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Nick_Tarleton comments on What are our domains of expertise? A marketplace of insights and issues - Less Wrong
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I agree (NB: also computer scientist, not physicist) with the premise that civilizations probably expand at near-c, but there's a problem with this. Since it seems that intelligent life like us could have arisen billions of years ago, if life is common and this is the explanation for the Fermi Paradox, we should be very surprised to observe ourselves existing so late.
You are right. The argument is not compatible with the possibility that life is very common, and this makes it much less interesting as an argument for life not being very rare. But it is not totally superfluous: We can observe the past of a 46 billion light years radius sphere of the expanding, 14 billion light years old Universe. Let us now assume that 4 billion years since the Big Bang is somehow really-really necessary for a maximally expanding civilization to evolve. In this case, my whole Fermi Paradox argument is still compatible with hundreds of such civilizations in the future of some of the stars we can currently observe. (You can drop hundreds of 10ly spheres into a 46ly sphere before starting to be very surprised that the center is uncovered.)
But you are right. I think my Fermi Paradox argument is an exciting thought experiment, but it does not add too much actual value. (I believe it deserves a sensationalist report in New Scientist, at least. :) ) On the other hand, I am much more convinced about the expansion speed phase transition conjecture. And I am very convinced that my original question regarding optimally efficient computational processes is a valuable research subject.
You're right, and I hadn't really thought that through — I had thought that this argument ruled out alien intelligence much more strongly than it does. Thanks.
Glad I could help. :) You know, I am quite proud of this set of arguments, and when I registered on LW, it was because I had three concrete ideas for a top-level post, and one of those was this one. But since then, I became somewhat discouraged about it, because I observed that mentioning this idea in the comments didn't really earn me karma. (So far, it did all in all about as much as my two extremely unimportant remarks here today.) I am now quite sure that if I actually wrote that top-level post, it would just sit there, unread. Do you think it is worth bothering with it? Do you have any advice how to reach my audience with it, here on LW? Thanks for any advice!
No, because if there is something like a Gaussian distribution of the emergence times of intelligent civilizations, we could just be one of the civilizations on the tail.
Exactly. The argument is that, since being on the tail of a Gaussian distribution is a priori unlikely, our age + no observation of past civilizations is anthropic evidence that life isn't too common.
We have no idea what the Gaussian distribution looks like. We don't necessarily have to be on the tail, just somewhere say one sigma away. No observation of civilizations just corresponds to us being younger than average and the other civilizations being far away. Or we could be older and the other civilizations just haven't formed yet. But none of this can imply whether life is uncommon or common.