Faster Than Light or is it

It's not.

John

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Remember: Once you go over the hill, you pick up speed

You still don't understand anything about signal reflections and termination.

-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.)

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The same old lying bullshit.

-- You can't fix stupid. You can't even put a Band-Aid? on it, because it's Teflon coated.

Ivor Catt, is that you? I wouldn't trust someone who can't get a screen dump off a digital scope.

Sorry,

I have nothing to do with it. Just found the link and wanted independent opinion. I personally do not see how propagation can be visualize in such setup. To me any wire is a delay line which propagates signals with velocities close to C but not equal or above.

Mathew Orman

You have to distinguish very carefully between a signal propogating with group velocity and wavecrests propogating at the phase velocity.

Looks like a reasonable introdution to common errors in FTL "signals" claims. It attracts cranks with monotonous regularity.

But in this case it is a bad experiment badly executed. Save your $5k.

Regards, Martin Brown

If you had a pair of scissors with blades that stretched between here and the Moon, it would be possible to close the blades together in such a way that the cut-point (shear nexus)moves faster than C.

Sometimes easier to conceptualize that than other analogies.

The scissors would have to be made of Neutronium or something. If you have a laser pointer, and project a spot at the moon, then quickly point it at Jupiter, how fast does the spot move? Does the beam curve on the way?

Cheers! Rich

No spot will be visible because the photons will be scattered across the motion path. No FTL can be created in such way. This is a classical bad example.

Mathew Orman

Yes, and the spot could move faster than light.

Another analogy is waves coming in to shore at an angle. As you make the angle more and more nearly parallel to the shore, the intersection of the wave with the shore line moves faster and faster without limit.

In principle it could be, nothing wrong with it as a thought experiment. If you insist on something physically realisable the "experiment" works with radar too, which *can* work at those distances,

Of course there is no violation of relativity here, there is no mass or information moving with these "superluminal" points.

--

John Devereux

The point where the laser spot hits a surface can be made to move at any arbitrary speed including FTL. Nothing physically moves faster than light in this experiment. This is a common misunderstanding about relativity. The relativity speed injunction is against a *signal* propagating at a speed greater than the speed of light in a vacuum.

One of the more impressive tricks is to demonstrate via Snell's law the refraction of precisely controlled monochromatic light on the extremely dispersive edge of the sodium D lines in sodium vapour. The phase velocity can be greater than c and the light beam bends accordingly. The catch is that as soon as you try to modulate it at all you suffer so much dispersion that the signal is totally scrambled.

The phase velocity of waves inside a waveguide is also higher than c, but the speed at which the signal moves along it is not. Any modulation necessarily creates sidebands and in a dispersive medium that limits the speed that signals can propagate unmolested.

v(group).v(phase) = c^2

Regards, Martin Brown

Well, I'd argue that if you're pumping out enough photos from the laser, you would see a spot at each point along the way (although it would tend to be motion-blurred).

Well, there's an important distinction to make here.

The position of the "spot" (the point of projection of the laser) can certainly be moved between two points, at rates which appear super-luminal. Simplified example: aim it at the moon just as it rises above the horizon, then whip it around and point it at a satellite orbiting at the moon's L3 point (i.e. near-equal distance from Earth, directly opposite the moon). You can probably re-aim (even by hand) in less than a second, and thus move the "spot" from the moon to the L3 satellite in that time... less than half of the time it takes light to travel between moon and L3.

That does not mean that you've created "faster than light" travel or communication. You cannot use this technique to transmit *any*

*information* from the moon to the L3 satellite "faster than light".

The reason is this: the decisions about where the spot "will be", is made at on Earth, at the instant you move the laser... more than a second before the beam hits either the moon or L3. Nothing which happens on the moon (in the instant before the spot "moves") can affect the portion of the beam which has already been directed towards L3.

So, you can use this process to *synchronize* timing and events which occur on the moon and L3, to within a very small time window... much smaller than would be required for direct communication between the two target areas. You must, however, make all of the decisions about the timing well in advance of the event, so that light has time to travel from you, to the most distant of the sites you are synchronizing. In the example above, the "lead time" required is somewhat over a second... you cannot decide (on Earth) to change the "spot position" in a way which will be effective a tenth of a second from now... it's already too late to do that.

A similar principle applies to a lot of the experiments which appear to show FTL signal transmission, by looking at transmission lines (coax cables, etc.) and showing the positions of pulses on the line. When looked at closely, these experiments turn out to be looking at "phase velocity" - the effect of constructive or destructive interference between two or more signals traveling down the transmission line (sometimes in the same direction, sometimes in opposite directions).

Although the "pulse" (or peak or ripple) may appear to be travelling faster than C, you find that you cannot use this effect to send information down the line faster than C... because you cannot "change your mind" about the individual signals which are making up the pulse, and have this change get down to the other end of the line at any speed faster than C.

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Not only that, but the "spot" would take like 22 seconds to get to Jupiter! That is regardless of how fast the dope on Earth (Grise) moved it from A to B. It doesn't get to B for quite some time, even though the "aim" of it has.

Closing two lines that are only apart by a single degree or two, however, WILL transition that nexus in the described manner.

His crack about what the scissor is made of was pretty meaningless too.

No, it cannot. It still has to get to point B.

Pointing it at Jupiter does NOT place the 'spot' there immediately.

It takes it 22 seconds to get there. Period.

Try again, flawed logic boy.

Wrong. As soon as you move the 'spot' at what you think is FTL, the place you think the spot is pointed at is no longer being struck. It takes the spot x number of seconds at C to get there. This is true even if the two spots are on either side of you. If they are near you and far from you (Moon and Jupiter), it should be easy for you to figure out that the spot pointed at Jupiter does not mean that ANY photons have reached there yet. So no matter how hard you try, you cannot get those photons to emit over here, then 'over there' and claim that you transitioned between the two at faster than C.

DOH!

Flawed logic.

22 seconds is a bit short. Jupiter is about 629e6 km from the earth at average closest approach. Light moves about 300e3 km per sec. So, it would take light about 2100 seconds to make the trip. But, you made your point.

How would *you* know? You can't even calculate (or look up) the length of time it takes for light to reach Jupiter from Earth.