Nothing faster than light!

[firecracker]

:lol:

Imagine a revolving light source somewhere on the surface of the earth. Instead of a lighthouse beam, let's make it a laser source. Let's locate a specific detector somewhere in space such that it can intercept the beam as it rotates. For simplicity, let's place it out there somewhere around1 million km so that the source doesnt have to rotate faster than, say, 1 RPM.

1. Can the beam swing faster (angular velocity) than 300,000 km/sec anywhere along its path?

2. If it does, can the beam be detected?

[snoopy]

Hi firecracker,

 

Im not sure if this is a serious question if the beam is only rotating at 1 revolution per minute how do you expect it to have an angular velocity in excess of the speed of light ?

 

Nothing can travel faster than the speed of light is the general consensus apart from some speculative ideas. There are a lot of good reasons for this

if your question is can anything travel faster than light then probably not 'relativity' gives you nonsense answers if you try and plug in veloctities above `c`. But some physicists and mathematicians have pointed out some `patterns` in theory could exceed the speed of light but its highly theoritical work. Nothing in nature has ever been detected going faster than light.

[Buffy]

This is actually an interesting question. Think about it snoopy: as the radius increases, the angular velocity does too. If your thinking were purely Newtonian, at some radius the angular velocity would reach and exceed c....

 

Hypothetically fast,

Buffy

[snoopy]

Errm not really sure im understanding you Buffy,

 

angular speed (omega) = 2*pi*f = v/r

 

where v = tangential velocity and r = radius of rotation

so if the radius increased the angular speed would decrease

to increase the speed the radius of rotation would have to be fractional and small.

 

But it is a bit of a non argument anyway as you cant use classical mechanics for things like light beams as you end up with crazy mixed up answers and you have to use quantum mechanics.

 

So could you explain what you mean or am I being dense ?

[Buffy]

We're using colloquial terms here (that's fair game on this site: we don't require people to be experts in order to discuss topics). In your equation, set v=c and solve for r...

 

A really long string,

Buffy

[snoopy]

Ahh so your saying Im being dense,

 

Thanks sigh....

 

Still not really getting why my tangential velocity is at c but ok I will stick around and play.... why not.

not much on TV anyway.

 

its still a bit like using K.E. = 1/2mv^2

and applying it to ions and because of their enormous speed then saying oh look I can power a small town out of a stream of ions (which you cant)

 

if your saying that individual particles inside the light beam could exceed the speed of light but bunch up and they even out as the speed of light maybe I could see where your coming from.

 

As it is im confused by what your saying but maybe you could explain it further how did my tangential velocity get so big and why am I using classical mechanics to describe the nature of light ?

[Buffy]

Its a problem where if one considers only classical mechanics that it would provide a mechanism for light to "exceed the speed of light."

 

The beam is revolving at 1 revolution per second. Your equation can be used to compute the tangential speed of a photon at one mile out which would be [math]2\pi [/math] miles per second. Using simple classical mechanics one could extrapolate this out to say that a photon at a radius of greater than [math]\frac {300,000} {2\pi} km[/math] has an tangential velocity greater than the speed of light (300,000 km/sec).

 

The game is to tell why with Special Relativity that this is not the case! :)

 

Paradoxically fast,

Buffy

[Pyrotex]

Good job so far guys.

I think what Firecracker was getting at is an old problem in Relativity 1 class. Don't think of the photons in the light beam. Think of the spot of light that it projects against an imaginary "wall". You know, like teasing your cat with a pocket laser. :)

 

If the "wall" is far enough away, (Buffy showed just how far) then our spot of light would appear (to us) to travel along the wall at a speed faster than c.

 

This is just high school geometry. Now the hard question is, how come that spot of light is traveling faster than c, but we say that nothing can travel faster than c????

 

One answer is that a spot of light is not a "something". Not an object with mass, not material in any way. The photons that hit our eyes as we watch the "spot" are all traveling at c, just the way they should. There is NO object, thing, that is traveling faster than c, just an apparition. Voila!! Special Relativity not required.

 

Something like an optical delusion. Firecracker? Any questions?

[Janus]

To add to what Pyrotex has already said. Not only is nothing material traveling across the wall, neither is any type of information. IOW, you can't use this method to communicate between two points on the wall between which the spot passes.

[CraigD]

Can the beam swing faster (angular velocity) than 300,000 km/sec anywhere along its path?

“Swinging” a beam of light is not like swinging a pole.

 

The photons in a beam of light, from a LASER or other source travel from their source at the speed of light. Rotating the source of that light changes only the direction of the photons being emitted, not their speed, or the speed or direction of any photons that have already been emitted.

If the "wall" is far enough away, (Buffy showed just how far) then our spot of light would appear (to us) to travel along the wall at a speed faster than c.

 

This is just high school geometry. Now the hard question is, how come that spot of light is traveling faster than c, but we say that nothing can travel faster than c????

 

One answer is that a spot of light is not a "something". Not an object with mass, not material in any way.

I agree with that answer.

 

We can actually produce an effect like this fairly easily. A sufficiently powerful laser rotated at a mere 10 rotations/minute will paint a bright mark across the Moon’s surface that appears to move at never less than 1.3 times the speed of light. 3500 RPMs would allow a laser on a 100 m tall platform to paint a dot across a point on the ground from 11000 m to 10000 m away in about .0000024 s, for an apparent speed of about 1.34 c.

 

A string of light bulbs connected to precise timers can give the appearance of a moving point of light that exceeds the speed of light.

 

The appearance of motion is not the same thing as a moving object.

[snoopy]

Yeah as I suspected when I read this thread its not really a serious question

just a game for you lovely nerds to harass poor newbies like me with

to Buffy

Im certainly not going to explain special relativity to you im just going to politely say because there is no `absolute` time and `distance` is relative

the light beam never exceeds the speed of light but I know you know this so as I say your just playing with me like a cat does with a mouse.

 

to Pyrotex

I honestly think you are having more of a laugh than Buffy was and as I said to Buffy above im not going through all the Lorentz transformations to explain why light cant travel faster than `c` no way. It would serve no real purpose as I suspect Firecracker was probably just having as much of a giggle in starting this thread as the moderators have in continuing it.

[firecracker]

Good job, Craig D.

But remember, I asked could a detector be aable to detect it. If it could, it wouldn't matter if it were massive. It would have been something, and it would answer whether anything could be faster.:)

[Buffy]

Yeah as I suspected when I read this thread its not really a serious question just a game for you lovely nerds to harass poor newbies like me with

Um, I hate to take the spotlight off of you, but every discussion that goes on in a thread that you particpate in is not an active conspiracy to bait you. Its nice to think that the world revolves around you, but it really doesn't, and sometimes, as you hopefully will find here, that's a good thing!

...but I know you know this so as I say your just playing with me like a cat does with a mouse.

We have a lot of people here who are learning, in fact it probably represents the majority. Just being told an answer is the *worst* way to learn, so a lot of what we do is practice the Socratic Method in various forms. Now as a matter of fact if you go back and look, I was trying to clarify the situation for not only you, but for everyone else who's reading this, and I was witholding the answer not from you--because you obviously understand the principles involved--but from everyone else who's following along so they can get some time to think about it themselves and figure it out on their own.

 

So really, its not all about you. You're not the only one here. You might want to look at the rest of the community and think about how their needs might be served by this approach.

 

Join in the fun: you might actually enjoy it.

 

All the world's a stage, :)

Buffy

[jungjedi]

Imagine a huge pair of scissors, with blades one light year long. The handle is only about two feet long, creating a huge lever arm, initially open by a few degrees. Then you suddenly close the scissors. This action takes about a tenth of a second. Doesn't the contact point where the two blades touch move toward the blades' tips much faster than the speed of light? After all, the scissors close in a tenth of a second, but the blades are a light year long. That seems to mean that the contact point has moved down the blades at the remarkable speed of 10 light years per second. This is more than 108 times the speed of light! But this seems to violate the most important rule of special relativity -- that no signal can travel faster than light. What's going on here?

[snoopy]

to buffy

 

I was actually only joking I dont think the world revolves around me and the rest just gets silly after that so i wont bother replying to it.

[jungjedi]

i think the world revoves around g.w.bush

[CraigD]

Imagine a huge pair of scissors, with blades one light year long.

…

What's going on here?

As the tip of the scissors blade approaches the speed of light, it’s mass approaches infinity. The power required to squeeze the handle approaches infinity. So nothing can close the scissors quickly enough to cause the tip of it to exceed c.

 

This is just a variation on the usual explanation of why any massive object can’t be accelerated to greater than or equal the speed of light.