In air, when there is light propagating in a direction, we can still see it even when it is not primarily travelling in our direction, because a small part of the light hits the air molecules, and changes its direction; it travels towards us.
Does this mean that, in a vacuum, you would not be ...
I remember, in 1988 or 1990, being totally baffled by demonstrations of "lasers" that made a bright spot but without any beam connecting the spot to the laser emitter. I knew that lasers had a visible beam in air and vacuum. I had seen it on G.I.Joe.
@rob I think we should get a law passed which requires any introductory laser demonstrations to be performed in a smoke-filled environment to make sure that it looks cool enough.
Best laser demo I ever saw was an 800W continuous wave CO2 laser that they had at the Ontario Science Center when I was a kid. I don't think smoke would have helped.
If you go to a museum to watch all those exhibits brightly lit, it's not easy to tell where the light is coming from and spotlights are placed. IMHO, even without vacuum you rarely can see light that's not shining directly at you, that is unless the air is filled with dust or smoke.
An answer considering photon-photon scattering (why impossible) and possible ultra-high-energy relativistic effects (null radiation causes curvature) would be much more interesting than the current "no, no atoms to scatter from" ones. I'm nowhere at this level though.
Agree with @Agent_L. Even in (ordinary, dry and clean) air it is hard to see a beam. You want droplets/fog or smoke or similar to make beams/rays evident. Image search.
@rob This just reinforces that you can't trust those pesky scientists - believe what you see on TV - I mean, would GI Joe lie to you? He's literally a hero.
