
New Property of Light Discovered - jchanimal
https://phys.org/news/2019-06-property.html
======
max_likelihood
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.

~~~
mNovak
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

~~~
max_likelihood
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](https://i.ibb.co/VDgS8yr/3beam.png)

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nabla9
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...](https://journals.aps.org/pra/abstract/10.1103/PhysRevA.45.8185)

>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](https://doi.org/10.1103/PhysRevA.45.8185)

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

~~~
abdullahkhalids
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...](https://journals.aps.org/pra/abstract/10.1103/PhysRevA.80.063831)

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mattnumbers
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?

~~~
abdullahkhalids
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.

~~~
bladedtoys
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.

~~~
abdullahkhalids
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.

~~~
mygo
> 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

~~~
abdullahkhalids
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...](https://en.wikipedia.org/wiki/Orbital_angular_momentum_of_light)

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.

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AnimalMuppet
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?

~~~
a3n
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.

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

~~~
raimonides
That is such a good question!

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qu1mby
Whoa. Anyone have any ideas for use cases? The article mentions manipulating
tiny materials - like what? How would that work?

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

~~~
madengr
Problem is OAM does not work for long distances.

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

~~~
madengr
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.

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tinborn
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.

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m463
I wonder if this will affect how we understand things about light, like the
two-slit experiment.

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angel_j
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.

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

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sand500
I think it is this one?:
[https://arxiv.org/abs/1901.10942](https://arxiv.org/abs/1901.10942)

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

~~~
sand500
Thanks, I didn't realize you could read a paper on Science without paying.

~~~
not2b
You can't, officially. scihub is the napster of scientific publications, and
it regularly gets shut down and moves.

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inflatableDodo
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?

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

Is StarTrek happening?

~~~
cmpb
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.

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

~~~
mNovak
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)

~~~
ryanthedev
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.

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

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ryanthedev
What does this mean for the double slit experiment?

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ASTP001
Can waves wave angular momentum? This is really neat.

~~~
nsajko
Transversal waves can:
[https://en.wikipedia.org/wiki/Angular_momentum_of_light](https://en.wikipedia.org/wiki/Angular_momentum_of_light)

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_burmer
TLDR they changed a doughnut into a croissant.

~~~
BillSaysThis
Did they use a cronut as the intermediate stage?

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

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

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andischo
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?

~~~
AnimalMuppet
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.

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ganzuul
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.

