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Next physics question of the day - phase velocity and FTL

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Juddog

Juddog

Diamond Member
Since phase velocity can be faster than light, why can't it be used for faster than light communication?

From the wiki page:
http://en.wikipedia.org/wiki/Phase_velocity

From an unrelated but similar page explaining more:
http://www.mathpages.com/home/kmath210/kmath210.htm

So normally it's assumed that the phase velocity can exceed the speed of light, but the information it carries cannot. I fail to understand why a signal beam couldn't be established beforehand and have the phase velocity signal layered on top of that? I would think that communication would be able to be layered on top of the existing radiation source in either direction.

The other thing that confuses me is this Cherenkov radiation:
https://en.wikipedia.org/wiki/Cherenkov_radiation

It seems that you're exceeding the speed of light within a substance causing a "sonic boom" of light. It also happens when astronauts are working in space and they get a stray particle that beams through their optic nerves:
https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena
 
The reason I ask is this: a radio station broadcasts carrier waves, and then sends the actual content over the modulated wave. Why wouldn't it be possible to do the same thing with something along the lines of cosmic radiation? The particles themselves don't have to exceed the light, but rather the pulse modulation would sit on top of the "broadcast" particles (for example riding on top of neutrino waves).
 
From your second link
As most people know from experience, the combination of two slightly unequal tones produces a "beat", resulting from the tones cycling in and out of phase with each other.
Click to expand...

I would estimate that less than 5% of the population knows that.
 
Juddog said:
Since phase velocity can be faster than light, why can't it be used for faster than light communication?

From the wiki page:
http://en.wikipedia.org/wiki/Phase_velocity

From an unrelated but similar page explaining more:
http://www.mathpages.com/home/kmath210/kmath210.htm

So normally it's assumed that the phase velocity can exceed the speed of light, but the information it carries cannot. I fail to understand why a signal beam couldn't be established beforehand and have the phase velocity signal layered on top of that? I would think that communication would be able to be layered on top of the existing radiation source in either direction.

The other thing that confuses me is this Cherenkov radiation:
https://en.wikipedia.org/wiki/Cherenkov_radiation

It seems that you're exceeding the speed of light within a substance causing a "sonic boom" of light. It also happens when astronauts are working in space and they get a stray particle that beams through their optic nerves:
https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena
Click to expand...

Just look at where the photons would be and how the signal would propagate.
 
Paul98 said:
Just look at where the photons would be and how the signal would propagate.
Click to expand...

The signal would propagate over the existing group velocity of the light / other medium (e.g. neutrino wave).
 
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Juddog said:
Impressive - how long have you been doing that for?
Click to expand...

Since I started playing guitar, 30 years maybe? It isn't hard, you just have to listen. It's really a "gimme", and a cheater method imo. I'm much more impressed with people that have perfect pitch, or even relative pitch. Danny Gatton used to tune is guitar to the 60Hz hum from fluorescent lighting. I find that impressive :^)
 
Juddog said:
Impressive - how long have you been doing that for?
Click to expand...

It's guitar tuning 101.

Get your E string right, then tune your A string down below an A, hit the harmonics on E and A (5th fret E, 7th fret A), and turn the tuning peg for A. You'll hear a fast beat that gradually slows as you sharpen the string and when the beat goes away, your E and A are in tune. Repeat all the way up.
 
Juddog said:
Since phase velocity can be faster than light, why can't it be used for faster than light communication?

From the wiki page:
http://en.wikipedia.org/wiki/Phase_velocity

From an unrelated but similar page explaining more:
http://www.mathpages.com/home/kmath210/kmath210.htm

So normally it's assumed that the phase velocity can exceed the speed of light, but the information it carries cannot. I fail to understand why a signal beam couldn't be established beforehand and have the phase velocity signal layered on top of that? I would think that communication would be able to be layered on top of the existing radiation source in either direction.

The other thing that confuses me is this Cherenkov radiation:
https://en.wikipedia.org/wiki/Cherenkov_radiation

It seems that you're exceeding the speed of light within a substance causing a "sonic boom" of light. It also happens when astronauts are working in space and they get a stray particle that beams through their optic nerves:
https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena
Click to expand...

The change in phase velocity (which is what you'd need to communicate information) can only propagate at the group velocity of your second wave.
 
Is this why no one can make good looking surface water in video-games was my conclusion.

Not really getting faster than light information travel out of it.
 
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