There's one thing I never quite got about these speculations. When you consider a ship leaving a solar system and accelerating, and the speed gained turning the "ambient hydrogen" into deadly radiation, why does it always seem to assume that hydrogen atoms are just kind of hanging out at low velocities relative to the home solar system and only turn into a problem when the bald captain says "engage"?
Its a big universe, might the hydrogen atoms already be traveling at these deadly speeds relative to the ship once it leaves Sol? Might there be a "galactic current" so that the ship can go with the flow?
I know its just for fun but its still silly to think that anyone who could propel a canned ape at .8c wouldn't be able fend off a few rouge hydrogen atoms.
In short, because the Milky Way disk sets a preferred rest frame. Other matter in the galaxy all rotates with approximately the same speed around the galactic center. Here are some numbers to give you intuition.
Tangential speed of the galactic disk, obtained by averaging over nearby stars: ~ 250 km/s
Speed of sun (and other nearby stars, in random directions) with respect to the galactic disk: ~ 15 km/s
Speed of Earth with respect to the Sun: 30 km/s
Speed of satellites in low-Earth orbit: 7.8 km/s
Speed of light: 300,000 km/s.
If you want more, this article discusses these speeds in the context of estimating the dark matter velocity distribution.
It's fairly accessible as far as physics articles go.
Even if you get outside the Milky Way, the cosmic microwave background sets a preferred rest frame. I can't find the exact speed of the Milky Way in that frame, but the speed of the Earth is ~360 km/s. (This means the Milky Way's speed is somewhere between 100 and 600 km/s.)
The take-home message is that the although the laws of physics have no preferred rest frame, the actual matter out there in the universe most certainly does.
The other take-home message is that all of the speeds in question are small relative to c. So trying to go with the flow won't get you anywhere very fast.
Because the hydrogen atoms we care about are bound in galactic orbit, and that means they have a relatively low maximum relative velocity to us. If they weren't, they would all fly out into the intergalactic void in geologically short timescales, star formation would cease, and we'd have no such radiation problems after all.
No worries (and now I feel bad for haranguing someone with dyslexia), I guess it just winds me up because I see it a lot - it slips past the spellcheck.
Its a big universe, might the hydrogen atoms already be traveling at these deadly speeds relative to the ship once it leaves Sol? Might there be a "galactic current" so that the ship can go with the flow?
I know its just for fun but its still silly to think that anyone who could propel a canned ape at .8c wouldn't be able fend off a few rouge hydrogen atoms.