Slowing down light

[rigadin]

Somehow scientists have slown down light. I didn't read the whole article but I saw the headline. how did they do that?

[swansont]

A really sharp resonance, from electromagnetically-induced transparency, because the index changes rapidly in the vicinity of resonances. We have discussed this before; you can use the search function to find the other threads.

[Riogho]

the speed of light is not constant as it moves from medium to medium. When light enters a denser medium (like from air to glass) the speed and wavelength of the light wave decrease while the frequency stays the same. How much light slows down depends on the new medium's index of refraction, n. (The speed of light in a medium with index n is c/n.) The index of refraction is determined by the electric and magnetic properties of the medium. For air, n is 1.0003, for ice, n is 1.31, and for diamond, n is 2.417. The bending of the light you mentioned upon entering a denser medium is how lenses work.

 

Although the speed of light is no longer constant when we think about different media, we do know that light always travels fastest in a vacuum. Nothing can reach speeds faster than c. Thus from our equation v=c/n, n must always be greater than 1. Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

[ydoaPs]

the speed of light is not constant as it moves from medium to medium. When light enters a denser medium (like from air to glass) the speed and wavelength of the light wave decrease while the frequency stays the same. How much light slows down depends on the new medium's index of refraction, n. (The speed of light in a medium with index n is c/n.) The index of refraction is determined by the electric and magnetic properties of the medium. For air, n is 1.0003, for ice, n is 1.31, and for diamond, n is 2.417. The bending of the light you mentioned upon entering a denser medium is how lenses work.

 

Although the speed of light is no longer constant when we think about different media, we do know that light always travels fastest in a vacuum. Nothing can reach speeds faster than c. Thus from our equation v=c/n, n must always be greater than 1. Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

 

Light does not slow down. It gets absorbed and re-emitted by the matter. This takes time. The light is still traveling at c, it just takes longer to get through the medium.

[Riogho]

But everything is a medium.

 

There is no vacuum.

[Fred56]

When light enters a denser medium (like from air to glass) the speed and wavelength of the light wave decrease while the frequency stays the same.

This means the velocity must decrease, according to E=hf (f is actually nu, it looks a bit like a v for velocity).

The idea that light (photons) 'hops' from atom to atom at c is a common misperception.

Ahem.

The misperception is possibly due to the 'naive' model of a solid as a collection of little spheres all packed together (like many Sciencey teachers still probably instruct their minions). This model is past its use-by date, because BECs tell us that matter can be all three (a gas/liquid/solid), so this casts a bit of doubt on the "balss-and-stickstogethu-bits" idea...

[swansont]

Light does not slow down. It gets absorbed and re-emitted by the matter. This takes time. The light is still traveling at c, it just takes longer to get through the medium.

 

Light slows down, i.e. the propagation time increases. Photons do not, as they always travel at c.

 

This means the velocity must decrease, according to E=hf (f is actually nu, it looks a bit like a v for velocity).

The idea that light (photons) 'hops' from atom to atom at c is a common misperception.

Ahem.

The misperception is possibly due to the 'naive' model of a solid as a collection of little spheres all packed together (like many Sciencey teachers still probably instruct their minions). This model is past its use-by date, because BECs tell us that matter can be all three (a gas/liquid/solid), so this casts a bit of doubt on the "balss-and-stickstogethu-bits" idea...

 

No, they are the classical and quantum-mechanical decriptions of what's going on. In the classical picture the wavelength (not the frequency) changes and the light slows, because ^{[math]c=\lambda\nu[/math]} . In the QM picture (quantum electrodynamics) the photons are occasionally absorbed in a virtual state of the atoms of the medium, delaying their travel.

[Fred56]

OK, I guess I stuck with the 'simplified' a bit, but effectively, even though photons still travel at c, in a dense medium (near condensed matter) isn't it 'true' that they have a further distance because space is more curved?

Also, solids are a quantised state, rather than a collection of individual bits of matter, I remember reading somewhere. It's spread out a bit unless it's affected by strong electric fields etc, I thought. BECs don't fit any of the other descriptions of matter, do they (or what)?

[Riogho]

I didn't say that the actual photon slowed down.

 

I said the speed of light did.

 

The equation for distance is a simple, D=RT

 

So therefore

 

Speed = Time / Distance

 

Therefore if it takes a few of those quanta packets of light longer to get to the other side, they have essenitally slowed down.

[Severian]

Light does not slow down. It gets absorbed and re-emitted by the matter. This takes time. The light is still traveling at c, it just takes longer to get through the medium.

 

This is true, but it is not that it is being delayed by the absorption and re-emission itself. In fact, the absorption and re-emission is instantaneous (I am not talking about exicting an electron up to a higher energy level, which is a different process).

 

You can think of it as a particle or a wave property, whichever you prefer. When thinking in terms of waves, imagine that the interaction is in fact altering the direction of the particle, and then another interaction will later on put it back on course. All the time, it travels at c, but has traveled further than you thought so appears slower.

 

However, the more correct way to think about it is via its wave property. The scattering scatters part of the wave backwards (and to the sides) and part forwards (a bit like the reflection/transmission of wavefunctions through barriers in basics QM classes). The backward flowing wave interferes with the forward flowing wave coming in (assuming it is a continuous beam of light) and makes it effectively slower. You can see this in experiments - for example, the first few photons of light will come out of the medium as if they had traveled the whole way at c, which of course they did, since they belong to the forward scattered part of the light (though the intesity will be a lot less).

[swansont]

OK, I guess I stuck with the 'simplified' a bit, but effectively, even though photons still travel at c, in a dense medium (near condensed matter) isn't it 'true' that they have a further distance because space is more curved?

 

Curvature doesn't enter into the discussion, AFAIK.

[Fred56]

from physicsforum.com FAQ:

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted.

 

This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

 

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other.

 

Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

The speed of light in 'condensed matter' is a function of 'available states' (of vibration). Something to do with phonons (sound quanta). If there are no available vibrational states, the photon is 'absorbed' and emitted, so there is accumulation of delays. This is an explanation, or a model, of what happens (and I don't know that it can't be picked apart by someone)...

[Riogho]

But graphite and diamonds are bonded differently.

 

Clarification:

 

Graphite is made of Carbon. The carbon is however only bonded three time, leaving 2 unused valence electrons.

 

Diamond is bonded 4 times. That is why it is stronger.

[Fred56]

But graphite and diamonds are bonded differently.

That's what this guy says causes the different refractive properties...

both are made up of carbon atoms. In fact' date=' if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction.[/quote']

 

Curvature doesn't enter into the discussion, AFAIK.

I think it isn't being dismissed, either, though.

There's a paper about relativistic rest-mass being able to be negative, or something. I can't put my finger on it yet, but I think that he's saying there's a problem with the material mass (condensed), and the observation that a photon can acquire a rest mass (in a superconductor, e.g.)

[Riogho]

"the index of refraction is different along different crystal directions."

 

I would think different molecular bonds of the same atom would make that different, yes.

[swansont]

I think it isn't being dismissed, either, though.

There's a paper about relativistic rest-mass being able to be negative, or something. I can't put my finger on it yet, but I think that he's saying there's a problem with the material mass (condensed), and the observation that a photon can acquire a rest mass (in a superconductor, e.g.)

 

Yes, as I recall there is a model in which photons interacting in a medium can be viewed as massive, and thus travel slower than c. Still not related to gravity and space curvature.