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.
- 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.
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.
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.
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.
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
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.
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.
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.
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.