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Stop, Light. 254

Posted by michael
from the let-there-be-darkness dept.
parvati writes: "The New York Times is reporting that two separate research teams, both from Cambridge, MA, have managed to slow, stop, and then reconstitute light. The ability to stop and then accurately restore a beam of light has implications for quantum computing and communication in that it may provide a mechanism to store the information coded by single photons."
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Stop, Light.

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  • by Anonymous Coward
    The key being classically speaking. I tend to champion string theory which suggests that there are no such point-like singularies.
    It doesn't, however, suggest that electrons have event horizons or are black holes.
    Among other things, it gets rid of the paradoxes when combining gravitational relativity to quantum physics.
    Some of them.
    My Information comes from "The Elegant Universe" by Tom Green. So take that as you will.
    Brian Greene, and your understanding of Greene's book leaves something to be desired.
    Never mentioned gravity.
    Did too. Event horizons are a gravitational effect.
    Einstein showed that all forces are indistinguishable
    Einstein showed no such thing. That's not even really true in unified field theories such as string theory -- the different forces unify at high energies, but are quite different at low energies.
    Gravity is obvious, but charge is the same, assuming they are attractive.
    Nope. Coloumb's law looks like Newton's law of gravity, but there's more to electromagnetism than Coloumb's law and more to gravity than Newton's. The full theories (Maxwellian and electromagnetism and general relativity) are quite different.
    Likewise with the nuclear force (at least the strong one;
    Certainly not. The weak and electromagnetic have been unified into electroweak; the strong hasn't, nor has gravity. (Unless you subscribe to string theory, but even then the previous comments apply.)
    As for a lack of ability to escape the event horizon. There is already a strong belief that quantum particles radiate away from the horizon through quantum fluxuation (black-body radiation).
    Hawking radiation produces a thermal spectrum. Emission of photons from electrons doesn't.
    Particle does not necessarily denote shape..
    "Particle" has a well-defined meaning within particle physics, and what I said is true.
    From the book, mass was really just a side effect that is completely cancelled out in the case of bosons.
    As I said, mass arises as an effective concept via the Higgs mechanism, and doesn't apply to some particles (such as photons). Of course there are also massive bosons (the carriers of the weak force).
  • by Anonymous Coward
    try slashdot2000/slashdot2000

    Lameness filter encountered. Post aborted.
    la de dah dah stupid lameness filter

  • Based on an article that I read a couple of years ago (and nothing else, as I have no background in physics, let alone quantum phyisics) the principle of teleportation revolves around twining particles. When you have two particles, A and B, and you twine B around A, B becomes an exact opposite of A. If you them twine a third particle C around B, it becomes an exact opposite of B, hense an exact duplicate of A. In the process, though (and this is the way that I remember it, not necessarily the way it really is), A and B are effectively destroyed, leaving you only with C. By effectively destroyed, I'm assuming that they lose whatever properties they had that made them distinctly A or distinctly B. The big plus point here is that it apparently doesn't matter how far appart B and C are from each other. They could be a few feet or a few light years (though I immagine there would be problems being a few light years away).

    However, the researcher interviewed in this article noted that he immagined that transporting a person by this means would be a bit like being ripped appart one particle at a time. Not the way I would want to start my vacation!

    So, by inference here, I'm assuming that when the above poster said that the original would be destroyed, I don't think he meant destroyed in the sense that you destroy a rabid dog, but rather, the original would effectively disipate. Of course this all brings up the philisophical question of "am I made up of more than a bunch of random synaptic firings organized and chained in such a way as to give me personality and self?" to which I can answer a resounding and joyfull "Yes". But hey, that's the fun of Christianity.

    Oh, for more on the idea of knowing that you are a copy (an idea put forth in the recent Swartzegnager [what kind of sick person gives their kid a name that hard to spell?] flick "the Sixth Day") check out the Orson Scott Card short story "Fat Farm". I think that its in the Cruel Miricles collection. There was a hardback copy of all those books put together, but I don't think its in print. I don't think the smaller books are either.

  • I don't think they way they did it would apply to space travel, but what about time travel? I suppose you could encase something in this gas, and achive time travel. But then again, if space and time are relavent, I guess it would be possible to travel with it... I'm not up on this subject at all, but does anybody care to post a real educated view on this?
  • Despite the fact that this perfect replica would be absolutely indistinguishable from you, it would only be so to anyone BUT yourself. To me, this is almost like the scientific proof of something similar to a soul.

    Actually, I would think it would be quite the opposite. Most religion's concepts of a soul is something non-material/measurable/detectable, created by the "Divine Breath", and not something mankind can ever create or copy. If we can copy a person by copying its physical presence, and that copy is to all perceptions the same as the original, that would set off some serious theological alarms. Would killing the copy be murder, or just making an obscene clone fall? And how do you tell which is which? And does the copy have a soul, and if so does that mean we are just the sum of our matter or have we become Gods?
  • When do we get the damned spaceships?!! :)

    This just serves to reinforce the position that NASA is grossly underfunded. In its heyday NASA went from having NO launch capabilities to the Apollo moon missions in 15 years.

    What have we spent the last 15 years doing? Servicing the same damned shuttles and only going into low orbit on each trip. There should be a Moore's law for space technology. I think we've progressed computers far enough to keep us happy for a few years, why not concentrate on the space program?
  • This is where heisenbergs uncertainty principle comes in... you can't measure the transmission (read-out) without affecting the transmission. So when you have quantum communications which are eavesdropped on, _BOTH_ sides know it instantly because the quantumlink is disturbed so far for the eavesdropping where eavesdropping is defined as listening to two parties without the 2 parties _knowing_ it...
  • This idea was remotely touted with in a Star Trek: The Next Generation episode where Riker accidentally faces himself after a transporter fsckup.

    The "duplicate" Riker (left alone on an abandoned planet for years) would be, in this scenario, the Riker that would have been "killed" (destroyed, dismantled, atomically-disassembled or whatever).

    Though, being a ST:TNG episode, they instead focussed on his love affair with counsellor what's-her-name-again.

    Karma karma karma karma karmeleon: it comes and goes, it comes and goes.
  • counsellor Troy.

    Karma karma karma karma karmeleon: it comes and goes, it comes and goes.
  • Reading the article, I alternate between thinking that the atoms of the gas is store just information and thinking they store energy as well (the actual light). Can anyone clarify?
    --
  • Thanks for all your help. I appreciate it.
    --
  • Well, I'm pretty sure space and time are relevant.
    --
  • Um, I don't know much about quantum entanglement, although from what I do know it sounds cool, but the Stargate uses controlled wormholes, which aren't quantum effects on any level, merely distortions of space/time.

    Now, there are actually teleportation devices on the Stargate series, like the Go'uld (sp?) ring teleporter thing and the Asgard's beam teleporter, which could use quantum effects, but the Stargate itself just manages to create a wormhole at both ends (That's one wormhole, with two ends.) ...witness the episode where the Stargate at the other end fell into a black hole, thus causing the non-local end of the wormhole to fall into the black hole. (Which is not really a good idea, but it was fairly funny to see them repelling down the floor towards it.)

    -David T. C.

  • From the article ...

    Quantum computers could crank through certain operations vastly faster than existing machines; quantum commmunications could never be eavesdropped upon.

    Could NEVER be eavesdropped on? .. I'm not sure I understand this .. if there's a way to transmit something, then there can always be someone/something in the middle that can intercept it. Anyone with any insight into this care to explain or was this a case of the reporter getting carried away?

    --
  • Citing restrictions imposed by the journal Nature, where her report is to appear, Dr. Hau refused to discuss her work in detail.

    Two years ago, however, Nature published Dr. Hau's description of work in which she slowed light to about 38 miles an hour in a system involving beams of light shone through a chilled sodium gas.
    This really bothers me, how scientists can't talk about their own work because of some exclusive contract with some journal. It's no big deal if they have an exclusive contract to be the first to publish their papers, but not even talk about it? That's ridiculous.
  • I can stop light, then send it off in a whole new direction! It's called a "mirror." I will apply at the US Patent Office, and make millions!

    --
  • Unfortunately, the problem with this is that the probability of this happening is of course so astronomically remote that it will never happen anywhere in the expanse of the universe within whatever lifespan the universe may have.

    Nitpick: that's not an entirely true statement. As long as the universe does not collapse upon itself in a Big Crunch, then the lifetime of the universe is effectively infinite. (That is not to say life itself is infinite, as at a certain point the universe becomes too cold and dead to continue to maintain life.)

    One of the tenants of probability theory is that, given enough time, an event will occur. Thus, given an infinite period of time, _all_ events that are possible will eventually occur. In other words, if you wait an infinitely long time, you'll eventually teleport to the inner ring of Neptune.


    --
  • I want the first person who tries to make an artifcial black hole hauled off and shot. The second person would pray to have the fate of the first person. I am not sure if most people understand the danger of an artifical black hole.

    Black holes do damage through tidal forces the smaller they are they more damaging they are to the immediate area. Also there is not way to control it. There is nothing that can hold it in one place. If you try to hold it then you feed it mass and it gets bigger. Right now with our tech if you make a black hole the world is over. The thing will eat the entire planet in fairly short order and as it undertakes simple harmonic motion through the earths core growing as it goes.

    IE this is a one way death sentence. If they want to experiment with artificial black hole at least do it around the orbit of pluto. Then if we fsck up there we will have time to leave the solar system. Still not a good option but a better one.
  • ...in refrigerators. Hell, Burroughs talked about how the Martians used this to levitate their airships around a hundred years ago. There's a movie to be made: "A Princess of Mars.:
  • I just enter what I would if I had to register.. user aaaaa, pass aaaaa. Someone else back yonder had the courtesy to create that. =)
  • > Okay, you're pulled apart and your entire > quantum makeup is sent somewhere else. What > stops you from becoming a pile of goo?

    Ask Jeff Goldblum, he figured it out in "the Fly"
  • So for all of the Sci-Fi authors out there, we finally have FTL Travel - We don't need to bend space, create wormholes or anything requiring any massive energy... Just open the front door and walk !!! ;-)

    So would some kind soul care to explain how 'c' is a constant that is used throughout physics, but we can still slow/stop light? My physics isn't what it should be these days :-(

    Layman's terms appreciated, but not mandatory.
    --
  • As others have pointed out, what's really happening here is not that photons are somehow stopped (they have zero rest mass) but that they are storing their information in the spin of the rubidium atoms. The light beam is stored with the help of a second beam used to excite the atoms.

    This is analogous to a hologram, which manages to store phase information from light, using a reference beam (normally split off from the illuminating beam). This slow light is more thn just a hologram, though, because of all the extra information that is archived.
  • Artificial black holes, eh? Would make nice portable garbage disposals, wouldn't they?

    Sure, no problem. The first step is converting your garbage to light, then the light can be disposed of in the black hole.

  • That does sound yucky.


    http://www.nara.gov/exhall/charters/declaration/ declaration.html

  • That ratio may be different, but that ratio no longer has anything to do with the definition of pi. Curved spacetime doesn't change the fact that pi = 4*(lim(n->inf) sum(k=0..n of S(k))) where S(n) = (1+2n)^-1*(-1)^n
  • He may have been referring to something like this page's discussion of anisotropy in EM interactions [rochester.edu] which doesn't have to do with different velocities of light but might have been misremembered as such.
  • It can detect eavesdropping between Alice and her conversation partner, but it still provides no guarantee that her conversation partner is Bob; Dr. Evil can still play man-in-the-middle.
  • by Anonymous Coward
    If I remember my QED lectures properly, the slowing of light in any medium, is an apparent, not actual effect. We should think of the path of the light beam as a series of absorptions and reemissions. The longer the excited states (absorptions) remain before decaying (emission), the slower light takes to travel through the medium. Of course this is a huge simplification of the process.

    These experiments appear to be creating long-lived absorptions of the incoming light, followed by decay of the excited state by the second laser beam. In fact the article even mentions that the incoming light is effectively stored in the spin of the gas. A great leap forward for atomic gas physics, but a far cry from modifying the fundamental structure of space-time!

    One of the reasons why this is exciting, is because it demonstrates large scale QM phenomena.

  • I would look for the body.

    But you are right, for interference phenomenae to work the way they do, you need to have identical particles.

    But this is important only if you want to understand how their clever trick works. (And you need to know more -- you mostly need to know electronic energy levels, what transitions are allowed, and how waves can interfere.) Understanding the importance of the result is simpler. Light has two properties: polarization and momentum. For a particular band of momentum (corresponding to the Rubidium line they are using) the polarization state is effectively recorded into the atoms of the vapor. And this is what is used in quantum encryption, "teleportation", and other newfangled ideas.

    Of course, the atoms will lose this information as they collide with each other and the vessel walls, but for a thin vapor the time scale for this is over a microsecond which these days seems like an eternity.

    Recording polarization may sound simple, but it's not. And this is an indirect recording... you can't look at it to tell what the state is, but the state is preserved. I do have a clumsy explanation which hopefully some readers may be able to decipher:

    You know light can be linearly polarized.. horizontal is x, vertical is y, diagonals go like (x + y) and (x - y), etc. Well, if someone hands you a beam of light, you can measure the polarization of it with a sheet of polaroid. Perpendicular to the plane of polarization no light is transmitted, parallel almost all gets throught, at 45 degrees the intensity is half. So, this seems easy.

    But how do you measure the direction of polarization of a single photon? The answer is: you can't. The beam of light is an ensemble of photons that are (by assumption) in the same polarization state. When we hold it diagonally, we see 50% intensity because each photon individually has a 50% chance of making it through or being blocked. When we have say 1e6 photons, we can see that 5e5 made it through and say, ah yes, we are diagonal. But if we have only one photon to go on, it either makes it through, or it doesn't. So if it makes it through, the polaroid may be aligned to the polarization, or it may be any amount off. All that we do know for sure is that it is not perpendicular.

    Also, add to this an additional subtlety that coefficients describing polarization are actually complex. e.g., you can have have a polarization in the direction (x + iy) which may be circularly polarized clockwise. (x - iy) would be counterclockwise. For this light the probability of being transmitted is 50% no matter how you orient the polaroid. But I digress... The point is, quantum mechanics is a wonderful description of the world, and this new trick will be very helpful in taking advantage of quantum mechanics in engineering applications.
  • um. you just solved the RIAA problem.

    Soon, all music will be distributed this way. You will only be allowed to listen to that Madonna single once, and one time only, for each $5.00 automatically extracted from your bank account :)
  • The problem is that as time go on, this artificial blackhole gathers mass, and by gravitational pull, attracts more mass, until it goes nova.

    That could be a problem while driving your car.

    Karma karma karma karma karmeleon: it comes and goes, it comes and goes.
  • you would have a read-once hologram, of dubious usefulness.

    Not to M.I.A. agents...

    Karma karma karma karma karmeleon: it comes and goes, it comes and goes.
  • That makes no sense...

    It was either Faraday or Maxwell... or somebody else around there which did some work which strongly indicated that c was a constant. Looking into that is on my list of things to do... this work prompted Michaelson and Morley to perform their experiments showing light moved at a constant speed in all directions.

    And if you're interested in the "cosmic" speed, there were those pesky calculations involving the speed of light based on the orbits of the moons of Jupiter. This leveraged the width of the earth's orbit against Newtonian physics and the observed position of Jupiter's moons. Not horribly precise, but nothing is known to an infinate number of significant digits.

    The constant 'C' is defined as the speed of light in a vacuum, as one poster here worded it so eloquently, the speed of light in different medium is due to absorption and retransmission.

    But I'm only saying that 'C' is constant. I'm not saying what 'C' is. Just like PI is a constant, only known to a million or so significant digits (in an unaccelerated reference frame).

    Stopping a photon is probably just some media spin.

  • I hate it when somebody says something irrefutable. And here I was trying to avoid somebody splitting hairs by telling me that pi was not a constant depending on your reference frame.

    More precisely, the ratio of the circumference into the diameter of a circle varies in an accelerated reference frame regardless of the position, velocity or acceleration of the observer. That is an important distinction, I'm glad you pointed it out.

  • How would you detect [a quantum-teleported copy]?

    It would be obvious. The subject would continually complain about having his "molecules scattered about the universe," and perpetually goad his more logical, scientific colleagues with illogical references to emotion and heritage.

  • I wonder if this could also be used for holography: freeze the interference pattern into the material, and read it out later

    Yes, and not necessarily for recording. In fact, I think it would lend itself more to spectroscopic analysis, especially at low light levels. Freeze light as it enters, integrating signal until you've collected enough to build up a useful signal to noise ratio. And the extremely high indices of refraction in the materials used would give you all the spectral resolution you ever need. Add a third dimension using holography and you have enough basis to do solid state hyperspectral imaging [nasa.gov].

    A big problem with spectroscopy and spectroradiometry is that when you get high spectral resolutions, you need insanely high signal to noise ratios, on the order of 10^3 or even 10^4, to do chemical analysis. This kind of phenomenon allows you to increase your signal without adding noise (well, beyond the inherent Poisson noise, N = S^0.5).

  • Yes, and you can also get 'zero point enegery' from it, which is basically infinite energy free of charge. On the other hand, you might literally 'pop' the space-time bubble of the universe, like someone inside a balloon tunneling out of it to the low pressure outside. If you did that, the entire universe would be destroyed as the faster-then-light repressurization wave swept over it. So it's not really that great an idea. ;)

    -David T. C.
  • This makes me think of the 'Stargates' in the movie and television series.
    Could that be IT???

    --

  • I'm afraid not, no. That's not how quantum entanglement works. Even though you can get instantaneous action at a distance, there's still no way to transmit information faster than light. Basically, all entanglement tells you is that when you measure thingy A, then whatever answer you get, instantly you know thingy B is in the same state.

    Okay, two questions...

    1: (not FTL, but I'm curious)

    Suppose you have a particle that has a 50% chance of changing state after time T. Actually, suppose you have two such particles, identical, entangled. (I may be talking out of my ass here- I don't know if particles can degrade like atoms do or if entanglement can work at the scale of a whole atom. IANAP.)

    One particle goes on a near-lightspeed journey and comes back. The other particle doesn't. Time dialation applies. The age of the travelling particle is less than T, while the age of the particle that stayed home is greater than T.

    So when you measure one of the entangled particles, is the probability that the particles have changed state higher or lower than 50%? Does it matter which one you measure? Is it even possible to send only one of the particles on a two-way trip without breaking the entanglement?

    2:

    Suppose you want to send one bit of information to Alpha Centauri. You need it to get there FTL. Fortunately, the Grays have a space station roughly half-way between Alpha Centauri and Earth - but slightly closer to Earth - that regularly sends out a pair of entangled particles, one to Earth and the other to Alpha Centauri, at the speed of light (or as close to it as possible).

    One of these entangled particles reaches Earth. You measure either it's momentum or it's position, depending on whether you want to send a 0 bit or a 1 bit. Shortly thereafter, the other entangled particle reaches Alpha Centauri, where they attempt to measure it's position.

    According to the uncertainty principle, the more you know about a particle's momentum the less you know about it's position, and vice versa. If you've measured one entangled particle's position, the good folks at Alpha Centauri should have no trouble also measuring it's position, right? On the other hand, if you've measured it's momentum, the other guys shouldn't be able to measure their particle's position, right? If it is possible for the folks at Alpha Centauri to determine whether or not the measurement was successful then they have received a bit of information from you that traveled at just under twice the speed of light. I suppose it is not possible for the folks on Alpha Centauri to determine whether or not the measurement was successful? Would they just get a measurement and be unable to determine it's accuracy?

  • Basically this discovery exactly remembers a
    pattern of light and replicates it.
    This memory device would be entirely optical
    and fit into photonic computing systems.
    I'd guess density would be pretty good.

    The other optical memory schemes I've seen
    involved continous loops, set and read.
    Another is holographic alteration of material
    optics.
  • In theory, there is already a non-zero possibility that as you read this you will be spontaneously teleported in entirety to the inner ring of Neptune. Unfortunately, the problem with this is that the probability of this happening is of course so astronomically remote that it will never happen anywhere in the expanse of the universe within whatever lifespan the universe may have. So the only way we would ever achieve teleportation of a complete human-sized object would be if we found some way to modify the quantum probability of where an individual particle's position ends up, so that we could say with certainty what the end position would be in order to keep every particle in a human together, while at the same time, keeping the human from exploding in a huge burst of particles flying in every direction. In other words, we have to find a way to violate everything we've found to be true about quantum mechanics, but keep your fingers crossed, I have hope. :)
  • The key word in what you said is that for heisenberg's uncertainty principle to be relevant you must "know" the position or momentum of a particle. In other words, that knowledge must escape to the outside world. If that knowledge is just stored inside of an opaque bose-einstein condensate, then no knowledge has escaped to the outside world, and hence, the wavefunction does not collapse.
  • Not in a super cold bose-einstein condensate. Putting anything close to a super-intense pulse of laser light into it would overheat the gas beyond the condensate point, and all the magic would leak out.
  • The Phys Rev Letters paper will be available online (though isn't there yet) at:

    http://link.aps.org/abstract/PRL/V86/P783

    The official citation is Phys Rev Letters, Volume 86, p. 783 (published 29 Jan 2001).
  • gravitational pull, attracts more mass

    The problem is, a black hole would need to be awfully large to have more than negligible gravitational attraction.

    Black holes only have as much attraction as the mass they contain. A black hole that weighs as much as your car is going to have the same gravitational attraction as... your car. How much gravitational attraction does a skyscraper have? (Negligible.) How much gravitational attraction does an aircraft carrier have? (Negligible.)

    A black hole would have to have tremendous mass to make even the slightest effect on nearby objects, and then you still have problems because black holes are incredibly dense - which means even "large" black holes are going to be incredibly small (if I remember correctly, a black hole with the mass of the Earth is about the size of a marble - how small is a black hole that only has the mass of an aircraft carrier?). How much matter it can consume is limited by the size of the black hole.

    Not to mention, too, that Stephen Hawking proved that black holes do give off some radiation, and thus will shrink if they do not consume enough mass to stay stable (and if they shrink past a certain limit, they simply explode).

    --
  • Eavesdropping by definition means a third party listening in on a conversation between two other parties.

    With quantum communications, any intermediate listener would cause the signal to be modfied (or garbled) and one party would know the conversation was tapped, hence, they are no longer eavesdropping...
  • How would you transport you hologram?

    Personally, I'd use a small, beeping robot.

  • I seem to recall this idea being discussed in the very first Star Trek novel, "Spock Must Die". It was a very chilling concept in the context of a world where matter transporters are common.

    I haven't read the book in about 20 years, but a lot of the philosophy behind your topic is discussed in it, since the story revolves around a transporter accident.

    Rick "Putting Plastic Pointy Ears Back in the Drawer Now" Gutleber

  • No need to actually register yourself into their spam machinery.
    -----
  • No,
    Light IN a vacuum moves at a MAXIMUM of C - It CAN and does move slower, especally in other mediums.
  • I forgot to mention one thing.. If photons move in both the direction of time AND in some 3D spatial direction (with the speed C), then their total speed is HIGHER than C, isn't it?
  • by MrP- (45616)
    I can stop light too!

    ::gets out flash light::
    On
    Off
    On
    Off

    See? Now give me a Nobel prize or something, I'm a freaking genius!
  • Many people still surf with misconfigured /etc/hosts files. In order to browse NY Times, you need to add an entry for it to this file.

    208.48.26.217 www.nytimes.com

  • [...] no-one remembered to make c final so it's a variable.

    Actually, that's not true. The speed of light in a vacuum is exactly 299,792,458 metres per second by decree.

    Unfortunately nobody remembered to make the length of a metre final.

  • Unless you can use optical fibers to transport the read-out laserbeam. Now this would be interesting, if you could maybe put this inside an optical router?

    That was pretty much what I was trying to say.. The problem I had was that it's not too different than a black-box hard drive that stores an email and then sends it out on the internet then deletes the message.. The only difference would be that you'd garuntee it's deletion once it was sent.

    However if this is fast enough, it still might have applications in the routing world.

    -Michael
  • Anyway, my REAL point: what about heisenberg's uncertainty principle? As the photon slows shouldn't it's position become more and more indeterminable? And when it stops, how do they know where it is?

    Except that we're not dealing with a single photon, but a sea of photons. The only times we've ever been able to monitor single photons is when they've collided with a photo-cell. We could say, yup, there it was at that moment in time in that exact place, but good luck finding that particular photon ever again.

    -Michael
  • Well, first of all this is speculation into the realm of sci-fi.. We have no technology even close to what it might take, so all we can do is look at new discoveries and contemplate the possibilities.

    In this case for example.. What we seem to have is the ability to "capture" the quantum states (mainly the wave-front) of a volley of photons inside the spins of gaseous atoms. Essentially this sounds like taking a picture of the macro-scopic cloud of light, then reanimating it. Well, to my mind that's a building block towards teleportation of any type. In this case it's really only a time-shifting. But the fact that the information was reproduced amid the regular quantum fluxuations is astounding enough for my imagination to go active.

    In my comment, I even went so far as to discount amplification of the information signal (which essentially rules out almost any form of analysis, which would include information dessimation that would otherwise violate Hisenburg's principle). The "signal" in my mind was such a photographic plate as this gaseous stop-light. That this concept could ever be applied to anything other than light is pure speculation - albeit a fun one.

    -Michael
  • When you stop light, you have to rename it heavy.

    It seems to me that the photons are not physically stopped.. In fact there is little physical difference between this and regular obsorbtion of light by matter.

    The main difference seems to be that instead of giving wrought energy to the electrons / nucleus, they're exclusively affecting the spin (???). Supposedly this means that the wave-front is captured instead of just a raw packet of energy. Normally an atom obsorbs a photon, then at a later time ejects either it, or some combination of photonic energy in random directions (kind of like scattering). But what I believe is happening here is that the wave-front is reconstituted by possibily analogously gyroscopic-inertial forces (I know it's not really spin, but never truely understood it) in the exact same direction.

    So basically it's no different than your common everyday sun-light off a white tee-shirt sort of event except that there's no scattering, and you can use a trigger instead of random quantum fluxuations for the retransmittion.

    As a final response to your statement, since the light isn't actually stopped, it isn't "heavier", much like the undetectible additional mass of an atom when it obsorbs photons. Beyond that, the slowing of matter makes it lighter, rather than heavier (according to the theory of relativity and the lorenz factor).

    -Michael
  • Untrue. Electrons don't have event horizons -- classically speaking, they're "naked singularities"

    The key being classically speaking. I tend to champion string theory which suggests that there are no such point-like singularies. Among other things, it gets rid of the paradoxes when combining gravitational relativity to quantum physics. My Information comes from "The Elegant Universe" by Tom Green. So take that as you will.

    I believe that how they capture photons in the first place.
    No, it's definitely not a gravitational effect..


    Never mentioned gravity. Einstein showed that all forces are indistinguishable, and string theory suggests that at the right temperature and pressure, they are litterally the same force.

    Each force that is attractive to another paticle has an event horizon, provided that repulsive forces do not counter it. Gravity is obvious, but charge is the same, assuming they are attractive. Likewise with the nuclear force (at least the strong one; I don't really understand the weak nuclear force, though it's been tied into electro-magnetism).

    As for a lack of ability to escape the event horizon. There is already a strong belief that quantum particles radiate away from the horizon through quantum fluxuation (black-body radiation). On a smaller scale, the fluxuations are significantly more likely to have a profound affect.

    String theory doesn't really say much on this topic, so I'm really just speaking out of my hat.

    Actually, string theory suggests that there fundamentally aren't such things as particles, just strings.

    Particle does not necessarily denote shape.. Singularity would be the correct term for classical quantum particles.

    Additionally, there are branches of string theory that suggest the existence of multi-dimensional undulating blobs. I believe M-theory takes care of all of this, so a multi-dimensional vibrating particle would probably be the best generic description.

    From the book, mass was really just a side effect that is completely cancelled out in the case of bosons. Whether they're initially massless or massive is just a frame of reference; duals like the wound and unwound string.

    -Michael

  • The book is by Brian Greene, not Tom Greene.
    Perhaps I should qualify, from memory, and then specify when I'm leaving the book as a source of reference.

    You seem to have gotten some mistake impressions from Greene's book, though

    Well, the event-horizon stuff isn't in the book; that's for sure. (At least not for anything other than classical black-holes).

    But I don't think that really distorts the content too much (assuming that I'm wrong, which I'm not convinced that I am).

    -Michael

  • This is another case of the NYTimes screwing up the technical details and making something have totally different implications than one it sounds like. They are NOT stopping light AT ALL IN ANY WAY. I like the NYTimes, but the BBC reports this tech stuff better. They are not stopping light, just copying it's parameters into the gas, then recalling them.

    Read the posts by zCyl and Ferzerp to see why.
  • in saying it's a snapshot, i mean, information of the light is stored and new light is emitted from that information. The article states that the light exitting the device does not have the exact same characteristics of the light entering. So, it is changed.
  • One of the practical problems with quantum encryption is that the receiver must receive the same photons that were sent by the transmitter, or the message will be undecipherable. No repeaters can be used. (Repeaters are used to maintain the strength of optical signals over long distances.)

    If the beam could be reconstructed with a higher intensity than the original beam, but the same properties (spin etc.), stop-light chambers could be used as repeaters for quantum-encrypted signals.
  • So they use the glass in the wall of a bathroom to make a nice calm peacfull setting. What hapens when they house is sold? or 20 years in the future you get to watch some guy take a leak.
  • When light is 'stopped', does it decrease in intensity after a while or can it just be stored indefinitely?

    There will be some degradation, but if you put the pattern enhancers into a repeating diagnostic loop, it can save the original energy pattern for hundreds of years.

    This is a good way to save yourself if you ever crash land on a Dyson Sphere. 8^D

  • However, I wonder if this could also be used for holography: freeze the interference pattern into the material, and read it out later, reconstructing the image. In theory, since the material could record the interference pattern in three dimensions rather than two (like a photographic plate), this might allow for more detailed holograms.
    Better yet, the ultimate in secure messaging. Encode your message in the beam, freeze it, ship the entire apparatus as a package & unfreeze the beam at the other end. Once reconstructed, the beam is read out, the message is read, and the imprint left behind is destroyed. Thus, the message can be read only once.
  • The speed of light in a medium is inversely proportional to the refractive index of the medium. The refractive index of air is about 1.0008, of water is 1.33, of quartz is 1.54, and diamond is 2.42. When light crosses a boundary between media with different refractive indices the path changes (Snell's Law).

    As far as relativity is concerned, this doesn't cause any problems. Relativity only says you can't go faster than the speed of light in vacuo. As others above have pointed out, Cerenkov radiation is emitted when particles exceed the speed of light in a medium - those particles are not exceeding the speed of light in vacuo. Relativistic mass gain is due to velocity, not acceleration (review those Lorenz transforms). A (relatively) easy way to think of this is to view the Lorenz transforms as mathematically compressing Newton's mechanics so that what Newton would have called an infinite velocity is instead perceived as the speed of light. Think of taking the straight line from velocity = acceleration * time (Newton's model) and bending it so that it approaches the vertical speed of light line asymptotically (special relativity). Some extremely counterintuitive things occur, but they have actually been observationally verified. (Now I'm rambling, too, so I'll stop...)
  • From my understanding of that work it was not intended to actually create black holes, but by slowing light down enough you could create a vortex in the material that would suck the slow moving light in, in the same way as a black hole.

    This would allow lots of interesting studies of the effects, and would be a lot safer (read: less terminal) than actually creating a black hole.

    This was from a New Scientist article relating to the same research.

    Lots Of Love

    Bill
  • I just hate it when my car goes critical, and there ain't a service station for miles around....

    ---
  • You know what this means.

    It is now possible to say that everyone has performed the miraculous feat of Faster-Than-Light travel!

    ---
  • ...is that no matter what frame of reference you're in, light will always travel at c. You're standing still? It travels c. You're running alongside a beam of light at 299,999 mps? It travels c.

    But uhh yeah, like has been said, velocity is a measure involving time. My stupid way of thinking is velocity measures the change of everything else, xyz, in fixed positions of time. So would moving in time measure (eg) zyt, in fixed positions of x?

    Head hurting...

    Good thread btw, I'd mod it up if I could

  • I can imagine this being useful, especially if they can paint a car with some sort of light deadening material. It would render laser speed detection devices obsolete.

    Such material already exists; it's called thin film technology, and I really don't know much about it, but I understand it's used on the surfaces of stealth vehicles the render them almost invisible to radar.

    No doubt it's way too expensive (currently) for regular automobiles, just to avoid traffic tickets.

    But I say, if you feel the need for speed, put yourself on a closed race track, don't put others in danger...

    --

  • Wouldn't that cause problems with light entering from different angles? If for a piece of glass, light takes 20 years to filter through at a zero degree angle of incidence, what about a 45 degree angle? Since it has to pass through about 1.4 times the amount of material as the zero degree angle, would that mean it takes 28 years to bump through?

    Of course, that might cause a cool quantum fishbowl effect . . . think of the sci fi applications. :)

  • Correct me if I am wrong, but are they not, in essense, just taking a snapshot of a photon and then recreating it?

    According to the Heisenberg Principle, that's not possible. If they had 'taken a snapshot' that would have serious ramifications for the way we perceive the world... (insert beethoven's 5th)
  • Hmmm. Artificial black holes, eh? Would make nice portable garbage disposals, wouldn't they? So, assuming black holes exist in all sizes, you just create a "quantum" black hole, then put it in a mason jar large enough to be just beyond the hole's event horizon. And with the intense radiation from the hole's perimeter, you could also attach a steam turbine and power your car or PC off it.

    I smell a new startup. Venture capital! I need venture capital!
  • First off, c is the speed of light in a vacuum, and light travels at differents speeds in different media.

    Correct so far.

    This means that when moving from a medium into one in which the speed of light is slower than the first, the lightwaves/photons (whatever) need to release some energy, (As energy=hf, and f=speed of light/wavelength [sorry, couldn't find lambda sign]so there would be a difference in energy after going into a "slower" medium) which is given off as, I believe, flashes of visible light (yes?, no? maybe? I'm unsure)[Also, then how does light speed up going into faster media? Does it? What accelerates it if it does? Anyone?)

    When moving into a denser medium (as in one with a higher index of refraction so it slows down the light travelling through it), it's wavelength is also shortened. It's frequency, however remains the same. Since f=v/wavelength, and v and wavelength are both reduced by the same factor (to do with the ratios (ratii??) of the refractive indices of the two media), then f remains constant.
    Another way to see this is to think about the light wave as it enters the medium. Imagine the peaks and troughs of the wave as they cross the boundary. For each peak entering, there must be a corresponding peak leaving, hence the frequency remains constant.
    When light enters a medium in which it can travel faster, the inverse happens. This still doesn't break any rules about faster then c, unless you go from a vacuum, to something less dense, but I guess that ain't possible.
    As far as flashes being observed, these are simply due to the fact that as the light crosses a boundary between two substances with differing refactive indices, some reflection occurs. This reduces the intensity of the light, but does not affect the frequency. This leads on to things such as antireflective coatings, where you go through intermediate layers to 'soften' the effect of reflections, to things such as impedence matching in wires (hence why some people spend money on 'matched' cables for AV systems, trying to improve quality by reducing reflections along the cables).
    Or something like this.

    Banni

  • i thought that light always moved at c

    It does, but no-one remembered to make c final so it's a variable.

    Nothing is immutable - except flux.

  • This reminds me of the "slow glass" stories. For the uninitiated: Slow glass is just like regular glass but the thicker it is, the longer it takes for light to pass through. So for a 1 inch thick piece it may take, say, 20 years. They put the glass out in the forest for 20 years, then install it in a house. Now the inhabitants have a 20 year forest scene streaming in the window. Other stories put the glass to other uses.

    Anyway, my REAL point: what about heisenberg's uncertainty principle? As the photon slows shouldn't it's position become more and more indeterminable? And when it stops, how do they know where it is?
    --
    MailOne [openone.com]
  • C is a constant defined as "the speed of light in a vacuum". You can't just take any light you happen to have lying around (in gas chambers or underwater or wherever). Light slows down going through ANY substance.
    --
    MailOne [openone.com]
  • -snip- .- Questions: Does anyone know what happns when two photos, with the same frequency, the same fase (and generally the same energy) and different directions hit one another?

    Not exactly sure, but I know that it's a good way to get rid of Stay Puff Marshmelow Man.
  • The transistors in the memory of your computer store a value which is nearly identical to the "1" or "0" which was written to it. If a high voltage is written it is drained over time through parasitic capacitances. Periodically the value is refreshed (brought back up to the correct level) through refresh.

    When you read from memory you're essentially reading through a high gain buffer which restores any signals which are almost high to a high signal or which are almost low to a low signal.

    As long as there is some means to detect the appropriate value of a signal with a high enough probability of success nearly is good enough.

  • First off, c is the speed of light in a vacuum, and light travels at differents speeds in different media. This means that when moving from a medium into one in which the speed of light is slower than the first, the lightwaves/photons (whatever) need to release some energy, (As energy=hf, and f=speed of light/wavelength [sorry, couldn't find lambda sign]so there would be a difference in energy after going into a "slower" medium) which is given off as, I believe, flashes of visible light (yes?, no? maybe? I'm unsure)[Also, then how does light speed up going into faster media? Does it? What accelerates it if it does? Anyone?)
    (mmmm, a bit offtopic, aren't i?)

    Relativity (or some part of it anyway) means, at least for this case, that nothing can travel faster than the speed of light in that medium. And most massive objects (like ships) cant come anywhere near close. Please, correct me if i am wrong, but as an object (not light) accelerates toward fairly fast speeds (or velocities, if you prefer, but I just want the scalar) it gets more and more massive, and hence acceleration gets harder and harder (a=F/m)
    Very much out of my depth here, considering only formal education in physics so far is high school level, but does the above have anything at all to do with the wave behaviour of non-light particles? I mean, how can an object gain mass due to acceleration, unless looking at kinetic mass (?) being calculated by energy and velocity, energy being calculated by that formula given above.

    Please correct me, i am rambling. May as well mod me down now.
  • So what would happen if you dropped your black hole? Would it careen through the planet, slowly gathering mass until it got big enough to do some real damage?
  • by coreman (8656) on Thursday January 18, 2001 @03:55AM (#499537) Homepage
    Intensity is just the number of photons as percieved by your eyes. More intensity means more photos hitting your receptors. So, if the photons are conserved, just stopped and then restarted and redirected in the same direction, it would be percieved as the same intensity. You do have to ask if the number of photons is modified and/or increased by the slow down/stop/speed up process.
  • by EricWright (16803) on Thursday January 18, 2001 @10:15AM (#499538) Journal
    Except that relativistic velocities are not simply additive... v_net != v_1 + v_2. Rather, v_net = (v_1 + v_2) / (1 + v_1*v_2/c^2). That way, even if v_1 = v_2 = c, the equation becomes v_net = (c + c) / (1 + c*c/c^2) = 2c/2 = c. At speeds that are not a significant fraction of c, the equation gives v_net \approx v_1 + v_2, as the denominator is approximately 1 (well, 1 + epsilon, which as we all know, is 1).

    Your flaw is trying to add things with dissimilar units... when you consider time as a fourth dimension, you are really doing calculations with ct (a length), not t (a time). That way, you can express the distance through spacetime as ds^2 = (ct)^2 - r^2 (or r^2 - (ct)^2, depending on which metric you use).

    Eric

    Eric
  • by PigAlien (29865) <pigalien@yah[ ]com ['oo.' in gap]> on Thursday January 18, 2001 @06:04AM (#499539)
    This makes me think of the 'Stargates' in the movie and television series.

    Would it be possible to create a huge bose-einstein condensate, break it in half and flatten it out?

    If so, then you would merely need to transport the two 'gateways' whereever you wanted and teleport between the two locations.

    The theory being that when you walk into one of the portals, your entire quantum makeup would be absorbed and transmitted to the other portal because of quantum entanglement.

    Anyone think this would work? How would you stimulate the portal to 'release' your energy?
    http://www.nara.gov/exhall/charters/declaration/ declaration.html

  • by macpeep (36699) on Thursday January 18, 2001 @04:09AM (#499540)
    Think about a football field that is 100 meters from end to end. Now think of a guy that runs in a straight line from one end to the other, along the sidelines. If he runs 10 meters per second, it takes him 10 seconds to run the entire length of the field. Now make the guy run from one corner to the other. He still runs at 10 meters per second, but it will take him more than 10 seconds to reach the other end. By spending some of his motion in the width-direction of the field, his motion in the direction of the length of the field becomes slower.

    Think for a moment that time works just like a spatial dimension. You have a specific absolute speed in a specific direction.. time. When you start moving to some spatial dimension (one of the three traditional ones), your motion in the direction of time becomes slower because you are no longer fully "commited" in that direction.

    Now think about photons. They move with ALL of their speed in some spatial dimension. Does this mean that photons stand still in time? If you slow down light, does this mean that time actually starts ticking for them? Could it be that the fading of the light has something to do with the fact that time runs for them? If they are in an absolute vacuum, light doesn't fade because time stands still, no matter how far you shine the light. Introduce "dust" and it slows down and fades.

    I'm sure my theory is very flawed but I'm not exactly sure at what point. I mean time DOES slow down when you move, but am I looking at this the wrong way?
  • by maraist (68387) <michael.maraistN ... .com ['n0s' in g> on Thursday January 18, 2001 @05:23AM (#499541) Homepage
    I wonder if this could also be used for holography

    Better yet, it could be used for read-once messages. Albeit bulky and not as cool as flash-paper, or exploding sun-glasses. :) At first glance, I can concieve of doing with this as people have hypothized about quantum computers.. Send encryption keys that can only be read once down a fiber line, etc. The stumbling block I see here is that only the gas medium holds this property.. Transmittion down the fiber is still subject to existing hackability. But perhaps a short-distance "secure" channel of gas could be used?

    Maybe not.. :)

    -Michael
  • But uhh yeah, like has been said, velocity is a measure involving time. My stupid way of thinking is velocity measures the change of everything else, xyz, in fixed positions of time. So would moving in time measure (eg) zyt, in fixed positions of x?

    I recommend "the elegent universe" by Tom Green. He very beautifully discrbes Einsteins relativity and multi-dimentionality in lay-terms and with lots of colorful analogies.

    Essentially time can be thought of as the interval of a regular periodic event (such as a frictionless bouncing ball or pendulum.. or even a photon bouncing between two mirrors). The trick is that the event is periodic with respect to a single frame of reference. A man on a space-ship flying past you will observe a seperate period in your time-piece.

    Light has a fixed velocity and is timeless in free space because it has almost no mass (there's still debate about whether it's truely massless) and thus can not "Do anything" that would be measurable (such as the position of a ball reflecting light to the observer). As the author suggested, by traveling diagonally across the foot-ball field, you're traveling in two dimensions. The hypotenus is the traveled path, and the sine or cosine is the observed path and velocity from either the point of view of time or any of the spacial dimentions. Its simple geometry.

    However, when the light's wavefront interacts with neighboring atoms, they are achieving some sort of event that is non-instantaneous.. If they pass into an electron's event horizon and contribute to it's energy, that takes time. Likewise when they radiate out of the electron and continue along some new path (scattering), or reconstitute their original form (transparency).

    There is massive speculation that the wave-nature of light (and all material for that matter) is such that the photon really travels all possible directions and interacts with all material proportionately (evidenced by single-photon refraction patterns). Thus the more matter nearby, the more interaction, thus the more timeable events, thus the more traveling through time, thus the less traveling through space, thus the slower apparent probagation of light through a non-vacume.

    The "stop-light" above quite possibly is such a time machine for light where it travels sole-ly through time (or any of the other hypothesized micro-dimensions) until it's released. There is nothing totally radical about this, since this happens whenever a fermion captures a photon for an extended duration. The bizzarness is in the exact replication later on.

    -Michael
  • Even an electron has an event horizon, when the radius is small enough.. I believe that how they capture photons in the first place. Isamov's black-body radiation probably explains how the photons escape.

    Light travels at different speeds in different mediums (due to permiability and permittivity values), so in a very slow probagation medium, it's possible that quntum particles get a boost in the sizes of their event horizons.

    Of course this has little to do with the capture of other _atoms_ which is what initiates the common concept of a massive black hole.

    It might be possible to pack so much energy into a quark that it's event horizon could capture another fermion. But of course it would be so "hot" that it would break through just about anything.

    In theory, you could boost a particle with enough photons simultaneously that it becomes another particle (along the lines of string theory). It's the same basic idea of atom-smashers. If you were to find a big enough quantum particle, then it would have rest energy sufficient to maintain a suitible event-horizon withtout being too hot. Unfortunately the only way we currently know to give a particle enough energy is through larger and more expensive colliders.

    Perhaps a focused "stop-light" could provide enough simultaneous photonic energy that the target quantum particles will rematerialize into something larger just long enough to collect neighboring particles in a mini-black hole.. Don't worry though, the most likely result will be a decay of that macro-particle in short order.

    Quantum physics doesn't lend much room for ultra-massive particles. BUT, string theory suggests that all particles are inherently massive (a plank-mass - the wieght of a grain of salt) and that through vibration of 11 different dimentions their apparent mass is reduced in several discrete levels.

    My idea is this. Take a massive spherical chamber filled with high a temperature gas that will act as this light-trajectory-storage medium. Shine bright light radially inward with the "store" light turned on... The light at the center will be dimmed and ultimately too low an intensity to wreck havoc. Then after sufficient time and energy is stored in this huge volume of now super-highly energetic particles. Trigger the light's release. Since the wave-front is stored, it should reproduce the original direction of the light and thus flash the center with energies approaching if not exceeding atom-smashers.

    The high temperature gas will help obsorbe any reactions that might occur since their expansion should be minimized.

    -Michael
  • by wowbagger (69688) on Thursday January 18, 2001 @03:54AM (#499544) Homepage Journal
    Obligatory no log in link [nytimes.com]

    However, I wonder if this could also be used for holography: freeze the interference pattern into the material, and read it out later, reconstructing the image. In theory, since the material could record the interference pattern in three dimensions rather than two (like a photographic plate), this might allow for more detailed holograms.

  • ...we sacrifice the idea that a particle has an individual identity...

    That is the key concept that is poorly conveyed within the Times article. It's obvious that even good science reporting is not necessarily understandable by the masses without the teaching genius of a Sagan or the like.

    This brings up an interesting topic, the subject of many late-night, coffee-fueled debates around here: If you could teleport a human through some means, would this property of "no-unique-identity" actually allow you to create an EXACT COPY of the teleported human (who is unaware that he/she/it is even a copy), while, in fact, you KILLED the original? How would you detect this?

  • by pseen (219746) on Thursday January 18, 2001 @04:01AM (#499546)
    I believe you forgot an 0 there! Login : slashdot2000 pass : slashdot2000 Worked for me!
  • by dachshund (300733) on Thursday January 18, 2001 @06:16AM (#499547)
    Yes, you're right. Just found the Discover article [discover.com], which does indeed say that the lab-grown black holes wouldn't be the real thing. They would instead be "a small and completely safe vortex of cold atoms" that would let researchers study the effects of black holes.
  • by dachshund (300733) on Thursday January 18, 2001 @03:57AM (#499548)
    I've heard that this capability might allow scientists to create artificial black holes. Apparently if you can slow light down enough, you might be able to create a situation in which a singularity comes into existence. I wish I had more information on this-- I think I read it in Discover a few months back. I have no idea if this discovery would make such a thing possible. Anyone with more information? I'm obviously fairly ignorant in this area, but the article I read seemed to take the possibility seriously enough.
  • In order to reproduce your inner-most quantum states (e.g. the electrical synapses currently coursing through your system), you'd have to flash every particle some-how. Whenever you detect a particle, you disturb it significantly.

    It seems to me that the only way you could teleport would be to 'flash' the host, then radiate their profile. It _might_ be possible to analyze the profile so as to reproduce multiple targets. But my belief is that the discretization of this profile information would render it useless. Additionally, analog amps / splitters could quite possibly introduce disturbences which would defore the target.

    I still don't think teleportation will ever be practical for life-forms, but it might work for the simple transport of raw minerals (with pure substances). Perhaps, for example, you could energize the minned metal on the moon into a super-plasmic or photonic state which could be tunnelled. Alternatively the wave-properties in cooled matter might be of more use - Instead of super-heating, perhaps super-cooling is what is necessary. Course in either manner, the atomic structure is disrupted, so the usefulness is minimized.

    Heck it would be useful just to condence matter to alleviate gravitational weight for greater space-transport.

    Oh well, fun to brain-storm.

    -Michael
  • by Ferzerp (83619) on Thursday January 18, 2001 @03:57AM (#499550)
    From reading the article, it sounds to me like the light is being destroyed and then new *nearly* (from the article it says it's not the same) identical light is emitted. While interesting, this phenomena is no where near as much of a breakthrough as if they had actually stopped light.

    Correct me if I am wrong, but are they not, in essense, just taking a snapshot of a photon and then recreating it?

    I would go in to some of the implications of actually stopping light (instantaneous communications, etc), but it is too early in the morning for my mind to work that deeply :)
  • by zCyl (14362) on Thursday January 18, 2001 @04:15AM (#499551)
    Technically, taking a quantum snapshot of a photon and then recreating it is the same thing as stopping it and restarting it. When we get down to such a level, we sacrifice the idea that a particle has an individual identity, and instead only acknowledge the existence of a set of properties for the particle. If the experiment simply resulted in light of the same frequency being emitted, then this would still be interesting as a means of optical storage, but by no means would it be as interesting from a theoretical perspective. What makes it interesting is that the imprint of the light is stored in the quantum spin states of the gas atoms, which means there is a theoretical possibility (which can't be determined too well from a nytimes article) that all the "uncertainty information" inherent in the photon is preserved across the restart. That would make this a true stopping and restarting of a photon.

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