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Science

Scientists Solve Century-Old Optics Mystery 265

Posted by CmdrTaco
from the all-out-of-joementum dept.
evan_arrrr! writes "From the article: Since the early 20th century physicists have known that light carries momentum, but the way this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric: one suggesting it pushes the surface in the direction light is traveling; the other suggesting it drags the surface backwards towards the source of light. After 100 years of conflicting experimental results, a team of experimentalists from China believe they have finally found a resolution."
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Scientists Solve Century-Old Optics Mystery

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  • Google cache... (Score:5, Informative)

    by OG (15008) on Monday January 12, 2009 @11:52AM (#26418735)

    Since it's already slahshdotted, here's [74.125.47.132] the cached version.

    • by plasmacutter (901737) on Monday January 12, 2009 @12:31PM (#26419373)

      Since it's already slahshdotted, here's [74.125.47.132] the cached version.

      Page wont load in google cache either. Google cache has been slashdotted.

      • by tepples (727027) <tepples@[ ]il.com ['gma' in gap]> on Monday January 12, 2009 @12:48PM (#26419661) Homepage Journal

        Since it's already slahshdotted, here's the cached version.

        Page wont load in google cache either. Google cache has been slashdotted.

        That's because your web browser is trying to pull the CSS and images from the (now slashdotted) original server before it lays out the page. Click "Text-only version" to view the page without CSS and images.

        • by Jabbrwokk (1015725) <grant.j.warkentin@ g m ail.com> on Monday January 12, 2009 @01:55PM (#26420767) Homepage Journal
          Experiment resolves century-old optics mystery

          Since the early 20th Century physicists have known that light carries momentum, but the way this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric: one suggesting it pushes the surface in the direction light is travelling; the other suggesting it drags the surface backwards towards the source of light. After 100 years of conflicting experimental results, a team of experimentalists from China believe they have finally found a resolution.

          Weilong She and his colleagues from Sun Yat-Sen University have studied the effect of light at the interface of air and a silica filament and they find that light exerts a push force on the surface (Phys Rev Lett 101243601) "This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light" said Ulf Leonhardt a researcher in transformation optics at the University of St Andrews, UK.

          The authors suggest this finding could now pave the way for new applications like highly efficient fusion using laser 'compression'.

          100 year riddle

          Hermann Minkowski had proposed in 1908 that light momenta is proportional to a material's refractive index then the following year, another German theorist, Max Abraham proposed the opposite -- momentum is inversely proportional to a material's refractive index.

          It was suggested that this debate should be resolved experimentally but it proved to be notoriously difficult to record the momentum of light in a dielectric. In the seventies it seemed like the mystery was finally solved using a simple experiment involving an air-water interface. Conservation of momentum inferred that if Minkowsi was right, the water surface would compress slightly as light rays pass through, but if Abraham was correct it would bulge. A bulge was witnessed and Abraham was declared the victor.

          Unfortunately, later in the same year further analysis showed the bulge to be the result of an unrelated optical effect; the debate was once again thrown open.

          21st Century makeover

          She and colleagues have now finally overcome these difficulties by replacing the water surface with a nanometre silica filament. "We report direct observation of a push force on the end face of the silica filament exerted by the outgoing light" said She. Given this result, Minkowski has been declared the new winner and light momenta is directly proportional to the material it is travelling through. "The experiment represents a modern form of a beautifully simple idea" said Leonhardt.

          One application that may spring from this knowledge is a more precise technique for laser-induced inertially-confined fusion: a method of producing fusion energy by compressing a fuel capsule made to high density. A series of incoherent laser beams incident on a transparent dielectric ball in a vacuum would cause it to shrink under pressure to achieve nuclear fusion.

          Mansud Mansuripur from the University of Arizona recognizes the potential of radiation pressure for inertially-confined fusion but he warns that She and colleagues have only considered electromagnetic pressure without taking account of mechanical forces. "A correct accounting for the deformation of the silica filament in the reported experiments would have required a complete balancing of the momenta" he said.

          About the author

          James Dacey is a reporter for physicsworld.com

          • by Arthur B. (806360)

            Weird... the slower the light the higher the momentum then ? What was Minkowski argument ?

  • Slashdotted already (Score:3, Informative)

    by dj015 (680676) <darryl.sailingaway@co@uk> on Monday January 12, 2009 @11:52AM (#26418743) Homepage
    Google Cache [74.125.45.132] for anyone interested in reading it
    • Re: (Score:3, Insightful)

      by Cowmonaut (989226)
      Timestamps people! Be nice to your fellow posters. If its redundant to a post with the same timestamp just ignore it!
  • by fprintf (82740) on Monday January 12, 2009 @11:53AM (#26418761) Journal

    Does this article help explain how those little lightbulb things with the rotating black/white cards work? I always loved those as a kid... in fact I was shocked to find them at Home Depot the other day in a demonstration of why LowE glass can be a good thing. They had two of them, but the one behind the low E glass was barely rotating when exposed to a lightbulb while the other behind regular glass was whizzing around.

    • by corsec67 (627446) on Monday January 12, 2009 @11:59AM (#26418879) Homepage Journal

      Nope, a radiometer [wikipedia.org] depends on the air inside the bulb to function. If it was a complete vacuum, it doesn't work.

      It works by the air on the black side of the vanes expanding, while the air on the light side doesn't, moving the vane towards the light side. If it was powered by momentum, it would move the other direction, since absorbing the light should impart less momentum than bouncing the light.

    • by nategoose (1004564) on Monday January 12, 2009 @12:02PM (#26418941)
      No. Those work because the black side of the squares absorbs light which produces heat which makes air touching it heat up which causes that air to expand which creates a pressure difference between that side and the other side of the card which causes the thing to spin.
      The actual force produced is minuscule.
    • by Hatta (162192)

      I might as well ask my physics question here. How is it that light has momentum when it has no mass?

      • by GospelHead821 (466923) on Monday January 12, 2009 @12:42PM (#26419545)

        Light has zero rest mass, but it has an effective momentum and, therefore, an effective mass but only while it's moving (which is always.)

        • Re: (Score:2, Informative)

          by azenpunk (1080949)

          another (much more generic) way to think about it is that momentum gives direction to energy. if you have energy that's not heat, you'll likely find momentum along with it.

          • Re: (Score:3, Insightful)

            by gardyloo (512791)

            It's a nice way of thinking about it. Almost by _definition_, heat is simply energy for which we don't bother to quantify momentum.

        • by nschubach (922175)

          But 0 times anything is 0, so doesn't that mean that Light has a very minuscule mass? (ie: lower than we could measure?)

          • by blueg3 (192743) on Monday January 12, 2009 @02:23PM (#26421225)

            No. The formulas for momentum and energy that are simply a product of mass and velocity are nonrelativistic equations, approximately correct for bodies with rest mass at "slow" speeds.

            There are two quantities when discussing "mass". What we generally refer to as "mass", an intrinsic property of an object, is rest mass. Light has no rest mass (and never exists at rest). Objects with nonzero rest mass can have speeds between 0 (inclusive) and c (exclusive). Objects with zero rest mass have velocity c only.

            The momentum carried by a photon with energy E is p = E / c.

        • by Hatta (162192)

          It's not meaningful to talk about the rest mass of something that's never at rest.

      • by shking (125052) <babulicm@cuug.a b . ca> on Monday January 12, 2009 @12:50PM (#26419701) Homepage

        I might as well ask my physics question here. How is it that light has momentum when it has no mass?

        It has energy, and energy is equivalent to mass [wikipedia.org] according to this formula: e=mc**2. Some guy named Al [wikipedia.org] figured it out at the beginning of the 20th century. He became quite famous.

        • by CopaceticOpus (965603) on Monday January 12, 2009 @02:04PM (#26420909)

          Exactly. This is why a charged battery is heavier than a dead battery (a fact you'll be thankful for if you ever have to push-start a car!)

          Also, have you ever noticed how dust tends to accumulate on a window sill? As sunlight pours down through the window over time, a very tiny fraction of the light is converted from energy to mass. It happens too slowly to observe, but eventually it will accumulate into dust particles.

          Different surfaces will result in different rates of mass conversion. I painted my house with a specially formulated paint with a very low rate of mass conversion, provided by a friend who has military contacts. It sure wasn't cheap, but worth it for all the time I save on dusting!

        • by Hatta (162192)

          So your answer is that light in fact does have mass?

      • by JustinOpinion (1246824) on Monday January 12, 2009 @01:01PM (#26419891)

        How is it that light has momentum when it has no mass?

        For the same reason that speeds don't strictly add up linearly: relativity. In Newtonian mechanics, momentum is p = m*v where m is the mass and v is the velocity. But when you take relativity into account, the proper definition [wikipedia.org] is actually p = gamma*m*v. For a photon, you might think m = 0 would mean p = 0, but when v=c (the speed of light), gamma = 1/0. So you have an equation p = c*0/0. Obviously something is wrong, and in a careful analysis it turns out that for massless objects (which travel at c) p = E/c (where E is total energy, and c is speed of light).

        So, basically the momentum of massless particles arises from taking into account relativity. The fact that we can actually measure photon pressure is an interesting proof that the math "works."

        • by Hatta (162192)

          Obviously something is wrong, and in a careful analysis it turns out that for massless objects (which travel at c) p = E/c (where E is total energy, and c is speed of light).

          But E=m*c^2. So p = m*c*c/c = m*c. And we're back where we started from, p=mv. Plug the measured momentum of a photon in there, and you'll find the mass of the photon.

      • by TeknoHog (164938) on Monday January 12, 2009 @01:04PM (#26419951) Homepage Journal

        I'm not sure if this answers your question, but consider a photon hitting an electron. The electron starts to move a little faster, as it gains some of the photon's energy. But because the motion of the electron changes, there must be some momentum transfer involved, and it must have come from the photon.

        It's really only changes in momentum that can be directly measured. It isn't meaningful to consider momentum (or likewise energy) as an inherent property of the object.

        The weird thing about the photon-electron collision is that the photon won't slow down at all. It can only move at c, or not exist at all. When it loses energy, its frequency decreases. A loose analogy could be an aircraft that's flying at a constant speed, but as it's burning its fuel, the mass is decreasing, and so is p = m*v.

  • "Experimentalist" (Score:4, Insightful)

    by girlintraining (1395911) on Monday January 12, 2009 @11:57AM (#26418833)

    What happened to good old "Scientist"? It's a nice, nine letters long, and respected. "Experimentalist"... It sounds like what a social deviant might call themselves. Like some weird cult that was rejected by the mainstream sect of Scientist, so they had to add an extra six letters to their name to make up for their lack of membership. Maybe more letters makes it sounds more smart? -_-

    • by jc42 (318812) on Monday January 12, 2009 @12:47PM (#26419649) Homepage Journal

      What happened to good old "Scientist"? It's a nice, nine letters long, and respected. "Experimentalist"... It sounds like what a social deviant might call themselves. ...

      Of course, the more common term is "experimental scientist", as opposed to someone like Albert Einstein or Stephen Hawking, who were/are mostly known as theoretical scientists.

      But "experimentalist" is a valid English word, makes sense in context, and has fewer syllables than "experimental scientist" while still emphasizing the experimental nature of their work.

  • by starglider29a (719559) on Monday January 12, 2009 @11:59AM (#26418861)
    And the winner is... "pressure!"

    Dang it!!! There goes my bet with Hawking about making a tractor beam. But wait... if we could use a photon emitted from NEGATIVE MASS it would have NEGATIVE MOMENTUM!!! Ok, Stephen... it's ON!
  • Mirrored (Score:4, Informative)

    by dj015 (680676) <darryl.sailingaway@co@uk> on Monday January 12, 2009 @12:01PM (#26418921) Homepage
    Text Only Mirror [dazneale.com]
    • Re: (Score:3, Insightful)

      by azenpunk (1080949)

      i read 'text only mirror' and my first thought was 'how in hell do they choose what gets reflected?'

  • and the winner is... (Score:4, Informative)

    by Digitus1337 (671442) <`moc.liamtoh' `ta' `sutigid_kl'> on Monday January 12, 2009 @12:02PM (#26418939) Homepage
    "We report direct observation of a push force on the end face of the silica filament exerted by the outgoing light" said [Weilong] She."

    TFS left it out; this was the result.
  • Slashdot Effect (Score:2, Insightful)

    by mfh (56)

    We need to get these guys working on the Slashdot Effect, next.

  • by Thelasko (1196535) on Monday January 12, 2009 @12:08PM (#26419049) Journal
    The mystery is whether or not giving your child the same name as a feminine pronoun is confusing.

    The answer is, yes, it's very confusing.
  • I think it all depends from which side you look at it. From the light's perspective, or from the surface.

    • Re: (Score:3, Insightful)

      by gardyloo (512791)

      I think it all depends from which side you look at it. From the light's perspective, or from the surface.

      So you're saying that from one perspective a surface will be attracted to the direction from which the light came, and from another perspective it will be repelled? That is *not* a relativistically viable effect :)

  • Push me Pull me (Score:2, Insightful)

    by drewsup (990717)
    Seeings how we are already experimenting with laser driven propulsion, i would have though the answer was obvious..
  • by malignant_minded (884324) on Monday January 12, 2009 @12:22PM (#26419227)
    The article is unclear to me, maybe I missed something

    ...Weilong She and his colleagues...

    Ok so we are talking about a guy right?

    This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light" said Ulf Leonhardt a researcher...

    hmm not sure article doesn't indicate one way or another

    ...Hermann Minkowski had proposed in 1908 that light momenta is proportional to a material's refractive index then the following year, another German theorist, Max Abraham proposed the opposite...

    Still guys right?

    21st Century makeover

    She and colleagues have now finally overcome these difficulties by replacing the water surface with a nanometre silica filament.


    Wait who is a she???
  • by Muerte23 (178626) on Monday January 12, 2009 @12:24PM (#26419249) Journal

    http://arxiv.org/abs/cond-mat/0502014

    This paper from MIT showed conclusively through experiment (almost 4 years ago) that in a refractive material the medium temporarily gives up its momentum to the photon, so that the momentum of the photon in the medium is nhk.

    It's too bad that this new experiment didn't cite the prior art.

    • Re: (Score:3, Funny)

      by plasmacutter (901737)

      nhk

      ah the joy of field specific acronyms nobody understands.

    • Re: (Score:3, Funny)

      by jc42 (318812)

      ..., so that the momentum of the photon in the medium is nhk.

      Hmmm ... a bit of googling ...

      NHK could be Nihon Hohsoh Kyokai, the Japanese broadcasting company.

      NHK could be Nederlandse Hervormde Kerk, the Dutch Reformed Church.

      But somehow, I suspect that neither was what was meant. Got a better expansion that makes sense in context?

      • Re: (Score:3, Informative)

        by blueg3 (192743)

        Wavenumber (n), wavevector (k), and Planck's constant (h).

        E = nhk = hf = hbar*omega

    • by waxigloo (899755) on Monday January 12, 2009 @12:44PM (#26419585)
      If you actually read the article, you would see that it does cite the reference that you point to.

      So much for posting accurate comments.

      • by Muerte23 (178626)

        Wow, you are correct. When I downloaded the paper and scanned the references, I somehow didn't see the reference listed.

        My mistake.

        I guess I have to change my point to "how is this new now?" But I guess a nice experiment in any case.

        • Re: (Score:2, Informative)

          by waxigloo (899755)
          It may have to do with the fact that the paper you cited is measuring recoil momentum in a cold atom cloud and not a traditional dielectric material. But I am not sure.
    • by mbkennel (97636)

      Indeed, especially given that the abstract of the PRL says that the conclusions are the opposite, i.e. supporting Abraham's theory, whereas nhk is Minkowski's side.

      "There are two different proposals for the momentum of light in a transparent dielectric of refractive index n: Minkowski's version nE/c and Abraham's version E/(nc), where E and c are the energy and vacuum speed of light, respectively. Despite many tests and debates over nearly a century, momentum of light in a transparent dielectric remains con

  • I knew I read about this quite some time ago: http://arxiv.org/abs/0806.2442 [arxiv.org]

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