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

[paddys09]

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?

 

[Dr Lots-o'watts]

Hi paddys09,

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

 

[paddys09]

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.

 

[Dr Lots-o'watts]

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).