# Physicists detect the enigmatic spin momentum of light

Ever since Kepler's observation in the 17th century that sunlight is one of the reasons that the tails of comets to always face away from the sun, it has been understood that light exerts pressure in the direction it propagates. Radiation pressure is produced by the momentum carried by light, and it plays a crucial role in a variety of systems, from atomic to astronomical scales.

In a recent theoretical paper, a group from the RIKEN Center for Emergent Matter Science in Japan showed that momentum density in non-uniform optical fields has an unusual component, which is orthogonal to the propagation direction of light and is proportional to the optical spin, which means the degree of circular polarization. They predicted that this spin momentum would produce a transverse spin-dependent optical force, a few orders of magnitude weaker than the usual radiation pressure.

Now, based on the theoretical work, a group from RIKEN, the University of Bristol, and other institutions have used an extremely precise technique to experimentally verify that light does in fact exert the extraordinary perpendicular force, which is determined by the polarization of the light. The research has been published in *Nature Physics*.

To measure the new type of optical momentum and force, they used an extremely sensitive nano-cantilever, capable of femtoNewton resolution—meaning it could measure a force even smaller than the force gravity exerts on a single bacterium—which was immersed in an evanescent optical field directly above a total-internal-reflecting glass surface.

According to Konstantin Bliokh, the corresponding author of the paper, "Our findings revisit fundamental momentum properties of light and, revealing a new type of optical force, enrich optomechanics."

Looking to the future, Franco Nori, who led the research team, says, "Our group's investigations integrate relativistic field-theoretical, quantum-mechanical, and optical aspects of the dynamical properties of light. They offer a new paradigm which could provide insights into a variety of phenomena: from applied optics to high-energy physics."

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**More information:**Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever,

*Nature Physics*, DOI: 10.1038/nphys3732

Konstantin Y. Bliokh et al. Extraordinary momentum and spin in evanescent waves, *Nature Communications* (2014). DOI: 10.1038/ncomms4300

**Citation**: Physicists detect the enigmatic spin momentum of light (2016, April 25) retrieved 26 August 2019 from https://phys.org/news/2016-04-physicists-enigmatic-momentum.html

## User comments

Nik_2213Da SchneibHyperfuzzyDa SchneibNoumenonIf you speak of photons you must use quantum theory in which it is expressed by use of operators .

Do you mean by "present theory", Maxwell's equations?

HyperfuzzyI show the field has the velocity vector of the particle and is continuously updated. Speed of light relative to the particle, i. e Maxwell. You might think of the field as being permanently attached to the particle with a speed of light update. Relative to other points we see frequency, motion, etc., Vector arithmetic. I can go on but the superposition is obvious. Though a media, i.e other charges, field to particle and particle to field ... update description of poynting vector as a 4D vector

NoumenonPlease send Phys.Org a request to disable comment ratings. Their associate site has done this.

Da SchneibIn the mathematical treatment, I see the E and A fields, which is Maxwell, and I see that the SAM of a single photon is +/- hbar, which is QM.

As for OAM, see https://en.wikipe...of_light which discusses helically polarized light. This article also discusses both Maxwell and QM.

Da SchneibCaptain Stumpyit won't stop the sock-puppets... it won't stop the trolling... it won't stop unsubstantiated clams...

in fact, it won't *do* anything except undermine the ability to diagnose a problematic individual and increase the hardships in tracking socks (as then it would require a bot to search the site)

the problem with disabling the ratings is that it allows the trolls to flourish

(which is why i downrated your post)

best choices:

either disable the comments entirely (cheapest)

or moderate the site (still cheap, minimal changes to programming of profiles and permissions, free MOD's per then system i forwarded)

*******

it would be smarter and more effective to disable the comments entirely and allow other sites to pick up the comment business slack and only track shares, reading of articles or times visited for interest or popularity

Da SchneibMy first question is, why does gauge invariance apply here?

My second question is, if we can detect both circular and helical polarizations (and anybody who has any idea how a satellite antenna works knows we can- once again, a matter of engineering, never mind physics), and we have both Maxwell and QM descriptions for them in the beam and particle domains, what's the problem?

My third question is, if gauge invariance is at odds with experiment, why is it not anathema in the particle physics community? And I assume you mean theoretical particle physics, since experimental particle physicists are generally more interested in experiment.

Da SchneibAnother question.

This seems highly questionable since the spin of bosons is what distinguishes them from fermions in QM. Unit-spin bosons' Law of Spin and Statistics makes them able to cohere; half-unit-spin fermions' Law of Spin and Statistics makes them obey Pauli exclusion and prevents them from cohering. This is extremely basic QM. I can't imagine someone claiming a photon has no spin, or even that it cannot be measured since it obviously is.

Can you resolve this conundrum for me?

Da Schneib@Cappy

I suggest more liberal use of the mute button. I stopped complaining about commercials when I started using it many years ago, and it's even better now that I have a fast forward on my DVR, and the same applies here. If no one talks to them they will eventually give up and go away. Then this will be a better place.

Captain Stumpyi see your point, and sometimes i agree

i just don't think it's the best way to combat the pseudoscience or fearmongering rant of stupidity like certain "pretend" engineers

also, there is this to consider

https://www.youtu...EwjBXlZE

i will take your argument under advisement

i still think moderation is the best and most effective means to create meaningful dialogue between opposing viewpoints while supporting the rule of law (equally) and allowing the ability to present conjecture about potential new ideas (considering the evidence, that is) in scientific discourse

you don't watch commercials?

wow

i rather like commercials (when i see them)

IMHO - they're an indicator of societal beliefs and structure plus much more

HyperfuzzyGive up idea of the photon and you will see that the field is a dynamic part of the particle. Don't confuse the wave property and the particle property. In fact the particle or any particle need not exist. It is only a point with a set of given attributes. So first design the conversation upon absolutes. The whole idea of particles is bogus in QM and the idea of discretes is Bogus when talking about the continuum. Albeit, you may find some truth but also some non-truth. why confuse oneself. Keep a physics that is reality. Don't argue over the nonsense.

If the above looks like nonsense, trust me, it is.

HyperfuzzyName of God rev 3.14

HyperfuzzyDa SchneibWhydening GyreFollowing your discourse in this and another article (testing of relativity with a satellite)has been (almost exasperatingly) fascinating (due to my lack of education in these topics). Though I am following you (barely..:-)) in a broader sense, would it sound stupid if I said you guys are making me feel kinda stupid?

Da SchneibUsing QFT, we can show that since the photon is the exchange particle of the EM force and the EM force has a + and a - charge, the spin of the photon must be odd because even spins give only a single polarity; and due to renormalizability arguments (and we know QED is renormalizable thanks to Feynman, Tomonaga, and Schwinger) spins greater than 2 are not possible. There is only one odd number less than 2. Nice and simple.

Whydening GyreAhhhh... The ineluctable nature of science esoterotica...

Almost as good as porn..:-)

Da SchneibIn other words, you can't say there isn't any SAM along z; and since L and S are not conserved you can't say there isn't any L either. Because r × P doesn't work in QM.

Da SchneibThe equivalent to classical angular momentum is the quantum property OAM, also called L. But unlike classical mechanics, L is not conserved. Instead, it is co-conserved with an exclusively quantum property called SAM, also called S, which has no equivalent in classical mechanics and is also not conserved. The sum of these quantities, J, or TAM, is conserved, but this means quantum angular momentum can switch back and forth between L and S as long as J stays the same. Thus, the equation @Phys1 is using, L = r × P, and no I'm not going to try to explain cross products here, is not correct for S, though it is correct for L; and thus, angular momentum as we normally conceive of it (classical physics) does not appear to be conserved.

This is leading to an apparent paradox, and to indigestion for @Phys1.

HyperfuzzyDa SchneibI think you and I have different views of Maxwell's equations. I would place them right at the dividing line between classical mechanics and quantum mechanics; they were, and remain, THE field theory of EM. The quantum theory of EM was started by Planck and confirmed by Einstein; and the quantum field theory of EM, QED, was discovered by Dirac and put in its modern form by Feynman, Tomonaga, and Schwinger.

I see Maxwell's equations as the same thing for EM that the Einstein Field Equations are for gravity. We're at the same stage with gravity as we were in the latter half of the 19th Century with EM.

Da SchneibAlso, L is conserved in classical mechanics; it is not in Maxwell's equations, either for light or ED.

I should be really clear: when I say "classical mechanics," I mean Newton and Galileo, up to Maxwell. I draw the line there. I would accept arguments that Maxwell might be classical, but they probably wouldn't change my opinion. I don't consider relativity to be classical either.

I wasn't looking in ED, i.e. Maxwell's equations, I was looking at classical mechanics as I define it, i.e. Newton and Galileo where we're talking about L in terms of rotating extended bodies and spinning and orbiting planets and like that. I see S in Maxwell's equations and I'm scrambling to catch up. Now it's my turn for some indigestion. ;)

[contd]

Da SchneibKeep in mind we have at least three and maybe four versions of L here:

True classical mechanics, i.e. Galileo and Newton et al.,

Maxwell equation optics, where L is the angular momentum density vector,

Maxwell ED, where L is the canonical angular momentum,

and QED where L is the orbital angular momentum of particles.

We also have two versions of S:

Maxwell equation optics, where S is the Poynting vector, the linear momentum density of the EM field,

and QED where S is the spin angular momentum, a quantized parameter of individual particles.

So this is going to get very difficult very fast unless we keep these straight, or unless you can relate them to one another.

[contd]

Noumenonjohn berry_hobbesThat's rich coming from a promoter of religion. Is that some kind of denial where you see exactly what you're wallowing in, but only when you can project it onto something else?

Stupid, liar and bullshit are the pillars of religion.

NoumenonI did not post " I am downvoted by the usual psychos.",.... that was Phys1. How embarrassing for you.

As far I know, you and the pack of troll-raters, have zero understanding of any point made by me where I may have referenced Kant. Your "characterization" of what I believe is not based on your own demonstration of competency.

Da SchneibI'm sorry, that's all I have time to post right now; I'll definitely get back to this later.

HyperfuzzyDude, try decaf. There is no God. However the belief has an object, Amen, Amon, Amun, Rev. 3.14. knowing this does not make me a religious freak. you need to chill, juz say'n

HyperfuzzyI know. Nor anyone else. LOL

Whydening GyreI wouldn't comment on that, either.

Too many psychos out there...

HyperfuzzyAlso, inspect the universe of discourse for the logic of relativity. Don't be surprised when you see an undefined logic loop. Which of course does not fit into Formal Logic. Also note the non-axiomatic axioms. Speed vs a propagation constant. Now, is it simply nonsense or genius?

HyperfuzzyI'm trying to visualize the particle motion that will create the field we observe. With the field we are able to unlayer a set of possible motions if not the exact motion. A spiraling electron will have a time varying poynting vector displaying the changes in direction. Is this what we are trying to see? Considering the plane wave and wither perpendicular to the motion or directly behind or in front. Same thing?

Da SchneibDa Schneibcomposeviko_mxHyperfuzzyYeah, neat pictures without causality. Please

Da SchneibThe conserved quantity in quantum mechanics is J, not L.

viko_mxI do not know what kind of ideas this person is shared with the world, nor care how you interprete them. But I аm sure that the physical and moral laws of the Creator that support His order in the universe does not change with time. True science explores the facts obtajned by obsevation and experiments and does not deal with fashion trends. Because fashions are evanescent human fancies, but the Creator's laws are eternal.

Da SchneibSpin is not continuous. It can only have the values,

... 2, -3/2, -1, -1/2, 0, 1/2, 1, 3/2, 2, ...

It is discrete.

More technically, only symmetries that are describable as a Lie group have corresponding conserved quantities under Noether's theorem. That is, symmetries that are differentiable.

Let us examine a circle and an equilateral triangle. The triangle can only be rotated to three different orientations and remain unchanged (for lurkers, symmetries mean that things remain unchanged under some operation, in this case rotation); the triangle has a discrete symmetry. However, the circle can be rotated to any angle and will still be a circle; the circle's symmetry therefore is continuous.

[contd]

Da SchneibHmmm, I don't see why having both SAM and OAM quantized- therefore discrete- prevents J from being continuous.

And L is also discrete; it is the reason for the existence of orbitals in atoms, where there are only certain permitted orbitals and other values are never seen.

The continuity of J comes in when we examine many different atoms; each has its own distinct set of permitted values of L. Thus, each different species has a different spectrum. And in each permitted value of L, there are two alternatives: the alternatives of S, one +, one -.

MRBlizzardKonstantin Y. Bliokh, Franco Nori

http://arxiv.org/...04.03113

Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever

M. Antognozzi, C. R. Bermingham, R. L. Harniman, S. Simpson, J. Senior, R. Hayward,

H. Hoerber, M. R. Dennis, A. Y. Bekshaev, K. Y. Bliokh, & F. Nori

http://arxiv.org/...06.04248

john berry_hobbesFuck you.

Da SchneibConservation of energy is due to the symmetry of physics over time. By Noether's theorem.

Da SchneibSymmetry of physical results over position is dual to conservation of momentum.

Symmetry of physical results over time is dual to conservation of energy.

Symmetry of physical results over rotation is dual to conservation of angular momentum.

You will find all three of these symmetries and their dual conservation laws in the Wikipedia article on Noether's theorem. There are others but they are more esoteric.

Da SchneibCheck the definition of Noether's theorem. You don't even need a textbook; it's on Wikipedia.

Da SchneibNoether's theorem only links conservation laws to continuous symmetries, not discrete ones.

These facts are clearly stated (and well sourced) in the Wikipedia articles on angular momentum and Noether's theorem. I'm sorry, man, you've just plain got this wrong. I had it wrong long ago, and someone who had serious particle physics chops proved it to me; that's how I know.

I should perhaps have said that Noether's theorem does not apply to *all* continuous functions. Only to functions that show related continuous symmetries that form a Lie group.

Da SchneibThat's very interesting. Tell me more about the vector symmetry of the EM field. I suspect it will turn out to be a symmetry over time, which will relate to conservation of energy, rather than to conservation of any type of angular momentum.

Da SchneibI'd ask more about your paper but personally I wouldn't put any personally identifying information on here and I don't expect you would either. In fact I'd discourage it; trolls are.

Da SchneibDa SchneibWorth mentioning as well that because the B field is the relativistic correction, it must be a vector field in order to account for possible varying frames from which the scalar E field might be observed.

HyperfuzzyOK, but how do you apply this $hit?

Da SchneibI'll point out that symmetry over inversion is inherently a discrete symmetry, whether of space or time. Symmetry over space means things remain unchanged over spatial position, which no field associated with EM can do since EM falls off by the inverse square law, and symmetry over time means if I measure it now and wait a minute and measure it again I'll get the same value (all other things remaining equal). Finally, we know that T (time reversal symmetry) and P (parity symmetry) are not individually symmetric; this was proven by the neutral kaon interactions decades ago. It is only CPT symmetry that results in Lorenz invariance, indicating that charge, parity, and time must all be reversed together for the symmetry to hold. You can't reverse just one.

Da Schneib1. CPT symmetry/Lorentz symmetry.

2. J conservation where J=L+S corresponds to rotational invariance/symmetry.

Da SchneibcomposecomposeDa SchneibSAM is a discrete symmetry; a given particle can only have + or -, not any intermediate value, and all particles can only have values that are multiples of a half-integer, either even multiples (bosons) or odd ones (fermions).

Da SchneibDa SchneibHyperfuzzyHyperfuzzyPlease sign in to add a comment. Registration is free, and takes less than a minute. Read more