Light Momentum: How Can Light Have Momentum if its Speed is Constant?

paddys09
Messages
14
Reaction score
0
Speed of light and momentum?

Hi all,

I have just started reading about quantum physics as my uncles cousin was the late John Bell (Bells Theorem), so got interested in the subject, purely as a hobby! My question is probably very simple, but I have only started reding about this last week so go easy on me haha. How can light have a momentum if its speed is constant? Is it the behaviour of the waves that have momentum? and not the actual light traveling through a space?
 
Physics news on Phys.org


Hi paddys09,

The higher its frequency (or the lower its wavelength), the more it has momentum. The mathematical relation is actually quite simple.
 


Thanks for the quick response, can continue reading this book now! starting to understand it now, its a slightly different form of momentum than in classical physics because it isn't based on mass? So I take it that's how we convert light into electricity, by catching the electrons that are released when a photon hits a metal object? Sorry for the dumb questions I am more practical than mathmatical and need to see things work before i can understand them, I have a feeling this is going to be a problem understanding this subject haha.
 


In fact, for a single particle of light, a photon, the momentum is p = hf/c, where h is a constant (Planck's constant), c is the constant speed of light, and f is the frequency. It is the actual light, each individual photon or all collectively, that has momentum. This isn't really distinct from "the behavior of the wave".

It is the same momentum, but it is calculated differently because of a photon's properties (such as it's lack of mass and constant speed). The momentum p gained by a mass (a perfectly black surface for example) absorbing a photon completely is strictly mv = hf/c.

That is roughly correct for light conversion to electricity, although special conditions are required to "catch" the electrons, (and especially to release them).
 
Last edited:
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In her YouTube video Bell’s Theorem Experiments on Entangled Photons, Dr. Fugate shows how polarization-entangled photons violate Bell’s inequality. In this Insight, I will use quantum information theory to explain why such entangled photon-polarization qubits violate the version of Bell’s inequality due to John Clauser, Michael Horne, Abner Shimony, and Richard Holt known as the...
Not an expert in QM. AFAIK, Schrödinger's equation is quite different from the classical wave equation. The former is an equation for the dynamics of the state of a (quantum?) system, the latter is an equation for the dynamics of a (classical) degree of freedom. As a matter of fact, Schrödinger's equation is first order in time derivatives, while the classical wave equation is second order. But, AFAIK, Schrödinger's equation is a wave equation; only its interpretation makes it non-classical...
I asked a question related to a table levitating but I am going to try to be specific about my question after one of the forum mentors stated I should make my question more specific (although I'm still not sure why one couldn't have asked if a table levitating is possible according to physics). Specifically, I am interested in knowing how much justification we have for an extreme low probability thermal fluctuation that results in a "miraculous" event compared to, say, a dice roll. Does a...
Back
Top