How does force figure into the photon in QM theory? Is it as per Newton's F=ma or what?

Question

If its F=ma then what is the mass of the photon?

Answer 1

The modern definition of force is "the rate of change of momentum" (you can determine for yourself that this is the same as F=ma when you're talking about classical physics).

Photons don't really have "mass," but they do have momentum, equal to h/λ. When they slam into something "black" (i.e. are absorbed), they transfer momentum h/λ to it. When they slam into something "white" (i.e. are absorbed & re-emitted) they transfer momentum 2h/λ to it. By measuring the rate at which the photons hit, you can calculate the rate of momentum transfer; i.e., the force the photons exert.

Answer 2

QM, and classical mechanics professionally, is performed using energy. The basic operational equation is E^2 = p^2c^2 + m^2c^4. For a photon, there is no rest mass and this reduces to E^2 = p^2c^2 or E = pc. There are QM operators that you can use to write such as an equation. The former form leads to a d'Alembertian operator and the Klein-Gordon equation.

Essentially though, higher physics uses energy and its Hamiltonian formulation as a description of nature. A force really is usefully equivalent to a potential anyway.

Answer 3

the photon is massless by nature...it does however have a mass equivalent which a result of the energy contained within the electric and magnetic fields

the energy of a photon is a function of its frequency E = hf
the mass equivalent of this is E = mc^2

therefore the mass equivalent of a photon is m = hf/c^2
gravity acts on the photon because of this mass....and results in gravitational lensing etc

btw this isnt treating the photon from a quantum mechanical perspective...for that you need to look at the "second quantization" or gauge boson theory

Answer 4

Modern physics does not use the concept of force, because F=ma is a circular definition of force in terms of mass and mass in terms of force. We exclusively use energy formulations. In other words, we do not speak of forces when discussing QM. Instead, we talk about potential energy functions.

Answer 5

Force does not exist in quantum mechanics.
The Schrodinger equation is in terms of the classical potential, but there is no force term. The same is true of the Dirac equation and, I believe, every other quantum theory. You are solving for quantum states and the concept of force is inappropriate.