On Sun, 06 Jul 2003 22:35:55 GMT, wrote:
In article , (Alan Anderson) writes:
(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?

Before you even start getting to relativistic physics, the point
missed by many is that even in classical physics momentum *is not*
defined as mv (yes, there is quite a lot of physics beyond high school
physics). Momentum is defined as a gradient of the Lagrangian (yes, I
know this doesn't mean much to whoever didn't study it but, as I said,
there is lots of physics beyond high school physics). In the
particular case of a classical massive particle, this *evaluates* to
mv but that's a result, not a definition. For other entities you get
different result. Thus, even within classical physics electromagnetic
waves carry momentum even though they're massless.

When you're not so busy you need to write a book, Mati. Something
like "All the Physics you Learned Wrong" or something.

After all this practice you could probably write it half asleep with your
eyes closed.



-Laurel