Register to reply

Light and negative-speed

by AspectProteus
Tags: light, negativespeed
Share this thread:
AspectProteus
#1
Jan18-08, 03:56 PM
P: 3
Alright. I think, perhaps, I might have a handle on this. Maybe. I read this article a few months ago, but if someone could explain it a little better I'd appreciate both your effort and candor.


A negative-speed pulse of light acts much the same way. As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward. The reversed pulse not only propagates backward, but it releases a forward pulse out the far end of the fiber. In this way, the pulse that enters the front of the fiber appears out the end almost instantly, apparently traveling faster than the regular speed of light. To use the TV analogy again—it's as if you walked by the shop window, saw your image stepping toward you from the opposite edge of the TV screen, and that TV image of you created a clone at that far edge, walking in the same direction as you, several paces ahead. -from the article
The article itself- http://www.rochester.edu/news/show.php?id=2544

My question is, how accurate is this? Is it possible FTL as it purports, a circumvention of relative limitations, or merely another victim of sensationalist reporting?

Insofar as it's [the pulse] not being able to carry information, what exactly does 'information' mean in this context? The information describing this pulse as 'light' for instance, or that of a signal transmission, or...
Phys.Org News Partner Physics news on Phys.org
Engineers develop new sensor to detect tiny individual nanoparticles
Tiny particles have big potential in debate over nuclear proliferation
Ray tracing and beyond
Cthugha
#2
Jan19-08, 06:20 AM
Sci Advisor
P: 1,658
Quote Quote by AspectProteus View Post
My question is, how accurate is this? Is it possible FTL as it purports, a circumvention of relative limitations, or merely another victim of sensationalist reporting?
It is sensationalist reporting. All they do is sending light through a nonlinear medium, which changes the shape of the light pulse, so that the peak of the pulse appears to travel backwards. As the peak is not associated with the speed of light or transport of information, there is no sensation here at all.

As a more easily understandable toy model imagine a train full of people. The train starts and people are distributed evenly all over the train. Now all the people start going to the back of the train with different speeds. You will notice, that the position of the "peak" of the number of people changes. Depending on the speed of the train, this position might even move backwards. Nevertheless this has absolutely no influence on the speed of the train.
AspectProteus
#3
Jan21-08, 10:56 PM
P: 3
Sorry, been a while since I’ve gotten the chance to come back here. Wasn’t expecting to find the thread moved, but perhaps it does fit a bit better beneath the classical umbrella.

cthugha said:
All they do is sending light through a nonlinear medium, which changes the shape of the light pulse, so that the peak of the pulse appears to travel backwards. As the peak is not associated with the speed of light or transport of information, there is no sensation here at all.

As a more easily understandable toy model imagine a train full of people. The train starts and people are distributed evenly all over the train. Now all the people start going to the back of the train with different speeds. You will notice, that the position of the "peak" of the number of people changes. Depending on the speed of the train, this position might even move backwards. Nevertheless this has absolutely no influence on the speed of the train.
Yeah… At first glance I figured it was something about the erbium laced fiber. However, after some thought it registered that he was discussing something a little different, hence my asking about the possibly sensationalistic language of the original article.

To point some out [emphasis mine]:

"It's weird stuff," says Boyd. "We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end.
So, I considered that one, figuring that the reporter had left out some important information, namely that it was only the leading edge that was exiting the other end of the fiber before the 'peak' of the pulse had even entered. But that didn't make sense. Boyd, and the reporter, were clearly suggesting something else.

By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially 'reconstructs' the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber."
Reconstruct, Boyd says, in a direct quote [given, of course, that the reporter hasn’t taken any liberties] I started to wonder about what was really going on.

…sent a burst of laser light through an optical fiber that had been laced with the element erbium. As the pulse exited the laser, it was split into two. One pulse went into the erbium fiber and the second traveled along undisturbed as a reference. The peak of the pulse emerged from the other end of the fiber before the peak entered the front of the fiber, and well ahead of the peak of the reference pulse.
Ok. With nothing in the article stating otherwise, I thought perhaps, perhaps the erbium laced fiber is encased in another medium, an atmosphere, so to speak, that also inhibits the speed of the 'twin' pulse outside of the fiber. Maybe the interior of the fiber isn’t quite as stifling as that without.

It would explain the discrepancy in speed, but...

Choosing to avoid the awkwardly posed ‘funhouse mirror’ explanation near the end of the article, I’ll instead bring into relief the descriptions of the four-stage graphic that illustrates the process.

i. as the initial pulse of light approaches the glass, a new pulse forms at the far end.
ii. the new pulse splits in two, one travelling backward in the glass, the other already exiting.
iii. the backwards pulse meets and cancels out the initial pulse.
iv. only the final pulse remains
I understand that the speed of the laser through the fiber doesn’t equal c, and this is what I mean by asking if it was sensationalist reporting- but is it, as you say, merely a fluid squash-and-stretch of one pulse?


:WARNING, RECKLESS LOGIC JUMPS AHEAD:


Hypothetically speaking, and giving the reporter’s journalistic integrity a big benefit of the doubt here-

What if somehow the interior of the fiber [no matter the ‘actual’ speed of the pulse through it] is akin to a superfluid, where all ‘quanta’ throughout the ‘fluid’ are acting in unison- a ‘one for all, and all for one’ kind of thing.

Could it, or a phenomen like it, account for the reported propagation of the ‘same’ pulse at the exiting end of the fiber as well as the ripple-like reverberation of the split-pulse backward into the initial?

Could this as well, account its exiting speed relative to the speed of its twin reference travelling outside the fiber?




~*~


Register to reply

Related Discussions
Can photons be accelated or do they always travel at one speed (speed of light)? General Physics 25
Light speed...Impossible Faster than light..Probable? General Physics 5
Real time effects of light, light speed, and the sun? Astronomy & Astrophysics 4
Speed of light Schmeed of light: things can travel faster Special & General Relativity 13
Speed after body slows down with negative acceleration. Introductory Physics Homework 2