Re: Photons and a spacecraft.

I don't quite understand why photons "surrounding" the
spacecraft would make it massless.  If you were in a car that
was surrounded by bubbles, which are very light, would this
also make your car very light?

It is true that you could make your craft accelerate using photons.
If you shined a beam of light (like a laser) on it, the momentum
imparted by these photons would indeed push it forward.  This is
the principle behind the "solar sail", a type of propulsion system
in which a huge sheet of lightweight material is used to harness
the "propulsive power" of the light of the Sun.  However, you could
never get to the speed of light, because this would take infinite
energy, and no matter how many photons you use, they won't have
infinite energy.

-Aaron