Chances of hitting something in intergalactic travel

[Gerinski]

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 traveling 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 traveled 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!

 

[mpv_plate]

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.

 

[mpv_plate]

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.

 

[OmCheeto]

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:

 

[pmacias]

I would say that there are a few key factors to consider when discussing the chances of hitting something during intergalactic travel. The first is the sheer size of the universe. The observable universe is estimated to be around 93 billion light-years in diameter, and it is likely that the actual size of the universe is much larger. This means that the chances of hitting something along a straight line journey are already incredibly small, as the vast majority of space is empty.

Secondly, the speed at which we would be traveling is also a crucial factor. While traveling at superluminal speeds may seem like a way to minimize the risk of hitting something, the reality is that even at these speeds, the likelihood of hitting anything significant is still extremely low. The average distance between galaxies is about 3 million light-years, so even at superluminal speeds, it would take millions of years to reach the nearest galaxy. And as you mentioned, the chances of hitting something smaller than a hydrogen atom are even smaller.

Furthermore, the fact that we can see light from the most distant sources in the universe without it being significantly absorbed or scattered is a testament to the emptiness of space. If there were a significant amount of matter in the way, the light would not be able to travel such vast distances.

However, it is important to note that there is still a possibility of encountering objects during intergalactic travel, especially if we are not traveling in a straight line. There are still objects such as comets, asteroids, and rogue planets that could potentially cross our path. But even in these cases, the chances of a collision are still very low.

In conclusion, while the chances of hitting something during intergalactic travel are not zero, they are incredibly small and would not be a significant concern for hypothetical superluminal travel. The vastness and emptiness of space make it unlikely for any significant objects to be encountered along the way.