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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."
Since it's already slahshdotted, here's the cached version.
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!
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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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.
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".
well doesn't everyone have to read the article first in order to comment on it?
Wait.. what the hell am I thinking?
That's what *SHE* said!
So much for posting accurate comments.
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.
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.
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.
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."
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.
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.
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
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!
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.