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Light so Fast it Travels Backward 415

Posted by CowboyNeal
from the there-and-back-again dept.
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.
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Light so Fast it Travels Backward

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  • by DrJimbo (594231) on Thursday May 11, 2006 @08:30PM (#15314092)
    Slashdot ==> Charlie Brown

    Stupid Science Stories ==> Lucy & the football

    • by Anonymous Coward on Thursday May 11, 2006 @08:33PM (#15314126)
      And... Linus ==> Linus

      AARRGGH!!
    • by marshall_j (643520) on Thursday May 11, 2006 @08:44PM (#15314188) Homepage
      not true. i find slashdot funny on occasions.. :P
    • "I believe the word you're looking for is, 'Good grief."."

      (It's a West Wing quote).

    • This nonsense depends on an equivocation on "velocity". It is easy to get phase velocities that are not just faster than light, but infinite. It is impossible to get group velocities that are faster than c (the speed of light in a free vacuum, a universal constant.) Information travels with the group velocity.

      For a scientist to report this as "faster than light" is simply dishonest, a means of grabbing headlines and attention in the hopes that it will bolster the next grant application.

      The world is full
      • by ZombieWomble (893157) on Friday May 12, 2006 @07:15AM (#15316314)
        It is impossible to get group velocities that are faster than c (the speed of light in a free vacuum, a universal constant.) Information travels with the group velocity.

        This statement, and your criticism of the experiment, is based on out of date (or simply ill-researched) information, and it worries me that it got modded up to 5.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 [rp-photonics.com] - definition of group velocity

        [2] http://www.rp-photonics.com/superluminal_transmiss ion.html [rp-photonics.com] - article on superluminal transmission, including a reference to situations where the group velocity is greater than c.

  • Reverse? (Score:5, Funny)

    by Anonymous Coward on Thursday May 11, 2006 @08:31PM (#15314106)
    .gnola evoM .ereh ees ot uoy rof gnihtoN
  • quote (Score:5, Insightful)

    by Lord Ender (156273) on Thursday May 11, 2006 @08:31PM (#15314109) Homepage
    "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.
    • Try reading further down the article.


      Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test.
  • /Obvious (Score:5, Funny)

    by ThomK (194273) on Thursday May 11, 2006 @08:32PM (#15314112) Homepage Journal
    I've done this too, it's called 'a mirror'.

     
  • by visgoth (613861) on Thursday May 11, 2006 @08:32PM (#15314116)
    So, basically, what scientists have shown is that reverse light (darkness) is faster than light!
    • by Anonymous Coward on Thursday May 11, 2006 @08:47PM (#15314211)
      So Vader was right. You don't KNOW the POWER of the dark side!

      I wonder, could this be used to make an unlightbulb? I've always wanted one of those. Too bright in the room? Don't walk over to pull the curtains, just switch on the unlight and voila, light just gets sucked into it and darkness speads into every corner of the room. Even better you get paid by the grid for the electricity you generate. Goodbye suncream, I've got my unlight with me, no need for trees to make shadows just hang it up and relax back in the shade. Imagine the tricks you could play on people with a 3000W unspotlight! Mwaahahaha. Who said science was boring?
      • Interestingly, the governing wave equation for light, d'Alembert's equation, has two fundamental impluse solutions, or Green's functions.

        One, the retarded Green's function, is a wave front emitted from the source and travelling outwards in time. The other, known as the advanced or acausal Green's function, is a wave front travelling inwards in time, which is absorbed by the source.

        The unlightbulb is not as fanciful as you might think.
    • Terry Pratchett already showed that. Everywhere light goes, darkness was there first.
    • [oblig. Dilbert reference]
      Well, why don't you go tell Phil, the Prince of Insufficient Light [unitedmedia.com] about it ? ;-)
  • Speed of what? (Score:3, Interesting)

    by hunterx11 (778171) <hunterx11@@@gmail...com> on Thursday May 11, 2006 @08:33PM (#15314124) Homepage Journal
    "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.

    I hate it when headlines use the semantics of "the speed of light" to sound sensational. "The speed of light" is just used to refer to the maximum speed of information propagation because light in a vacuum travels as that speed. I can change the speed of light by wearing glasses; while experiments similar to the one in TFA are much more complex and interesting, the point is that neither one is affecting the speed of information at all.

    • The colloquial "speed of light" is the speed of light in a vacuum. These experiments are causing light & thus information to travel faster than that benchmark.

      So, yes - The speed of the information is being affected.
      • Um, no. The speed of information is not being affected. Read the article.
      • Re:Speed of what? (Score:3, Informative)

        by cyclopropene (777291)

        The colloquial "speed of light" is the speed of light in a vacuum. These experiments are causing light & thus information to travel faster than that benchmark.

        So, yes - The speed of the information is being affected.

        No. In fact neither the speed of light nor the rate of trasmission of information exceed the speed of light in a vacuum. It is only the position of a relative maximum along the length of a light pulse that is accelerated or slowed. A light pulse (gaussian, say) consists of the sum of wav

    • Re:Speed of what? (Score:5, Informative)

      by tm2b (42473) on Friday May 12, 2006 @02:41AM (#15315746) Journal
      Er, no. It has to do with the idea that cause must preceed effect in all reference frames.

      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 [virginia.edu] 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:
      dx^2 + dy^2 + dz^2 - dt^2 = 0
      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".
  • by Harry Balls (799916) * on Thursday May 11, 2006 @08:33PM (#15314125)
    From a 1985 paper http://www.npl.washington.edu/av/altvw08.html/ [washington.edu] :

    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...
  • by Anonymous Coward on Thursday May 11, 2006 @08:34PM (#15314127)
    Last Post
  • Perhaps this interesting effect could be used somehow to cause light-speed spam to reverse upon itself, causing spammer inboxes to convert to pure energy, which in turn annihilates the spammers.

    Hey, a fellow can dream, can't he now?

  • by HyperTiger (898038) on Thursday May 11, 2006 @08:35PM (#15314143)
    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?
    • Photons are pulled from neighbouring universes.
    • by PhysSurfer (872187) on Thursday May 11, 2006 @09:48PM (#15314523) Homepage
      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.
  • FP! (Score:5, Funny)

    by Digital Vomit (891734) on Thursday May 11, 2006 @08:36PM (#15314148) Homepage Journal
    I can't wait to harness this technology. I'll be able to make First Posts without actually having to be the first poster. I will rule Slashdot!!!
  • Dupe (Score:5, Funny)

    by mattOzan (165392) < vispuslo@mattozanSTRAW.net minus berry> on Thursday May 11, 2006 @08:37PM (#15314154) Homepage Journal
    The first story was rushed out so fast it hasn't gotten here yet.
  • by Fruny (194844) on Thursday May 11, 2006 @08:37PM (#15314155)
    It has to explained out all over again every time an article of that type gets posted: phase velocity [wikipedia.org] can exceed the speed of light in a vacuum, group velocity [wikipedia.org] cannot.
  • Once an for all (Score:2, Informative)

    by Arthur B. (806360)
    Group velocity [wikipedia.org] is what carries information, faster than light phase velocity [wikipedia.org] is perfectly ok with Einstein's theory.
  • by mobby_6kl (668092) on Thursday May 11, 2006 @08:38PM (#15314166)
    They got Chuck Norris to roundhouse kick the regular light until it started moving backward!
  • by hsmith (818216)
    light traveling backwards, wtf
  • If they can accelerate late by making it go backwards..

    I guess we can expect less lag in the future. But as a side effect you find yourself getting fragged in quake than reciving the data of the rocket that hit you moments later.
  • Nothing new (Score:3, Informative)

    by brian0918 (638904) <brian0918@@@gmail...com> on Thursday May 11, 2006 @08:49PM (#15314223)
    This is nothing new. Materials with negative indexes of refraction have been used in experiments before. Here is the abstract of their article in Science:

    "Simultaneous Negative Phase and Group Velocity of Light in a Metamaterial"

    "We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample."
    • "We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the

  • Overflow (Score:3, Funny)

    by phasm42 (588479) on Thursday May 11, 2006 @08:51PM (#15314234)
    The Matrix must be using two's complement arithmetic and the overflow must not've been caught.
  • The main problem is that physics research is being more and more geared towards being appealing to people who don't know physics, hence all the BS taking advantage of phase/group velocity confusion, wanking about various string theories, etc. Sure, it's nice to let people know what's going on in physics, but in the end if they get the impression that most physicists are excited or even remotely interested by simple tricks like this, I don't think it bodes well.
  • by Danuvius (704536) on Thursday May 11, 2006 @08:52PM (#15314242)
    I've been curious about this for a while... so someone please explain where I am missing the obvious.

    Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?

    I'm sure I'm missing something... so please, rip apart the above over-simplified statement. I hope to learn something by observing the process. ;-)
  • by NearlyHeadless (110901) on Thursday May 11, 2006 @08:58PM (#15314268)
    See original press release [rochester.edu] with animations.
  • Dupe! (Score:5, Funny)

    by geekoid (135745) <dadinportland@@@yahoo...com> on Thursday May 11, 2006 @08:58PM (#15314269) Homepage Journal
    This was posted next week.
  • by tktk (540564)
    I've seent this before.

    Wasn't it called the Ferris Bueller Ferrari Effect?

  • by Rakshasa Taisab (244699) on Thursday May 11, 2006 @09:04PM (#15314303) Homepage
    This does not disprove Einstein's theory, it only exposes a flaw in the implementation. For some reason the idiot who implemented it didn't use a large enough data-type to store c, causing it to overflow in certain situations.
  • by overacid (604542) on Thursday May 11, 2006 @09:04PM (#15314310)
    http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html [netspace.net.au]
    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.
  • by Allnighterking (74212) on Thursday May 11, 2006 @09:13PM (#15314356) Homepage
    You see I can picture a car going forward and back. It has a front and a back. This plus the convention of designation means that one is forwared and one is Reverse. However with light this seems a bit odd. I mean if you had a perfect mirror and held it at exactly 90 degrees to the beam of light would it be going backwards or forwards to where it came from. I suppose if I think of it as obsorbtion, in that the origination source takes back the energy it pushed out it could be considered backwards. But then wouldn't this make a black hole a reverse Sun? In short. This is most likely why I'm in applied Physics (EE) not Theoretical Physics.
  • To understand how light's speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal, but as you pass the curved portion in the center, your reflection stretches, with the far edge seeming to leap ahead of you (the reference walker) for a moment. In the same way, a pulse of light fired through special materials moves at normal speed until it hits the substance, where it is stretched out to reach and exit the material's other side [See "f
  • If you put a laser on an rotating mount on an artifical satillite halfway between the earth and the moon, and pointed it such that the dot it generates appears on the earthward side of the moon at one point of it's cycle, and on the surface of the earth at the opposing point of the cycle, and the cycle took less than 2.5 seconds, the dot would be 'moving' faster than the speed of light.

    That's why you can play with a cat and a hand-held laser pointer and the dot can move faster then the cat, but smack a cat
  • Wierd idea (Score:2, Interesting)

    by randomErr (172078)
    We know that we exist in multi-dimensional universe. Not like monster from a parallel dimension, but rather dimensions such as width, length, height, and time. Is it possible that they accidentally skewed the photon of light slightly off the four dimensions we can perceive and went back on the time axis?
  • Misconceptions! (Score:3, Insightful)

    by Wilson_6500 (896824) on Thursday May 11, 2006 @09:26PM (#15314412)
    Folks, let's PLEASE keep in mind that you don't actually change the speed of light. What you're changing is the _apparent_ speed of light. Light appears to slow in a medium because stuff is absorbing and re-radiating it, holding it for a short while and changing its apparent speed. You never actually make photons move any slower.
  • by Falcon040 (915278) on Thursday May 11, 2006 @10:03PM (#15314592)
    " But researchers in New York now say they've pushed light into reverse. "

    Ah, when I were a lad, back in the days before this backwards superluminal light was deeply researched it was known more commonly as reflection.
  • by jfern (115937) on Thursday May 11, 2006 @10:11PM (#15314628)
    Where you can send an electron faster then the speed of light. Now let me explain. Speed of light in that medium is about 0.6c, where c is the speed of light in a vaccum. Electrons go about 0.8c. Relativity says nothing about whether you can break the speed of light, what it says it that you can't exceed the speed of light in a vaccum.
  • Wait... (Score:2, Funny)

    by Anonymous Coward
    Whatever happened to just going 88mph?
  • by nanepul (974398) on Friday May 12, 2006 @12:28AM (#15315277)
    I'm a Physics grad student who just happens to be doing my Master's project on negative index materials (or commonly known as NIM). I'm not an expert in this subject but our reseach group actually discussed this same subject last week. The point here is that the individual photons are not moving faster than light. In fact (what I was told by my professors and others) is that the pulse going in is NOT the same pulse going out. It's the front end 'tail' of the pulse which 'piles up' to become a new pulse which is seen coming out the other end in the shape of the original pulse. The incoming 'peak' of the pulse collapses (actually a portion of it gets reflected which for some reason doesn't ever show up in these simulations of the phenomenon) so only a portion of it exists after going in (I see this in my 1D FDTD simulation all the time). There is actually alot of distortion of the pulse at the interfaces (and inbetween) to the point that it's hard to say what is the original pulse and what isn't. In fact, if you just send light in with no 'peak' you will still detect a 'peak' coming out.

    • There are also two other things to consider:

      1) Their experiment detected the "other" solutions to maxwell's equations - in other words, the advanced wave. It's counter-intuitive, but a viable mathematical solution to the equations.

      2) Maybe the "backwards pulse" is a not-yet-understood result of a superluminal group velocity in the fiber materials studied. Superluminal group velocities happen quite often. And by definition the group velocity is the speed at which the modulation of the wave's ampli
  • by C10H14N2 (640033) on Friday May 12, 2006 @12:24PM (#15318558)
    The ENTIRE article was ripped from the university site. Not a single added value--in fact, it was negative value as I had to go to the original for the animations. In these cases, can we please bypass to blogospammers and just get the real deal? Pretty please?

    http://www.rochester.edu/news/show.php?id=2544 [rochester.edu]

It is contrary to reasoning to say that there is a vacuum or space in which there is absolutely nothing. -- Descartes

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