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Scientists Solve Century-Old Optics Mystery
evan_arrrr! writes "From the article:
Since the early 20th century physicists have known that light carries momentum, but the way this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric: one suggesting it pushes the surface in the direction light is traveling; the other suggesting it drags the surface backwards towards the source of light. After 100 years of conflicting experimental results, a team of experimentalists from China believe they have finally found a resolution."
Hmm. If you're like most people, I'm guessing that you don't have a
lot of experience with physics. (Well, not in the formal sense,
anyway! Most people are expert physicists on an intuitive level, with
such remarkable skill that they can lift objects, understand
reflections in a mirror, and even catch a flying ball!) So I'll try to
keep this frosty-posty on a very basic level, and I apologize in advance if I
start spouting jargon or go too fast. If you _do_ have a bit of a
physics background, my apologies for the simple explanation that
follows. I'll label some particularly important paragraphs with "***".
For the record, you won't need any "scholarly journals" here unless you
want to get very cutting edge indeed (or unless you want to go back
many decades or centuries to the original writings that discussed the
concept). The vast majority of what we know about momentum can be
found in textbooks: it is one of the most basic concepts in physics.
Also, keep in mind that while momentum is a fundamental part of
physics, the word has many (related) meanings in colloquial English,
too. I'd guess that a play named _Momentum_ will draw on a wide range
of those.
*** So, what is momentum? The first and most basic statement of the
concept of momentum comes from Newton's First Law of Motion: "An object
in motion will remain in motion unless acted on by an outside force."
(That's sometimes called the "Law of Inertia"; "inertia" and "momentum"
are closely related concepts.) In physics, an object's "momentum" can
be thought of as the "amount of motion" that it has: the greater its
momentum, the harder it is to stop it or to turn it in another
direction.
*** What makes an object harder to stop? Well, the faster it's moving,
the more you have to slow it down, so momentum must depend on speed.
(In fact, it turns out that the direction is important, too; physicists
call speed in a specified direction "velocity".) And the heaver it is,
the harder you have to push to slow it down, so momentum must depend on
"mass" (which is a physicist's technical term for what we normally
think of as weight).
***The formal mathematical definition of momentum (in classical
physics) is the product of those two quantities:
momentum = mass * velocity
To give a few examples, a flying gnat is fast but its mass is very low,
so it doesn't have much momentum. That means that it's easy for a gnat
to turn around and buzz in another direction (which you've probably
seen firsthand). On the other hand, a slowly rolling car still has a
lot of momentum because it's so very heavy: it would be hard to push
one to a stop even at very low speeds. As yet another example, a
bullet is pretty lightweight, but when it is fired from a gun its
enormous speed gives it very high momentum (and if a person tragically
gets in its way, the effort of absorbing all that momentum will break
their flesh and bones).
*** Now, as I mentioned earlier, "velocity" implies not just speed but
direction. So since momentum is proportional to velocity, momentum
always has a direction, too. That's a very fundamental fact about
momentum! Changing an object's direction can be just as hard as
stopping it completely.
*** Another remarkable fact about momentum is that the _total_ amount
of momentum in a system will never change. Physicists call this rule
"Conservation of Momentum", and they say that "momentum is conserved".
For example, if you're playing pool and you hit the cue ball into the
eight ball, when the cue ball slows down the eight ball will start
moving to make up the difference. If your shot is perfectly straight,
the cue ball may stop moving entirely while the eight ball rolls away
with the same velocity that the cue ball used to have. (It would
_have_ to be the same velocity: because the balls have the same mass,
conservation of momentum m
Is it just my observation, or is eldavojohn an idiot?
Since it's already slahshdotted, here's the cached version.
Google Cache for anyone interested in reading it
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.
If I have nothing to hide, don't search me
Text Only Mirror
"We report direct observation of a push force on the end face of the silica filament exerted by the outgoing light" said [Weilong] She."
TFS left it out; this was the result.
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.
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.
Experimentalists, as opposed to theorists.
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.
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.)
Virtue finds and chooses the mean.
Aristotle, Ethica Nichomachea
Since it's already slahshdotted, here's the cached version.
Page wont load in google cache either. Google cache has been slashdotted.
That's because your web browser is trying to pull the CSS and images from the (now slashdotted) original server before it lays out the page. Click "Text-only version" to view the page without CSS and images.
It 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.
-- "At Microsoft, quality is job 1.1" -- PC Magazine, Nov. 1994
Within physics, there is a difference between theorists (people who do try to prove things using math) and experimentalists (people who do experiments to test the theorists' theories).
Most physicists see themselves as either one or the other, and often the two do not get along. Theorists see experimentalists as being corrupted by real world problems when really all the problems can be solved by a little hard thought (and maybe some math). They think experiments shouldn't be called "science" but "engineering". Experimentalists see theorists as having pointless jobs because nothing they ever do will ever produce something useful to the human race, by their very nature.
In reality, of course, they are dependent on each other, because without the theorists' theories the experimentalists have nothing to test, and without the hope of some kind of payoff from experimentalists, theorists will never get funding.
Also, as a non-physicist, it can be fun to pit theorists and experimentalists against each other in battles to the death and watch what happens.
Being a computer scientist means you tell people how computers should work, not that you know how they actually work.
How is it that light has momentum when it has no mass?
For the same reason that speeds don't strictly add up linearly: relativity. In Newtonian mechanics, momentum is p = m*v where m is the mass and v is the velocity. But when you take relativity into account, the proper definition is actually p = gamma*m*v. For a photon, you might think m = 0 would mean p = 0, but when v=c (the speed of light), gamma = 1/0. So you have an equation p = c*0/0. Obviously something is wrong, and in a careful analysis it turns out that for massless objects (which travel at c) p = E/c (where E is total energy, and c is speed of light).
So, basically the momentum of massless particles arises from taking into account relativity. The fact that we can actually measure photon pressure is an interesting proof that the math "works."
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.
Escher was the first MC and Giger invented the HR department.
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.
Only if you can reasonably approximate a theoretician as a sphere, though.
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.
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.
I'll never make that mistake again, reading the experts' opinions. - Feynman
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)
If I have nothing to hide, don't search me
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.