I’m not sure I follow this. I think if there were extraterrestrials who were going to stop us from spreading, we’d likely see signs of them (e.g., mining the stars for energy, setting up settlements), regardless of what speed they traveled while moving between stars.
regardless of what speed they traveled while moving between stars
Adding to my other reply to your other comment I just made, let me just clarify that the model I’m working with is the “fast colonization” model from 25:20 of this Stuart Armstrong FHI talk, in which von Nuemann probes are sent directly from their origin solar system to each other galaxy, rather than hopping from galaxy to galaxy (as in the “slow colonization” model used by Sagan/Newman/Fogg/Hanson according to Stuart’s slide).
So if >0.99c probes are possible, then I think the hypothesis I described is at least plausible, since civilizations indeed wouldn’t see other expanding civilizations until those civilizations reached them.
To clarify, I am pointing out that if extraterrestrials exist that are mining stars for energy and doing other large-scale things that we’d expect to be visbile from other solar systems or galaxies, and if those extraterrestrials are >X light-years away from us and only started doing those large-scale things <X years ago, then we would not expect to see them because the light from their civilization would not yet have had time to reach us.
So the speed of expansion of their civilization isn’t a necessary aspect of why we can’t see them.
However, if the nature of our universe is such that extraterrestrials are likely to have arisen elsewhere in our galaxy (meaning <100,000 ly from us), then what’s the explanation for why they arose in the last <100,000 years and not in the billions of years before that? That sould seem improbable a priori.
One (partial) explanation for that coincidence is if we hypothesize that the nature of our universe is such that any civilization that arises and reaches a point of doing large-scale things that would be visible from many light-years away also expands at near the speed of light beginning as soon as it starts having those large-scale effects. If we further assume that such expansion reaching our solar system before now would have prevented us from existing today (e.g. by extinguishing life on Earth and replacing it with something else), then this serves as a (partial) explanation for the above coincidence by introducing an observation selection effect where we only exist in the first place because no other extraterrestrials have arisen within X ly of us in the last X years.
Note that I called this (“intelligence expands at (near) light speed once it starts having effects that would be visible from light years away”) hypothesis a “partial” explanation above (for lack of a better word) to note that while it could explain why it’s not surprising that we don’t see signs of extraterrestrials mining stars (even conditional on them existing), it is also a hypothesis that we find ourselves in a very rare world (simulation possibilities aside)--one in which intelligence arose more than once in our vaccinity, but at almost exactly the same time (e.g. 13.79995 and 13.8 billion years after the big bang if some other civilization in our galaxy started expanding 50,000 years ago), which a priori is unlikely.
I’m not sure I’m fully following, but I think the “almost exactly the same time” point is key (and I was getting at something similar with “However, note that this doesn’t seem to have happened in ~13.77 billion years so far since the universe began, and according to the above sections, there’s only about 1.5 billion years left for it to happen before we spread throughout the galaxy”). The other thing is that I’m not sure the “observation selection effect” does much to make this less “wild”: anthropically, it seems much more likely that we’d be in a later-in-time, higher-population civilization than an early-in-time, low-population one.
The other thing is that I’m not sure the “observation selection effect” does much to make this less “wild”: anthropically, it seems much more likely that we’d be in a later-in-time, higher-population civilization than an early-in-time, low-population one.
That’s a good point: my hypothesis doesn’t help to make reality seem any less wild.
Could it be that we don’t see extraterrestrial species spread throughout our galaxy because civilizations spread at near-lightspeed?
I’m not sure I follow this. I think if there were extraterrestrials who were going to stop us from spreading, we’d likely see signs of them (e.g., mining the stars for energy, setting up settlements), regardless of what speed they traveled while moving between stars.
Adding to my other reply to your other comment I just made, let me just clarify that the model I’m working with is the “fast colonization” model from 25:20 of this Stuart Armstrong FHI talk, in which von Nuemann probes are sent directly from their origin solar system to each other galaxy, rather than hopping from galaxy to galaxy (as in the “slow colonization” model used by Sagan/Newman/Fogg/Hanson according to Stuart’s slide).
So if >0.99c probes are possible, then I think the hypothesis I described is at least plausible, since civilizations indeed wouldn’t see other expanding civilizations until those civilizations reached them.
To clarify, I am pointing out that if extraterrestrials exist that are mining stars for energy and doing other large-scale things that we’d expect to be visbile from other solar systems or galaxies, and if those extraterrestrials are >X light-years away from us and only started doing those large-scale things <X years ago, then we would not expect to see them because the light from their civilization would not yet have had time to reach us.
So the speed of expansion of their civilization isn’t a necessary aspect of why we can’t see them.
However, if the nature of our universe is such that extraterrestrials are likely to have arisen elsewhere in our galaxy (meaning <100,000 ly from us), then what’s the explanation for why they arose in the last <100,000 years and not in the billions of years before that? That sould seem improbable a priori.
One (partial) explanation for that coincidence is if we hypothesize that the nature of our universe is such that any civilization that arises and reaches a point of doing large-scale things that would be visible from many light-years away also expands at near the speed of light beginning as soon as it starts having those large-scale effects. If we further assume that such expansion reaching our solar system before now would have prevented us from existing today (e.g. by extinguishing life on Earth and replacing it with something else), then this serves as a (partial) explanation for the above coincidence by introducing an observation selection effect where we only exist in the first place because no other extraterrestrials have arisen within X ly of us in the last X years.
Note that I called this (“intelligence expands at (near) light speed once it starts having effects that would be visible from light years away”) hypothesis a “partial” explanation above (for lack of a better word) to note that while it could explain why it’s not surprising that we don’t see signs of extraterrestrials mining stars (even conditional on them existing), it is also a hypothesis that we find ourselves in a very rare world (simulation possibilities aside)--one in which intelligence arose more than once in our vaccinity, but at almost exactly the same time (e.g. 13.79995 and 13.8 billion years after the big bang if some other civilization in our galaxy started expanding 50,000 years ago), which a priori is unlikely.
I’m not sure I’m fully following, but I think the “almost exactly the same time” point is key (and I was getting at something similar with “However, note that this doesn’t seem to have happened in ~13.77 billion years so far since the universe began, and according to the above sections, there’s only about 1.5 billion years left for it to happen before we spread throughout the galaxy”). The other thing is that I’m not sure the “observation selection effect” does much to make this less “wild”: anthropically, it seems much more likely that we’d be in a later-in-time, higher-population civilization than an early-in-time, low-population one.
That’s a good point: my hypothesis doesn’t help to make reality seem any less wild.