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Scientists Solve Century-Old Optics Mystery

CmdrTaco posted more than 5 years ago | from the all-out-of-joementum dept.

Science 265

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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265 comments

Slashdot Troll Discovers First Post (-1, Troll)

Anonymous Coward | more than 5 years ago | (#26418687)

'nuff said.

First Post AND Slashdotted Already (-1, Redundant)

Phrogman (80473) | more than 5 years ago | (#26418697)

I go to the article with zero comments posted and the server is already unavailable. Talk about preparation in advance...

Re:First Post AND Slashdotted Already (0)

Anonymous Coward | more than 5 years ago | (#26418775)

Not prepared enough my friend

Re:First Post AND Slashdotted Already (1)

AKAImBatman (238306) | more than 5 years ago | (#26418891)

This is getting ridiculous. Back in 2000, a Slashdotting was a serious issue. But with today's fast servers and abundant bandwidth, you have to be hosting on your home DSL line to get killed this easily. Hell, I've had both $8.00/mo shared hosting boxes (PHP/static HTML) and $120/mo dedicated boxes (ASP/J2EE/PHP) both survive proper Slashdottings/Diggs.

There's no excuse for going belly-up this easy. An over-bandwidth message I could see, but otherwise... :-/

Re:First Post AND Slashdotted Already (2, Funny)

rehtonAesoohC (954490) | more than 5 years ago | (#26419295)

Maybe it's hosted in China and the government deemed it questionable content.

Re:First Post AND Slashdotted Already (5, Funny)

allcoolnameswheretak (1102727) | more than 5 years ago | (#26419723)

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.

Re:First Post AND Slashdotted Already (0, Insightful)

Anonymous Coward | more than 5 years ago | (#26419731)

I think that posting the slashdot article should mean uploading the post to the slashdot server. Then we'd at least be able to view it there for a while. Maybe after a few days when the site becomes available again, we'd use the real link...

Re:First Post AND Slashdotted Already (5, Funny)

BattleApple (956701) | more than 5 years ago | (#26419061)

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

Wait.. what the hell am I thinking?

Re:First Post AND Slashdotted Already (3, Funny)

philspear (1142299) | more than 5 years ago | (#26419477)

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!

hah! (-1, Offtopic)

Anonymous Coward | more than 5 years ago | (#26418701)

Letting AC's see posts 'in the future' really helps with trolling.

Thanks.

A little background is apropos me thinks... (2, Informative)

Smidge207 (1278042) | more than 5 years ago | (#26418711)

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 must imply equal velocity before and after.)

If your shot is a little off center, the cue ball and the eight ball
will both bounce away at different angles. But you'll find that their
"sideways speeds" are exactly equal, because the total "sideways"
momentum before they hit was zero. (Remember, momentum has direction!
If there was no momentum to the left or right before the balls hit,
there can't be any total momentum left or right afterward, either.)

What if one mass is bigger than the other? Well, imagine a bowling
ball: when you roll the heavy ball down the lane, it has a lot of
momentum. When it hits a one of the light pins, the ball slows down a
little bit but the pin goes flying! That's because the ball is so much
heavier than the pin: because the total momentum has to be conserved, a
small decrease in the ball's speed leads to a big increase in the pin's
speed. "MASS * velocity = mass * VELOCITY". That's also why you don't
want to get hit by a car! You're so much lighter, the car will throw
you across the road and hardly slow down at all.

Conservation of momentum is always true, although sometimes it can be
hard to see. For example, if you skid to a stop in a car you might
think that its momentum has vanished, but what's actually happened is
that the friction between the tires and the road has transfered the
car's momentum to the Earth itself (which is so big that we don't even
notice the change). Forces like friction and air resistance can make
it _look_ like momentum is going away in many situations, when really
it's just being transfered somewhere that we can't easily see it.

*** Another interesting fact is that light carries momentum, even
though light has no mass. It's not _much_ momentum, of course, but
it's certainly there! You might have seen a science demonstration of
this at some point, where a little black and white pinwheel is placed
in a vacuum jar. When bright light shines on the pinwheel, it starts
to spin. (This only works if the details are set up correctly, of
course.) For light, the usual "mass * velocity" equation doesn't
apply, since light has no mass and all light has the same velocity.
Instead, the momentum carried by light depends on its intensity or
brightness (which sort of corresponds to mass: it's the "amount" of
light) and its _color_!

It turns out that blue light has a lot more momentum than red light
(other colors are mostly in between; it corresponds precisely to the
colors of the rainbow). And special kinds of "light" like X-rays are
in a sense even more blue than we can see: they carry a _lot_ of
momentum (at least as compared to other light), which is why too much
exposure to X-rays can be dangerous: when they hit your body tissues,
all that momentum can make the molecules in your cells break, leading
to burns or even cancer. That's why we get sunburns from ultraviolet
light, too: it's more blue than blue, though not as bad as X-rays.

*** That's the basics of momentum in classical physics. When you get
to the 20th century, things get a bit more interesting. In quantum
mechanics we learn that it is impossible to perfectly measure an
object's position and its momentum at the same time. This is called
the "Heisenberg Uncertainty Principle". For example, if you measure
its position very carefully, you won't be able to tell how fast it's
going. This is often explained by trying to imagine how you would look
at the object: usually, you'd shine light on it, but as I just said,
light carries momentum! If the light bounces off of the object and
into your eye, that will change the momentum of the object (just like
the cue ball changed the momentum of the eight ball). That's just a
rough example, but the position/momentum difficulties hold true in
general.

Einstein's theory of special relativity has things to say about
momentum, too. For instance, it turns out that "mass * velocity" is
only _approximately_ true. If the velocity is very high, close to the
speed of light, then the momentum grows much faster than this:

        momentum = gamma * mass * velocity, where
        gamma = 1 / (square root of[ 1 - (velocity/speed of light)^2 ])

That's a messy formula, I know, and you probably don't need to
understand it. The point is, momentum is more subtle than Newton and
the other classical physicists thought it was. In fact, Einstein
showed that momentum was closely related to "energy" (that is, roughly,
an object's stored capacity to do work), and that energy can actually
be thought of as "momentum in the direction of time". (And the
connection is an important one: "Conservation of Energy" is another
very important law of physics, just as important as "Conservation of
Momentum".)

Whew! That may be enough to get you started, anyway. If you want more
details, practically any introductory physics textbook will discuss
momentum at great length. Even if you aren't mathematically inclined,
I'm sure that most good "Physics for Poets" type books will discuss
momentum in some detail (and probably more clearly than I've managed
here).

Re:A little background is apropos me thinks... (2, Funny)

Yvan256 (722131) | more than 5 years ago | (#26419537)

Still using a 40-columns monitor, I see.

Scientists have yet to solve... (-1, Offtopic)

Anonymous Coward | more than 5 years ago | (#26418731)

the mysterious attractiveness of the first post!

Google cache... (5, Informative)

OG (15008) | more than 5 years ago | (#26418735)

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

Re:Google cache... (3, Funny)

plasmacutter (901737) | more than 5 years ago | (#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.

Click "Text-only version" (3, Informative)

tepples (727027) | more than 5 years ago | (#26419661)

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.

Re:Google cache... (0)

Anonymous Coward | more than 5 years ago | (#26419437)

OMFSM! Even the cache is slashdotted now! (Well, almost)

The end is nigh!

Don't leave me in suspense! (0)

Anonymous Coward | more than 5 years ago | (#26418737)

What is the answer?

push (0)

Anonymous Coward | more than 5 years ago | (#26419729)

it's a push

Slashdotted already (3, Informative)

dj015 (680676) | more than 5 years ago | (#26418743)

Google Cache [74.125.45.132] for anyone interested in reading it

Re:Slashdotted already (3, Insightful)

Cowmonaut (989226) | more than 5 years ago | (#26420099)

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

No Complaints (-1, Offtopic)

zwekiel (1445761) | more than 5 years ago | (#26418747)

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!

I'm livin' on Chinese lights (0, Offtopic)

Gizzmonic (412910) | more than 5 years ago | (#26418755)

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!

No physics background here (3, Interesting)

fprintf (82740) | more than 5 years ago | (#26418761)

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 (4, Informative)

corsec67 (627446) | more than 5 years ago | (#26418879)

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.

Re:No physics background here (4, Informative)

nategoose (1004564) | more than 5 years ago | (#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.

Re:No physics background here (1)

Hatta (162192) | more than 5 years ago | (#26419447)

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

Re:No physics background here (3, Informative)

GospelHead821 (466923) | more than 5 years ago | (#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:No physics background here (2, Informative)

azenpunk (1080949) | more than 5 years ago | (#26420201)

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:No physics background here (3, Informative)

shking (125052) | more than 5 years ago | (#26419701)

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.

Re:No physics background here (5, Informative)

JustinOpinion (1246824) | more than 5 years ago | (#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."

Re:No physics background here (3, Informative)

TeknoHog (164938) | more than 5 years ago | (#26419951)

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.

Re:No physics background here (0, Troll)

Il128 (467312) | more than 5 years ago | (#26420257)

Light has infinite mass not zero mass.

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



Scientists have been contradicting themselves for decades. They all know light has mass. They also know that to travel at the speed of light requires infinite mass.


Since no one can figure out how infinite mass and a light particle would work (and it does), scientist just pretend light has zero mass... It's so much simpler than acknowledging that mass radically changes at the speed of light and that infinite mass is zero mass.

Re:No physics background here (1)

vonsneerderhooten (254776) | more than 5 years ago | (#26420395)

Why then, do rays of light not knock us over when they hit us? /always wondered

Re:No physics background here (1)

Il128 (467312) | more than 5 years ago | (#26420471)

Because mass and energy are one and the same thing. At the speed of light mass is infinite and infinite mass is the same thing as zero mass.

And in English... (0, Redundant)

Dzimas (547818) | more than 5 years ago | (#26418813)

"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.

Actually... (0)

Anonymous Coward | more than 5 years ago | (#26418905)

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.

Re:Actually... (1)

rickb928 (945187) | more than 5 years ago | (#26419511)

So in which specific disipline do 'Experimentalists' actually work in?

Re:Actually... (1, Funny)

Anonymous Coward | more than 5 years ago | (#26419751)

So in which specific disipline do 'Experimentalists' actually work in?

Superfluous prepositions?

Re:Actually... (5, Informative)

Neeperando (1270890) | more than 5 years ago | (#26419795)

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.

Re:Actually... (5, Funny)

SBacks (1286786) | more than 5 years ago | (#26420155)

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.

Re:Actually... (5, Funny)

Neeperando (1270890) | more than 5 years ago | (#26420401)

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

Re:Actually... (1)

pjt33 (739471) | more than 5 years ago | (#26420223)

There are few more irritating phrases than "Scientists have solved/discovered/invented". The headline "Physicists Solve Century Old Optics Mystery" would be a bit better, but really "Century Old Optics Mystery Solved" would be best because it's truly non-tautologous.

Re:And in English... (1)

jellomizer (103300) | more than 5 years ago | (#26418917)

Don't you watch Sci-Fi. It only takes 10 minutes from Theory to Practical application, and it works all the time.

Re:And in English... (1)

LandDolphin (1202876) | more than 5 years ago | (#26419481)

It does not work all of th time, but dont worry. You'll have a proper solution within the next 30 min.

Re:And in English... (1)

camperdave (969942) | more than 5 years ago | (#26420005)

It only works first time if it is the last five minutes of the show. If it is the first twenty, it will fail in some spectacular yet non-injury producing way... at least for the main characters. There may be the odd disposable character who gets turned inside out and explodes, or suffers plasma burns (thereby giving an excuse for the medical officer character to get his/her lines in).

Re:And in English... (3, Informative)

Kartoffel (30238) | more than 5 years ago | (#26419137)

Experimentalists, as opposed to theorists.

"Experimentalist" (3, Insightful)

girlintraining (1395911) | more than 5 years ago | (#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? -_-

Re:"Experimentalist" (1, Informative)

Anonymous Coward | more than 5 years ago | (#26419107)

Or its just an effective use of language to differentiate between multiple types of scientists (experimentalists & theorists) seeing as thats how academia tends to differentiate them.

Re:"Experimentalist" (1)

pjt33 (739471) | more than 5 years ago | (#26420295)

Not just that: it tells you that the story is almost certainly about an experimental result rather than a theoretical one. However, "experimental scientist" vs "theoretical scientist" isn't a very useful distinction per se. First say which branch of science, and then subdivide that if necessary: so here it should be talking about "experimental physicists".

Re:"Experimentalist" (1)

Samschnooks (1415697) | more than 5 years ago | (#26419393)

"Experimentalist"... It sounds like what a social deviant might call themselves.

I'm not supposed to say anything, but it's a journal for scientists who are into S&M, Golden Showers, etc....

Re:"Experimentalist" (5, Insightful)

jc42 (318812) | more than 5 years ago | (#26419649)

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.

Bye bye, Tractor Beam... :'( (4, Funny)

starglider29a (719559) | more than 5 years ago | (#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!

Re:Bye bye, Tractor Beam... :'( (0)

Anonymous Coward | more than 5 years ago | (#26419121)

That's no pressure, it's a tractor beam!

Re:Bye bye, Tractor Beam... :'( (2, Funny)

shakotah (785520) | more than 5 years ago | (#26419175)

Repulser blasts! *starts building Iron man suit*

Re:Bye bye, Tractor Beam... :'( (1)

Chrutil (732561) | more than 5 years ago | (#26419423)

if we could use a photon emitted from NEGATIVE MASS it would have NEGATIVE MOMENTUM!!!

Woahh - great idea!
And then if we really wanted to push something, we'd just run your negative mass photon emitter in a field of anti-time!

Re:Bye bye, Tractor Beam... :'( (3, Funny)

Evan Meakyl (762695) | more than 5 years ago | (#26419709)

if we could use a photon emitted from NEGATIVE MASS

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

Re:Bye bye, Tractor Beam... :'( (1)

darkonc (47285) | more than 5 years ago | (#26420111)

Captain, the repulsor beam is working!
Yes, but you've just vaporized the target.
Well, you can't have everything, eh?

Re:Bye bye, Tractor Beam... :'( (1)

aliquis (678370) | more than 5 years ago | (#26420551)

Make a black light laser!

that's... (-1, Troll)

Anonymous Coward | more than 5 years ago | (#26418877)

...what She said

What is the mystery resolution? (0, Offtopic)

aardwolf64 (160070) | more than 5 years ago | (#26418885)

My bet is 1280 x 1024. :-)

Re:What is the mystery resolution? (1)

rickb928 (945187) | more than 5 years ago | (#26419559)

Right now, I would settle for 1152x864.

Can there be no middle ground? Why just 1024x768 or 1280x1024? Is it too much to ask Intel to do the needful? Why?

Mirrored (4, Informative)

dj015 (680676) | more than 5 years ago | (#26418921)

Text Only Mirror [dazneale.com]

Re:Mirrored (3, Insightful)

azenpunk (1080949) | more than 5 years ago | (#26420555)

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

and the winner is... (3, Informative)

Digitus1337 (671442) | more than 5 years ago | (#26418939)

"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.

had to be done (4, Funny)

Kartoffel (30238) | more than 5 years ago | (#26419149)

That's what *SHE* said!

Re:and the winner is... (2, Funny)

street struttin' (1249972) | more than 5 years ago | (#26419465)

DUDE! Add a spoiler alert next time!

This is great! (-1, Offtopic)

nurb432 (527695) | more than 5 years ago | (#26418955)

Now i can sleep at night knowing this has been solved.

Ok, pure science is cool, but is it really front page news?

Re:This is great! (1)

Bigjeff5 (1143585) | more than 5 years ago | (#26419261)

Er, well, since this could lead to significantly more efficient Fusion, and other applications...

Yes?

I mean, it's news for nerds, right? It's a nerd problem that has taken 100 years to solve, right? Of all the things that show up on the front page, this is one of the articles that MOST deserve to be there, not least.

Cheers.

Server toppled by light via Slashdot medium (1)

elzbal (520537) | more than 5 years ago | (#26418977)

Slashdotted! Mirror here: http://www.spotlynx.com/node/2371 [spotlynx.com]

Slashdot Effect (2, Insightful)

mfh (56) | more than 5 years ago | (#26419009)

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

Re:Slashdot Effect (0)

Anonymous Coward | more than 5 years ago | (#26419129)

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

Have you clicked the link? They already are.

Re:Slashdot Effect (0)

Anonymous Coward | more than 5 years ago | (#26419391)

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

Next, maybe they can answer the age old question of whether the Slashdot Effect sucks or blows.

Ouch, I hurt myself when I thought of that.

Re:Slashdot Effect (3, Funny)

ArsonSmith (13997) | more than 5 years ago | (#26419965)

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

In simple terms... (4, Funny)

Thelasko (1196535) | more than 5 years ago | (#26419049)

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.

Re:In simple terms... (0)

Anonymous Coward | more than 5 years ago | (#26419297)

What would his son Her She think of that comment, you insensitve clod?

Re:In simple terms... (0)

Anonymous Coward | more than 5 years ago | (#26419523)

Don't know, but if he's the son, wouldn't he be from Mars [wikipedia.org] ? The daughter could be Her She [wikipedia.org] .

Re:In simple terms... (0)

Anonymous Coward | more than 5 years ago | (#26419543)

could have been worse. In chinese, "Thelasko" is a type of body excrement.

Re:In simple terms... (0)

Anonymous Coward | more than 5 years ago | (#26420287)

Uh...in case you didn't notice that's his LAST name. So not that confusing, srsly.

Relativity (1)

Fuzzums (250400) | more than 5 years ago | (#26419133)

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

Re:Relativity (2, Insightful)

gardyloo (512791) | more than 5 years ago | (#26419255)

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 (3, Insightful)

drewsup (990717) | more than 5 years ago | (#26419153)

Seeings how we are already experimenting with laser driven propulsion, i would have though the answer was obvious..

Wait, girl or boy? (3, Funny)

malignant_minded (884324) | more than 5 years ago | (#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???

Re:Wait, girl or boy? (1)

malignant_minded (884324) | more than 5 years ago | (#26419421)

Nevermind I'm an idiot. I only looked at the first name Weilong. It's She as in Weilong She. doh!

Re:Wait, girl or boy? (1)

Chris Burke (6130) | more than 5 years ago | (#26419483)

On the plus side, your original post is much funnier to me now that I know you were actually confused and not just making fun. :)

Re:Wait, girl or boy? (3, Funny)

mattcasters (67972) | more than 5 years ago | (#26419645)

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?

Re:Wait, girl or boy? (0)

Anonymous Coward | more than 5 years ago | (#26420389)

Or, indeed, the Fork 'andles [timesonline.co.uk] sketch by the two Ronnies.

Already demonstrated at MIT (3, Informative)

Muerte23 (178626) | more than 5 years ago | (#26419249)

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:Already demonstrated at MIT (2, Funny)

plasmacutter (901737) | more than 5 years ago | (#26419337)

nhk

ah the joy of field specific acronyms nobody understands.

Re:Already demonstrated at MIT (1)

Muerte23 (178626) | more than 5 years ago | (#26419385)

sorry.

n = (index of refraction)
h = (well, hbar, the planck constant)
k = (photon wavenumber)

Re:Already demonstrated at MIT (1)

Forbman (794277) | more than 5 years ago | (#26419853)

h == planck's constant.

h-bar == h*pi, or something like that... (yes, it's still a constant, but...)

Re:Already demonstrated at MIT (0)

Anonymous Coward | more than 5 years ago | (#26419725)

How about (ÂÃ--h)/Î instead of nhk, then? Ah the joy of non-utf comments.

Re:Already demonstrated at MIT (2, Funny)

jc42 (318812) | more than 5 years ago | (#26419573)

..., 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:Already demonstrated at MIT (5, Informative)

waxigloo (899755) | more than 5 years ago | (#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.

Re:Already demonstrated at MIT (1)

Muerte23 (178626) | more than 5 years ago | (#26419781)

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:Already demonstrated at MIT (2, Informative)

waxigloo (899755) | more than 5 years ago | (#26420033)

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.

Re:Already demonstrated at MIT (1)

rkowen (135560) | more than 5 years ago | (#26420147)

From reading the cached article it claims that Minkowsky won in that as the above poster cites that the photon momentum is proportional to the index of refraction "n". However, if that is true then wouldn't the substance move in the opposite direction of motion of the photon to conserve total momentum since the photon momentum is increasing since the index goes from 1 to >1 ?
This is opposite from what the fine article claims.

Just sayin'

For a physics newbie (0)

Anonymous Coward | more than 5 years ago | (#26419983)

who doesn't RTFA, how can light have momentum if momentum is mass*velocity. Do Photons have mass?

Experiment resolves century-old optics mystery (1)

soybean (1120) | more than 5 years ago | (#26420251)

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

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