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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.
.gnola evoM .ereh ees ot uoy rof gnihtoN
"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.
Way to read the article, CowboyNeal.
A slashdotter who didn't build his own computer is like a Jedi who didn't build his own lightsaber.
I've done this too, it's called 'a mirror'.
TK
So, basically, what scientists have shown is that reverse light (darkness) is faster than light!
My patience is infinite, my time is not.
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.
And... Linus ==> Linus
AARRGGH!!
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Well, the article says the light comes out the other end before the putting-in end has light going in, so that it goes backwards through the fiber (from the end it came out of, towards the end it came into).
What if you are about to put the photon in, and it comes out of the fiber at the other end, but you change your mind and don't put it into the going in end?
The first story was rushed out so fast it hasn't gotten here yet.
This was posted next week.
The Kruger Dunning explains most post on
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 article was pretty confusingly written so I can't be totally sure what is going on but i think this only sounds cool because we confuse the speed the actual photons travel and the speed the wave appears to travel.
It is perfectly possible to get *effects* from light that appear to travel at faster than the speed of light. Just take a flashlight in a super huge room and whip it around really fast. The spot of light on the wall may very well 'travel' faster than light but no actual photons traveled faster than light so this isn't a problem.
While this experiment is somewhat different I believe a similar confusion makes it sound way more interesting than it really is. In particular there are two different speeds one needs to talk about when you are talking about how fast light goes. There is the speed at which a crest of the wave advances and then there is the speed that a photon travels (probably some other ones too than I'm forgetting). I believe all this experiment is doing is making it so the crest of the wave appears to travel faster (or with negative speed?) than light even though all the photons in the light are not moving faster than light.
Thus it is a big analagous to the flashlight case where you have some effect (in this case the crest of the light wave) which appears to move faster than light even though no actual photons or information is really doing so.
To give an idea of how this could happen (though not the mechanism here) imagine a bunch of rods in a row like this:
_____ (time 0)
Now suppose we put activators under these rods to raise them at prearranged times. If we did this right we could get a 'wave' moving like this:
-______ (time 1)
--_____ (time 2)
_--____ (time 3)
__--___ (time 4)
Now if we timed the activators right we could make this 'wave' travel down the line arbitrarily fast (in principle even faster than the speed of light) even though no information or particle is actually being moved that fast.
While clearly the mechanism is different in this case I believe this is all that is happening. Namely the peak of electric field moves faster than light (or negative?) even though no real thing is doing so.
If you liked this thought maybe you would find my blog nice too:
The short answer is "no." The long answer is Einstein's Theory of Relativity.
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 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.