Faster than c communication?

Main Question or Discussion Point

On my web site (unnamed since I have been previously reprimanded for "promoting" the site!) I got a question, pretty straightforward, the essence of which is: Two long wires are not parallel and make an angle θ with each other; they cross at one point and one wire is moving with velocity v perpendicular to the first; the point where they cross moves with a velocity u.

It is easy to show that u=v/tanθ and u may be larger than c. But this point has no mass and carries no information so, no problem with special relativity (just like a sweeping laser spot moving across the moon with v>c). But then I got a followup question suggesting having an array of moving wires the spacing of which is coded information. What am I missing?

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FactChecker
Gold Member
If the distance between the wires changes, that change can not travel along the wire at infinite speed. In fact it is limited to c or less.

All wires but one are moving with the same velocity and are parallel. The distance between any two of them does not change.

FactChecker
Gold Member
Oh. I missed that. There is nothing that says that information set up earlier can not be read fast. The wires that you describe can not be set up over a distance faster than c. It would take a long time to set up the parallel wires of that length. Your example is just a more complicated case of sending information and storing it in a safe in a distant location. Then the safe can be opened and read instantly. That does not mean that the information was transmitted instantly.

Dale
Dale
Mentor
I second @FactChecker here. The information is transmitted at the time of setting up the spacing between the wires. The diagonal wire is actually completely irrelevant.

Ibix
No information is being transmitted in the u direction - it's being transmitted in the v direction. It's just that observers spaced perpendicular to that get the information with different delays. The only way to communicate in the u direction is to poke one of the wires as it passes, and then this is just a complicated variant on "can I communicate faster than light by pushing a rigid rod".

Mister T
Gold Member
But then I got a followup question suggesting having an array of moving wires the spacing of which is coded information. What am I missing?
The coding can't travel faster than ##c##.

DaveC426913
Gold Member
Though this is an idealized thought experiment: here's some practical cold water tossed on it:

The speed of transmission will be far slower than c; it is limited to the speed of sound in the material that the wires are made of.

Even if they were made of the hardest substance currently known - diamond - their best transmission speed will be only 12km/s - the speed of sound in diamond.