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Old 08-13-2011, 08:35 PM #17
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Default Re: Scientists drag light by slowing it!

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Originally Posted by HIMNL9 View Post
^ well, the photons still travel in linear path INSIDE THE ROD, but the rod is rotating, so when the light come out from the other side, the output point of the rod that is corresponding to the input one have shifted ..... the path is helical in the space, but is linear in the medium.

As easy example, imagine the rod as a bunch of linear tubes all together ..... shoot a ball in one of the tubes (except, ofcourse, the central one, that is the rotation axis ), and the ball go linearly from one side to the other .....
Thats where the analogy ends, i am affraid. When using a photons in a rod, you can very well shoot the beam through the central axis of the rod. Even at dead center, the crystal lattice still rotates around the photons (observed externally) causing both the delay and the change in polarization.

The last is the key difference: if you would view the setup as a sort of gattling-gun barrel spinning, it would account only for the time difference, but not for the shift in polarization of the light (and orientation of the exiting image).


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Old 08-14-2011, 03:36 AM #18
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Default Re: Scientists drag light by slowing it!

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If bicyclists in a line traveling at 36kilometers/hour spaced 10 meters from each other, they would hit a wall at the rate of one every second.

If they were to all slow down to 18kilometers/hour when they crossed a line a 100 meters before the wall, they would be spaced 5 meters apart instead, and they would still hit the wall at a rate of one every second.
Those poor bicyclists never stood a chance
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Old 08-14-2011, 03:22 PM #19
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Default Re: Scientists drag light by slowing it!

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each photon have its own energy, that depend from the wavelenght, NOT from the speed
I think you mean frequency. Plank's equation is Energy = plank's constant times frequency: E = hƒ.

λ=c/ƒ. The speed slows through a material according to its refractive index, and its wavelength increases or decreases but its frequency remains constant



If frequency remains constant, energy must also remain constant.

Power is energy/time. Since the energy per photon remains constant, and the number of photons per second remains constant, the power must also remain constant.
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Old 08-14-2011, 05:15 PM #20
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Talking Re: Scientists drag light by slowing it!

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Thats where the analogy ends, i am affraid. When using a photons in a rod, you can very well shoot the beam through the central axis of the rod. Even at dead center, the crystal lattice still rotates around the photons (observed externally) causing both the delay and the change in polarization.

The last is the key difference: if you would view the setup as a sort of gattling-gun barrel spinning, it would account only for the time difference, but not for the shift in polarization of the light (and orientation of the exiting image).
Sorry, but the spinning have nothing to do with the slowing factor ..... only density (optical, or refraction index if you prefer), of the object have something to do with sowing factor.

The spinning is only for have the ability to see the slowing factor in an easy way




@ Cyparagon: frequency is right, i say wavelenght cause i'm used to consider it as frequency, but this is valid only in vacuum

But i just thoughd another thing, about the cyclists example ..... is right that they hit the wall still one each second, but opposite to photons, if they travel at half of the speed, they transfer to the wall the half of the energy, in the second example .....

(also, but this is just a theoretical problem, how the photons act when they hit a target ? ..... as particles, or as waves ? )

Also:

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Originally Posted by Cyparagon View Post
Power is energy/time. Since the energy per photon remains constant, and the number of photons per second remains constant, the power must also remain constant.
This is basically right, but you have to admit that is right also that, if you have 1000 photons each meter instead one, the energy density is 1000 times higher (also if the energy of the single photons remain constant)

(i like paradoxes )
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Last edited by HIMNL9; 08-14-2011 at 05:21 PM.
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Old 08-14-2011, 05:46 PM #21
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Default Re: Scientists drag light by slowing it!

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Sorry, but the spinning have nothing to do with the slowing factor ..... only density (optical, or refraction index if you prefer), of the object have something to do with sowing factor.
It obviously does: If the rod were not spinning, the time for the light to travel through the rod would be much shorter. In a stationary ruby rod the light would slow down a factor of 1.7 to the outside observer (i.e. the optical density of ruby), not a factor of 1000 or more. Also, the polarization and image projection angle would remain unchanged.
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Old 08-14-2011, 06:22 PM #22
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Default Re: Scientists drag light by slowing it!

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Originally Posted by HIMNL9 View Post
if they travel at half of the speed, they transfer to the wall the half of the energy
Actually Ekinetic = mv²/2, so 1/4 the energy, but yeah. They would have had to lose that energy when they slowed, which isn't the case with photons.

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how the photons act when they hit a target? as particles, or as waves?
Yes.



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the energy density is 1000 times higher (also if the energy of the single photons remain constant)
I guess I would have to agree in the strictest sense.

Kind of like there's 10W of circulating power inside the resonator of a 100mW ion laser, although "it can't be tapped off for anything practical" - laserFAQ
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Old 08-14-2011, 08:39 PM #23
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Default Re: Scientists drag light by slowing it!

At least it could be theoretically tapped by removing one of the mirrors, at which point all energy bouncing in the cavity would be released. I'm not sure how that would work practically in an optical system, but in RF systems you can easily obtail all the energy in, for example, an LC tank by open or short circuiting it.
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Old 08-14-2011, 09:24 PM #24
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Default Re: Scientists drag light by slowing it!

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Originally Posted by Benm View Post
It obviously does: If the rod were not spinning, the time for the light to travel through the rod would be much shorter. In a stationary ruby rod the light would slow down a factor of 1.7 to the outside observer (i.e. the optical density of ruby), not a factor of 1000 or more. Also, the polarization and image projection angle would remain unchanged.
No the spinning drags the light, it does not affect the time it take for the light to travel through the rod. HIMNL9's analogy of rotating tubes would be good here. The speed trough the rod is the same whether it's rotating or not, the rotation rod only adds a velocity perpendicular to the propagation direction. The high index reached is a different effect and not linked to the rotation.

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Actually Ekinetic = mv²/2, so 1/4 the energy, but yeah. They would have had to lose that energy when they slowed, which isn't the case with photons.



Yes.





I guess I would have to agree in the strictest sense.

Kind of like there's 10W of circulating power inside the resonator of a 100mW ion laser, although "it can't be tapped off for anything practical" - laserFAQ
Actually, light always travels at the speed of light, nature just eems to work that way, as Einstien found out.. Look up what feynmann said about what makes a refractive index. It's not light traveling slower, it's the material oscillation with the material with a phase delay so the reradiated light is at a different phase (delayed) netto changing the propagation speed, but the speed of individual photons is still the light speed.

The flux inside the ruby rod would still be the same, but because of the higher refractive index which slows the propagation speed the optical path length has increased. This way there are more photons in total in the ruby rod than normal.

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At least it could be theoretically tapped by removing one of the mirrors, at which point all energy bouncing in the cavity would be released. I'm not sure how that would work practically in an optical system, but in RF systems you can easily obtail all the energy in, for example, an LC tank by open or short circuiting it.
You can do cavity dumping with lasers, just take a fast enough pockels cell.

Last edited by Bluefan; 08-14-2011 at 09:27 PM.
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