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Say you could move infinitely fast/gravity doesn't affect you. You move in a straight line. Do you hit anything, like stars or black holes? Not talking about cosmic dust.

Qmechanic
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Mondo Duke
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    "Say you could move infinitely fast.." First you should use more physically possible assumptions, one cannot travel infinitely fast. – Triatticus Apr 29 '22 at 21:38
  • And a more resonable assumption would be the limit of velocity which is light – Dirac Delta Yeah Apr 29 '22 at 21:48
  • ~90% of the cosmic microwave background has traveled through the Universe since most of its history without hitting anything. Those that did interact with particles, mostly did so when the Universe was younger and hence denser. If you were a photon departing today, you most likely would never hit anything. – pela Apr 29 '22 at 23:31
  • You could adapt this answer to get an answer to your question: https://physics.stackexchange.com/a/388809/27732. You would just essentially replace $d^2 R$ by $d R^2$. – Andrew Apr 30 '22 at 00:39
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    A comment on that answer actually does that calculation and concludes the answer to your question is "probably not." (Based on the number they got, you are roughly 10000 times more likely to win the lottery than hit a star.) – Andrew Apr 30 '22 at 00:41
  • If gravity doesn't affect you, how do you define a geodesic (i.e. straight line)? – Sandejo Apr 30 '22 at 08:06

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No.

The question is a bit vague, but I try to answer it in spirit. Remember Olbers' paradox, which asks why the sky is dark at night. In an infinitely large, infinitely old universe, one should see stars in all directions. From all we know, the universe may be infinitely large, but it is only 14 billion years old, so light from further away hasn't had time to reach us. Thus, the night sky is dark.

Turning this around, you can travel along any direction and with the same probability that the night sky is dark and not star-surface bright, not hit anything.

Now obviously there is any number of caveats... around here, there is about one star every parsec, and they have a million kilometers in diameter. If you start out near the center of a galaxy, the story may look different. Conversely, once you made it out of the Milky Way, the probability would drop drastically even more. You mention black holes, these are just old stars too in this context. Then there's planets but their surface area is tiny compared to that of stars, so we can neglect them. Gas and dust is more fun: you are guaranteed to encounter that! All along the way, essentially all the time. Another thing you are encountering all the time are cosmic rays of all kinds and energies, and of course, the background light from the cosmic microwave background.

rfl
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