Next physics question of the day - phase velocity and FTL

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Juddog

Diamond Member
Dec 11, 2006
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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
 

Juddog

Diamond Member
Dec 11, 2006
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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).
 

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
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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.

I would estimate that less than 5% of the population knows that.
 

Paul98

Diamond Member
Jan 31, 2010
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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

Just look at where the photons would be and how the signal would propagate.
 

Juddog

Diamond Member
Dec 11, 2006
7,851
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Just look at where the photons would be and how the signal would propagate.

The signal would propagate over the existing group velocity of the light / other medium (e.g. neutrino wave).
 
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lxskllr

No Lifer
Nov 30, 2004
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Impressive - how long have you been doing that for?

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 :^)
 

silverpig

Lifer
Jul 29, 2001
27,703
11
81
Impressive - how long have you been doing that for?

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.
 

silverpig

Lifer
Jul 29, 2001
27,703
11
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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 change in phase velocity (which is what you'd need to communicate information) can only propagate at the group velocity of your second wave.
 

OverVolt

Lifer
Aug 31, 2002
14,278
89
91
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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