You see the beam because the air has many dust particles in it and some of the light bounces off those particles heading various directions, including toward you.Although photons can bounce off each other, the process is extremely rare for visible photons in a vacuum or air. It plays no role in making the beam visible. Thus you won't be able to see the laser beam in a perfect vacuum.
By the way, no beam can travel "in a perfectly straight line" since the wave equation (or, if you prefer, the uncertainty principle) always gives some spread in the directions the beam is going.
Mike W.
(published on 02/12/2013)
