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Light so Fast it Travels Backward
An anonymous reader writes "Slowing down light used to be considered a neat trick for physics wonks. But researchers in New York now say they've pushed light into reverse. And as if to defy common sense, the backward-moving light travels faster than light." While there's not much use to come of it yet, it will be interesting if Einstein himself is proved wrong.
From a 1985 paper http://www.npl.washington.edu/av/altvw08.html/ :
When advanced-wave light travels from point A to point B it arrives at point B earlier than the time it left point A. Shortly after World War II, when radar was still new, a pulsed radar beam was first bounced off the Moon and reflected back to Earth. Measuring the round trip time of the radar pulse (about 2.5 seconds) became a very precise way of determining the Earth-Moon distance. If the same measurement were done with advanced radar waves the reflection from the Moon's surface would arrive back at the Earth 2.5 seconds before the pulse was transmitted.
From there, it isn't much of a trick to lengthen the interval with automatic repeaters which bounce the advanced waves many times, lengthening the look-ahead time from seconds to minutes or hours or even days. A computer could be hooked up to broadcast ASCII-coded advance-wave messages to the past and to receive and decode them when received. Such messages could be used in any number of schemes for fun, profit, or military preparedness. The reader who is interested in possible applications is referred to Isaac Asimov's pseudo-science-fact articles in the Astounding SF's of the 1950's concerning "thiothimoline", a kind of soluble organic crystal with the unique property that it dissolved slightly before water was added.
Guess we are almost there now.
Just think of the applications:
Knowing any stock price swing several minutes (OK, just give me one minute!) in advance.
Ah, the possibilities...
Dedicated Linux servers (root access) $45 p.M.
It has to explained out all over again every time an article of that type gets posted: phase velocity can exceed the speed of light in a vacuum, group velocity cannot.
See original press release with animations.
http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
No information ever acutally travels faster than the speed of light.
Nice visual explanation anywho.
The short answer is "no." The long answer is Einstein's Theory of Relativity.
Well, the article says the light comes out the other end before the putting-in end has light going in
False, if you read the article nothing comes out the output end until the proceding edge of the light enters the input. The proceeding edge contains all the information about the light pulse, so causality is never violated and your thought experiment would never work.
Special relativity starts with the notion that you will measure light as going C no matter how fast you're going, or what direction you're going. (Why? Because that's what experiments showed when they tried to find an absolute frame of reference - if there were one, you could find it by looking at how light behaves). Briefly, something going faster than C means that you can find a reference frame in which cause follows effect - time travel.
The way the math shakes out, all of special relativity is based upon the notion that light in a vacuum travels along the geodesic:and that simultaneity happens along those geodesics. C, the "speed of light in a vacuum" is critical as the normalizing factor for distance and time (in doing SR and GR, velocities are best expressed as fractions of C - so half the "speed of light in a vacuum" is the unitless 0.5 - unitless because time and distance have the same unit).
Now, if light travels slower than C in any particular medium, even in a vacuum, as long as it's consistent in all reference frames that's no great shakes for special relativity per se - it just means that light isn't as special to space-time as we thought and that the M-M experiments seemed to show. If light travels faster than C, *that* is what breaks special relativity and the definition of simultaneity. In essence, it means that you can define a reference frame in which an effect will preceed its cause.
If you want to learn more about it, google on terms (along with "special relativity") like "light cone," "simultaneity," "absolute past," "absolute future," and "absolute elsewhere." For the history of special relativity, start with the link I included earier, or "Michelson" and "Morley".
"It is our blasphemy which has made us great, and will sustain us, and which the gods secretly admire in us." - Zelazny
This site has a nice illustrative applet on group velocity which helps to visualise some of the points in the parent posts: http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
You can also use it to show why you can't transfer information faster than light.
This statement, and your criticism of the experiment, is based on out of date (or simply ill-researched) information, and it worries me that it got modded up to 5.
In this case, the group velocity is indeed faster than the speed of light - the form of the wavepacket peak (the speed of which is the definition of the group velocity [1]) travels through the fibre almost instantaneously, much faster than c. This is one of the two things about this experiment is interesting, as by the old-fashioned definition you are championing, information has just been transmitted faster than the speed of light (as has been done before [2], although I believe it was generally in quantum-tunneling type situations, rather than something as normal-seeming as a optical fibre.)
The significant point to take home from that part is that the "It's the group velocity that carries information" mantra is not strictly true. In this case, the leading edge of the pulse is all that is needed to reconstruct the whole thing, and then suddenly we're faced with a battle between our definition of information transportation at the group velocity (with the wave peak) and causality. Causality obviously wins, and information transportation needs a more complex definition than is covered in introductory optics courses.
References, cos I like that sort of thing:
[1] http://www.rp-photonics.com/group_velocity.html - definition of group velocity
[2] http://www.rp-photonics.com/superluminal_transmiss ion.html - article on superluminal transmission, including a reference to situations where the group velocity is greater than c.