Re: Radiation Pressure

[Leigh Palmer <palmer@sfu.ca>]

> This question gets right to the heart of one of introductory students'
> favorite questions, "If light doesn't have mass, how can it have
> momentum?"

One might as well ask "If light doesn't have mass, how can it have
energy? or angular momentum? or lepton number? etc.?" All of these
(mass, linear and angular momentum, energy, etc.) are concepts
related to independent properties of light, and they are all more
or less different concepts. The problem arises because the students
have been introduced to momentum and energy by examples which
linked those properties to a massive particle.

One might make the observation that light has no mass, therefore it
has no electric charge because we have always observed charge is
associated with particles that have mass. Reasoning of that sort
closes the mind to the objective consideration of the properties of
an as yet unobserved phenomenon.

If it is any consolation to you, when I was a student light was
said to have mass. Back in those days we distinguished mass from
rest mass, and the mass, m, of any system was simply related to its
total energy, E, by the Einsteinian relation E = m*c**2. In those
days light was said to have mass. Nowadays the convention is
different. "Mass" means what we used to call "rest mass" or "proper
mass". It is by that convention alone that light now is said to
be massless. In the olden days the momentum, p, of light was,
simply, p = m*c which yields p = E/c when one applies the Einstein
relation above.

Philosophically you should prepare yourself for the idea that
energy and momentum aren't "stuff" like mass is. They are
mathematical entities which play the role of constants in physical
system representations.

You should now go to an elementary E&M text and look up Poynting
vector and Poynting flux. Then we'll talk about how sunlight speeds
planets up (the Poynting-Robertson effect).

Leigh