I was poking around the toobz (looking for a citation or link to something about “slow light”) and ran across this press release from last year that made me clench and then start grinding my teeth. I have no idea who vets these things, but OMFG, it’s bad. The press latches onto these ideas that are just wrong, and use cutesy buzzwords and phrases to try and connect the story to the urban-legend version of physics that the popular-press readers know, partly because that’s what gets fed to them by the popular press. It becomes that much harder to undo the damage once the bad information gets ingrained, much like when superluminal physics gets reported, only to invariably find it’s anomalous dispersion, and nothing has “broken the lightspeed barrier” or in any way violated relativity.

Here’s the press release: Light and Matter United

Let me say, at the outset, that Lena Hau, et. al, do some amazing, quantum jaw-dropping atomic physics, and I’m not making any arguments or objections about that work. What I’m critiquing is how that work is being reported.

Lene Hau has already shaken scientists’ beliefs about the nature of things. Albert Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can’t be speeded-up or slowed down. But in 1998, Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic.

It’s well-known that light traveling through a medium does so at a speed slower than c, and the light that was slowed down wasn’t in a vacuum, so WTF? It was in a specially-prepared sample of a Bose-Einstein Condensate (BEC), called Electromagnetically-Induced Transparency (EIT), which creates a narrow window (in frequency-space) where light won’t be absorbed, and near a resonance you get a change in index of refraction. A rapidly-varying index of refraction, as you get here with a sharp resonance, will slow down the group velocity of light by a large factor. As one can read in the paper (pdf),

[W]e obtain a nonlinear refractive index of 0.18 cm²W^-1. This nonlinear index is ~10⁶ times greater than that measured in cold Cs atoms

So the experiment was way cool, but not something that shakes one’s belief about relativity, and the whole bit about the speed of light in a vacuum is a head-fake.

Here’s the next one: stopped light.

Two years later, she brought light to a complete halt in a cloud of ultracold atoms.

Here’s another phrase for the masses, which sounds once again like relativity has been foiled. What has happened is the light has been absorbed — poof! Gone! Hell, I can “stop” light. The neat thing about Hau’s experiment is that the information contained in the photons hasn’t been lost — it has been stored and is available for retrieval (another pdf article)

At this point, information originally carried by the photons (pulse shape, amplitude, length, polarization, etc.) is fully mapped onto the long-lived ground or spin states of the atoms

So the photons are absorbed, not “stopped.” They don’t exist anymore. What the experimenters then do is induce another transition that releases the stored energy, as photons, with all of that information intact. Wickedly neat, but not “stopped” light.

Ok, onto the last one:

She and her team made a light pulse disappear from one cold cloud then retrieved it from another cloud nearby. In the process, light was converted into matter then back into light.

It’s that second sentence that tightens my throat. Converting matter into light certainly happens in some situations, but it’s not happening here. One needs go no further than the abstract of the paper to piece together what’s going on:

The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this ‘messenger’ atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser.

To sum up (explaining would take too long): The light pulse is slowed and compressed, and then absorbed when the coupling laser is turned off (the BEC is no longer transparent) and the atoms absorb the light. This interaction gives them a momentum kick, because the photons with which they are interacting have momentum, so these atoms are now moving at some small speed (around 6 cm/s). They travel into a second BEC a short distance away and interact with it, “imprinting” their phase information. The coupling laser is turned on, which recreates the original light pulse in all its glory.

F’amazing physics. But at no point is light converted to matter or matter to light. That would be like describing writing as converting thoughts into matter (to steal a sentiment from a colleague).

Once this kind of press release is out, other journalists pick up on it, and it propagates, as in this article. Pretty much a rehash of the PR, but now the lousy physics has made it into the title, and people, even just seeing the title, will think that this is what’s going on.