Talk:Slow light

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request for expansion[edit]

I'd like to see this article expanded with some information about stopping light. From what I've read on various websites this is done with Bose–Einstein condensate. I've found some sources about it [1], but I hardly know anything about the subject so I'm not too eager on adding it myself.

Also some (past) records on minimum speed achieved would be nice to have added in. - Dammit 17:57, 7 May 2006 (UTC)

[2] I have a link here Mindule 08:36, 31 December 2006 (UTC)

acording to the anu pesopls they have stopted the longest (by a factor of 500 over the nearest competitor) using a laser to induce an opace material to become clear then after the light is in it to turn of the laser.. theoretically stoping it indefinatly ill get to referenceing it and stuff laterShinigami Josh 12:55, 28 May 2007 (UTC)

What Happens After the Slowdown[edit]

When light travels through a medium, its velocity decreases. But what about once it exits that medium...does its velocity "pick back up again?" Does its speed go back toward c?

Light always travels at the fastest it can, for each medium it is always at its fastest, so if you have a pane of slow glass in vacuum, the moment light leaves the pane it's already traveling at the full speed of light. --TiagoTiago (talk) 22:51, 14 November 2011 (UTC)

cut Unknown Worlds reference.[edit]

With some dithering, I deleted this paragraph: "Slow glass also appears in Unknown Worlds Of Science Fiction comics (1975). In these stories, the glass originates in the future, and its ability to show "...scenes from various locations in time, space and other dimensions..." is used as a framing device for the comic's stories. It's use was directly inspired by Bob Shaw's story.[3]". The reason I deleted it is that, if the piece of glass shows other locations in "space and other dimensions," it clearly isn't slow glass, but something quite different. (also, I note that the link is non-existent). Geoffrey.landis (talk) 15:40, 20 November 2007 (UTC)


Am I understanding this right? It seems to me that if one could stop light in midair then it would be possible to create free floating holograms? someone tell me if i'm wrong (talk) 20:39, 9 January 2008 (UTC)

You have to be able to see the hologram, so you can't just stop light. so then you start up the light again, but then its gone in a nanosecond. However if you can record where the light wave patterns are permanently you can then see them continuously. Graeme Bartlett (talk) 01:06, 10 January 2008 (UTC)
You'd think that if you had a "crystal ball" of "slow glass", that you could take this ball and carry it around and look at the light coming out from a day ago, having made the long passage through the glass. But such glass would have a ludicrously high refractive index, so it would be the sort of crystal ball that is not so easy to see an image in.
Still, it makes me wonder... how much light energy can you store in such a material? Can you surpass the limits of storage batteries and fuels? Wnt (talk) 02:38, 9 August 2010 (UTC)


I'm no quantum physicist, but it seems the explanation on this page of how light is slowed down physically is called incorrect on Speed of light

This page says: "In the case of absorption and re-emission, there is a finite amount of time for a material to absorb and re-emit a photon and this lag time will cause an effective "slowing" of the observed photon speed. Between absorptions and re-emissions, however, the photon is traveling at c."

while Speed of light#Interaction with transparent materials says: "It is sometimes claimed that light is slowed on its passage through a block of media by being absorbed and re-emitted by the atoms, only traveling at full speed through the vacuum between atoms. This explanation is incorrect and runs into problems if you try to use it to explain the details of refraction beyond the simple slowing of the signal."

Which is right? -- (talk) 03:22, 5 February 2008 (UTC)

You are right.
The features of wave transmission in a medium are refraction, phase velocity, group velocity, dispersion. These are the elements that are relevant to slow light.
These velocity effects have nothing to do with absorption and re-emission. Here transmission stops and restarts! The result is thermalization and black body radiation, or in other cases, fluorescence or phosphorescence. These processes transform the frequency and direction radically, effectively randomizing them or entirely rechanneling them. All the information gets scrambled and lost. There can also be delays.
Scattering lies somewhere in between, in that the photons interact more mildly with the atoms than in absorption or emission, not causing electron jumps to speak of, but nevertheless veering around and bouncing off of charges with more variety of behavior than the refraction and dispersion that characterize transmission through a crystal. Under scattering, direction is lost or transformed, polarization is transformed or manipulated, but frequency is essentially preserved.
Refraction and dispersion are in effect the simplest and most uniform kind of scattering, with easier formulas reflecting more predictable behavior, and less transformation of the information.
For the slowing of light, it's like this. In free space, the frequency as a function of wavenumber is given by ω=ck. In matter, the frequency gets affected by interaction between the E-B wave and a lattice of microscopic dipoles and becomes a new function ω = ω(k). This is the "dispersion relation" peculiar to the material it is passing through.
The phase velocity v_p is then given by ω(k)/k and is normally below c in matter such as glass, water, etc. The index of refraction is n = c/v_p > 1. So light (meaning a point of constant phase) travels "slower" than c in normal materials.
As we're told in the article, the radical slowing in these experiments concerns the group speed, not the phase speed. The group speed is normally also the message speed. (This holds as long as the group speed is below the phase speed and also below c, but the true criterion must be subtler than this.) The group speed is given by dω(k)/dk, so it's trickier to manipulate. But since it is a derivative, it is unstable and ultimately can be manipulated more radically than ω(k)/k. Evidently you can make it super small (like 17 m/sec or 9.7 km/sec!) for very narrow bands of ω, which you can then hit with two very close frequency waves and watch the beats roll forward so slowly you can almost hold them in your hand.
Detail would be welcome! I've even provided some that aren't in the article, but can be inferred. If I had time, I'd gladly turn it into a vanity article for the authors of these interesting experiments.
P.S. I can't rule out that the "absorption and re-emission" you cite does correctly describe some velocity-slowing process in QED. (talk) 00:39, 25 May 2015 (UTC)

Military uses[edit]

I have removed this paragraph:

In military use, stopping or considerably slowing down light could be used to camouflage vehicles. If the light were stopped, the vehicle would be invisible and would only appear as a blind spot in the eye of an observer. If the light were slowed down, the vehicle would reflect the light slower that the light reflecting off of the vehicle's surroundings, causing observers to see only a distortion in the air.

It sounds quite unbelievable to me, but it is also unsourced, and I can not even imagine a possible source for something like this. Also if light were stopped, the vehicle would appear black. --CyHawk (talk) 00:19, 9 March 2008 (UTC)

Fast Computing And 'Slow Glass'[edit]

Quantum Doughnuts Slow And Freeze Light At Will: Fast Computing And 'Slow Glass' [4] ScienceDaily 2009/03/09 Shenme (talk) 03:38, 10 March 2009 (UTC)

Stopped or stored light[edit]

I know of an expirment that managed to completely stop light, for a better descirption of it see here as this is were I origanally wrote about it; were I was recently redireced here. I have a reference of the expirment but at this point in time, t is not on me; I will find it and come back to you. Can any replies please be on my talk page. 'The Ninjalemming'' 18:22, 21 May 2009 (UTC)

When speed of light = speed of sound[edit]

What happens when the speed of light in a medium is precisely equal to the speed of sound in the same medium? Can the light be transformed to sound in some kind of "sonic boom"? Can the sound be transformed to light in some kind of sonoluminescence? Is there an equilibration of energy between the two transmission modes? I wonder if there's something to use in solid state refrigeration. (hmmm, that last should not be a red link). See also [5] - pity I can't access it and don't know Mandelstam-Brillouin scattering or what an optical-acoustic soliton is... Wnt (talk) 13:30, 4 August 2010 (UTC)

Short on specifics[edit]

I'd like to see the materials, the experimental setup, and the math all in this article -- concretely described, with empirical numbers. How big is it, how much power does it draw, what's it made out of. All that. There'e two experiments mentioned by name and university in the intro, with a speed. Let's see them with details in the body. (talk) 23:24, 24 May 2015 (UTC)

More details would be good, but go easy on the math. Wikipedia is an encyclopedia not a textbook. Mathematical treatment of a topic is appropriate only when it aids understanding of the concept; particularly one needs to consider what readers might be interested in a topic, and write the article at an appropriate level for the likely readers.--Srleffler (talk) 17:01, 25 May 2015 (UTC)

Was Hau's team the first?[edit]

If Hau and her team of researchers was the first to accomplish making slow light, the article should state this, provided a good source is cited. If she and her team were not the first, or if there were earlier efforts that had partial success, these should be mentioned. In addition, theoretical proposals for slow light prior to experimental realization should be covered. (talk) 21:01, 9 February 2016 (UTC)

Suggestion for the heart of the article[edit]

For me, the most interesting part of this article, by far, should be where it explains how it is possible to slow light. The first paragraph of the section "Different ways to achieve slow light" reads as follows:

"There are many mechanisms which can generate slow light, all of which create narrow spectral regions with high dispersion, i.e. peaks in the dispersion relation. Schemes are generally grouped into two categories: material dispersion and waveguide dispersion. Material dispersion mechanisms such as electromagnetically induced transparency (EIT), coherent population oscillation (CPO), and various four-wave mixing (FWM) schemes produce a rapid change in refractive index as a function of optical frequency, i.e. they modify the temporal component of a propagating wave. This is done by using a nonlinear effect to modify the dipole response of a medium to a signal or "probe" field. Waveguide dispersion mechanisms such as photonic crystals, coupled resonator optical waveguides (CROW), and other micro-resonator structures[6] modify the spatial component (k-vector) of a propagating wave. Slowlight can also be achieved by exploiting the dispersion properties of planar waveguides realized with single negative metamaterials (SNM)[7][8] or double negative metamaterials (DNM).[9]"

Who among us can understand that? Not I, despite having advanced training in technical fields. There is nothing *wrong* with including this information — as a supplement to this article, as a nice luxury. What is not a luxury is that this article must explain in terms that an ordinary person can understand, the answer to the question: What is the gist of at least one method used to slow light? (Once again: Describing multiple methods is fine, but it is much less important than describing *one* of them — preferably the one that the team used originally, or one that is commonly used now.2600:1700:E1C0:F340:F89E:9660:FD17:4502 (talk) 23:48, 26 April 2018 (UTC)