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Chances of hitting something in intergalactic travel

  1. Sep 1, 2013 #1
    I was discussing with some friends, in the hypotetical case that we could travel really large distances through space at superluminal speed (or that we did not care about the trip lasting millions or billions of years), whether the risk of hitting some object during the trip might be a significant concern. This referring to travelling through normal 3-D space in a straight line, not via wormholes or extra dimensions (for example with an Alcubierre drive, or even at light speed or slower).

    I argued that the chance of hitting anything bigger than a hydrogen or helium atom is negligible. After all we can see the most remote edges of the visible universe and the CMB, meaning that those photons have travelled the whole universe from their source until our telescope without having hit anything significant, or else they would have been absorbed. If the chance of hitting anything was significant, the sky would look much darker and possibly we would not even be able to see the CMB.

    As counterargument, it was mentined that precisely the fact that we detect all that radiation is proof that even the photons from the most distant sources, even those from the CMB, eventually hit something, in this case our telescope. If it was true that the chances for a photon to hit anything were negligible, it would mean that the chances of it hitting the screen of our tiny telescope would be basically zero.

    One would think that if a photon has crossed 12 billion light-years of space without hitting anything at all, the chances of it then suddenly hitting something so ridiculously small as a photon detector in an Earth telescope should be virtually zero. And yet it happens all the time, which might seem like a paradox, the seemingly virtually impossible happening all the time from every direction of the Universe.

    I guess that the answer is that although seemingly virtually impossible for any particular photon to hit something after a 12 billion light-years journey, the number of emitted photons is so inconceivably huge that many of them do actually experience the seemingly impossible, manage to travel for 12 billion light-years without hitting anything and yet then happening to hit a tiny speck of matter in a tiny planet called Earth.

    Any comments on these lines of reasoning? Thanks!
  2. jcsd
  3. Sep 1, 2013 #2
    I would agree with you. It is very unlikely for one single traveler to hit anything. But if you send out colossal amount of "travelers" to all directions from every point in space - like the CMB photons - small fraction of them will hit something. Overwhelming majority of them will not hit anything.
  4. Sep 1, 2013 #3
    It is like the lottery. It is very unlikely for a single person to win anything. But if a colossal amount of people is gambling, one of them will be the winner.
  5. Sep 1, 2013 #4


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    Superluminal? Does this belong in the sci-fi section?

    Anyways, I've heard that the density of hydrogen atoms in outer space is about 1 per cm3.*

    I can only imagine that flying through this stuff, approaching the speed of light, would be a bit like that thread I started back in July.

    *I'm either getting old and senile, or people are coming up with better guesses: 40 atoms per m3
    And did you see that? No one reminded me on Monday! :grumpy:
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