Velocity u > c? Faster Than Light Comm?

In summary, the conversation discusses the limitations of information transmission using moving wires. It is stated that the distance between the wires cannot change faster than the speed of light and that the information is actually transmitted at the time of setting up the spacing between the wires. It is also mentioned that the speed of transmission is limited to the speed of sound in the material the wires are made of, which is significantly slower than the speed of light.
  • #1
f todd baker
61
22
On my website (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.
fasterthanlife.JPG


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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  • #2
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.
 
  • #3
All wires but one are moving with the same velocity and are parallel. The distance between any two of them does not change.
 
  • #4
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.
 
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Likes Dale
  • #5
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.
 
  • #6
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".
 
  • #7
f todd baker said:
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##.
 
  • #8
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.
 

1. What is "Velocity u > c"?

"Velocity u > c" refers to the velocity of an object being greater than the speed of light, which is known as the speed limit of the universe. This concept is often discussed in the context of special relativity and the theory of general relativity.

2. Is it possible for an object to travel faster than the speed of light?

According to our current understanding of physics, it is not possible for an object to travel faster than the speed of light. This is due to the fact that as an object approaches the speed of light, its mass increases infinitely and it would require an infinite amount of energy to accelerate it further.

3. What happens if an object does exceed the speed of light?

If an object were to exceed the speed of light, it would violate the principles of causality and the laws of physics as we know them. This would lead to paradoxes and contradictions, making it impossible for the universe to function as it does.

4. Can anything travel at the speed of light?

Yes, according to Einstein's theory of special relativity, massless particles such as photons can travel at the speed of light. However, objects with mass can never reach this speed.

5. How does the concept of "Velocity u > c" impact our understanding of the universe?

The concept of "Velocity u > c" challenges our understanding of space and time and forces us to rethink the fundamental laws of physics. It has also led to the development of theories such as wormholes and warp drive, which propose ways to circumvent the speed of light barrier.

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