Bad Astronomy: Misconceptions and Misuses Revealed, from ...

250 BEAM ME UP
Galaxy. To be able to actually see a planetary nebula from up
close, or to watch the final seconds as two madly whirling neutron
stars coalesce in an Einsteinian dance of mutual gravitation: that’s
why we went into astronomy in the first place! But right now,
today, we know of no way to travel or even to transmit information
faster than light.
Therein lies the problem. Laser beams travel at the speed of
light, so there is literally no way to tell that one is headed your
way. There’s more: out in space, you can’t see lasers at all. A laser
is a tightly focused beam of light, and that means all the photons
are headed in one direction. They go forward, not sideways, so
you can’t see the beam. It’s just like using a flashlight in clear air:
you can’t see the beam, you only see the spot of light when it hits
a wall. If you see the beam, it’s because stuff in the air like particles
of dust, haze, or water droplets is scattering the photons in the
beam sideways. In laser demonstrations on TV you can see the
beam because the person running the demo has put something in
the air to scatter the beam. My favorite was always chalk dust, but
then I like banging erasers together. Anyway, if you’re in a laser
battle in your spaceship, you really won’t see the enemy shot until
it hits you. Poof! You’re space vapor (ironically, a second shot
fired would get lit up by all the dust from your exploding ship).
Sorry, but dodging a laser is like trying to avoid taxes. You can try,
but they’ll catch up to you eventually. And unlike lasers, the IRS
won’t be beaming when it finds you . . .
5. . . . who have come from a distant galaxy . . .
Even the awesome speed of light can be pitifully dwarfed by
the distances between stars. The nearest stars are years away at
light speed, and the farthest stars you can see with your naked eye
are hundreds or even thousands of light-years away. The Milky
Way Galaxy is an unimaginably immense wheel of hundreds of billions
of stars, over one-hundred-thousand light-years across—
—which in turn is dwarfed by the distance to the Andromeda
galaxy, the nearest spiral galaxy like our own. M31, as astronomers
in the know call it, is nearly three million light-years away.
Light that left M31 as you look at it in your spring sky started