New Property of Light Discovered | Hacker News Reader

New Property of Light Discovered

phys.org

307 pointsjchanimal71 comments

I did a project during my undergraduate degree in physics which involved interfering three planar waves at 60 degrees from one another to create a hexagonal intensity pattern. The interesting thing was that at each of the 6 corners of the hexagon was a singularity (optical vortex) where the phase was undefined. At these points, the phase space was shaped like a spiral staircase (screw dislocation) and particles suspended there could actually be rotated. It was like an “optical wrench” if you will.

On a small scale, planar waves can be modeled like flat sheets of paper traveling through space without any angular momentum (no twisting motion). Yet when these sheets hit an object from multiple angles with the right timing, they can actually cause the object to twist.

Very interesting. To clarify, 60deg phase offset or azimuth (e.g. converging at a single point)? Now I really want to plug this into an EM simulator

Sorry, I misspoke about the angle of separation (it's actually 120 degrees) and did a poor job of describing the orientation. If you imagine a vector perpendicular to the sheet indicating the direction of travel, then these three vectors would intersect at a single point and lie along the surface of a cone. The angle between the axis of the cone and each vector, the azimuth, was 30 degrees.

https://i.ibb.co/VDgS8yr/3beam.png

> On a small scale, planar waves can be modeled like flat sheets of paper traveling through space without any angular momentum (no twisting motion).

They certainly have angular momentum, it just depends on the choice of origin. If you pick an origin along the peak ray of the plane wave, there will be no twist around that point. Just like with a particle traveling in free space.

Unfortunately, I don’t know enough to intelligently comment on this. I was largely under the guidance of my professor. However, I can tell you that in my modeling I used the paraxial approximation and that the light was linearly polarized. It was my understanding that only circularly polarized light carried intrinsic angular momentum (https://en.wikipedia.org/wiki/Spin_angular_momentum_of_light...).

Also, I would agree with you that in a uniform electric field, a single E vector in isolation would appear to produce a torque on a point some distance away. But if the rest of the field is considered, the net torque at that point would be zero, right?

This is unbelievably fascinating. Can anyone go into more detail?

To make it clear:

Photons have two different forms of angular momentum. Spin angular momentum (SAM) and orbital angular momentum (OAM). Radiated photon carries not only spin but also orbital angular momentum.

That OAM must exist has been known some time but it was observed relatively recently (27 years ago). Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes https://journals.aps.org/pra/abstract/10.1103/PhysRevA.45.81...

>Abstract

> Laser light with a Laguerre-Gaussian amplitude distribution is found to have a well-defined orbital angular momentum. An astigmatic optical system may be used to transform a high-order Laguerre-Gaussian mode into a high-order Hermite-Gaussian mode reversibly. An experiment is proposed to measure the mechanical torque induced by the transfer of orbital angular momentum associated with such a transformation.

> Received 6 January 1992

>DOI:https://doi.org/10.1103/PhysRevA.45.8185

was OAM used in any way in applications even though it hasn't been observed properly ?

Yes, people have used OAM to demonstrate various quantum information tasks. See this as an example https://journals.aps.org/pra/abstract/10.1103/PhysRevA.80.06...

When we discovered that neutrinos oscillate, we knew that they had mass because we were able to observe a change from within our frame of reference, implying a finite time dilation. I thought that light was unable to change its physical properties in time (as observed from our frame of reference) due to infinite relativistic time dilation. How is it possible that we can observe torque, a change in angular momentum, for a particle moving at the speed of light?

All special relativity says is that if an object is moving at the speed of light, its motion will appear to be the same in every reference frame. That is all. It says nothing on what can happen to photons when they are interacted with other photons or or with other media. Photons have several internal properties and all of these can be manipulated, either in the lab or naturally. The polarization of photons can be changed by Faraday rotation for example. In cosmology, you might be aware of Hubble's law, which involves the changing of the frequency of light.

I haven't read this paper or intend to, but here is a key quote

> We demonstrate that the self-torque arises as a necessary consequence of angular momentum conservation during the extremely non-linear, non-perturbative optical process of high-order harmonic generation (HHG). In HHG, the interaction of an intense field with an atom or molecule leads to the ionization of an electronic wavepacket, which acquires energy from the laser field before being driven back to its parent ion, and emitting a high-frequency photon upon recollision.

What they are saying essentially is that by interacting the light with matter they are able to generate the said torque.

I think the question comes from the following understanding:

- change requires the passage of time.

- particles moving at the speed of light relative to us have infinite time dilation and so no time passes from our point of view.

- the article is talking about particles moving at the speed of light which under go change.

I have the same (mis?)understanding and am curious how this can work as well.

...so no time passes from the photon’s point of view.

Infinite time dilation means that a photon from a galaxy 3 billion light years away arrives here instantaneously from the photon’s perspective. Which means there’s actually no distance from the photon’s perspective - the entire universe is a singularity. Pretty mind-blowing IMO.

That would be true if you can find a frame of reference in which the photon's velocity is 0 (its rest frame -- a pre-requisite for associating that frame of reference with the photon in the first place). But special relativity does not allow photon velocity to be zero in any frame of reference, so there is no frame of reference associated with a photon.

Is that like saying that time & distance have no meaning for a photon?

More or less.

It goes on to mean that nothing really "has a meaning" for a photon, so it's hard to really talk about a photon's perspective one way or the other. We generally just say that the photon doesn't have a perspective, i.e. that any discussion of the photon's rest frame is meaningless.

So to the degree that your question has an answer, the answer is "yes". But a better answer is that it's misleading to think that the question even has a yes-or-no answer.

Take it a step farther and the photon isn't even truly nessicary. The recoil experience when the photon was emitted, and the push felt when it is absorbed should match exactly, and essentially correspond to a single, equal, collision event to the right observer.

Your second point is where all the trouble is arising. Special relativity is talking about one thing and one thing only - how an object moves from one point to another in space, and how fast this process appears. The position (and speed and acceleration) are external properties of an object, they depend on who is doing the describing of these properties. Special relativity is concerned mainly with these things.

Objects also have other internal (non-space/position) related properties. For example, light has polarization. These properties can and do change with the passage of time. For example, you can change the polarization of light with a strong magnet. Relativity can and does place some restrictions on how this evolution happens, in so far as something is moving in space, but that is a more involved story.

> Objects also have other internal (non-space/position) related properties. For example, light has polarization.

So the angular momentum of light would be put under the same category as polarization, which is a property that a single photon can have?

Or is it a property like wavelength, momentum, frequency, intensity, etc -- a property of light that can be observed only when the light is a system of lots of photons, instead of just an individual photon?

Also, sorry if my categories are wrong. If so, I hope it doesn't take away from my main question of whether it is a property of a single photon or a property of a system of photons

Wavelength = 1 / frequently and is directly related to photon's energy. You only need one photon to observe these.

Intensity is number of photons per unit area per unit time, so it takes many photons normally, but a single photon produces a non-zero intensity.

Every category you have named, wavelength/frequency, intensity (function of frequency), linear momentum (function of frequency), polarization are properties that individual photons have. It is possible to construct photon detectors that are able to resolve these properties for a single photon thrown at them.

Orbital angular momentum is also a property of single photons, related to the shape of their wave function. You can read about it and see some example shapes on the wikipedia page https://en.wikipedia.org/wiki/Orbital_angular_momentum_of_li...

I can't think of a property of light that is only in the bulk and not at the single photon level (somebody correct me if I am wrong). But I haven't read this paper in any detail to find if the "torque" is a bulk property or a single photon property.

And the third point is where that trouble can be resolved: light doesn't move through matter at c.

Photons only travel at the speed of light in a vacuum, when they're not interacting with anything. Throw interactions into the mix, and they go much slower.

The light produced by fusion in the sun's core can take up to 170,000 years to escape! Plenty of time for the angular momentum (or any other property) of the light to change.

Thanks for the explanation. I completely misunderstood the effect. Now, I misunderstand it a little bit less. Progress!

> I thought that light was unable to change its physical properties in time (as observed from our frame of reference) due to infinite relativistic time dilation.

This is not correct. What is correct is that the concept of a "rest frame" for a light ray doesn't make sense. But our rest frame, as observers watching the light, is perfectly well defined, and in our frame it's perfectly possible to see light changing with our time.

Do I understand this right? Light has angular momentum (long known). That angular momentum can change (new), which requires torque. But it's a beam of light putting torque on itself. Or, it's photons in a beam of light putting torque on other photons in a beam of light.

Worse, it seems to be (if I understood the article correctly) that different parts (front and back) of the beam of light are torquing each other. How can they do that while still moving at the speed of light?

The forward speed is the speed of light, but the angular speed is ... the angular speed. It's two different properties and measurements.

Disclosure: I probably don't know what I'm talking about.

Is there a limit to angular speed, just as there is a limit to forward speed?

That is such a good question!

Whoa. Anyone have any ideas for use cases? The article mentions manipulating tiny materials - like what? How would that work?

See "Laser Manipulation of Atoms and Particles" for an introduction to the subject: https://sci-hub.tw/10.1126/science.253.5022.861

Applied to microwaves in particular, modulating wave properties is how we transfer data. New ways to pack more bits per Hz are always useful.

Get your patents now...

Problem is OAM does not work for long distances.

Can you elaborate? I've seen OAM at the conferences but not super familiar

As the beam diverges, you need larger and larger receive antennas to discriminate between the modes, more aperture than you need to compensate for path loss. That seems to be why OAM research is now mmWave (better collimating) and more applicable to short range backhaul.

Looks like this could improve bandwidth of free space connections, like satellite or directed beams.

What are the limitations in applying this to mobile communication?

Very hard to track a beam between moving terminals

Quantum Computer.

There certainly seems to be a lot of research in the field of telecommunications : https://en.wikipedia.org/wiki/Orbital_angular_momentum_multi...

Google for Optical Tweezers

"manipulating tiny materials" - I thought of semiconductor manufacturing, for litography... but I'm not an expert at all.

I find this theory fascinating to consider: all physical objects/abstractions at each order of magnitude (atoms to laniakeas) in spacetime (shared reality) are holons governed by a natural and pragmatic system (platonic ideals) with some degree of freedom collapsing along quantum-information received by differential co-observations. Those objects with the most appearent freedoms and power to act, we traditionally call alive, yet it seems all objects at all orders of magnitude have some amount of autonomy, especially our own bodies. Light, electrons, and energy all exist within what appears to be the information and language layer between holons co-observing relational quantum-information (like spacetime) where time is the rate of subjective information observation, and the limited speed of information/light results in what we see in response time witnessed as observation-intention/reaction-action/physical change.

Question for any postmodernist: (I don’t know if there are any in the physics circles because it seems that postmodernism ideology which dominates the humanities has serverly diverged from physics in some fundamental theories, and I am trying to understand this gap in theory.) It seems postmodernism’s claims on subjective reality throwns out the theory of platonic ideals and renders it impossible to see the math and geometric uniformity of the universe; and cannot conceive of the law “information is indestructable” if all reality is subjective. Please help me understand what I fail to see.

To continue, if objective reality only exists within the spacetime dimensions and additionally a subjective reality exists as a set of dimentions within the information/language/feeling dimensions but collapses as observers understand each other via communication, this could find explain some of the gaps between social philosophy (humanities) and natural philosophy (science).

Note: I will likely come back to this post tomorrow to rewrite it.

I wonder if this will affect how we understand things about light, like the two-slit experiment.

I had a wild thought, not 6 months ago, that angular momentum in light could account for all the missing dark stuff in the universe. I didn't know science didn't know light had angular momentum.

Science did know it. It just didn’t realize that the angular momentum could change with time.

I think it is this one?: https://arxiv.org/abs/1901.10942

The Science version has much more readable/viewable formatting and has a bit more references, though.

https://sci-hub.se/10.1126/science.aaw9486

Thanks, I didn't realize you could read a paper on Science without paying.

You can't, officially. scihub is the napster of scientific publications, and it regularly gets shut down and moves.

heh, Sci-hub is not Science

Made me think, I wonder if the photon pressure from a vortex beam of light will spin a target, as well as push it away?

Is this torque in the same sense of the word as “ability to move objects”?

Is StarTrek happening?

No, it seems that I’m this case, the angular momentum of the light beam itself changes over time, a process that is typically referred to as (or driven by?) torque in classical mechanics, and so they used that term here as well. They call it “self-torque” (I believe) because it’s the light changing it’s own angular momentum. But I’ve not been involved with physics academia in several years, so take it with a grain of salt.

You can push objects with light, check out https://en.wikipedia.org/wiki/Solar_sail

This sounds like refraction. Two beams traveling through the cloud. If you use a double prisim, you could achieve the same type of results.

Refraction is a linear effect. This is nonlinear because two waves enter the gas at f1 and leave at f2 (the photon momentarily energizes an electron in the gas, which collapses and releases a photon at another frequency)

You cannot guarantee the angles at which they enter the gas.

They also stated to using different wavelengths out of sync. Which would cause the angle of refraction to be different.

Different angles of refraction lead to different changes of speed as they enter one medium to another.

Also I'm not sure what you mean "the photon momentarily energizes". The photon has a constant amount of energy. A beautiful balance of kinetic vs potential energy.

Could this encode additional classical bits (optical transceivers) and/or qubits (single-photon quantum key-distribution protocol)?

What does this mean for the double slit experiment?

Can waves wave angular momentum? This is really neat.

Transversal waves can: https://en.wikipedia.org/wiki/Angular_momentum_of_light

TLDR they changed a doughnut into a croissant.

Did they use a cronut as the intermediate stage?

Ok I'm upvoting both of you, I don't care what HN says.

I could listen to Laura Rego Cabezas talk about physics all day long.

Might be some stupid questions, but maybe someone can enlighten me:

Could this explain the Schrödingers cat experiment, i.e. the light moving through both of the two slits? If light has a self torque, a single beam of light could travel through both slits (almost) simultaneously depending on the torque. Also, can someone clarify whether every beam of light has self-torque? Or do you need to create a beam of light in a special way to achieve this property? If every beam of light has that property, wouldn't that mean that light doesn't travel in wave form, but rather in a wave-cylindrical form?

That's the two-slit interference experiment. Schrodinger's Cat is completely different.

And no, this has nothing to do with the double slit experiment.

And no, not every beam of light has self-torque. You need to create it in a special way.

They rediscovered circular polarization?

ED: No, apparently polarization is called spin angular momentum in this context. OAM can take a range of twist-rates while spin is either right- or lef-handed.

And to the mute person who thinks downvoting questions is helpful: please just go away.

Reply on news.ycombinator.com