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Scientists Solve Century-Old Optics Mystery
Posted by
CmdrTaco
on Mon Jan 12, 2009 11:47 AM
from the all-out-of-joementum dept.
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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Google cache... (Score:5, Informative)
Since it's already slahshdotted, here's [74.125.47.132] the cached version.
Re:Google cache... (Score:4, Funny)
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.
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Click "Text-only version" (Score:4, 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.
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Re:Click "Text-only version" (Score:5, Informative)
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
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Slashdotted already (Score:3, Informative)
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No physics background here (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.
Re:No physics background here (Score:5, Informative)
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.
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Re:No physics background here (Score:5, Informative)
The actual force produced is minuscule.
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Re:No physics background here (Score:4, 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.)
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Re: (Score:3, Insightful)
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.
Re:No physics background here (Score:5, 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.
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Re:No physics background here (Score:4, Informative)
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.
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Re:No physics background here (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!
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Re:No physics background here (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 [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."
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Re:No physics background here (Score:4, 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.
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Re: (Score:3, Insightful)
are you mad??? if light had infinite mass, then every torch would act like the Half-Life 2 gravity gun.
no... if you turned the torch on, you would be instantly destroyed.
there is no such thing as "infinite" in reality - every case where "infinity" turns up is a case where the existing maths is not quite up to describing reality.
Re:No physics background here (Score:4, 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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"Experimentalist" (Score:4, 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? -_-
Re:"Experimentalist" (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.
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Bye bye, Tractor Beam... :'( (Score:5, Funny)
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!
Re:Bye bye, Tractor Beam... :'( (Score:4, Funny)
You're speaking about dark light, aren't you?
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Mirrored (Score:4, Informative)
Re: (Score:3, Insightful)
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)
TFS left it out; this was the result.
had to be done (Score:4, Funny)
That's what *SHE* said!
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Re: (Score:3, Funny)
In simple terms... (Score:5, Funny)
The answer is, yes, it's very confusing.
Wait, girl or boy? (Score:3, Funny)
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
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???
Re:Wait, girl or boy? (Score:4, 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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Already demonstrated at MIT (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.
Re: (Score:3, Funny)
nhk
ah the joy of field specific acronyms nobody understands.
Re: (Score:3, Funny)
..., 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)
Wavenumber (n), wavevector (k), and Planck's constant (h).
E = nhk = hf = hbar*omega
Re:Already demonstrated at MIT (Score:5, Informative)
So much for posting accurate comments.
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Re:First Post AND Slashdotted Already (Score:5, Funny)
Wait.. what the hell am I thinking?
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Re:First Post AND Slashdotted Already (Score:4, 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!
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Re: (Score:3, Funny)
Re:First Post AND Slashdotted Already (Score:4, Insightful)
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Re:First Post AND Slashdotted Already (Score:5, Funny)
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Re:And in English... (Score:4, Informative)
Experimentalists, as opposed to theorists.
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Re:Actually... (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.
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Re:Actually... (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.
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Re:Actually... (Score:5, Funny)
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Re: (Score:3, Insightful)
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 :)
Re: (Score:3, Funny)
Still using a 40-columns monitor, I see.
Re:A little background is apropos me thinks... (Score:4, 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.
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Re:Slashdot Effect (Score:4, Funny)
Does the slashdot effect push the server over or pull it down?
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Re: (Score:3, Funny)
What is the answer?
The answer is that slashdotting provides a positive force on the server regardless of the medium.