Question about the speed of light

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Discussion Overview

The discussion revolves around the implications of the speed of light in relation to rotating objects, specifically a string with a weight and a laser emitter. Participants explore whether the end of a rotating string can exceed the speed of light and the behavior of light spots projected from a rotating laser. The conversation touches on theoretical limits, the nature of light propagation, and the mechanics of forces within materials.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that the end of a rotating string could theoretically exceed the speed of light, raising questions about the mechanics involved.
  • Another participant counters that angular velocity is limited by the speed of light, implying that the end of the string cannot exceed this limit.
  • There is a discussion about the forces holding the molecules of the string together and how quickly these forces can transmit information along the string.
  • A participant raises a related question about a rotating laser emitter and whether the dot it projects can move faster than light, proposing that it could under certain conditions.
  • Some participants argue that while a spot of light can appear to move faster than light, it does not violate relativity since it is not a physical object capable of transmitting information.
  • Concerns are raised about the time lag in light reaching a target, which could affect the perceived speed of the light spot on a wall.
  • One participant asserts that the water spot from a hose can theoretically move faster than light, challenging previous arguments about speed limits in fluid dynamics.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the speed of light in relation to rotating objects and the behavior of light spots. There is no consensus on whether the end of a rotating string can exceed the speed of light or if a laser dot can move faster than light without violating relativity.

Contextual Notes

Participants highlight the importance of considering time delays in the propagation of light and forces within materials, which may affect the conclusions drawn about speed limits. The discussion remains open-ended with various assumptions and conditions not fully resolved.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in physics related to motion, light propagation, and the limits of speed in theoretical scenarios.

adam84
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If the speed of light is 299792458 m / s and the speed of light is the maximum speed an object can travel.

If i have a piece of string with a weight on the end of it and i rotate the string the velocity differers along the length of the string

e.g the end of the string is traveling at a faster speed than a point of the string close to the rotation point.

Now giving the above facts is it not true that there would be a point at which the end of the rotating string would rotate faster than the speed of light given a length of string?
 
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I would say that given the radius of your string, the angular velocity it's limited because w·R < c.
 
What type of a force do you think holds the molecules/atoms of the string together? And how fast do you think this force transfers from one end to the other?

Zz.
 
I'm having a debate on one another forum about something like this too. The question was "if you rotate a laser emitter around it's axis, would the dot projected on the wall (let's say the emitter is in the center of a cylindrical room) move faster than light?".

I thought that the stream of photons would form a hyperbolic spiral when the emitter is spinning fast enough, is that the case? Some other person said that it would move faster and linked me this http://en.wikipedia.org/wiki/Angular_velocity

So then I came here because the people on this forum seem to know a tad more than I.
 
You can already get an idea what's going on here simply using water coming out of hose. If you move slow enough, the spot where the water its the ground (or wall), moves at the same speed as how fast you are turning. But try moving faster and faster. You'll see the spot lagging behind more and more. Why? Because it takes time for the water particles to travel from the tip of the hose to the spot on the ground.

The mistake often made in this type of scenario is the explicit assumption that the light from the source instantaneously gets to get to the wall. This is false.

Zz.
 
ZapperZ said:
You can already get an idea what's going on here simply using water coming out of hose. If you move slow enough, the spot where the water its the ground (or wall), moves at the same speed as how fast you are turning. But try moving faster and faster. You'll see the spot lagging behind more and more. Why? Because it takes time for the water particles to travel from the tip of the hose to the spot on the ground.

The mistake often made in this type of scenario is the explicit assumption that the light from the source instantaneously gets to get to the wall. This is false.

Zz.

So it doesn't move faster than light?
 
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Xendance said:
So it doesn't move faster than light?

Right.
 
spots of light CAN move faster than light speed

Xendance said:
I'm having a debate on one another forum about something like this too. The question was "if you rotate a laser emitter around it's axis, would the dot projected on the wall (let's say the emitter is in the center of a cylindrical room) move faster than light?".
You can certainly have a spot on the wall move faster than the speed of light. (If the wall is big enough and far enough away.) The classic example is sweeping a laser beam across the surface of the moon.

Zapper's point about there being a time lag between when you sweep the laser and when the light reaches the target is correct. But when the light finally reaches the target, it will sweep across at the speed with which you swept the beam. Say I set up a sweeping laser beam to sweep across the moon in 1/1000 of a second. The light takes whatever time it takes to get to the moon, then the spot sweeps across the diameter of the moon in 1/1000 of a second.

Xendance said:
So it doesn't move faster than light?
It certainly can, but this does not contradict relativity since a spot of light is not a physical object. Further, that moving spot can't be used to send a signal from one side of the moon (or room) to the other.
 
Even the dot can not move at the speed of light. Then the dot will be not the dot, it's discrete.
 
  • #10
For a virtual object (i.e. not physical) that would hold true (referring to Doc Al's comment).

Now giving the above facts is it not true that there would be a point at which the end of the rotating string would rotate faster than the speed of light given a length of string?

However, the OP was asking about a string with some object attached to the end being twirled around, that presumably has mass. I would think in this case that it is impossible for that object to surpass or even get remotely near the speed of light.

Is that not a correct assumption?
 
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  • #11
stewartcs said:
However, the OP was asking about a string with some object attached to the end being twirled around, that presumably has mass. I would think in this case that it is impossible for that object to surpass or even get remotely near the speed of light.

Is that not a correct assumption?
Of course: no part of the string will get moving anywhere close to the speed of light.

But Xendance asked a different question. That's what I was answering.
 
  • #12
Oh sorry, didn't read what he was quoting!
 
  • #13
Doc Al said:
You can certainly have a spot on the wall move faster than the speed of light. (If the wall is big enough and far enough away.) The classic example is sweeping a laser beam across the surface of the moon.

Zapper's point about there being a time lag between when you sweep the laser and when the light reaches the target is correct. But when the light finally reaches the target, it will sweep across at the speed with which you swept the beam. Say I set up a sweeping laser beam to sweep across the moon in 1/1000 of a second. The light takes whatever time it takes to get to the moon, then the spot sweeps across the diameter of the moon in 1/1000 of a second.


It certainly can, but this does not contradict relativity since a spot of light is not a physical object. Further, that moving spot can't be used to send a signal from one side of the moon (or room) to the other.

I had an interesting thought about this. If the spot traveling faster than the speed of light
hit a sensor causing a chain of events on one side of the moon and then did the same on the other side of the moon, which chain of events would occur first?
 
  • #14
The chain triggered by the first sensor would obviouly happen first. The two events, however, cannot interact with one another at anything greater than c. It would still be faster (essentially simultaneous) to trigger both events with a broad-beam radio.
 
  • #15
ZapperZ said:
You can already get an idea what's going on here simply using water coming out of a hose. If you move slow enough, the spot where the water its the ground (or wall), moves at the same speed as how fast you are turning. But try moving faster and faster. You'll see the spot lagging behind more and more. Why? Because it takes time for the water particles to travel from the tip of the hose to the spot on the ground.

The mistake often made in this type of scenario is the explicit assumption that the light from the source instantaneously gets to get to the wall. This is false.

Zz.

I don't think that's quite right. The water spot where it impacts the ground CAN in theory move faster than the speed of light (not just faster than the speed of water through air), even though the water in the stream itself is moving nowhere near the speed of light. And the fact there's some delay if the source is rotating - that's pretty much irrelevant, the spot can still travel extremely fast. So I'm pretty sure it's a flawed argument.

Similarly like when a plane water wave arrives on a beach at an angle. The speed of the point of impact of the crest can move a lot faster than the speed of the water waves themselves. As far as I can tell, there's no logical reason that point of impact can't travel at any speed, even beyond the speed of light. -Since there's no causal link between any two points of impact along the beach (one can't cause the other to happen, nor stop it from happening).

Another example is when you switch a lamp on in a room. The boundary of the rapidly growing bright circle of light cast on a nearby wall will definitely travel faster than the speed light itself...
 
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  • #16
YellowTaxi said:
I don't think that's quite right. The water spot where it impacts the ground CAN in theory move faster than the speed of light (not just faster than the speed of water through air), even though the water in the stream itself is moving nowhere near the speed of light. And the fact there's some delay if the source is rotating - that's pretty much irrelevant, the spot can still travel extremely fast. So I'm pretty sure it's a flawed argument.
ZZ Wasn't saying that the spot can't travel fast. It usually seems that what confuses people is that they envision a straight laser beam tracing out an image on the moon. The laser beam will be bent (in a spiral if you spin fast enough), and the spot on the moon/out in space can move faster than C.
Similarly like when a plane water wave arrives on a beach at an angle. The speed of the point of impact of the crest can move a lot faster than the speed of the water waves themselves. As far as I can tell, there's no logical reason that point of impact can't travel at any speed, even beyond the speed of light. -Since there's no causal link between any two points of impact along the beach (one can't cause the other to happen, nor stop it from happening).
Yes, that's basically the same phenomena.
 
  • #17
ZZ Wasn't saying that the spot can't travel fast. It usually seems that what confuses people is that they envision a straight laser beam tracing out an image on the moon. The laser beam will be bent (in a spiral if you spin fast enough), and the spot on the moon/out in space can move faster than C
He said the bending is proof that the spot traced at the tip can't move faster than c.
The (apparent) bending doesn't prove that at all. That's precisely why it's not a good example to use if you're trying to answer the initial question.
 
  • #18
YellowTaxi said:
He said the bending is proof that the spot traced at the tip can't move faster than c.
I addressed this issue of spots of light moving faster than light speed in post #8.
 

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