except you, but you saw something (the intersection) appear to move from the bottom of the monument, to the top, in the time it took you to shift your weight slightly to one side. That’s pretty fast. How can we use the apparent movement of the intersection that only you perceived, by coincidence of your position to send information from the bottom of the distant monument, to its top?
A: We can’t.
Here’s a diagram representing four points in time during the monument/flagpole example above - the intersection of the monument and the flagpole is marked in each frame by a yellow star - and it appears to move swiftly up the monument.
But nothing real actually moved up either flagpole or monument.
Sure, but that scenario isn’t what happening with the “scissors”.
In the scissors, there is an actual point of contact that is changing position.
Now, if you say that this isn’t a good example, than I’ll go back to my original statement, which is that this point can’t move FTL.
It doesn’t make any difference. In the scissors, the movement of the point of contact is as unreal as the movement of the intersection of the flagpole and the monument.
But I see what I think might be hanging it up for you - if you’re applying force to the handle of the scissors only, to close them, then you can’t make the intersection appear to move any faster than the speed at which the force propagates along the metal of the scissors - because until that force has propagated to a given section, that section can’t move to close up.
But the point of intersection between the flagpole and the monument can appear to move FTL, but only because nothing is actually moving along the perceived path.