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Science

Light Traveling at 38 Miles an Hour 156

the-empty-string writes "A Danish physicyst and her team managed to slow down a beam of light to an astonishing speed of only 38 miles per hour. It's on the front page of the New York Times." Update: 02/18 12:20 by H :One of our more scientifically inclined readers sent in a better explanation-click below to read it.
Mike Schiraldi writes "There has been so much misinformation and confusion on /. regarding slowing down the speed of light.. So many comments are along the lines of "Wow, i thought the speed of light was constant. I can get a car with slow headlights and drive faster than the beams!"

You can't let so many people be misinformed. Please let them know:

The speed of light IN A VACUUM is the constant c.

Science has been able to slow light down for centuries by making it travel through a different medium. The speed of light in air is less than c. The speed of light in water or glass or plastic is less than c. This is what makes lenses work. (check your encyclopedia for more information)

The reason this is a breakthrough is simply because they've been able to slow light down to a much lower speed than anyone else has ever been able to do. They have not changed c, the speed of light in a vacuum. The beam of light is only slowed down when it's in the condensate. Once it leaves the condensate and is travelling through air again, it returns to a speed of 186,000 miles per second. This does not contradict any law of physics. "

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Light Traveling at 38 Miles an Hour

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  • From what I can tell, the speed of the light travelling through the BEC is dependent on the temperature of the BEC. Now, a few billionths of a degree above 0K is really damn cold. It take an enormous amount of energy to get something that cold (that's why they call it high energy physics!).

    I can see using the properties that we learn elsewhere, but don't expect to ever see slow light yourslef...:)
  • The way I understand it is that mass increases as speed approaches c. If you actually went 100% of the speed of light, your mass would be INFINITE. But since acceleration takes force, it would be harder and harder to apply force to get you up to the speed of light. Eventually, just short of c, it would be impossible to get you going any faster.
  • woof :)
  • by Edge ( 640 )
    How would slow light make waking up in the morning easier?

    You aren't going to be able to control the speed of the sun's light.

    You could set up lights in your room to come on at a set time in the morning, (if it were still dark outside). But that would not benefit from slow light either.. You would have to control the power going to the lighting fixture, not the speed at which the light eminating from the fixture would reach you.

  • Posted by NeuroNomad:

    For years we have tried to come up with a way to travel faster than the speed of light, hehehe, could not do that so now we have just decided to slow light down. They even are going to try to slow it down slow enough that a turtle could out walk a beam of light.

    That is funny.
  • Posted by Tester TW:

    Here's the URL
    http://www.nature.com/cover/cover0218.html
  • Posted by tdibble:

    No, the first poster was right. They cooled the tramsmitting medium such that the time between absorbsion/emmission was dramatically heightened. I look forward to seeing a video of this, personally.
  • Posted by tdibble:

    This isn't a hard fact. There is a fringe branch of physics which is looking into *increasing* the "c" constant. The trick here is that a vacuum is devoid of matter, but not of energy ond forces; it is this which constrains the speed of light, and modifying the constraints may allow light to travel faster than the cvac.

    In other words, the finality of the maximal speed of light is accepted generally, but not completely, and there is some evidence in favor of the vocal few.

  • Posted by skitzo:

    hmmm...that's very cool.
  • Posted by Bill, the Galactic Hero:


    The phase velocity is equal to the optical frequency divided by the magnitude of the wave vector (i.e., if optical freq. = omega & wave vector = k, vphase = omega/|k|. Remember omega and k from exp[ i ( omega t - k.r) ], the active ingredient in a wavefunction).

    The group velocity is equal to the derivative of the optical frequency with respect to the magnitude of the wave vector.

    Where the relationship between the optical frequency and the wave vector's magnitude isn't strictly linear, that's where the phase velocity and the group velocity differ.

  • Posted by AnnoyingMouseCoward:

    There is a little problem - it's called zero point energy. This permeates all of space/time, and was predicted by the Dutch physicist Hendrik Casimir in 1948. It's existence has been experimentaly proven ( refer to "Physical Review Letters, vol 78, p 5 ") by Dr Lamoreaux at the Los Alamos labs back in 1997.

    Zero point energy prevents any physical particle from ever being completly still. It's the residual energy left over from the Big-Bang. Acording to the American physicist Richard Fenyman, the ZPE content of a volume the size of a coffee cup would be sufficient to completly vaporise the Earths oceans.

    ZPE manifests as the constant production of particle/anti-particle pairs, the duration of their existence being inversely proportional to their energy ( in accordance with the Heisenberg uncertainty principle ). This is the source of Hawkins radiation around black-holes and the main reason why cyrogenic preservation is limited to a few thousand years, even if you could get to absolute zero ( which the third law of thermodynamics says you can't ).
  • Turing used cycling mercury delay lines on his first computer. Anyone want to calculate how much this thing can store as a cycling optical signal? Include an overhead for error correction...
    --
  • unfortunately, this article requires a subscription.

    Does anyone have a url of a version of this which is freely viewed?

    -herb
  • Now we can drive at light speed or faster. That should cut down on the commute. ;>
  • Weird that I first heard about her on slashdot.

    It should tell something, either about me or about the Danish media.
  • So how exactly does the medium slow down light? Do the photons bounce off the molecules? If so, it sounds to me like light isn't really slowing down, but rather taking a longer path.


    --
    Timur "too sexy for my code" Tabi, timur@tabi.org, http://www.tabi.org
  • To a decent approximation, the speed of light (measured locally) is not affected by gravitational fields - the only thing that changes is the apparant frequency of the light as seen by different observers (known as gravitational redshift).

    As others have noted, the speed of light in a non-vacuum medium is slower by a ratio known as the refractive index, which is less than 2.5 for most materials (I think diamond is 2.42 or something) and is exactly 1 in a vacuum. So, the real news story here is the creation of a material with a *huge* refractive index (which is really neat).

    Depending on how you manipulate the equations, it *is* possible to have a refractive index less than 1 (and therefore a "speed of light" faster than in vacuum). These conditions can occur in an ionized gas, such as the layers of our atmosphere which bounce radio signals around the globe. However, it also turns out that you can't transmit any information through this medium faster than in vacuum, so it's really just a mathematical curiosity.

  • >What effect does this have on E=mc^2?

    -----BEGIN RANT BLOCK-----
    version 2.10a

    None whatsoever; "E=mc^2" will continue to be a widely-parroted simplification of the more general definition of the total energy of a particle, (E^2) = (m0c^2)^2 + (pc)^2, where m0 is the particle's rest mass and p is its momentum, and c is the speed of light _in vacuum_ (which hasn't changed). This news story is about some unique optical properties of a new type of matter, but it doesn't affect electromagnetism or relativity theories.

    E=mc^2 may have been a big deal back when the "wireless" and the "horseless carriage" were on the top of every geek's Christmas wish-list, but these days it's just a routine part of physics like Ohm's law (V=IR) is in electronics. It's not a big deal. The rest of Einstein's work is much more interesting (general relativity, stimulated emission (lasers), the photoelectric effect, etc).

    "E=mc^2" may look good on a T-shirt, but apart from that there's little reason why this particular Thought McNugget deserves to be so widely quoted when so much more of the really interesting physics is completely ignored.

    By the way, nuclear fission was observed in the lab by people like Otto Hahn, without the need for "E=mc^2" to explain where the energy came from. Nuclear bombs are just a matter of banging rocks together, once you have the right rocks - who cares if the total mass of the bomb fragments is slightly less after the blast? Your city still ends up as kitty litter thanks to the binding energy in the nucleus.

    ------END RANT BLOCK------

  • Oh dear... I'm confused. I thought speed of light was a constant ( c )? Or is that speed of light in a vaccuum? So what's the speed of light in vaccuum compared to the speed of light in air or water, then?

    Feel free to flame away, I have absolutely no clue what I'm talking about and would appreciate it if someone could explain it in armchair terms.

    At least now I know what computer illiterates feel like when I talk to them.

    (it ain't my fault, my physics teacher was too attractive for 16-year-old boys to actually pay attention to what she *said* :-)
  • For some strange reason too many people don't see the difference between

    1. Raping children, recording it, trying to squeese out as much money as possible.

    2. Beeing a pedophile and arousing yourself with sexual images of children, not produced using real children.

    For me, people can have whatever strange sexual ideas they like, as long as they don't hurt people by, say, raping them.

    So, what should be outlawed is of course not 'child porn' (wich can, for example, be drawn or computer animated) but raping or otherwise hurting children.

    (Unlike here in Sweden where we have a new law since 1999 outlawing even the owning of any kind of child porn. Oh, they are so stupid! And over half the 'liberal' party voted for it (twice) and the largest 'conservative' voted against it. Just shows (again) that all they care of is votes/money/power and that ideologies and morals definetly comes in second place. Sigh.)
  • Einstein's 1905 work has been confirmed in many different contexts, from radio transmission to particle decay. The ironic thing about your statement is that physicists probably know more about light than any other phenomenon!
  • Any fun time dialation involved?


    --
  • I wonder what this means for fringe physics?

    Like, if they get it going really slow, what's that mean for going faster than light?

    -- haaz, playing Lead into Gold today..

  • remember, indians a century ago put their ear on a railtrack so before they saw the train, they know if there was one or no (my english is not very good), they even were able to know the distance!

    anyway, speed of sound is greater in steel that in air, as light speed is slower in water than vaccum, etc.
    --
  • Years ago a science fiction author called Bob Shaw
    had a book predicated on a material called slow
    glass. Basically it took light years to travel
    through the medium, so you took a piece of glass
    set it somewhere scenic for a year or two and then
    hung it on your wall and enjoyed the view for
    the same couple of years. A few more orders of
    magnitude and we will be there..
  • I'm way out of my league in evaluating the use of slow light. So please, someone explain the benefit of slowing down light. It just seems a little weird to intentionally slow down the fastest known thing in the universe.
  • ...and a monkey might shoot out my butt.

    After reading about just how much work they went through to make a BEC (that's Bose-Enstien Condensate), they then suggest that this could be used for night vision goggles.

    Does anyone else NOT want a substance at about a billionth of a degree kelvin near their head? This stuff makes liquid nitrogen look toasty by comparison.

    BEC's have all kinds of "odd" properties that make them interesting, but I can't get excited about strapping one over my eyes. Call me silly, but that just strikes me as a Bad Idea.

    If they can duplicate the same quantum states that allow this high index of refraction in something a little more stable, this could actually lead to something practical, but I'm not holding my breath.

    Also of interest, someone used a BEC to create a coherent beam of matter. I don't remember the URL though.
  • No, the actual wave packets, or photons, that carry the light are still travelling at 298,000 km/s (or whatever exact figure it is) it is just that they're being absorbed and reabsorbed. When a beam of light containing billions of these photons enters the condensate the condensate will absorb them and the atoms which then are excited by absorbing these photons re-emit them after a certain time.
    It isn't that difficult to comprehend unless you try and understand how they are abosrbed and re-emitted. THen it gets trickier to explain in normal words. :-)
  • What about in the cold of space? Could this be used to trap solar energy? Space is near 0K and you would not need the energy requirements to maintain the low temp of the media. I see this as a very useful application of this technology.
  • I was thinking the same reading this article, if i remember correctly the book ended on a somwhat '84ish note, slow glass of different "speeds" (was'nt it calld retard or something) being used for surveillence and all
  • Somebody up there mentioned using BECs to make a kind of laser that shoots atoms instead of photons. Well, here's MIT's page on doing exactly that: ufn.ioc.ac.ru:8200/news/eng/1997/0397.html
  • ...in car headlights. Sorry. Couldn't resist.
  • No, high energy physics is more usually used when talking about particle accelerators, etc. The energy requirements aren't huge - all you're doing is gently nudging a few billion atoms around with light.
  • Let's face it - you can't fully describe what happens to light in a transparent medium (of any considerable density) without looking at the quantum mechanics.

    As a first approximation, what happens is that the light enters the material, is absorbed by an atom and then re-emitted after a tiny delay and in the same direction. It then gets absorbed by another atom, etc etc. The delay before being retransmitted is what causes the reduction in the speed of light.

    Something similar to what you describe occurs in a translucent medium - here the photons are re-emitted in random directions (and after a longer delay), so they do indeed seem to bounce off the atoms.

    Note that in a BEC, there's no such thing as "an atom", so the actual mechanism is almost certainly nasty enough to require a PhD to understand.
  • Light entering the medium gets held up in an optical "traffic jam" (not to be confused with "optical molasses"...), so a volume of BEC left in a beam laser light at the right angle and polarisation for a given time will contain more light than an equivalent volume of some other transparent medium. You cannot "suspend" light in a BEC because the BEC would have to be at absolute zero, and thus infinitely large (thanks to Heisenberg's uncertainty relation).
  • Would it neccessarily heat up? I think it would depend on the transparency of the material. I could be way off base here, but there are lenses for extremely high power lasers which do not melt when used. If the glass 'slowed' the light down, via absorbing energy, then you wouldn't be able to see anything out of it anyways.. (not in the visible spectrum at least)..

    Maybe this hypothetical slowglass would make a great battery?
  • Sounds like he got his information in a college physics class; where were you when they covered optics and electrostatics?

    The higher the index (i.e. slower the speed of light) in a material, the thinner the lens. That's why I can get 1/8" thick polycarbonate lenses for my glasses that do the same job as 1/2" thick glass.

    Spectral dispersion is a problem in this application, though... however, if you're doing spectroscopy, a high-dispersion prism is useful... so therefore spectroscopy may be one of the first applications of a condensate prism.

    As for the capacitor issue - The permittivity (capacitance/thickness) of a material is established by the speed of propogation of an EM wave (e.g. light) through the material. So a Bose-Einstein Condensate could make an outstanding dielectric for a capacitor, assuming its breakdown potential and leakage properties are acceptable.

  • So if your driving at 38mph with the headlights on...

  • Yes, if you could push a mirror through the condensate faster than 38 mph you would definitely get some pretty cool Cherenkov radiation. This is essentially what Cherenkov radiation is, only Cherenkov radiation is produced by charged particles moving faster than 38 mph. Of course, moving a mirror through this gas would pose certain difficulties.

    I heard an interesting story once regarding Cherenkov radiation. Apparently in some of the older accellerators, in order to line up the detector with the beam of electrons (or whatever), the experimentalist would put his/her eye in the path of the beam and look at the Cherenkov radiation (typically blue) in order to see precisely where the beam was. The electrons were travelling (once in the eye) faster than the local speed of light, so they spit off Cherenkov radiation. But I'm not sure I'd really want them shooting on through my brain!
  • Actually, the neutrinos can't create Cherenkov radiation, since they're neutral. The neutrino detectors detect the charged particles created when the neutrinos decay. But you are right about them using Cherenkov radiation to do it.
  • Does anyone else NOT want a substance at about a billionth of a degree kelvin near their head? This stuff makes liquid nitrogen look toasty by comparison.

    Actually, the BEC has a very tiny head capacity, so it probably wouldn't freeze your eyes... Of course, the vacuum chamber might break your neck...

  • my car was faster than a beam of light!
  • Well, yeah, I guess...But since the slowing occurs in a lab environment, and probably inside some rather spooky equipment, I don't think it's gonna help much.

    A bomb on the lab is still going to vaporize the whole shebang.

    How did we get on this topic, again? I can't believe I'm talking about bombing a danish phyics lab.
  • So did anyone else think of that Bob Shaw short story 'Light of Other Days'?
  • c I believe was the speed of light in a vacuum that is constant speed of light through anything else isn't
  • Yeah but I'm too tired to punctuate....Actually I'm too lazy cause I never do it when I'm awake either..
  • Hmm... Here's an odd thought.. For a moment, assume that technology has advanced to a point where size no longer matters. Power no longer matters. Heat no longer matters, etc.

    Ok, everyone has seen the light sabers in Star Wars. What if the handle contained a SUPER high powered laser and one of these devices. Then, when activated, the light would slowly extend until it reached a set limit. Then the device would get to 0 degs K and hold the light still.. Ok.. So I'm crazy.

    Maybe I just like light sabers too much.

    :-)
  • by Bearpaw ( 13080 )
    If light can be slowed to a certain speed, does that also mean it's -possible- to speed it up?

    I don't think it's implied by this approach.

    I've never really believed that the "maximum speed" of anything is the speed of light. It just seems odd placing a limit on something that we don't really know everything about.

    Not really. I don't know everything about my car, but I have a pretty good idea what its limits are. We may not know everything about light, but we know some things. More importantly, we know quite a bit about the behavior of mass as it approaches lightspeed -- it acts in a way that very strongly supports the idea of a lightspeed limit.

    It *may* be possible to somehow "sidestep" the lightspeed limit, but that's another question.
  • This is, sort of, the first thing that popped into my mind too. Consider fiber optic networking.
    There is a speed limit (not a bandwidth one) imposed on these networks because we can not pump data into them fast enough. But once data is in there, it's moving real fast.

    It's moving so fast in fact, that a single bit of data takes up a whole lot of space. You need a very long piece of cable to 'pipeline' a very short burst of data. Hence, the lengths of cable in an optical FDDI ring must be pretty significant, else a data packet can start arriving at the original station before it's finished being sent. This poses a problem.

    Now, with a EBC fiber, with light moving REAL SLOW, you can pump in individual photons for individual bits... So the data density per linear distance increases significantly, as the rate of transmission decreases proportionally. Not exactly a good solution for a networking medium...

    But, consider a very short (distance) EBC optical buffer that data is stuffed into; and once full, a switch opens and a massive burt of data (oh, a GB) blasts down the pike at the speed of light.
  • About ten years ago, I went to Florida on vacation. On the Ft. Lauderdale boardwalk, I bought a little tin can, roughly the size of a Coke can, labelled "Canned Florida Sunshine". A typical tourist novelty - like sand in shorts.

    I've never openned it for fear of the sunshine getting out. But now, if I have some of this supercooled atom soup, I can open the can, capture the sunshine, and enjoy it during the New England winter. :)
  • by TA ( 14109 )
    The maximum speed of information is the speed of
    light in vacuum. If information went faster you
    would be messing up the sequence of cause and effect.
    The light speed in non-vacuum hasn't got anything
    to do with this though.
    (and note that if it's not information it *can* go
    faster than light, but it's useful for nothing anyway so..)
  • There's nothing stopping you from going faster than
    light speed in non-vacuum (by that I mean faster than
    38 miles/second in this case). It happens all the time in
    physics experiments. It's a bit like going faster than the
    speed of sound, it will be a light "boom" which is visible as a blue
    corona. I'm not 100% sure but I believe the blue color
    of the water in nuclear reactors are caused by this.
    In any event I have seen many photos of the effect.
  • As far as I understand it: no. The "speed of light" that everyone hears about is the accepted speed of photons through space, with nothing to slow 'em down. The speed of light goes down as the photons pass through various substances, much like a person trying to run through water is much slower than on dry land. This is what causes all those nifty keen prismatic effects when light goes through glass and water at the right angle.

    Now, just as the speed of light in space is a certain amount, so too is the speed of light in this substance the scientists are playing with. And just like you cannot exceed the speed of light while travelling through space, you can't exceed it if you were travelling through that super-cooled substance. Therefore, no object could accelerate even close to 38 miles per hour, inlcuding radio waves, fast cars, or Microsoft lawyers on their way to court with their latest faked video, while travelling through that substance.

    Or something like that...
  • A nice black peice of paper will do that, you'll end up with a hot black peice of paper..

    ;-)
  • "Other Days, Other Eyes" was the name of the book. As I recall it ended with a scene of crop dusting airplanes seeding the entire countryside with slivers of slow glass, each a different thickness and thus having a different time constant. Talk about invasion of privacy . . .

    VonSlatt
  • It runs Windows 98. :)

    ...David
  • As well, as long as you BEC isn't perfectly transparent, you'd get heat deposited in there. Which might make it somewhat difficult to keep it at 50E-9 K...
  • There was a Wired article on some guy that said he could reflect gravity. It involved a superconducting disk, and the Wired article made him out to be a loony, but they didn't discount the possibility.

    The idea being that the superconducting disk would diminish the force of gravity between any objects on opposite sides of the disk.

    Probably BS, but whatever.
    --
  • as username and password.

    Petrus Vectorius
  • Oh, man, I read that story in highschool -- loved it.

    One thing I brought up was that the slow-glass farms would have to be really secure against tresspassing, else pranksters could moon the glass (only to be seen 10 years later), or worse.

    Also: once installed, light from inside the house would start traveling throught the glass. 10 years later, someone could watch what was going on in your house. (Imagine the year is 2006 and we finally get to see OJ kill his wife?)

  • I remember seeing some japanese show that was doing that with big solor collectors on building roofs, and distributing it indoors via fiber optics. Bummer when it's cloudy out, tho.
  • C is the speed of light in a vacum. What light does in gas, water or Bose-Einstein condensates is of zero consequence to relativity.
  • Is there anyone who could give us a more elaborate summary than this ? I can't get at the article either... (can you say /. effect ?)

  • See, this is why Anonymous Cowards need to go away. This one couldn't be bothered to read the article before posting a stupid and arrogant comment. Besides, no one implied that c3e8; the slowing of light as it passes through other substances is a well known phenomen among bright, interested 12-year-olds.
  • The speed of light does not slow at all. It remains at the constant velocity of c, BUT takes a longer path through mediums other than vacume. It's like driving your vehicle 70mph from point a to point b, then driving it again at 70mph, but taking a detour to point c before arriving at b, taking twice as long and claiming the speed at being 35mph. The speed does not change on bit, but the distance travelled does. Just goes that the distance travelled through a medium is longer than trough a vacume so it would take longer for the light to travel through the medium, but the velocity of light is still the same.
  • Slowing the speed of light won't change wavelength. I.E. you can't change visible light into x-rays because the wavelength remains the same. Wavelength velocity. x-rays and visible light travel at the same velocity.
  • Hey, anyone know alot about Absolute Zero?
    From what I remember when the temperature reaches Absolute Zero the atoms just stop moving?
    Anyone have any info on it?
  • Speaking of neat visual effects, I wonder what the light looks like exiting the einstein-bose material.

    When a laser is shot through a glass of light it refracts and generally keeps moving as a ray. Cherenkov radition creates a nice blue glow. The question being, when light exits the Danish material, does it exit as a general glow or as a a slightly diffracted beam?
  • Not quite. Cherenkov radiation is the shock wave (of light) produced by something travelling faster than the *local* speed of light -- e.g., something moving 50 MPH through our Bose-Einstein condensate with a 38 MPH local lightspeed. It's analogous to the shock wave produced by a supersonic object (bullet, jet, etc.) moving through the atmosphere faster than the local sound speed. But the "superluminal" object is still limited to travelling slower than the *vacuum* speed of light (c). (I suppose a tachyon *could* produce Cherenkov radiation, but you don't need one.)

    Experimental physicists use this effect in things like neutrino detectors, such the AMANDA detector at the South Pole: fast-moving subatomic particles created by interactions between neutrinos and atoms inside the earth go zipping through South Polar ice faster than the speed of light *inside ice*, creating a flash of Cherenkov radiation. If you have light detectors embedded in the ice, you can pick up the flash; if you have enough detectors and good clocks, you can track the flash moving through the ice and figure out which direction the particle came from, which tells you where in the universe the parent neutrino came from...

    I suspect the original poster's idea *won't* work because it relies on getting light to travel faster than the local light speed, which by definition may not be possible...but I'm not up on my relativistic shock wave theory.
  • Seems to me that the people that slowed light down didnt get enough attention from their parents as children or just wanted to be in the news because I don't see any use for this! Do you? (feel free to flame...I like flames...see the nickname!!!)

    If you are going to do something like that, do something thats useful! (besides filling up space in the sunday paper!)
  • So you get this material with a mega-galactic refractive index, so the light passing through must presumably travel in some sort of spiral or helix pattern. Therefore the distance it must travel is MUCH further.

    So how do you measure the speed?

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