Artemus

The superluminal propagation claims made in the article to which the link you give refers was debunked immediately after it was published. There was a column in IEEE Spectrum (the trade magazine for members of the Institute of Electrical and Electronics Engineers) shortly afterward. The problem is they used a bogus definition of information transfer velocity.(Something which isn't "defined" anyway, it is a physical quantity.) Notice that they say the pulse shape was "essentially unchanged", but don't say what that means. The best example I can come up with to describe what they did is to imagine an automobile traveling at a given velocity, and defining the position of the auto as the position of its center of mass. The catch is, as the auto travels you are continuously dropping bits and pieces of it off, always working from the back, but always retaining a smaller, but essentially car-shaped mass. When you measure the change of position of the center of mass over a given time you calculate a velocity that is faster than the true velocity that anything is traveling. If you look at the very leading edge of the car-mass, the fastest speed you can possibly measure is the true velocity.

Similarly, in experimental results the true velocity can only be reliably meaured by looking at the time of the first response to the time of the first excitation. (The 50% levels they mention obviously occur well after the first response.) This has always been measured at or below the speed of light. This still doesn't keep the claim from being made every few years, with the media jumping on the bandwagon immediately but then forgetting to report the debunking.

wade-w

Thanks for the clarification, Artemus.

Jesus Tap-Dancin' Christ

Yeah, I just went back over that part when studying for my finals this week. Should've put that there.

Thanks for catching that and reminding me. Probably worth a few points on my final.

Shadowy Man

Is this strictly true? I see two problems with this.

One: atoms tend to absorb and re-emit discretely.

Two: how do you maintain phase information if you are constantly destroying the photon?

Am I missing something here?

Bettawrekonize

Light speed
<a href="http://www.nature.com/nsu/000720/000720-9.html" target="_blank">http://www.nature.com/nsu/000720/000720-9.html</a>

<a href="http://www.ldolphin.org/constc.shtml" target="_blank">http://www.ldolphin.org/constc.shtml</a>

<a href="http://www.orbits.00space.com/vsl.html" target="_blank">http://www.orbits.00space.com/vsl.html</a>

<a href="http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html" target="_blank">http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html</a>

Artemus

Here is a good debunking of superluminal information transfer in the optical tunneling case, which is mathematically the same as quantum tunneling.<a href="http://elec-engr.okstate.edu/utol/papers/paper66.pdf" target="_blank">Physical Review reprint</a>

{edited to fix link - sci}

[ December 17, 2002: Message edited by: scigirl ]</p>