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From: Timo Nieminen on 12 Jun 2010 00:39
On Jun 12, 11:24 am, "Sue..." <suzysewns...(a)yahoo.com.au> wrote:
> On Jun 11, 4:17 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
>
> > For a beam of light, the radiation force is the change in the momentum
> > flux when the beam is reflected, refracted, or absorbed. For a photon,
> > the impulse imparted to a reflector, refractor, or absorber, is the
> > change in the photon momentum when it is reflected, refracted, or
> > absorbed.
>
> The notion certainly didn't come from the sci-fi
> shelves. It may  be instructive to examine the
> differences in two concise statements.
>
> <<Since the force on a dielectric object is given
> by the change in momentum of light induced due to
> refraction of the light by the object, the total
> force on the object is the difference between the
> momentum flux entering the object and that leaving
> the object. The total force on an object due to
> refraction of light is therefore >> eq3,p2
> "Optical Tweezers: Measuring Piconewton Forces"http://www.biophysics.org/Portals/1/PDFs/Education/williams.pdf

In an ideal world, one clearly distinguishes between "(total)
momentum", "momentum density", "momentum flux", and "momentum flux
density". In practice, we are sometimes too lazy. This aside, it all
just comes done to conservation of momentum, or, if you prefer to
express it longhand, Newton's 3 laws of motion.

> I get  the impression the method is an approximation
> using kinetic theory to speed up calculations where
> molecular dynamics (many bodies) is more accurate,
> but too intense computationally for a laboratory
> tool. Just one more reason I hesitate to scale
> it up to larger objects.

Matter, ultimately, isn't a continuum characterisable by a simple
epsilon, mu, and sigma (or mu and complex epsilon). "Atomic theory",
in the old sense of the word, kinetic theory, the existence of
discrete atoms (well, they were assumed to be a-tomic, literally) is
the first of our non-classical, quantum, theories. Classical theories
assuming continuous matter are just an approximation. Where the
approximation is sufficiently good so as to be exact past the point
where we stop bothering to write down digits, it's a pretty safe
approximation to make.

Same for the EM field. If we're interested in the time-averaged force,
whether the time-average over 1 optical cycle, or merely an average
over a time orders of magnitude shorter than the time-scale on which
the measurements we're comparing with, classical EM theory is a very
convenient approximate method for dealing with, e.g., 10^20 photons
per second.

Worry instead about failure as we scale down, not up!

> Can one learn about the Rayleigh regime and the
> Mie regime anywhere near books about the Mao regime?
> (Spudnik wanted me to ask because he is too shy)

Despite Mao's rhetoric on Western physics, he didn't have anything
against the core concepts in particle physics. Quite friendly to them,
even, since positive and negative charge merely demonstrates that the
validity of materialist dialectics extends to the physics world. He
knew of, read, and had published in Chinese, work by Sakata Shiyouchi.
Apart from his "Talk on Sakata's article", he mentioned Sakata's work
elsewhere. Perhaps the would-be philosopher of dialectic materialism
who wants something from Rayleigh and Mie might find it in E vs H, TE
vs TM. Considering the latter, and the consequent affront that a third
polarisation would be to the core principles of dialectic materialism,
we find a sound socialist affirmation of div(E) = 0 = div(H). Plasma,
therefore, is deeply reactionary, as well as potentially quite
reactive.

http://www.starpathvisions.com/enchanted.html
From: Sue... on 12 Jun 2010 07:04
On Jun 12, 12:39 am, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
> On Jun 12, 11:24 am, "Sue..." <suzysewns...(a)yahoo.com.au> wrote:
>
>
>
> > On Jun 11, 4:17 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
>
> > > For a beam of light, the radiation force is the change in the momentum
> > > flux when the beam is reflected, refracted, or absorbed. For a photon,
> > > the impulse imparted to a reflector, refractor, or absorber, is the
> > > change in the photon momentum when it is reflected, refracted, or
> > > absorbed.
>
> > The notion certainly didn't come from the sci-fi
> > shelves. It may be instructive to examine the
> > differences in two concise statements.
>
> > <<Since the force on a dielectric object is given
> > by the change in momentum of light induced due to
> > refraction of the light by the object, the total
> > force on the object is the difference between the
> > momentum flux entering the object and that leaving
> > the object. The total force on an object due to
> > refraction of light is therefore >> eq3,p2
> > "Optical Tweezers: Measuring Piconewton Forces"
http://www.biophysics.org/Portals/1/PDFs/Education/williams.pdf
>
> In an ideal world, one clearly distinguishes between "(total)
> momentum", "momentum density", "momentum flux", and "momentum flux
> density". In practice, we are sometimes too lazy. This aside, it all
> just comes done to conservation of momentum, or, if you prefer to
> express it longhand, Newton's 3 laws of motion.

In this less than ideal world there is still some
expectation that displacements will at least have
the correct signs. The "push" of a pressure
won't become the "pull" of a suction just by
small changes in and experiment's size or changes
in a dominate mode. The scale of your tweezing
example is specifically where the effects of
London forces would cause such an inversion of
force.

http://en.wikipedia.org/wiki/Lennard-Jones_potential

And the mode alters the displacement as well:

<<a number of other beam types have been used to
trap particles, including high order laser beams
i.e. Hermite Gaussian beam (TEMxy), Laguerre-Gaussian
(LG) beams (TEMpl) and Bessel beams.

Optical tweezers based on Laguerre-Gaussian
beams have the unique capability of trapping
particles that are optically reflective and
absorptive. Laguerre-Gaussian beams also possess
a well-defined orbital angular momentum that
can rotate particles>>
http://en.wikipedia.org/wiki/Optical_tweezers

>
> > I get the impression the method is an approximation
> > using kinetic theory to speed up calculations where
> > molecular dynamics (many bodies) is more accurate,
> > but too intense computationally for a laboratory
> > tool. Just one more reason I hesitate to scale
> > it up to larger objects.
>
> Matter, ultimately, isn't a continuum characterisable by a simple
> epsilon, mu, and sigma (or mu and complex epsilon). "Atomic theory",
> in the old sense of the word, kinetic theory, the existence of
> discrete atoms (well, they were assumed to be a-tomic, literally) is
> the first of our non-classical, quantum, theories. Classical theories
> assuming continuous matter are just an approximation. Where the
> approximation is sufficiently good so as to be exact past the point
> where we stop bothering to write down digits, it's a pretty safe
> approximation to make.
>
> Same for the EM field. If we're interested in the time-averaged force,
> whether the time-average over 1 optical cycle, or merely an average
> over a time orders of magnitude shorter than the time-scale on which
> the measurements we're comparing with, classical EM theory is a very
> convenient approximate method for dealing with, e.g., 10^20 photons
> per second.
>
> Worry instead about failure as we scale down, not up!

I think we can *worry* without scaling up or down.

This thread seems to have wandered out of the
frying pan in to the fire.

Loss of Newton's first and second laws are a
small price to pay for the simplicity of
~an absorber heats the traversed volume once,
a reflector heats the traversed volume twice.

The discussion wandered into something far
more interesting so I shouldn't belabour the
point.

If Newton's first and second laws can replace intensive
molecular dynamics to make a tool easier to
use, that's fine with me. But I don't see
the basis for sweeping statements that apply
with any great generality.

>
> > Can one learn about the Rayleigh regime and the
> > Mie regime anywhere near books about the Mao regime?
> > (Spudnik wanted me to ask because he is too shy)
>
> Despite Mao's rhetoric on Western physics, he didn't have anything
> against the core concepts in particle physics. Quite friendly to them,
> even, since positive and negative charge merely demonstrates that the
> validity of materialist dialectics extends to the physics world. He
> knew of, read, and had published in Chinese, work by Sakata Shiyouchi.
> Apart from his "Talk on Sakata's article", he mentioned Sakata's work
> elsewhere. Perhaps the would-be philosopher of dialectic materialism
> who wants something from Rayleigh and Mie might find it in E vs H, TE
> vs TM. Considering the latter, and the consequent affront that a third
> polarisation would be to the core principles of dialectic materialism,
> we find a sound socialist affirmation of div(E) = 0 = div(H). Plasma,
> therefore, is deeply reactionary, as well as potentially quite
> reactive.
>
http://www.starpathvisions.com/enchanted.html

Ouch. Yes, I deserved that.
Be careful what you ask for, right? :-)

I don't believe your equations are from Kansas.
http://www.starpathvisions.com/imageAHB.JPG

Sue...


From: Sam Wormley on 12 Jun 2010 16:27
On 6/11/10 11:27 PM, kado(a)nventure.com wrote:
> On Jun 8, 4:00 pm, Sam Wormley<sworml...(a)gmail.com> wrote:
>>
>> Do you understand that the frames of the twins are not both
>> inertial frames of reference?
>>
>> Physics FAQ: The Twin Paradox
>>
>> http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html
>
>
> So please just tell me which twin is younger than the other.
>

The twin that experienced the accelerations is the younger of the
two when they are back together. See: The Twin Paradox: The Spacetime
Diagram Analysis.

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_spacetime.html
From: Timo Nieminen on 12 Jun 2010 16:46
On Jun 12, 9:04 pm, "Sue..." <suzysewns...(a)yahoo.com.au> wrote:
>
> In this less than ideal world there is still some
> expectation that displacements will at least have
> the correct signs. The "push" of a pressure
> won't become the "pull" of a suction just by
> small changes in and experiment's size or changes
> in a dominate mode. The scale of your tweezing
> example is specifically where the effects of
> London forces would cause such an inversion of
> force.
>
> http://en.wikipedia.org/wiki/Lennard-Jones_potential

A force between what and what? Take a sphere in a uniform medium. If
the medium pushes on the sphere more or less ("pull" if you want, with
the usual cautions on "pull", vacuum "sucking" etc), what does this do
to the motion of the sphere? (OK, a medium with a temperature gradient
isn't uniform, see below.)

Where sphere meets sphere, or sphere meets cell, or sphere meets
substrate, this is a different story. These things stick, and vdW/
London forces are among those at work. These don't scale with trapping
beam power; enough power can unstick the stuck. It's a known practical
issue, sometimes useful, sometimes really bad, sometimes avoidable. (L-
J isn't the same, and isn't relevant here beyond a not-needed-for-the-
current-purposes explanation of why real matter isn't as compressible
as an ideal gas.)

(Salt (i.e., NaCl) crystals are _really_ sticky!)

> And the mode alters the displacement as well:

Yes, it does. In the way expected from the usual theory of photon
momentum/radiation pressure.

> Optical tweezers based on Laguerre-Gaussian
> beams have the unique capability of trapping
> particles that are optically reflective and
> absorptive.

This isn't true. If one is fussy and demands 3D trapping, then LG
beams generally don't give you 3D trapping of reflective or absorbing
particles; in the typical experiment, the particles are trapped in 2D,
and pushed against a surface. If this is sufficiently trapped to be
called "trapping", then this isn't unique, since you can do the same
with a Gaussian beam. Low-index particles are another story (even if
still a 2D story), but that isn't the wiki-claim.

> If Newton's first and second laws can replace intensive
> molecular dynamics to make a tool easier to
> use, that's fine with me. But I don't see
> the basis for sweeping statements that apply
> with any great generality.

There are two kinds of approximations we can make here. First, we can
use convenient approximate theories, such as continuum models of
matter, classical electrodynamics, simple semi-classical models of
atoms for atom trapping. Using conservation of momentum _isn't_ an
approximation of this type; as far as we know, it is correct at all
scales. If the usual classical (i.e., continuum) approximations work
for tweezers, they should be fine for larger scales, too. Be careful
when going down to trap nanoparticles, quantum dots, molecules, and
atoms.

The second kind of approximation is ignoring effects that are
(hopefully) not relevant. This is where one can go wrong much more
easily. What to do? Know what you need to include, know what can be
safely excluded, and measure or observe what you aren't sure about.
What else? Be careful, because previous measurements might have been
made at quite different size scales; you need to know how these things
scale.

For radiometer forces in particular, how can we do this? Measure
directly, iirc this has been done by Ashkin, in his pre-tweezers
(1970-1986) optical force work; in a gas, so not very relevant to
trapping in liquid. Vary refractive index or reflectivity while
keeping absorption and heating the same, or at least not varying in
the same pattern. Done, e.g., R. V. Jones (forces on things in various
liquids), lots of tweezers experiments on objects with varying
refractive index (and therefore varying reflectivity). Even some of
the optical torque experiments are relevant - the ones with absorbing
particles (e.g., Friese, Enger, et al., 1996). Vary the absorption and
keep other stuff the same. Done, e.g., trapping in D2O vs H2O, thermal
damage to specimens as function of wavelength, slightly absorbing
particles. Directly measure forces due to thermal gradients in liquids
(a.k.a. thermophoresis), also done. The lack of temperature dependence
of the usual tweezers forces, the "scattering" force and the gradient
force, has been adequately shown.

Not to leave out the really conclusive ones: look at atom trapping,
where the effects of the background gas is catastrophic to the
trapping.

Next, does the known physics that is included in the model explain
what is seen? If so, either all of the important stuff is included, or
non-included stuffs almost exactly cancel each other out, or non-
included stuff cancels an error or other problem with the included
stuff (including, of course, non-existence of the included stuff,
everything being due to the non-included stuff). Also done.

There was an old optical tweezers pre-print claiming that the forces
seen in optical tweezers are thermal. (On arxiv? I can't find it
there.) You might be interested in looking for this. We can discuss
here, if you wish.
From: NoEinstein on 12 Jun 2010 20:26
On Jun 11, 11:34 pm, NoEinstein <noeinst...(a)bellsouth.net> wrote:
>
Correction: Make that WENT where! — NE —
>
> On Jun 11, 4:17 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
>
> Dear Timo:  You know enough about physics to bluff with the best of
> them.  But you lack sufficient focus to understand very many of the
> particulars.  It should suffice to say that the energy IN must = the
> energy OUT.  Figuring out what portion of the energy when where is
> madness.  All high energy particle physics, like that fiasco in
> France, is concerned with "matter" which doesn't really matter!  —
> NoEinstein —
>
>
>
>
>
> > On Jun 11, 9:38 pm, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote:
>
> > > On Jun 10, 10:31 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
> > > > On Thu, 10 Jun 2010, Tim BandTech.com wrote:
> > > > > On Jun 10, 3:19 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
> > > > > > On Jun 10, 9:43 pm, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote:
> > > > > > > On Jun 9, 3:40 pm, Timo Nieminen <t...(a)physics.uq.edu.au> wrote:
> > > > > > > > On Jun 10, 4:10 am, "Tim BandTech.com" <tttppp...(a)yahoo.com> wrote:
>
> > > > > > > > > If we had, say, a large lead ball hanging on a
> > > > > > > > > string in stasis, and sent a tiny steel ball into the lead ball at
> > > > > > > > > high velocity then we would observe some heating, but too achieve the
> > > > > > > > > level of heating that light achieves will be quite some trick to mimic
> > > > > > > > > in the terms of massive collisions.
>
> > > > > > > > Not at all. Wikipedia tells me that the energy of a typical 5.56mm
> > > > > > > > NATO bullet when fired is 1.7kJ. Shoot them into a massive target at a
> > > > > > > > little under 1 round per second, and you achieve approximately the
> > > > > > > > same heating. (Not the same force as with light! Just the same
> > > > > > > > heating.)
>
> > > > > > > Here again I see your obfuscatory tactic. Firstly you falsify and in
> > > > > > > your conclusion you agree.
>
> > > > > > You were quite specific: "level of heating that light achieves will be
> > > > > > quite some trick to mimic". This level of heating is easy to mimic.
>
> > > > > > > The amount of heating that light is capable
> > > > > > > of when absorbed versus the work that can be done mechanically due to
> > > > > > > that absorption are remarkable in comparison to your NATO bullet.
>
> > > > > > So, you want to change "level of heating" to "amount of heating versus
> > > > > > work"? If you meant this in the first place, you weren't clear enough.
> > > > > > To criticise my reply to your actual original words on the basis of
> > > > > > your _changed_ version lies somewhere on the scale from weaseling to
> > > > > > complete bullshit.
>
> > > > > I'll have to own here that I should have used the word 'relative'
> > > > > within the context, but I see it is fairly easy to interperet since
> > > > > the context of the whole argument is still present. Hell, I can match
> > > > > the heat of sunlight rubbing some steel on a rock. The lead of the
> > > > > bullet will melt on impact. Your interpretation of my writing is
> > > > > clearly not coherent at many levels.
>
> > > > Your original argument wasn't coherent, then. As far as I could tell, you
> > > > thought it improper that almost all of the energy should go into heating,
> > > > not work.
>
> > > Jeeze. I pity any onlooker who attempts to decode your own position.
> > > Now, via double crossing, I must admit that I do agree with your
> > > statement above and that this is exactly what I've been discussing all
> > > along.
>
> > So, you agree that "improper that almost all of the energy should go
> > into heating,
> > not work" is a fair summary of your original argument?
>
> > Then what complaint do you have against the bullet example? In the
> > bullet example, almost all of the energy goes into heating. Surely
> > this is just as "improper". If it's OK for most of the KE of a bullet
> > to end up heating the target, rather than doing work, why is somehow
> > wrong for a photon?
>
> > Note well that in both the bullet and photon cases, you get a force if
> > the bullet/photon is reflected with no loss of energy. It's simpler to
> > consider the reflection case, since you avoid the complications you
> > get into below.
>
> > > > We see the same thing in Newtonian mechanics for a light object in
> > > > inelastic collision with a heavy object. Qualitatively, the same type of
> > > > thing, most of the energy going into heating.
>
> > > It is a peculiar thing to discuss an inelastic collision this way.
> > > There are more dynamics here and the options as to what happens with
> > > the energy are numerous. For instance, if we could design a spring
> > > with a ratchet to capture the projectile then we would only heat by
> > > the inefficiency of the spring device, rather than by the objects
> > > entire kinetic energy. Likewise as I stated before, this capture
> > > mechanism could as well generate electricity, and of course this is
> > > the quest of many right now; to capture electricity from the sun, and
> > > to do it at a better margin than 20%. We cannot grant the heat clause
> > > unconditionally, and instead want the mechanism of that heating. It is
> > > not so difficult to see electron disturbance; literally microcurrents
> > > flowing in short circuit fashion which provide the heating effect.
> > > That these charge concerns are so nearby to the Nichols device is a
> > > caveat on the operation of that device. That your light traps are so
> > > behaved is likewise a caveat. Some would like to free these electrons
> > > to take a larger path. Others would like to ignore them for their
> > > inconvenient truth.
>
> > Yes, we can explain the heating when an EM wave is absorbed in terms
> > of electrodynamics. Are you seeking to attribute the observed forces
> > to electron ejection? The atom trapping experiments show it isn't so,
> > very clearly. Given that the observed forces are predicted by
> > conventional theory to within the experimental error (i.e., better
> > than 10% in the less accurate experiments, 0.1% in good experiments),
> > it doesn't appear to be significant in the macroscopic experiments
> > either. Does the magnetic force on a current-carrying wire depend on
> > electron ejection? If not, why should the forces due to an EM wave?
>
> > > > > The context of the discussion for me revolves around the photon energy
> > > > > and how we can come to attribute the photon momentum to the photon
> > > > > energy without concern for such things as angular momentum.
>
> > > > We're not attributing the photon momentum to the photon energy.
>
> > > > For a moving bullet, does the kinetic energy cause the momentum? Does the
> > > > momentum cause the kinetic energy? Do we attribute one to the other?
>
> > > > For the photon, consider a spin +1 photon. How much of its energy is "in"
> > > > the angular momentum?
>
> > > > Be specific: consider a 500nm photon, with hbar angular momentum. We can
> > > > write down 3 numbers: its energy, its momentum, and its angular momentum.
> > > > How are these 3 related to each other, showing appropriate concern for
> > > > angular momentum?
>
> > > > How about for a 5Hz photon? Do the same. Does this mean that a 5Hz photon
> > > > shouldn't have any momentum?
>
> > > From what I've read of the quantum stuff the angular momentum is not
> > > so easy to observe. They even go so far as to grant a photon an
> > > orbital angular momentum, though the orbital context is not present in
> > > freely propagating light. And yet the light as an oscillation is still
> > > a coherent concept, and so the rotation momentum that I am considering
> > > is more like your NATO bullet spinning.
>
> > > As I recall the 5Hz photon will have the same quantum angular momentum
> > > than will the 500nm photon, but will have less total energy via e=hf.
>
> > Keep it simple,. don't worry about orbital angular momentum. If you
> > can't answer the simple question, why make it more complicated.
>
> > Don't get sidetracked into pretty pictures of photons, either, or long
> > philosophical tracts. Answer the simple question first.
>
> > If the momentum and/or angular momentum are attributed to the energy,
> > how much of each, in each case, for the 500nm photon and the 5Hz
> > photon? Angular momentum hbar, E=hf, p=h/lambda. Write down the
> > numbers, and say how much of the energy is "in" the angular momentum
> > (or whatever you mean, when you say that having angular momentum means
> > that less energy must be "available" for the momentum).
>
> > (There are 2 potential nuggets to be had here, if you bother to do
> > it.)
>
> > [moved]
>
> > > > The ratios or values of energy, momentum, and angular momentum, as
> > > > commonly stated for photons, come straight out of classical
> > > > electromagnetic theory. Are these ratios or values wrong? Yes or no, no
> > > > handwaving, no waffle, just a straight answer.
>
> > > As you seek a straight answer I must ask you how crooked is the path
> > > of modern theory? Yes, we'd all like a straight answer, yet we do not
> > > actually have one yet. This is not to say that we should give up. We
> > > seek a straight answer, but I must admit that I do not yet have one.
>
> > If you can't say straight-out that it's right or wrong, from what
> > information are you arguing?
>
> > It's simple in a way. We can, directly from Maxwell's equations, by
> > finding the induced currents and dielectric polarisation, find the
> > force acting on the current and polarisation, via the Lorentz force,
> > and find the work done on the current by the field (which is the
> > heating).
>
> > > beyond this it seems that in the accumulation the overlap of radiation
> > > pressure with photon momentum has gotten lost in the shuffle.
>
> > No. Or if it did, stop shuffling it. It isn't an overlap, one is the
> > rate of change of the other.
>
> > For a beam of light, the radiation force is the change in the momentum
> > flux when the beam is reflected, refracted, or absorbed. For a photon,
> > the impulse imparted to a reflector, refractor, or absorber, is the
> > change in the photon momentum when it is reflected, refracted, or
> > absorbed.
>
> > If light has momentum, there must be a force when this momentum
> > changes. If light can exert
>
> ...
>
> read more »- Hide quoted text -
>
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