[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.

[mattnumbers]

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

[pdonis]

> 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.