A thought Experiment on velocity greater than C

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Discussion Overview

The discussion revolves around a thought experiment involving a high-speed motor and a laser in a large cylinder, exploring the implications of the laser's point of light moving along the cylinder walls potentially at speeds greater than the speed of light. Participants examine concepts related to the speed of light, information transfer, and the nature of light and motion in this context.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the rotation of the laser could cause the point of light to appear to move faster than the speed of light along the cylinder walls.
  • Others argue that while the point of light may seem to move faster than light, no physical object or photon is actually exceeding the speed of light.
  • A participant questions whether this implies that information can travel faster than light, to which others clarify that the information from the laser to the wall travels at the speed of light.
  • Some participants express confusion about the implications of the thought experiment and seek clarification on the nature of the movement of the laser point and its relation to information transfer.
  • There are references to semantic arguments and the limitations of the thought experiment, suggesting that the reasoning may not hold under scrutiny.

Areas of Agreement / Disagreement

Participants generally agree that no physical object can exceed the speed of light, but there is disagreement about the implications of the laser point's movement and whether it constitutes faster-than-light travel or information transfer. The discussion remains unresolved regarding the interpretation of these concepts.

Contextual Notes

Some participants highlight that the reasoning may involve semantic issues and that the thought experiment does not necessarily reflect physical reality. There are also mentions of assumptions regarding the nature of information and the movement of light.

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Supoose at the center of A huge cylinder of radious say 1km we place a high speed motor of say angular speed w > 3E5 . And in the motor we attach a laser directed towards the walls of the cylinder. When we rotate the motor with the laser lighted, won't the laser point move along the cylinder walls at speed greater than the speed of light?

(I have attaced a drawing)
 

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thecritic said:
Supoose at the center of A huge cylinder of radious say 1km we place a high speed motor of say angular speed w > 3E5 . And in the motor we attach a laser directed towards the walls of the cylinder. When we rotate the motor with the laser lighted, won't the laser point move along the cylinder walls at speed greater than the speed of light?
Yes. "Points" are free to move at any speed. Of course each photon travels at c, and no photons (or masses) are moving "along the cylinder wall".
 
THEN this means "something" can travel faster than light as opposed to the famaous quote
"NOTHING CAN TRAVEL FASTER THAN LIGHT"
 
thecritic said:
THEN this means "something" can travel faster than light as opposed to the famaous quote
"NOTHING CAN TRAVEL FASTER THAN LIGHT"

Correct, that famaous quote is wrong. It is an oversimplification.
 
thecritic said:
THEN this means "something" can travel faster than light as opposed to the famaous quote
"NOTHING CAN TRAVEL FASTER THAN LIGHT"
That famous quote is simply not true in SR or GR. It's just a faulty paraphrase.

The speed limit of c is for particles relative to inertial reference frames only. In your example, no particle exceeds the speed of light.
 
Sorry I didn't understand the experiment quite clearly. But acc. to my knowledge optimum speed is the speed of light.
 
rozan977 said:
Sorry I didn't understand the experiment quite clearly. But acc. to my knowledge optimum speed is the speed of light.
In common language "optimum" means "best"! In any case everyone here is agreeing that no physical object (as opposed to a spot of light on a wall) can move faster than light.

If I aim a very, very bright laser at the moon, and then move it through a small angle, the spot of light it makes on the moon will appear to move across the face of the moon faster than the speed of light. That's the basic concept of the initial post. But neither the laser nor the photons making up the light has moved faster than the speed of light.
 
thecritic said:
THEN this means "something" can travel faster than light as opposed to the famaous quote
"NOTHING CAN TRAVEL FASTER THAN LIGHT"

What is the "something" that is traveling faster than c? The points at which the photons strike the cylinder? That's not really a "something".
 
Wait a minute. If we consider the spot of light as some sort of information,
Doesn't this also implies that information has traveled faster than light ?
 
  • #10
No, the information in the spot has traveled from the laser to the wall at the speed of light. No information traveled from one point on the wall to another.
 
  • #11
thecritic said:
Supoose at the center of A huge cylinder of radious say 1km we place a high speed motor of say angular speed w > 3E5 . And in the motor we attach a laser directed towards the walls of the cylinder. When we rotate the motor with the laser lighted, won't the laser point move along the cylinder walls at speed greater than the speed of light?

(I have attaced a drawing)

These types of reasoning are largely referred to a gorgian arguments. It's like saying "If you are a father, then you must be the father of everything, because you cannot not be a father." It's semantic.
 
  • #12
thecritic said:
Wait a minute. If we consider the spot of light as some sort of information,
Doesn't this also implies that information has traveled faster than light ?

Think of it this way. One have person standing where the spot of light starts and one that is standing where it moves to. There is no way for the person at the first position to affect the spot in any way that would allow him to use it to send a signal to the second person.
 
  • #13
Yeah, I have now understood this. Thanks Janus.
 
  • #14
Think of it this way. One have person standing where the spot of light starts and one that is standing where it moves to. There is no way for the person at the first position to affect the spot in any way that would allow him to use it to send a signal to the second person.

What about if there weren't two people, but 1guy moving the laser and on the other side some sort of a digital sensor-platform for example, that would recognise the movement of the laser from one point to the other as one bit of information?
 
Last edited:
  • #15
RogiNicba said:
What about if there weren't two people, but 1guy moving the laser and on the other side some sort of a digital sensor-platform for example, that would recognise the movement of the laser from one point to the other as one bit of information?
So? What about it?
 
  • #16
Anyway one that's not transferring information faster than light. No one on the cylinder wall can control the laser.
 
  • #17
Sorry, I didn't read the whole thread. I read this:

If I aim a very, very bright laser at the moon, and then move it through a small angle, the spot of light it makes on the moon will appear to move across the face of the moon faster than the speed of light. That's the basic concept of the initial post. But neither the laser nor the photons making up the light has moved faster than the speed of light

and I thought that if you'd have a laser point fixated on the moon, then even a slow movement of the laser on Earth would make the point move very fast(possible FTL) on the moon. And if something would recognise just the smallest movement of the laser in one direction as information, then it(info) would travell FTL. But rereading what HallsofIvy said, the laser only appears to be moving FTL but that doesn't acctualy happen on the face of the moon? Could somebody clear this up for me?
 
  • #18
Do one thing. Arrange two bulbs at a separation of 3e8m. Light one bulb and then turning this off, light another bulb at t=0.5 sec.
You are now reasoning that the information from one bulb has traveled faster than light to the other bulb.
 

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