# What is the specific photon-particle interaction by which momentum is transferred in radiation pressure?

Updated Preface (in response to comments).

Per the title, this question is focused on understanding "the specific photon-particle interaction by which momentum is transferred in radiation pressure".

In the original question I included a preface which gave an example of what I meant by "radiation pressure": solar sails. That was for an example only. The question is not about solar sails, and not about the classical theory of how to calculate radiation pressure. "Solar sails" are mentioned only as orientation and evidence that there is a real macroscopic phenomena involved that does not have an obvious description at the detailed level of specific photon-particle interaction. My apologies if that mention has been misleading.

The information I seek is a description of what happens at the level of specific photon-particle interaction to generate radiation pressure.

What follows is the original post, verbatim:

In this question "radiation pressure" means the term as used in describing the behavior of light sails. "specific photon-particle interaction" means exactly what particles the photons interact with to impart momentum to the impacted object. There are formulas to calculate radiation pressure, however, they do not describe the photon-particle interaction that produces the pressure. There are the photon-electron interactions of reflection and refraction, however, these are 100% elastic, meaning no energy is lost or gained, so it is difficult to see how they could transfer the energy of increased momentum to the to the impacted object.

A conjecture is that the photons impact the quarks in the hadrons of the impacted object. However, I have not seen any obvious (i.e. through googling) papers that describe this.

So the question: what is the specific photon-particle interaction by which momentum is transferred in radiation pressure, and what are links to papers that describe this?

• Wait, why are reflection and refraction "photon-electron interactions"? Those are macroscopic phenomena. Also, elastic collisions transfer momentum all the time. If a moving billiard ball collides with a stationary one of equal mass, the first ball transfers all its momentum to the second. In any case, you might want to start by asking what the interactions responsible for reflection and refraction are. – probably_someone Jan 28 '18 at 19:03
• Photons and particles are both the hard way and the less useful way to think about this question. Seriously. Treat it in classical electrodynamics and be done with it. – dmckee Jan 28 '18 at 19:06
• The question is aimed at understanding possible photon/quark interactions involving relatively low energy photons. Radiation pressure as a phenomena is a potential way of getting at knowledge of that type of interaction, since that knowledge seems to be otherwise not readily findable. – wayfarer Jan 29 '18 at 20:31
• What the classical theory tells you is that the interaction is dominated (by many orders of magnitude) by the electrons and then the ratio of strength between the EM and strong interaction tells you that the interaction between the wave and the nucleus is dominated by coherent interaction with the whole nucleus (and the remnant is dominated by interaction with individual nucleons). The quarks just don't come into the problem. What you are proposing is like trying to understand the economy of a large nation by examining the transactions of a child's lemonade stand. – dmckee Jan 30 '18 at 17:49
• @dmckee Classical theory is a set of language metaphors that abstract a set of mathematical formulas that provide relatively accurate predictions while leaving out details such as the precise way in which photon momentum is transferred to the mass-bearing quark/gluon soup in the hadrons in order to make them move. I'm beginning to conclude that the answer is "not yet covered by current accepted science". Fission may have seemed unimaginable in 1880; by 1945 they had figured out what to do with the lemons. – wayfarer Jan 31 '18 at 18:51

An individual photon carries energy $h \nu$ and momentum $h \nu / c$. When a photon is absorbed by material, the energy and momentum are transferred to the absorber. If a photon is subsequently emitted by any process, then energy and momentum is carried away by the emitted photon.

If we consider photons in a beam directed at a perfect planar mirror along its normal vector, then in the mirror frame each photon is contributing 2 $h \nu / c$ in momentum to the mirror where $\nu$ is the photon frequency as measured in the mirror frame. This causes the mirror to accelerate.

In the initial rest frame of the mirror, the frequency of the incident photons remains the same, but the reflected beam becomes increasingly redshifted owing to reflection off of the receding mirror. In this way, the photons are transferring both energy and momentum to the mirror. Schematically this is how a solar sail might operate.

None of the above discussion depended on the nature of the absorption or emission processes. As commenters have already indicated, the interaction cross section of photons with electrons is vastly greater than that of their interaction with hadrons. Practically speaking, the hadrons don't make an important contribution to the absorption or emission of the photons.

• "each photon is contributing 2 hν/c in momentum to the mirror". However at the particle level there is no "mirror", there is only a collection of particles. "the interaction cross section of photons with electrons is vastly greater than that of their interaction with hadrons." And yet the hadrons have most of the mass, to which the momentum has to be applied. Is the momentum transferred to the electrons, and then to the hadrons? I'm trying to see this in terms of specific photon-particle interaction. Such interaction MUST occur; the question is, exactly how? – wayfarer Feb 3 '18 at 21:37
• The photons first transfer momentum to the electrons. Then, the electrons transfer momentum to the nuclei via the coulomb force (the positive charges are pulled along by the negative charges). – kleingordon Feb 3 '18 at 23:39
• that initiated a search that led to this: Vol. 25, No. 17 | 21 Aug 2017 | OPTICS EXPRESS 20803 which contains this quote: "While in the field of theoretical quantum mechanics there are various debates about the behaviour of the momentum of the light at the edge of the light-matter interaction [39–41], the other fields of science and technology consistently provide supportive evidences on application and utilization principles of some of its effects". i.e.. accurate measurements, but the precise explanation is still being debated. Still seems unsettled. – wayfarer Feb 5 '18 at 0:12
• There is an accessible link to the full text of that article here: osapublishing.org/DirectPDFAccess/… – wayfarer Feb 5 '18 at 0:13
• If you want the quantum mechanical description, a textbook on quantum electrodynamics is probably where you need to start. You ask about Feynman diagrams, and they provide a component of the answer, but in fact the photons are in a sense interacting with all the electrons in the material at once, and the final result is computed via a path integral. Again, this is overkill if you're just trying to understand how something like a solar sail operates. Also see the top-voted answer to this question: physics.stackexchange.com/questions/1909/… – kleingordon Feb 8 '18 at 18:52