(Alex Terrell) wrote:

If photons have mass, and if they travel at c, how come they don't
have infinite mass? Since they don't have infinite mass, they can have
no mass, and therefore no momentum.

You were doing fine until that last part. How do you conclude that no
mass means no momentum?

Please don't misintepret me. I do "believe" in solar sails. I'm just
trying to figure out how it works, because, in the normal world:

Momentum = mass * velocity

If mass = 0 and velocity = 3E8, then momentum = 0

Please tell me what I'm missing?

You're missing the difference between "the normal world" and "Newtonian
physics", that's all. The relativistic formula for momentum has an extra
factor which increases as velocity approaches c, becoming infinite when
v=c, making that formula inapplicable to photons. What is zero times
infinity?

The true value of a photon's momentum "in the normal world" is
proportional to its frequency.