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."

Comments

Google cache...

[OG]

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

Re:Google cache...

[plasmacutter]

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

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

Slashdotted already

[dj015]

Google Cache for anyone interested in reading it

Re:Slashdotted already

[Cowmonaut]

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

No physics background here

[fprintf]

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

[corsec67]

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.

Re:No physics background here

[nategoose]

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

[Hatta]

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

[GospelHead821]

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

[shking]

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.

Re:No physics background here

[JustinOpinion]

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."

Re:No physics background here

[TeknoHog]

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

[azenpunk]

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

[vonsneerderhooten]

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

Re:No physics background here

[Il128]

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.

Re:No physics background here

[kae_verens]

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

[Il128]

Zero mass is infinite mass. It's the same thing.

Re:No physics background here

[Hatta]

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?

Re:No physics background here

[nschubach]

But 0 times anything is 0, so doesn't that mean that Light has a very minuscule mass? (ie: lower than we could measure?)

Re:No physics background here

[gardyloo]

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

[Hatta]

It's not meaningful to talk about the rest mass of something that's never at rest.

Re:No physics background here

[CopaceticOpus]

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!

Re:No physics background here

[nschubach]

Or is the mass of a light particle .000~001 where "~" replaces so many 0s that we cannot measure and would so minuscule that the formula appears to work? (Kind of like using PI in Math. You can get pretty close, but not quite accurate.)

Re:No physics background here

[Hatta]

So your answer is that light in fact does have mass?

Re:No physics background here

[Hatta]

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).

But E=m*c^2. So p = m*c*c/c = m*c. And we're back where we started from, p=mv. Plug the measured momentum of a photon in there, and you'll find the mass of the photon.

Re:No physics background here

[kae_verens]

paraphrased: "nothing == everything"

you are an idiot.

Re:No physics background here

[turbidostato]

"Zero mass is infinite mass. It's the same thing."

No, it isn't.

Re:No physics background here

[blueg3]

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.

Re:No physics background here

[shking]

No. Light has energy, which is equivalent, but not the same thing

Re:No physics background here

[Lurching]

Yes indeed light has mass. How much depends on the color of the light.

Shorter wavelengths have more energy and therefore more mass. m = E / c**2

Light has no REST mass, but since it cannot rest, that is irrelevant.

Re:No physics background here

[Requiem18th]

[citation needed]
please

Re:No physics background here

[blueg3]

You're conflating two terms for mass. A photon does indeed have "mass". However, it does not have rest mass, which is what is usually meant by "mass", and is certainly what is meant when the photon is referred to as a "massless particle". (Since energy and mass are the same, it's rare in physics to see non-rest mass referred to as "mass" at all. It's energy -- or relativistic energy.)

Re:No physics background here

[VisceralLogic]

But light doesn't have its velocity increased... it starts off at the speed of light already. It's probably more akin to E=mc^2. Photons have a defined energy, so it could be converted to a mass value calculated using Einstein's equation, which you could then multiply by the velocity to find momentum... I think.

Re:No physics background here

[Bananenrepublik]

This formula is only valid in a particle's rest frame. A photon (= light particle) doesn't have a rest frame, as it moves at the speed of light c in every frame. The general formula for frames (moving at constant velocity yaddayadda) is
E^2 + p^2 c^2 == m^2 c^4,
which reduces to the formula you quoted for p = 0, which is of course the definition of the rest frame.
OTOH for massless particles, m = 0, it follows
E = p c,
i.e. energy and momentum are the same for a massless particle.
Now, what is the meaning of the momentum for a photon? A photon can hit an electron, losing its momentum to the electron (plus a recoil, say, nucleus, which is needed for energy-momentum-conservation, yaddayadda), and the maximum momentum it can transfer to the electron is the momentum of the photon.
There are two things to note:
- the momentum p of a photon is related to its frequency f by p = 2 pi f h (h Planck's constant), and
- since momentum changes under Lorentz tranformation, the frequency of light does likewise (think "red-shift" [neglecting gracitational shifts yaddayadda])
Hope this cleared some misconceptions.

Re:No physics background here

[Bananenrepublik]

The first formula should have a minus sign between the first two terms, sorry.

Re:No physics background here

[nschubach]

I still don't understand why light and matter cannot be made of the same stuff. "at rest" matter being a particle not moving at the speed of light and light being matter traveling at this speed. "at rest" matter would be bumping into each other causing it to slow down and not emit energy from striking something else at speed and light being emitted at speed until it collides with something causing an expense of energy. It would mean that light could be so minuscule that it flies between some particles, reflects and refracts through others. The idea that light (aka: really fast matter in this case) has infinite mass to me means that we just don't have the tools to measure it's mass and speculate with an approximated formula/number. Light at rest in my mind would just be matter.

I don't know. The hard rule never made sense to me.

Re:No physics background here

[khallow]

Another application is to traffic law. I've never been caught running a red light though I have been caught exceeding the speed limit by a significant fraction of the speed of light.

Re:No physics background here

[Kent Recal]

Maybe light rests only when nobody's looking?
Just like we browse slashdot at work only when nobody's looking?

Re:No physics background here

[Hatta]

What exactly is the difference? Does non-rest mass not increase the gravitational force exerted by a particle? Does non-rest mass not affect the inertia of a particle?

That photons have no rest mass is obvious, since photons cannot rest.

Re:No physics background here

[bishiraver]

It's accurate, just not precise. http://en.wikipedia.org/wiki/Accuracy_and_precision

Re:No physics background here

[blueg3]

It's not a "hard rule" so much as a result of deeper theory that's confirmed by experiment.

Light does not have infinite mass. We do have the tools to measure its properties (momentum, energy, and rest mass).

The reason, by relativistic theory, that a particle with nonzero rest mass cannot move at the speed of light is that its energy is E = m_0 * c^2 / sqrt(1 - v^2/c^2). The energy diverges as v approaches c. (Note that this equation is undefined if v = c and m_0 = 0.) The kinetic energy, E - m_0 * c^2, must be supplied to the moving body. As available energy is finite, massive bodies are constrained to v c.

Massless bodies have energy determined solely by their momentum. (Without relativity, it's known that electromagnetic radiation carries momentum and energy.) Their energy is not divergent at v = c.

Re:No physics background here

[Chris Burke]

I still don't understand why light and matter cannot be made of the same stuff.

Uh, they are made of the same stuff. Energy.

The idea that light (aka: really fast matter in this case) has infinite mass to me means that we just don't have the tools to measure it's mass and speculate with an approximated formula/number.

We have a formula, and it shows that an object's relativistic mass approaches infinity as its velocity approaches c. For arbitrarily small values short of c, we have arbitrarily large masses. Which makes more sense: That despite the fact that mass dilation means you can never accelerate an object with mass from some speed below c to the speed of light, if something is actually traveling at c then the asymptotic behavior ceases and mass is instead some tiny value? Or that no object with mass can travel at c, the only thing that does travel at c, light, has no mass, but that the two are nevertheless related via mass-energy equivalence?

Re:No physics background here

[blueg3]

No, energy and mass are exactly the same thing.

However, rest mass is a different thing than "total mass". It's fairly similar to "heat" versus "kinetic energy" in that the rest mass for objects usually is hiding many different energies that we're not tallying individually (strong and electromagnetic binding energies, for example). The rest mass of an object is the mass it has with v=0. This is the same as rest energy, sure. It's not the same as total energy / mass.

Some physicists even refer to the quantity as mass-energy for clarity's sake. I prefer the convention that rest mass is always referred to as "mass", and total energy is referred to as "energy". Adding "rest" to "mass" and "relativistic" to "energy" doesn't hurt, though.

Re:No physics background here

[volpe]

The actual equation is:
E^2 = (pc)^2 + (mc^2)^2.
When all of the energy is due to mass (i.e. momentum is zero), you get the familiar equation E=mc^2. When all the energy is due to momentum (i.e. for photons), we get E=pc. Since we also know that for photons, E=h*nu, we can solve for p and get p = h*nu/c.

Re:No physics background here

[Chris Burke]

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.

Wow, man, that's deep. I wasn't sure, so I went and asked a physicist friend to tell me the truth. He denied it, and said it's not true, light has no mass, just energy. I told him it was okay, I knew the truth that infinite mass is the same as zero mass. He not only wouldn't admit this, he said it was wrong!

Which means, damn, you're right! Science already knows this but just pretends its not the case! They are lying to the public in some kind of conspiracy, and my friend is in on it! But don't worry, with the new season of 24 starting I'll have plenty of inspiration to help me get the truth from him.

Re:No physics background here

[blueg3]

The problem is you're referring to mass as an intrinsic property. While rest mass is intrinsic, total mass isn't.

For all things, total mass is the relevant quantity, not rest mass -- so photons do, in theory, exert gravitational force.

Inertia, though, can only be thought of as some separate quantity in nonrelativistic mechanics -- which obviously doesn't apply to light. For any relativistic particle (particularly light, which has no rest mass and so cannot be treated at all with Newtonian mechanics), you need to use relativistic equations of motion.

Re:No physics background here

[blueg3]

As it turns out, your stunning revelation is actually readily answered by a fairly small amount of education in physics.

Hint: you are referring to two different quantities using the same term, "mass".

Re:No physics background here

[Petaris]

You said it would produce minuscule force but would it scale? For example if you put a really huge version of lone in space and hooked it to a generator could it turn it or would it only have enough force (even at that scale) to turn itself? Just seems like it could be an interesting twist on solar power.

Re:No physics background here

[Obfuscant]

But light doesn't have its velocity increased... it starts off at the speed of light already.

That's like saying that a car can't have it's velocity increase because it always travels at "the speed of car". The "speed of light" is not a constant. The "speed of light in a vacuum" is.

The speed of light varying in a medium is one expression of the refractive index. The larger the refractive index, the slower the speed of light. Light travels slower in a medium with a higher refractive index.

Re:No physics background here

[nschubach]

I was thinking more so that light has a very tiny mass that we cannot measure at this time. I just don't buy that something has 0 mass. If it did, it would be nothing.

Re:No physics background here

[Chris Burke]

I was thinking more so that light has a very tiny mass that we cannot measure at this time. I just don't buy that something has 0 mass. If it did, it would be nothing.

No, it'd be purely energy. Why don't you think a wiggling electromagnetic field can't be said to be pure energy, and must involve tiny masses flying back and forth? I assure you that the electromagnetic field itself is a "thing" even though it has no mass.

And the problem is that regardless of our ability to measure tiny masses is that light having any mass at all, regardless of how tiny, causes all our equations to blow up. Equations that have been tested quite well on actual masses. To me, the idea that light is simply a form of energy with no mass makes a lot more sense than the idea that light does have mass, it just doesn't follow the same rules as every other kind of mass and can be accelerated to c without mass dilation. At the point where you're saying something has mass, but you don't actually put anything into any of the equations involving mass because they don't apply, that doesn't sound very useful to me other than to make you feel better. Isn't it easier to just accept that light has momentum but no mass?

Re:No physics background here

[blueg3]

I don't mean to imply that you're flat-out wrong, only that you're treading in dangerous territory with your understanding.

I should clarify that photons don't simply have "no rest mass". They have a rest mass, and it is exactly zero. (If I recall correctly, particles with zero rest mass are conveniently constrained to move at v=c, and particles with nonzero rest mass are always capable of moving at v=0 in some frame.)

Photons certainly do have mass, since they have energy, and the two are the same. In fact, if you had a perfect-mirror box, you could put five pounds of light in a box.

Most fundamental particles (quarks and leptons, for example) have some nonzero rest mass that appears to be a fundamental quantity. Everywhere else, "mass" is a euphemism for energy that isn't carefully-accounted for. The "rest mass" of a brick is made up of the fundamental rest masses of its constituent particles, plus lots of binding energy (between quarks, between nuclear particles, between electrons, et cetera). In the same way, we call kinetic energy, abstracted away in a statistical fashion, "heat". At a fine-grained level, there is no heat. Likewise, at a fine-grained level, there is only fundamental rest masses and energy. (And energy is the same as mass.)

So your five pounds of light in a box could suddenly become accounted away as box with a rest mass of five pounds (plus mass of empty box).

The danger is allowing any relativistic quantity to touch a non-relativistic equation or view of the world. It's sure to lead you to very wrong ideas. (See, for example, "zero mass is the same as infinite mass", elsewhere in this discussion.)

Re:No physics background here

[khayman80]

For the same reason that very weak breezes don't knock us over: they carry momentum, but a very small amount of it. Our legs are more than capable of reacting to this small force. In fact, the momentum carried by light is so small that its detection requires sensitive instruments. But light does carry momentum, which is the basis of a solar sail.

Re:No physics background here

[Khyber]

A similar thought would be coating wind turbines with solar panel material. And that can scale to some degree, given enough atmospheric pressure and wind, plus sun exposure. Deserts would be ultra-ideal for this.

Re:No physics background here

[nschubach]

How can you push/propel something if there's nothing to push against? Pretty much everything I read about energy states that it requires a medium to travel in, meaning (to me) that energy is the movement of mass (or chain reaction of mass pushing mass, thus resistance, degradation, et al.) and not an object itself.

That would mean, to me, that outer space is just a collection of tiny particles that have tiny mass and no inherit movement. Kind of like a really light (as in near weightless) soup. Objects traveling through space would push these tiny molecules out of the way momentarily as light travels at such a speed that it never deflects because of it's momentum.

Err, I'm getting off track, but I don't buy that energy and matter are separate entities. I have no proof, scientific study, or experiments I can try to prove it with, but I haven't really sat down to try. I just can't fathom that we are right. I don't know why.

Re:No physics background here

[corsec67]

Just seems like it could be an interesting twist on solar power.

Not really. If you were to put a giant reflector like that in space, solar winds would move it more than radiation pressure, and that would be a uniform pressure away from the sun.

If you wanted to generate electricity, it would be much better to curve that reflector and concentrate the lights on a collector that runs a turbine or similar heat powered generator. (This design has been used on earth before)

Re:No physics background here

[Gospodin]

...outer space is just a collection of tiny particles that have tiny mass and no inherit movement.

This idea was disproved in 1887. Time to get into 20th century thinking, dude!

Re:No physics background here

[DMUTPeregrine]

In space? No. It requires air to function.
And the huge version will have too much inertia/friction to even get started.

Re:No physics background here

[Chris Burke]

How can you push/propel something if there's nothing to push against?

Since when is energy 'nothing'?

Pretty much everything I read about energy states that it requires a medium to travel in.

They disproved this theory a long time ago. Look up the Michelson-Morely experiment. It was only by extrapolation from things like sound waves that it was assumed a medium was required for electromagnetic waves.

That would mean, to me, that outer space is just a collection of tiny particles that have tiny mass and no inherit movement. Kind of like a really light (as in near weightless) soup. Objects traveling through space would push these tiny molecules out of the way momentarily as light travels at such a speed that it never deflects because of it's momentum.

Interesting, but this has the same problems as the luminiferous aether, in that it should have effects visible on the orbits of planets and so on, unless you're using "really light" as a synonym for "doesn't behave as though it has any mass at all" again. Eventually the only way to make the aether work was to assume it was massless -- that's right, even when a medium for electromagnetism was thought to exist, it was considered massless.

Err, I'm getting off track, but I don't buy that energy and matter are separate entities.

They aren't. They're the same thing. Or more specifically, mass is just a form of energy. Light is a form of energy which is not mass.

E=mc^2 is an amazing equation in part because it's so tough to grasp its full implications (I certainly won't claim anything close). It also makes much more sense than light having a really really teeny tiny mass that doesn't obey the rules that every other mass does. I don't understand why it makes you more comfortable to say light must have a mass because that means it's not "separate" when obviously it is separate in some way because nothing else that has mass can travel at c, and our equations regarding mass say it would be impossible for anything with mass to do so.

Re:No physics background here

[Bryan Ischo]

Perhaps you'd like to define what you mean by 'mass'. What exactly is 'mass'?

If you say that mass is the stuff that makes objects solid, then what's 'solid'? If you say 'solid' means that nothing can penetrate it, then that's obviously wrong - neutrinos penetrate massive objects. Or do you think that there is some infinitesimally small little sized thing that can't be made any smaller, that nothing can penetrate, and that you'd call 'mass'?

Let me give you my take on mass. Mass is:

- a point from which gravitational forces are exerted

Mass is nothing more than a coordinate in space - a coordinate in space from which we can define gravitational effects originating. If something 'has mass' then it has a coordinate point that we can use in gravitational equations to determine how much gravitational force is being exerted from that point.

That's it. That's all mass is. It's the coordinates from which gravitational forces are exerted.

Light doesn't exert gravitational force on anything. It has no coordinates from which one would calculate the exertion of gravitational forces on other objects. Ergo, light has no mass.

Once again, if you are so hung up on this concept of 'mass' and insistent that objects must have it in order to 'push' other objects, then please define exactly what 'mass' is. If I were to build a microscope capable of magnifying to an infinite degree, what exactly would I see when I "zoomed in" on 'mass'?

Re:No physics background here

[Chris Burke]

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

Inertial dampeners, of course. Life on earth was pretty difficult until we had them installed.

Re:No physics background here

[profplump]

Congratulations. You've just (re)discovered the luminiferous aether.

Re:No physics background here

[shking]

Great explanation. Thanx

Re:No physics background here

[Hatta]

Since we also know that for photons, E=h*nu, we can solve for p and get p = h*nu/c.

Since we know that p=mv and v=c, we can write:

mc = h*nu/c
m = h*nu/(c^2)

So photons have mass proportional to their wavelength. Or does p=mv not apply at relativistic speeds?

Re:No physics background here

[nschubach]

If that were the case, then everything is energy. All energy interacts with other energy and creates what we believe to be matter by deflecting different amounts of energy. A steel plate is "energy" that cooperates in such a way to deflect a grouping of energy called a lead bullet and we are nothing more than energy (and not "space dust".) There would be no matter. By what you are telling me, matter is just a collection of energy that deflects or absorbs other energy. Nothing more than just plain old energy.

In the case of a wooden door, it's a collection of energy that deflects energy that composes a person's skin and reflects certain types of light energy giving it a brown color to our eyes and looking like a solid door. It's there, as a grouping of energy, that we cannot instantly "dissipate" with the tools available simply by touching it. We could sink it in water and let the collision of water energy slowly degrade the bonds in the door and let loose the energy to be re-used somewhere else, but it would still be energy that once formed a door. What makes it re-bond to other wood energy and not iron or steel energy? (I'm talking generalities here...Wood is made from molecules of certain types of known chemicals... are these molecules simply patterned energy and not teeny atomic particles orbiting a nucleus?)

Why can energy not be a particle orbiting another particle creating "energy"? Why could there not be one distinct building block "item" for all this to occur?

** I have used particle as mixed meaning a few terms in here, not on purpose, but because that's the only thing I know to call them.

Re:No physics background here

[Obfuscant]

And in medium, light is propogated by the absorption and re-emission of these electromagnetic waves by the atoms making up the medium, isn't it?

No. Were this true, there could never be a sunbeam. Or an image in a camera. Or transparent glass. When a photon is absorbed by an atom and then re-emitted, it can be going any direction. Random. (Under high-field conditions, like a ruby laser, that's no longer true.) If every photon (or all EM radiation) were absorbed by the medium and then re-emitted, the very first entry into a medium would result in a complete scattering of the radiation in all directions.

Now, SOME light is absorbed and some is re-emitted. There's a whole field of analytical chemistry dealing with both atomic and molecular absorption. Helium was discovered by, umm, forget his name, noticing that there were missing spots in the spectrum produced by a prism. This missing light either is scattered (by re-emission other directions) or lost (stays as higher energy atom or re-emits at other wavelengths).

What IS absorbed depends on the energy of specific electron transitions in the medium, or on vibrational/rotational states of a molecule.

So no, light does not "remain at c" when traveling through a medium. It slows to the speed of light, and of all electromagnetic radiation (including radio) in that medium. And no, speed of light in a vacuum and speed of light in a medium are not two different concepts.

Re:No physics background here

[Hatta]

I don't mean to imply that you're flat-out wrong, only that you're treading in dangerous territory with your understanding.

I should clarify that photons don't simply have "no rest mass". They have a rest mass, and it is exactly zero. (If I recall correctly, particles with zero rest mass are conveniently constrained to move at v=c, and particles with nonzero rest mass are always capable of moving at v=0 in some frame.)

Of course this is dangerous territory, in fact I'm pretty sure I'm wrong. But that's what makes it interesting. I think my major problem is in understanding how something can have a rest mass when it never comes to rest. It seems to me that if you have gravity and inertia, you have mass.

Most fundamental particles (quarks and leptons, for example) have some nonzero rest mass that appears to be a fundamental quantity.

Or you can think of it as the particle being composed of a certain amount of energy. Instead of 'rest mass', lets call it 'energy of composition' or something (in analogy to energy of formation). You can't make a particle without x amount of energy, just like you can't make a bond without y amount of energy. In which case we could say that all particles except photons have a fundamental energy of composition, whereas a photon's is dependent on its wavelength.

Does that make sense? You could give energy of composition its own letter, so that solving E=mc^2 for m doesn't draw people to the wrong conclusions.

Likewise, at a fine-grained level, there is only fundamental rest masses and energy.

In my eyes, it's even simpler. At a fine grained level there is only energy. All energy has mass, even that of a photon.

Re:No physics background here

[Il128]

Because the mass of a photon at light speed is basically zero because it is infinite. I know it's hard to grasp. Why don't photons escape black holes? Where does the momentum of everything entering a black hole going?

Re:No physics background here

[V!NCENT]

Light is a combination of mass and energy.

Re:No physics background here

[blueg3]

I think my major problem is in understanding how something can have a rest mass when it never comes to rest.

It doesn't and can't have rest mass. But saying that the rest mass is undefined because it can't come to rest, knowing a priori that it can't come to rest, is unsatisfying. Various equations include rest mass; one should know that this is zero for a photon. (Unfortunately, particles with no rest mass are called "massless". Technically, something with no mass whatsoever has no energy, and as such is not a "thing" at all.)

There's actually a lot of significant to "unable to be at rest". Massive particles are constrained to have speeds less than c. This means that there exists some inertial reference frame where the particle has speeed zero (or, for that matter, any other arbitrarily-chosen velocity). Massless particles exist only at speed c. As such, in all inertial reference frames, their speed is still c. (This fact is really at the heart of relativistic mechanics.)

Does that make sense? You could give energy of composition its own letter, so that solving E=mc^2 for m doesn't draw people to the wrong conclusions.

Your understanding here of mass and energy seems correct. For mostly historical reasons, relativistic terminology isn't very good. Real physicists (tm) do separate correctly rest mass / rest energy and total energy. What terms you use can very depending on what sort of objects you're talking about.

Note that unfortunately, this ideal view of things where you have only "fundamental matter-energy" and various binding energies is inconvenient, as relativity doesn't (yet!) work very well for subatomic physics. (The other place relativity is really useful is at the astronomical scale, where such a treatment wouldn't be possible anyway. You have to abstract things like stars into "giant balls of mass" without knowing too much about their real internal composition.)

The big confusion is that laypeople view mass as an intrinsic property of an object and they understand (for a well-educated layperson) Newtonian mechanics. To a physicist, rest mass is an intrinsic property of an object, and mass is just energy (which is rest mass + energy from other sources). Despite being able to roughly interpret some relativistic principles, laypeople generally don't have much practice never mixing relativistic concepts with Newtonian concepts.

But yes, as I was telling my wife earlier today, someone should be punished for terminology in relativity. Mass and energy should be renamed weird, arcane terms that can't possibly be confused with other things.

Re:No physics background here

[quanticle]

Indeed, scientists have even developed materials that stop and store photons.

Re:No physics background here

[Bryan Ischo]

Thank you for clearing that up for me.

It's my understanding that light is a series of alternating electric and magnetic fields, propogating through space. A changing electric field produces a changing magnetic field, which produces a changing electric field, which produces a changing magnetic field, and so on, and so on, forever, until something interrupts this process. And somehow (I can't remember how) each field is created at some infinitesimally small distance away from the previous one, after an infinitesimally small time has passed. And it's the ratio of the infinitesimally small distance over the infinitesimally small time which produces the constant speed of light, c. Is this overly simplistic definition at all accurate?

If so, then what causes this propogation to occur at a different rate in a medium? Is it just a fact of physics that the rate at which electric fields cause magnetic fields and vice verse is slower in the proximity of gravitational mass? Glass is composed of atoms with large gaps in between, in what 'parts' of the medium does light travel at 'c' and in what 'parts' does it travel at less than c? Does it travel at less than c when the light is passing within the electron shell of the atom? Or just sufficiently close to it? And whatever effects the medium has on light, why does it only have these effects when light is 'inside' it? Why isn't it like gravity, where the force is exerted an arbitrary distance away from the source of gravity, just with exponentially smaller and smaller effects? Shouldn't the speed of light be affected by whatever the medium is composed of even *outside* the medium? I mean, if I have a diamond the size of Jupiter, why does its effect on the speed of light only start the moment that light crosses the 'surface' of the medium?

Re:No physics background here

[WCguru42]

(If I recall correctly, particles with zero rest mass are conveniently constrained to move at v=c, and particles with nonzero rest mass are always capable of moving at v=0 in some frame.)

It's not just a convenience, without this constraint those zero rest mass particles would have zero energy and therefore not be particles at all. For something to exist it must have energy of some sort, whether that be purely it's rest mass for something at 0K in its own reference frame or pure energy with no rest mass as in the case of a photon. Everything else falls in-between.

Re:No physics background here

[amorsen]

If every photon (or all EM radiation) were absorbed by the medium and then re-emitted, the very first entry into a medium would result in a complete scattering of the radiation in all directions.

Mirrors don't scatter the radiation in all directions.

Re:No physics background here

[amorsen]

Wouldn't it then be powered purely by momentum and moving the other direction?

Theoretically yes, if you could get the light source strong enough and the friction low enough. In practice that would be hard.

Re:No physics background here

[jbatista]

The momentum carried by a photon with energy E is p=E/c.

To generalize to things other than photons:

E^2 = p^2c^2 + m^2c^4

Photons have no rest mass, so E=pc. Objects in their rest frame have p=0 by definition, so E^2=m^2c^4 or the oft-quoted E=mc^2. (You can make c=1 if you work in High Energy Physics or similar, to write shorter equations.)

If a photon fully dissipates on an object, both that equation and conservation of energy yield the object's final energy (squared) as

E_total^2 = p_photon^2c^2 + m_object^2c^4

As an interesting exercise for low-velocity (i.e. non-relativistic) approximation of the first equation, you can use the sqrt(x) Maclaurin series up to first order, and get the familiar

E = p^2/(2m)

Re:No physics background here

[drinkypoo]

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.

I am not a physicist, but... it is my understanding that light only moves at that speed in a vacuum.

Re:No physics background here

[Obfuscant]

Mirrors don't scatter the radiation in all directions.

Mirrors don't absorb, they reflect. Reflection is different than absorption/re-emission.

If you want a demonstration of what absorption/re-emission would look like were it the method of propagation of an EM wave in a medium, look at milk. Milk is actually a scattering system, but it looks the same. Similarly, fog.

Air is a medium. If light was absorbed and re-emitted as the means of moving through air, then we'd be living in a perpetual dense fog. We'd be able to see nothing, not even our hand in front of our face.

Re:No physics background here

[Obfuscant]

A changing electric field produces a changing magnetic field, which produces a changing electric field, which produces a changing magnetic field, and so on, and so on, forever, until something interrupts this process.

I have not heard it described in that way, and I do not believe that is correct. I know it is possible to work with systems where the E and M fields are separate -- where the same input results in different field strengths depending on the shape and kind of an antenna, for example. If one field created the other and vice versa, then a strong E would always have a strong M component, and I don't think this is necessarily true. I'm not a theoretical physicist, however, just a ham and an analytical chemist, so I deal with practical results and not always the underlying physics.

If so, then what causes this propogation to occur at a different rate in a medium?

The short answer is "mass". I don't have a good explanation. For this you will need a real physicist.

I mean, if I have a diamond the size of Jupiter, why does its effect on the speed of light only start the moment that light crosses the 'surface' of the medium?

Well, the MASS of the object does cause curvature of space in the vicinity -- a gravitational effect that has been observed when stars are occulted by the sun, so there is some effect before it crosses the surface. I think the rest of the answer probably involves the difference between strong and weak forces.

Re:No physics background here

[amorsen]

Mirrors don't absorb, they reflect. Reflection is different than absorption/re-emission.

How exactly do you believe reflection works?

Re:No physics background here

[Obfuscant]

Not by absorption and re-emissions.

Once a photon is absorbed by a material, the information about what direction it was going is lost lost lost. The electron moves to a higher orbit, or the bond starts vibrating slightly faster or whatever. What comes out goes whatever direction it happens to go.

The only exception to this is in a high-field condition that caused stimulated emission.

Re:No physics background here

[Scott Carnahan]

Let me give you my take on mass. Mass is: - a point from which gravitational forces are exerted

This does not agree with the definitions of modern physics. Mass is an inertial property, i.e., resistance to acceleration under action of a force.

Light doesn't exert gravitational force on anything. It has no coordinates from which one would calculate the exertion of gravitational forces on other objects. Ergo, light has no mass.

This isn't true. Light does exert gravitational force on nearby things, because it contributes to the stress-energy tensor in general relativity. However, that force is very small, so you don't feel it in everyday situations. In the standard model of particle physics, light has no mass because the photon doesn't interact with the Higgs field.

Re:No physics background here

[Bryan Ischo]

Ah, I see you actually know something, unlike me who just makes stuff up to complete the little bit that I do know.

Do you understand the point I was trying to make though? At one point I also believed that mass was some physical concrete thing. One day I realized that there are no physical concrete things at the atomic level and below; it's not really possible to describe things at that level using terms applicable to our macro world.

I get the feeling that the O.P. wants to believe that light must have mass because he's trying to reason using concepts that make sense at the scales that we can perceive, but those concepts do not apply at the atomic and subatomic scales.

Re:No physics background here

[volpe]

Or does p=mv not apply at relativistic speeds?

Correct. That equation is correct only in the newtonian low-velocity limit.

Re:No physics background here

[Mattsson]

are these molecules simply patterned energy and not teeny atomic particles orbiting a nucleus?

You seem to have misunderstood several concepts regarding mass, energy, light, matter and how they relate to one-another.
A startingpoint would be to skim through the wikipedia-entries on subjects like:
The theory of relativity
Mass
Matter
Energy
and also read up on subjects that relate to those subjects.

"Experimentalist"

[girlintraining]

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"

[Anonymous Coward]

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"

[Samschnooks]

"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"

[jc42]

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.

Re:"Experimentalist"

[pjt33]

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".

Bye bye, Tractor Beam... :'(

[starglider29a]

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... :'(

[shakotah]

Repulser blasts! *starts building Iron man suit*

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

[Chrutil]

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... :'(

[Evan Meakyl]

if we could use a photon emitted from NEGATIVE MASS

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

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

[darkonc]

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... :'(

[aliquis]

Make a black light laser!

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

[flyingfsck]

A pusher beam would be more useful, since it would not cause collisions with the pusher.

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

[rusl]

This is repulsive!

Re:First Post AND Slashdotted Already

[AKAImBatman]

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:And in English...

[jellomizer]

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

Mirrored

[dj015]

Text Only Mirror

Re:Mirrored

[azenpunk]

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

Re:Mirrored

[JWSmythe]

Well, first you get a whole bunch of vampires...

    Aw, the joke just isn't worth it today.

    [dawns his vampire hunting hat, and starts walking East]

    They'll be awake soon. I have work to do.

Re:Mirrored

[dangitman]

So, my ASCII art not only makes a nice wall hanging, but I can use it to shave, too?

Re:Mirrored

[MiniMike]

My first thought was a mirror that reflects everything in ascii art. I really need more coffee...

and the winner is...

[Digitus1337]

"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.

Re:and the winner is...

[street struttin']

DUDE! Add a spoiler alert next time!

Re:and the winner is...

[TheCarp]

Well duh, why do you think he encrypted it with ROT-26 ? :)

-Steve

Server toppled by light via Slashdot medium

[elzbal]

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

Slashdot Effect

[mfh]

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

Re:Slashdot Effect

[ArsonSmith]

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

Re:Slashdot Effect

[JWSmythe]

    Naw, Slashdot can't hold a tune. It sounds more like a dying cat in a blender.

Re:Slashdot Effect

[JWSmythe]

    Ya, but they come out too clean, and make my underpants smell like dead cat.

In simple terms...

[Thelasko]

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:First Post AND Slashdotted Already

[BattleApple]

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

Wait.. what the hell am I thinking?

Relativity

[Fuzzums]

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

Re:Relativity

[gardyloo]

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:And in English...

[Kartoffel]

Experimentalists, as opposed to theorists.

had to be done

[Kartoffel]

That's what *SHE* said!

Push me Pull me

[drewsup]

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

Wait, girl or boy?

[malignant_minded]

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?

[malignant_minded]

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?

[Chris Burke]

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?

[mattcasters]

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?

[ozbird]

Wait who is a she???

"She" is the cat's mother.

Already demonstrated at MIT

[Muerte23]

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

[plasmacutter]

nhk

ah the joy of field specific acronyms nobody understands.

Re:Already demonstrated at MIT

[Muerte23]

sorry.

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

Re:Already demonstrated at MIT

[jc42]

..., 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

[waxigloo]

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

[Muerte23]

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

[Forbman]

h == planck's constant.

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

Re:Already demonstrated at MIT

[waxigloo]

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

[rkowen]

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'

Re:Already demonstrated at MIT

[marcosdumay]

h-bar == h / (2 * pi)

Re:Already demonstrated at MIT

[mbkennel]

Indeed, especially given that the abstract of the PRL says that the conclusions are the opposite, i.e. supporting Abraham's theory, whereas nhk is Minkowski's side.

"There are two different proposals for the momentum of light in a transparent dielectric of refractive index n: Minkowski's version nE/c and Abraham's version E/(nc), where E and c are the energy and vacuum speed of light, respectively. Despite many tests and debates over nearly a century, momentum of light in a transparent dielectric remains controversial. In this Letter, we report a direct observation of the inward push force on the free end face of a nanometer silica filament exerted by the outgoing light. Our results suggest that Abraham's momentum is correct."

Re:Already demonstrated at MIT

[blueg3]

Wavenumber (n), wavevector (k), and Planck's constant (h).

E = nhk = hf = hbar*omega

Re:Already demonstrated at MIT

[jc42]

Huh? You used scientific algebraic notation? On slashdot? And you expected people here to understand you?

You must be new here ...

Re:Already demonstrated at MIT

[mbkennel]

Probably 'n' is index of refraction, which is related to the dielectric parameter.

Re:Already demonstrated at MIT

[blueg3]

Indeed. I'm misremembering electromagnetics a bit. :-) For one, wavenumber is generally written with a nu (although n is sometimes used), and it's really k-vec (wavevector) or nu*k-hat (wavenumber * wave direction).

Re:This is great!

[Bigjeff5]

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.

Re:First Post AND Slashdotted Already

[rehtonAesoohC]

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

Re:First Post AND Slashdotted Already

[philspear]

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!

Re:And in English...

[LandDolphin]

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

Re:Actually...

[rickb928]

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

Re:A little background is apropos me thinks...

[Yvan256]

Still using a 40-columns monitor, I see.

Re:What is the mystery resolution?

[rickb928]

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?

Click "Text-only version"

[tepples]

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:Click "Text-only version"

[Jabbrwokk]

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

Re:Click "Text-only version"

[Arthur B.]

Weird... the slower the light the higher the momentum then ? What was Minkowski argument ?

Re:Click "Text-only version"

[Hurricane78]

Am I right in understanding that

"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.

this makes the whole experiment worthless?

Re:Click "Text-only version"

[emtilt]

The article suggests that Minkowski's formulation is supported by this result, but reading the paper seems to suggest that Abraham's formulation is supported. The abstract states (and the paper's results confirm): "Our results suggest that Abraham's momentum is correct."

Re:Click "Text-only version"

[Fieryphoenix]

You know, that sounds about right... the more a material "drags" light to a lower speed, the more momentum the light imparts to the material. Can anyone comment if that is a fair nontechnical summation of the phenomenon?

Re:Click "Text-only version"

[Arthur B.]

The light *itself* is supposed to have more momentum...

Re:First Post AND Slashdotted Already

[allcoolnameswheretak]

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:Actually...

[Anonymous Coward]

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

Superfluous prepositions?

Re:Actually...

[Neeperando]

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:And in English...

[camperdave]

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:Actually...

[SBacks]

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.

Kinda old

[rrohbeck]

I knew I read about this quite some time ago: http://arxiv.org/abs/0806.2442

Re:Actually...

[pjt33]

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.

Experiment resolves century-old optics mystery

[soybean]

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

Re:Actually...

[Neeperando]

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

Re:Don't leave me in suspense!

[TemporalBeing]

42

Re:Actually...

[Anonymous Coward]

Only if you can reasonably approximate a theoretician as a sphere, though.

Re:And in English...

[SQLGuru]

Oh Crap! I'm wearing a red shirt!

http://www.genesjournalcomic.com/?p=104

Re:Actually...

[Jabbrwokk]

Maybe it's technically better, but your suggestion uses passive language. There's no real subject and makes for a bad headline from a news perspective, kind of like "Man struck" (by what, or by whom?)

The article's original "Experiment resolves century-old optics mystery" headline is probably best because it's active, avoids the scientist/researcher/experimentalist/alchemist problem and gets all the important information in there.

I am putting the grammar nazi hat away now.

Re:Actually...

[Anonymous Coward]

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.

Doesn't that mean that there's only a 25.7% chance that you're right?

Re:And in English...

[nschubach]

It's like a philosopher and a politician.

Theorists don't actually do anything. they just sit around and make up stuff. The Experimentalists actually try to make it work for their benefit.

Re:And in English...

[Rary]

It's like a philosopher and a politician.

Theorists don't actually do anything. they just sit around and make up stuff. The Experimentalists actually try to make it work for their benefit.

So.... in your philosopher/politician analogy, which one is the one that actually does something?

Re:For a physics newbie

[slew]

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

Short answer photons do not have mass.

The confusion for newbies is that p=m*v isn't the whole story. Photons always travel at the speed of light in a medium or inertial frame (whatever that may be). However, photons have no rest mass (or intrinsic mass which is what allows photons to travel at the speed of light given finite acceleration), but they do have momentum when they are moving at the speed of light. That's part of the wacky world of special relativity.

A more enlightening formulation to see how this works is E^2 = (p*c)^2 + (m0*c^2)^2. (p is the variable physicist usually use to indicate momentum)

In this formulation describing the conservation of energy, even things with no mass (like a photon) can have various amounts of momentum and take part in interactions that conserve energy.

Incidentally this formulation also simplifies to the famous E=m*c^2 for objects that don't have momentum.

Also to short circuit the next typical newbie question, the speed of light depends on the medium and isn't a universal constant (except the one that is quoted as speed of light in a vaccum). There are various interpretations about how light slows down in other medum(photons hit other objects and transfer momentum when not in a vaccum, are affected by various other localized electromagnetic fields of atoms, or take a non-linear path in those mediums), but these different speeds in different medium have nothing to do with momentum of a photon in a vaccum. Such a photon is always travelling at the speed of light in a vaccum and can still have any of a continuous range of momentum (ignoring quantum mechanics for a moment), yet the photon still has zero rest mass.

Note that some textbooks try to refer to apparent variability in mass as "relativistic inertial mass" which changes with velocity with a lorentz factor. A fudge-factor concept like relativistic non-constant mass has some attractive properties for objects that have non-zero rest mass, but unfortunatly it sort of turns into a zero divided-by zero handwave for something like a photon and is probably something to be best avoided if you want to understand how an object which has zero rest mass can still have momentum.

Re:And in English...

[nschubach]

That's the beauty! Neither! They get some intern to do it for them.

Damn you, Taco!

[PingXao]

You've gone and slashdotted the Institute of Physics! The Quantum Flux is not amused and the Strangelets are circling the wagons.

To China with love.

[Requiem18th]

Am I the only one who finds it amusing that we have just ddns a Chinese server?

Re: To China with love.

[evan_arrrr!]

It would be more amusing if it actually were a Chinese server that went down. The reality is that the PhysicsWorld.com server hosting the article went down.

Re:First Post AND Slashdotted Already

[shawb]

While that would be very nice for those few small sites that get hit, it would be copyright violation and get slashdot's pants sued off by anyone who makes money off of web hits.

Re:Actually...

[JWSmythe]

That's amazing! Through experimentation I've proven that 98.95% (rounded, of course, in a series of 100,000 trials) of mathematically proven statistics are yanked straight out of the posterior egesta expulsion orifice.

Re:A little background is apropos me thinks...

[bishiraver]

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.

Re: To China with love. Mandatory Meme

[FurtiveGlancer]

All your base is ours.

Re:This is great!

[rusl]

"More efficient Fusion..."

Re:First Post AND Slashdotted Already

[SolusSD]

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

Re:Don't leave me in suspense!

[Intron]

What is the answer?

The answer is that slashdotting provides a positive force on the server regardless of the medium.

Re:First Post AND Slashdotted Already

[v1]

what happened to mirrordot? it's currently hosting "0 articles". dur?

With apologies to Mary Poppins

[Amazing Quantum Man]

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!!

Re:A little background is apropos me thinks...

[Petrushka]

For those with a large monitor, the GP was doing us all a favor.

No, there's this marvellous new invention called "resizeable windows" that takes care of the issue you mention.

In effect, the GGP was overriding user settings and imposing his/her own settings on all browsers. It's about as polite as using a <font> tag in html.

Re:First Post AND Slashdotted Already

[hierophanta]

links to your Slashdottings/Diggs? nothing personal - but i just dont believe the claims you are making.

Re:push

[OeLeWaPpErKe]

The question is, given a dielectric medium, an interface between 2 substances with different refractive indexes, will a push or a pull will be exerted on the interface surface given that the surface the light is coming out of has a lower refractive index than the substance the light is flying into.

(stops for a breather)

And I can't find the full answer in the article. (the full answer will contain how the force exerted depends on the refraction indices, ie. just "push" or "pull" is not enough. That's like saying the clock rotates to the right : it's correct half of the time)

Ahhhh...

[dfsmith]

That explains why black cars get dirtier faster than lighter colored ones.

Re:First Post AND Slashdotted Already

[fugue]

Don't be absurd. That can't happen in the USA, because this discovery does not contradict any extant interpretation of the Bible.

Re:First Post AND Slashdotted Already

[HeronBlademaster]

Think again. Why do you think we tell people who are dying not to go toward the light? It's pulling on them!

~

Heh... tipical.

[crashfourit]

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...

Re:First Post AND Slashdotted Already

[jbezorg]

The server didn't go down. The undersea cables just went missing.

Anonymous Coward

[Anonymous Coward]

So the fucking sun is pushing us away!

Re:Actually...

[nodrogluap]

One of my favorite adages: "In theory, theory and practice are the same, but in practice they're not."

Re:First Post AND Slashdotted Already

[Migity]

The only time I REALLY wanted to read the article and it's not even there...jeez

Re:First Post AND Slashdotted Already

[ranulf]

How about we criticise the summary instead. It's experimenters not "experimentalists"... An experimentalist is surely someone who's prejudiced against experimental things...

Horrible writting

[NuShrike]

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.