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Author Topic: What would slowed light look like if you overtook it?  (Read 18245 times)

Offline yor_on

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What would slowed light look like if you overtook it?
« Reply #25 on: 04/01/2009 12:35:28 »
Bikerman Isn't it group velocity that enables a phase velocity?
So when 'quenching' group velocity you will 'quench' any information remaining?
Can there exist only a phase velocity??

Or am I misunderstanding you?
Or it??

http://en.wikipedia.org/wiki/Group_velocity
« Last Edit: 04/01/2009 12:43:27 by yor_on »
 

Offline lightarrow

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What would slowed light look like if you overtook it?
« Reply #26 on: 04/01/2009 16:00:33 »
I'm lost  ???
Where? Ah, yes, if you knew it, you wouldn't be lost... :)
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #27 on: 04/01/2009 16:07:18 »
I'm lost  ???
Where? Ah, yes, if you knew it, you wouldn't be lost... :)

I got lost when BEC was mentioned
 

Offline lightarrow

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What would slowed light look like if you overtook it?
« Reply #28 on: 04/01/2009 16:19:28 »
Bikerman Isn't it group velocity that enables a phase velocity?
So when 'quenching' group velocity you will 'quench' any information remaining?
Can there exist only a phase velocity??

Or am I misunderstanding you?
Or it??

http://en.wikipedia.org/wiki/Group_velocity
"Isn't it group velocity that enables a phase velocity?"
This phrase is obscure to me. Do you mean that a single sinusoidal wave cannot exist?
Phase velocity is the speed of a single sinusoidal wave. If there are more than one single wave, then the two or more waves interfere (overlap) producing another shape. This shape travels at a generally different speed (group velocity).

If:

Vph = phase velocity

Vg = group velocity

ν = frequency of a single wave

ω = 2πν = pulsation but is usually called frequency too

λ = wavelenght of a single wave

k = wavenumber = 2π/λ

then you have:

Vph = λν = ω/k  -->  ω = kVph

Vg = dω(k)/dk

The last formula means: if many frequencies propagates in a medium which, in general, can be "dispersive" = the various frequencies has different phase velocities in it = ω depends on k in a non-linear way (see blu formula), then the derivative dω(k)/dk gives the speed at which the shape of all the packet of waves travels.

Edit. The formula Vg = dω(k)/dk is valid of course even when the medium is not dispersive. In this case Vph is a constant so Vg = dω(k)/dk = d(kVph)/dk = Vph.
Example: for light in the void, Vg = Vph = c.
« Last Edit: 05/01/2009 14:59:14 by lightarrow »
 

Offline lightarrow

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What would slowed light look like if you overtook it?
« Reply #29 on: 04/01/2009 16:22:00 »
I'm lost  ???
Where? Ah, yes, if you knew it, you wouldn't be lost... :)

I got lost when BEC was mentioned
Bose Einstein Condensate. Is a cloud of extremely cold atoms, prepared and confined specifically, through which light propagates at a cyclist's speed. It's quite similar to the fact light travels at lower than c in glass or water.
 

Offline Mr. Scientist

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What would slowed light look like if you overtook it?
« Reply #30 on: 04/01/2009 16:27:48 »
My answer now, is that you would see it.

I needed some time to think about it. Hard question.
 

Offline Bikerman

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What would slowed light look like if you overtook it?
« Reply #31 on: 04/01/2009 17:07:59 »
Bikerman Isn't it group velocity that enables a phase velocity?
So when 'quenching' group velocity you will 'quench' any information remaining?
Can there exist only a phase velocity??

Or am I misunderstanding you?
Or it??
I was about to compose a length reply but I see that it has already been done above by lightarrow....basically, what he/she said  :)
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #32 on: 04/01/2009 17:15:56 »
Bikerman Isn't it group velocity that enables a phase velocity?
So when 'quenching' group velocity you will 'quench' any information remaining?
Can there exist only a phase velocity??

Or am I misunderstanding you?
Or it??
I was about to compose a length reply but I see that it has already been done above by lightarrow....basically, what he/she said  :)

He. Alberto.
 

Offline Bikerman

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What would slowed light look like if you overtook it?
« Reply #33 on: 04/01/2009 17:28:00 »
Ahh...OK. I normally make a point of not assuming gender on the internet (more out of courtesy than anything else)  :)
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #34 on: 04/01/2009 17:58:42 »
Ahh...OK. I normally make a point of not assuming gender on the internet (more out of courtesy than anything else)  :)

Same here. Unless their pic has a beard like SpeakerToAnimals at ScienceFile   :D
 

Offline Bikerman

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What would slowed light look like if you overtook it?
« Reply #35 on: 04/01/2009 18:02:04 »
Ahh...OK. I normally make a point of not assuming gender on the internet (more out of courtesy than anything else)  :)

Same here. Unless their pic has a beard like SpeakerToAnimals at ScienceFile   :D
LOL...yes, a cunning deception she uses :-)
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #36 on: 04/01/2009 18:02:59 »
Ahh...OK. I normally make a point of not assuming gender on the internet (more out of courtesy than anything else)  :)

Same here. Unless their pic has a beard like SpeakerToAnimals at ScienceFile   :D
LOL...yes, a cunning deception she uses :-)

 [:0]

I saw once that you referred to STA as "she" but I assumed it was a slip of the fingers on your keyboard.
 

Offline Bikerman

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What would slowed light look like if you overtook it?
« Reply #37 on: 04/01/2009 18:05:48 »
Speaker is in fact a she. She describes herself as a 'fat dyke'. She is also, in my opinion, a fabulous teacher and a very clever lady (PhD in particle physics). I have learned a great deal from Speaker and she is one of my favourite internet correspondents.
« Last Edit: 04/01/2009 18:12:48 by Bikerman »
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #38 on: 04/01/2009 18:12:05 »
Gawd! I owe her an apology.

I remember your saying that she had a PhD in particle physics.
 

Offline Bikerman

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What would slowed light look like if you overtook it?
« Reply #39 on: 04/01/2009 18:14:56 »
Gawd! I owe her an apology.

I remember your saying that she had a PhD in particle physics.
It is OK - she often gets mistaken for a 'bloke' and doesn't generally hold a grudge - in fact I think she quite enjoys revealing that she is in fact a large lesbian lady (doesn't half knock the religious bunnies off their stride in debates)
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #40 on: 04/01/2009 18:16:12 »
Gawd! I owe her an apology.

I remember your saying that she had a PhD in particle physics.
It is OK - she often gets mistaken for a 'bloke' and doesn't generally hold a grudge - in fact I think she quite enjoys revealing that she is in fact a large lesbian lady (doesn't half knock the religious bunnies off their stride in debates)

I can imagine!  ;D
 

Offline yor_on

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What would slowed light look like if you overtook it?
« Reply #41 on: 05/01/2009 09:18:23 »
Lightarrow:)
thanks.

I checked it out after you wrote and you are definitely correct.
I just looked at it out of the aspect of FTL.
Thats the problem with not having a good grounding in 'basic physics' I presume.
no matter how 'hungry' one are:)


Especially the way you set up and explained your equation/formula.
It's very refreshing to see it like that.
It makes it that much easier to put into context.

---
Lost DB? :)
To me you seem to 'find yourself' rather quickly::))
« Last Edit: 05/01/2009 09:25:08 by yor_on »
 

Offline lightarrow

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What would slowed light look like if you overtook it?
« Reply #42 on: 05/01/2009 14:51:29 »
Lightarrow:)
thanks.
You're welcome! For completeness, I have added a little consideration to that post.
 

lyner

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What would slowed light look like if you overtook it?
« Reply #43 on: 05/01/2009 15:05:50 »
yor_on
Quote
But if I choose to see photons 'particle-wise' and then question the way they will act if so?
Then that also works as far as I know.
It is not forbidden, is it?
Not 'forbidden' but I should say that you can't demand an answer in those terms. I don't know of a way to explain diffraction (photons, electrons or anything) if you don't take the wave approach. Likewise, I don't see an explanation can be made without thinking about waves here, either.
Why do you want to 'insist'? Standing up in a hammock is not the best way to make love, either.

I could get on my hobby horse here and say "how can you describe something like a photon unless you specify its characteristics in more depth than just calling it a particle?" If you can do that, then you may come up with the answer yourself.
« Last Edit: 05/01/2009 15:08:15 by sophiecentaur »
 

Offline yor_on

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What would slowed light look like if you overtook it?
« Reply #44 on: 05/01/2009 23:59:32 »
SC :)

I would say that it goes back to what we call matter and light?
And my own desire to understand it, same as yours I guess:)
You say that you are happy with a wave approach, and I agree that it makes a lot of phenomena more easily understood.
Like red shift when climbing up a gravity well. or tunneling.

But I am matter, and so are you, and I see waves every day, but that's not me.
To me there is a difference, so I want to see how far I can get it together:)

It's my mind game, it keeps me from getting bored, and let me discuss with interesting people what I find the most fascinating subject ever, spacetime.

Be warned (s)he who starts to think will easily find it becoming a habit.
Ah, sort of?

And its better than discussing your new car, don't you agree:)



« Last Edit: 06/01/2009 00:01:41 by yor_on »
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #45 on: 06/01/2009 09:59:26 »
I'm lost  ???
Where? Ah, yes, if you knew it, you wouldn't be lost... :)

I got lost when BEC was mentioned
Bose Einstein Condensate. Is a cloud of extremely cold atoms, prepared and confined specifically, through which light propagates at a cyclist's speed. It's quite similar to the fact light travels at lower than c in glass or water.

I think there's more to it than that. What about the Pauli Exclusion Principle? Don't the fermions in a BEC behave like bosons? That's a point - does the particles' spin change in a BEC?

I'm at a sort-of halfway house with BEC (as with a lot of other things). I've read and understand the low level stuff but I can't understand the mathematical treatments that explain it in depth. I wish there was a site that explained things in more depth but in idiot language (if that's possible).
 

lyner

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What would slowed light look like if you overtook it?
« Reply #46 on: 06/01/2009 10:11:39 »
Quote
But I am matter, and so are you, and I see waves every day, but that's not me.

There you go again!
If the electrons inside you were not waves, as well, then why don't they fall into the nuclei of their atoms?

You have to open your mind, `Grasshopper' or you will never come to terms with this stuff. :)
 

Offline yor_on

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What would slowed light look like if you overtook it?
« Reply #47 on: 06/01/2009 10:18:17 »
Yes Sempai :)

That is in fact a very good question.
If someone would like to do a 'take on it' from a 'particle' perspective I'm all ears.
(quite handy when it comes to flying)

Otherwise I will have to think...
A lot....

--------

But I never said that waves shouldn't be counted in, have I?
If I left that impression, I didn't mean it.

What I am unhappy with is explaining matter as waves and then leave it there.
It reminds me of alchemy in where you define numerous imaginative properties to objects and then from some lofty perspective say 'as above so under'.
It may describe relations better but it does not take a grip on what is the difference between matter and light.
I can't walk through a window pane.

(well I can, but not without breaking it:)
But I am trying to see what unify light with matter.
Like momentum
And spacetime geometry/geodesics.
« Last Edit: 06/01/2009 13:16:03 by yor_on »
 

Offline lightarrow

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What would slowed light look like if you overtook it?
« Reply #48 on: 06/01/2009 14:56:50 »
Bose Einstein Condensate. Is a cloud of extremely cold atoms, prepared and confined specifically, through which light propagates at a cyclist's speed. It's quite similar to the fact light travels at lower than c in glass or water.

I think there's more to it than that. What about the Pauli Exclusion Principle? Don't the fermions in a BEC behave like bosons? That's a point - does the particles' spin change in a BEC?

I'm at a sort-of halfway house with BEC (as with a lot of other things). I've read and understand the low level stuff but I can't understand the mathematical treatments that explain it in depth. I wish there was a site that explained things in more depth but in idiot language (if that's possible).

I tell you as I've (perhaps) understood it: at "normal" densities and temperatures the wavefunctions describing the atoms don't "overlap" in a significant way and so different atoms can be considered as "independent" on each other; every atom is a single entity. At very high densities and very low temperatures (see also Heisenberg principle and tunnel effect) those wavefunction can overlap so much that single atoms become indistinguishable; at that moment a couple, or even a large number of atoms becomes an only quantum system, which total spin is the sum of its constituents spins, so an even number of atoms will have an integer spin and so will become a boson. Now many of these bosons can condense in a single quantum state, that is, a single macroscopic particle. Now you have exquisitely quantum properties in a macroscopic object, not in a single very little particle.
« Last Edit: 06/01/2009 15:00:47 by lightarrow »
 

Offline DoctorBeaver

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What would slowed light look like if you overtook it?
« Reply #49 on: 06/01/2009 15:16:26 »
Grazie, Alberto. I actually managed to understand that!
 

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What would slowed light look like if you overtook it?
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