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Scientists Solve Century-Old Optics Mystery

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."

39 of 265 comments (clear)

  1. Google cache... by OG · · Score: 5, Informative

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

    1. Re:Google cache... by plasmacutter · · Score: 3, Funny

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

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

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  2. Slashdotted already by dj015 · · Score: 3, Informative

    Google Cache for anyone interested in reading it

    1. Re:Slashdotted already by Cowmonaut · · Score: 3, Insightful

      Timestamps people! Be nice to your fellow posters. If its redundant to a post with the same timestamp just ignore it!

  3. No physics background here by fprintf · · Score: 3, Interesting

    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.

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    1. Re:No physics background here by corsec67 · · Score: 4, Informative

      Nope, a radiometer 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.

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    2. Re:No physics background here by nategoose · · Score: 4, Informative

      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.

    3. Re:No physics background here by GospelHead821 · · Score: 3, Informative

      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.)

      --
      Virtue finds and chooses the mean.
      Aristotle, Ethica Nichomachea
    4. Re:No physics background here by shking · · Score: 3, Informative

      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 according to this formula: e=mc**2. Some guy named Al figured it out at the beginning of the 20th century. He became quite famous.

      --
      -- "At Microsoft, quality is job 1.1" -- PC Magazine, Nov. 1994
    5. Re:No physics background here by JustinOpinion · · Score: 5, Informative

      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 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."

    6. Re:No physics background here by TeknoHog · · Score: 3, Informative

      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.

      --
      Escher was the first MC and Giger invented the HR department.
    7. Re:No physics background here by Hatta · · Score: 3, Interesting

      Both zero and infinite mass work in all of the equations.

      Really? Momentum is p=mv. If the mass of a photon is infinite, then its momentum is too. Since momentum is conserved in a collision, when that photon collides with an object it transfers infinite momentum to that object. If the object is of finite mass, then p/m=v=infinity.

      So why don't I recoil with infinite velocity when I'm hit by a photon?

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    8. Re:No physics background here by CopaceticOpus · · Score: 5, Funny

      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!

    9. Re:No physics background here by blueg3 · · Score: 4, Informative

      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.

  4. "Experimentalist" by girlintraining · · Score: 3, Insightful

    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? -_-

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    1. Re:"Experimentalist" by jc42 · · Score: 5, Insightful

      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.

      --
      Those who do study history are doomed to stand helplessly by while everyone else repeats it.
  5. Bye bye, Tractor Beam... :'( by starglider29a · · Score: 4, Funny

    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!

    1. Re:Bye bye, Tractor Beam... :'( by Evan+Meakyl · · Score: 3, Funny

      if we could use a photon emitted from NEGATIVE MASS

      You're speaking about dark light, aren't you?

  6. Mirrored by dj015 · · Score: 4, Informative
    1. Re:Mirrored by azenpunk · · Score: 3, Insightful

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

  7. and the winner is... by Digitus1337 · · Score: 3, Informative

    "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.

  8. In simple terms... by Thelasko · · Score: 4, Funny

    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.

    --
    One of our competitors trademarked the term "hypothesis". From now on, we will call them "boneheaded ideas".
  9. Re:First Post AND Slashdotted Already by BattleApple · · Score: 5, Funny

    well doesn't everyone have to read the article first in order to comment on it?

    Wait.. what the hell am I thinking?

  10. Re:And in English... by Kartoffel · · Score: 3, Informative

    Experimentalists, as opposed to theorists.

  11. had to be done by Kartoffel · · Score: 4, Funny

    That's what *SHE* said!

  12. Wait, girl or boy? by malignant_minded · · Score: 3, Funny

    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???

    1. Re:Wait, girl or boy? by mattcasters · · Score: 3, Funny

      The confusion was unintentional I think. Perhaps the article was translated from Chinese?

      It's nothing like that famous cockpit conversation between captain Clarence Oveur, co-pilot Roger Murdock and nagivator Victor Basta in the movie "Airplane!" :

      Roger Murdock: Flight 2-0-9'er, you are cleared for take-off.
      Captain Oveur: Roger!
      Roger Murdock: Huh?
      Tower voice: L.A. departure frequency, 123 point 9'er.
      Captain Oveur: Roger!
      Roger Murdock: Huh?
      Victor Basta: Request vector, over.
      Captain Oveur: What?
      Tower voice: Flight 2-0-9'er cleared for vector 324.
      Roger Murdock: We have clearance, Clarence.
      Captain Oveur: Roger, Roger. What's our vector, Victor?
      Tower voice: Tower's radio clearance, over!
      Captain Oveur: That's Clarence Oveur. Over.
      Tower voice: Over.
      Captain Oveur: Roger.
      Roger Murdock: Huh?
      Tower voice: Roger, over!
      Roger Murdock: What?
      Captain Oveur: Huh?
      Victor Basta: Who?

      --
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  13. Already demonstrated at MIT by Muerte23 · · Score: 3, Informative

    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.

    1. Re:Already demonstrated at MIT by waxigloo · · Score: 5, Informative
      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.

  14. Re:First Post AND Slashdotted Already by philspear · · Score: 3, Funny

    1. Posing as someone else, post false news that own lab has made a breakthrough discovery
    2. Take down the faked article before any scrutiny can be applied and it is determined to be a fake
    3. ???
    4. Profit!

  15. Click "Text-only version" by tepples · · Score: 3, Informative

    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.

    1. Re:Click "Text-only version" by Jabbrwokk · · Score: 5, Informative
      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

  16. Re:First Post AND Slashdotted Already by allcoolnameswheretak · · Score: 5, Funny

    Don't underestimate the power of Slashdot. It's as if millions of mouse clicks suddenly jumped out into the internets and the servers where suddenly silenced.

  17. Re:Actually... by Neeperando · · Score: 5, Informative

    Within physics, there is a difference between theorists (people who do try to prove things using math) and experimentalists (people who do experiments to test the theorists' theories).

    Most physicists see themselves as either one or the other, and often the two do not get along. Theorists see experimentalists as being corrupted by real world problems when really all the problems can be solved by a little hard thought (and maybe some math). They think experiments shouldn't be called "science" but "engineering". Experimentalists see theorists as having pointless jobs because nothing they ever do will ever produce something useful to the human race, by their very nature.

    In reality, of course, they are dependent on each other, because without the theorists' theories the experimentalists have nothing to test, and without the hope of some kind of payoff from experimentalists, theorists will never get funding.

    Also, as a non-physicist, it can be fun to pit theorists and experimentalists against each other in battles to the death and watch what happens.

    --
    Being a computer scientist means you tell people how computers should work, not that you know how they actually work.
  18. Re:Slashdot Effect by ArsonSmith · · Score: 3, Funny

    Does the slashdot effect push the server over or pull it down?

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  19. Re:Actually... by SBacks · · Score: 5, Funny

    Also, as a non-physicist, it can be fun to pit theorists and experimentalists against each other in battles to the death and watch what happens.

    Wow, you just don't get it. There's no need to actually pit them against each other, I can provide mathematical proof that the experimentalists will win 84.3% of the time.

  20. Re:Actually... by Neeperando · · Score: 5, Funny

    Touche. But will the experimentalists be satisfied with your result, or will they want to have the fight anyway, just to be sure?

    --
    Being a computer scientist means you tell people how computers should work, not that you know how they actually work.
  21. Re:A little background is apropos me thinks... by bishiraver · · Score: 3, Insightful

    For those with a large monitor, the GP was doing us all a favor. It gets difficult for the human brain to read text with overly long lines: the optimal width is about 65 characters. Longer than that and the eye gets lost when traveling back to the left for the next line. Basic usability/readability knowledge.

  22. Re:First Post AND Slashdotted Already by SolusSD · · Score: 3, Insightful

    when it comes to scientific discoveries its more likely the US government would find it inappropriate.