Slashdot Mirror
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."
'nuff said.
I go to the article with zero comments posted and the server is already unavailable. Talk about preparation in advance...
"The first time I got drunk, I got married. The second time I bought a chimpanzee, after that I stayed sober" Arian Seid
Letting AC's see posts 'in the future' really helps with trolling.
Thanks.
Hmm. If you're like most people, I'm guessing that you don't have a
lot of experience with physics. (Well, not in the formal sense,
anyway! Most people are expert physicists on an intuitive level, with
such remarkable skill that they can lift objects, understand
reflections in a mirror, and even catch a flying ball!) So I'll try to
keep this frosty-posty on a very basic level, and I apologize in advance if I
start spouting jargon or go too fast. If you _do_ have a bit of a
physics background, my apologies for the simple explanation that
follows. I'll label some particularly important paragraphs with "***".
For the record, you won't need any "scholarly journals" here unless you
want to get very cutting edge indeed (or unless you want to go back
many decades or centuries to the original writings that discussed the
concept). The vast majority of what we know about momentum can be
found in textbooks: it is one of the most basic concepts in physics.
Also, keep in mind that while momentum is a fundamental part of
physics, the word has many (related) meanings in colloquial English,
too. I'd guess that a play named _Momentum_ will draw on a wide range
of those.
*** So, what is momentum? The first and most basic statement of the
concept of momentum comes from Newton's First Law of Motion: "An object
in motion will remain in motion unless acted on by an outside force."
(That's sometimes called the "Law of Inertia"; "inertia" and "momentum"
are closely related concepts.) In physics, an object's "momentum" can
be thought of as the "amount of motion" that it has: the greater its
momentum, the harder it is to stop it or to turn it in another
direction.
*** What makes an object harder to stop? Well, the faster it's moving,
the more you have to slow it down, so momentum must depend on speed.
(In fact, it turns out that the direction is important, too; physicists
call speed in a specified direction "velocity".) And the heaver it is,
the harder you have to push to slow it down, so momentum must depend on
"mass" (which is a physicist's technical term for what we normally
think of as weight).
***The formal mathematical definition of momentum (in classical
physics) is the product of those two quantities:
momentum = mass * velocity
To give a few examples, a flying gnat is fast but its mass is very low,
so it doesn't have much momentum. That means that it's easy for a gnat
to turn around and buzz in another direction (which you've probably
seen firsthand). On the other hand, a slowly rolling car still has a
lot of momentum because it's so very heavy: it would be hard to push
one to a stop even at very low speeds. As yet another example, a
bullet is pretty lightweight, but when it is fired from a gun its
enormous speed gives it very high momentum (and if a person tragically
gets in its way, the effort of absorbing all that momentum will break
their flesh and bones).
*** Now, as I mentioned earlier, "velocity" implies not just speed but
direction. So since momentum is proportional to velocity, momentum
always has a direction, too. That's a very fundamental fact about
momentum! Changing an object's direction can be just as hard as
stopping it completely.
*** Another remarkable fact about momentum is that the _total_ amount
of momentum in a system will never change. Physicists call this rule
"Conservation of Momentum", and they say that "momentum is conserved".
For example, if you're playing pool and you hit the cue ball into the
eight ball, when the cue ball slows down the eight ball will start
moving to make up the difference. If your shot is perfectly straight,
the cue ball may stop moving entirely while the eight ball rolls away
with the same velocity that the cue ball used to have. (It would
_have_ to be the same velocity: because the balls have the same mass,
conservation of momentum m
Is it just my observation, or is eldavojohn an idiot?
the mysterious attractiveness of the first post!
Since it's already slahshdotted, here's the cached version.
What is the answer?
Google Cache for anyone interested in reading it
The link is down! Now no one can complain, I haven't RTFA and have no idea what the hell I'm talking about.
Thank you server!
Web Hosting: Unlimited storage and bandwidth: $5/month
The lamp will glow once to bring death, once to bring life, and a third time to bring...power!
The Chinese scientists may have discovered the secret of Green Lantern's ring!
(-1, Raw and Uncut is the only way to read)
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.
This post brought to you by your friendly neighborhood MBA.
"a team of experimentalists from China believe they have finally found a resolution."
Here on earth, we have special words for 'experimentalists.' We refer to them as researchers or scientists. And they're not 'finding resolutions,' they're testing hypotheses.
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? -_-
#fuckbeta #iamslashdot #dicemustdie
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!
...what She said
My bet is 1280 x 1024. :-)
Actually, after a few years at college I realized that nobody had ever used the word "scientist." Laypeople use it to describe a thousand different kinds of professionals, but most (if not all) scientists refer to specific disciplines.
Text Only Mirror
"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.
Now i can sleep at night knowing this has been solved.
Ok, pure science is cool, but is it really front page news?
---- Booth was a patriot ----
Slashdotted! Mirror here: http://www.spotlynx.com/node/2371
We need to get these guys working on the Slashdot Effect, next.
The dangers of knowledge trigger emotional distress in human beings.
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".
I think it all depends from which side you look at it. From the light's perspective, or from the surface.
Privacy is terrorism.
That's what *SHE* said!
Seeings how we are already experimenting with laser driven propulsion, i would have though the answer was obvious..
The article is unclear to me, maybe I missed something
...Weilong She and his colleagues...
...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...
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???
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.
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.
it's a push
who doesn't RTFA, how can light have momentum if momentum is mass*velocity. Do Photons have mass?
I knew I read about this quite some time ago: http://arxiv.org/abs/0806.2442
thegodmovie.com - watch it
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) âoeThis paper is a beautiful piece of work and may become one of the classic papers on the momentum of lightâ 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 âcompressionâ(TM).
100 year riddle
Hermann Minkowski had proposed in 1908 that light momenta is proportional to a materialâ(TM)s refractive index then the following year, another German theorist, Max Abraham proposed the opposite â" momentum is inversely proportional to a materialâ(TM)s refractive index.
This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light Ulf Leonhardt, University of St Andrews
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. âoeWe report direct observation of a push force on the end face of the silica filament exerted by the outgoing lightâ 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. âoeThe experiment represents a modern form of a beautifully simple ideaâ 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. âoeA correct accounting for the deformation of the silica filament in the reported experiments would have required a complete balancing of the momentaâ he said.
About the author
James Dacey is a reporter for physicsworld.com
You've gone and slashdotted the Institute of Physics! The Quantum Flux is not amused and the Strangelets are circling the wagons.
Am I the only one who finds it amusing that we have just ddns a Chinese server?
But... the future refused to change.
All your base is ours.
Invenio via vel creo
Superhypertechnobabble really is so bogus
Even though the sound of it is something quite atrocious
If you say it long enough you'll always sound precocious
Superhypertechnobabble really is so bogus!!
Fascism starts when the efficiency of the government becomes more important than the rights of the people.
That explains why black cars get dirtier faster than lighter colored ones.
Which can be any number. It depends on how you get to zero.
E.g. X/X = 1 for all X, so lim X->0, 0/0=1
Alternatively,
(X*X)/X = X for all X, so lim X->0, 0/0=0
Or x/(x*x) becomes infinity when X->0 0/0=inf.
When I tried to access the link for this article... the server barfed and said... "Due to an unexpectedly high volume of traffic, this web page is temporarily unavailable. We apologise for any inconvenience this may cause, and hope you will visit again at a less busy time." Funny...
So the fucking sun is pushing us away!
That was both unnecessary and incorrect. How can you get a 7-word phrase so wrong?
Make up your mind!!!
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.
"Given this result, Minkowski has been declared the new winner and light momenta is directly proportional to the material it is travelling through."
And yet Abraham is declared the winner in TFA and Abraham's equation says it's inversely proportional.
'Objects with zero rest mass have velocity c only.'
This is a much over-simplified truth, not actually literally true. Light does not travel at c, not even in interstellar vacuum, as supernovas has been detected by neutrions slightly before the light reaches us. As for light travelling through glass, water and other transparant materials, light is travelling at a speed drastically lower than c due to changed permeability and permittivity. (Googling 'Cherenkov radiation' is highly recommended)