The key assumption here is whether advanced technological civilisation
(such as ourselves) is easy or difficult on the timescale of the age
of the universe (10^10 years).

Assuming that this is difficult (contra to your comments below),
solves the standard Fermi paradox (namely other advanced civilisation
are too far away to have reached us yet, and probably too far away to ever
reach us, unless the universe starts contracting again. 

So your Throgians are as good as mythical.

This also implies that such technological civilisations are also
rather diffuse within the Multiverse, _excepting_ of course those
which share part of their history with ours (eg the Nazis which won
WWII). We have some predictive power as to what those people would be
like, since they will be similar to us. 

You are no doubt aware of statistical studies of the fossil record
indicating that our own evolution was rather hard won (eg Robin
Hanson's "Hard Steps" paper).

So, I for one, would not discount Hal Finney's point.

                                        Cheers

Tim May wrote:
> 
> 
> In the interests of doing wild speculation here, which some folks are 
> asking for more of, here's a followup to a discussion we had a few 
> weeks ago:
> 
> The Fermi Paradox asks the question: "If extraterrestrial civilizations 
> are at all common, even at the level of a few dozen per galaxy per 
> galaxy life, why aren't they _here_? Or why don't we see artifacts, 
> relics, ringworlds, Dyson spheres, evidence of large engineering 
> projects, and so on?" Fermi used the succinct quip, "Why aren't they 
> here?," with the details about rate of spread even at slow, sublight 
> speeds, etc. implicitly obvious.
> 
> The Many Fermis Paradox is a variant of this: "If the MWI theory is 
> correct, AND if there is any communication or movement between the 
> worlds that is possible, why aren't they here?"
> 
> (Note that movement amongst branches is not part of any conventional 
> MWI interpretations, though it's used in novels and in some of the 
> wilder interpretations.)
> 
> Hal Finney pointed out that the same kind of Fermi Paradox argument is 
> used to argue against time travel: "If time travel is possible, why 
> aren't travelers from the future here?" (Arguments that they are here 
> but they only talk to some people, etc., are trumped by the sheer size 
> of the future, and by the "defector" argument to the "Cosmic 
> Quarrantine" solution to the Fermi Paradox: _lots_ of defectors would 
> violate the quarantine, etc.
> 
> Hal further raised the possibility that, like the time travel version, 
> perhaps the resolution has to do with the need for a "receiver": 
> neither time travelers nor MWI travelers are here, yet, because we have 
> not, yet, built the portals or receivers needed by them (opening  up 
> wormholes, a la Visser, etc.).
> 
> (The other resolution being, of course, that neither time travel nor 
> MWI travel is possible, which was Fermi's point about the absence of 
> aliens.)
> 
> I've thought about Hal's point and have this reply:
> 
> While we on this earth may not have built the necessary portals or 
> receivers for either (time travel or MWI travel), some other 
> civilization surely would have a billion or three billion years ago. 
> (Assuming the usual crude estimates of distribution of ages of stars, 
> non-special status of us as observers (a la Boostrum and others), and 
> crude estimates of likelihood of advanced civilizations elsewhere. If 
> we are the only form of life in our timelike region of the universe, 
> i.e., within a few billion light-years, then of course this makes the 
> odds of another receiver-builder nil.
> 
> In this fanciful scenario, the Throgians in a small elliptical galaxy 
> in Coma Berenices built a MWI receiver in their approximate equivalent 
> of 4500 A.D., by our measures. This was 2 billion years ago. Once this 
> MWI receiver was built, the Throgians were the beneficiaries and 
> traders of vast amounts of MWI knowledge...every discovery made in any 
> MWI universe they had contact with, including universes filled with 
> other civilizations, all manner of creatures, and so on.
> 
> Call this first construction of such a portal the "MWI Singularity."
> 
> (A similar situation, arguably much the same situation, ensues with the 
> construction of the first time machine portal.)
> 
> Steven Baxter has written a series of novels in his future history 
> which explore this very kind of idea.
> 
> With nearly any nonzero chance of building either a time machine or MWI 
> receiver, a distribution of civilizations would have done this billions 
> of years ago. (By the usual argument that there is nothing which makes 
> our emergence about 4.5 billion years after the formation/cooling of 
> the earth anything special in the cosmic scheme: there should have been 
> planets which similarly formed and cooled as many as 12 billion years 
> ago, giving some of them a 6-8 billion year "head start" on us.)
> 
> 
> As with other Singularities (AI, nanotech, time travel, MWI) in a 
> cosmos with civilizations with these kinds of head starts, "why aren't 
> they here?"
> 
> Perhaps in the MWI Singularity, the number of other universes expands 
> more rapidly than the number of occupants, making the movement of 
> travelers into any one branch not interesting. (Why would they come 
> here to our particular branch when endless diversity lies out there in 
> the multiverse? This was explored by John Barnes in "Finities," where 
> our branch "empties out" after MWI travel is discovered. Or they 
> evacuate a la Bear's "Blood Music." Or they do things we cannot 
> imagine.)
> 
> Perhaps there are other types of "cosmic censorship" involved.
> 
> But an issue we must face is this: if the MWI worlds--or the future, in 
> the time travel version--are forever inaccessible to us, then they are 
> essentially nonexistent to us, of no more value to our lives than 
> Lewis' "worlds where unicorns exist" or alternate history novels where 
> Germany won the Second World War.
> 
> For if such other branches _are_ accessible, then if we are not in some 
> special place in the universe these other branches were _also_ 
> accessible to the Throgians in that galaxy in Coma Berenices, enough 
> time ago for the radical implications of MWI travel/communication to 
> produce a massive infusion of knowledge and engineering capabilities.
> 
> Hal's argument that we have not seen either time travelers or MWI 
> traveler's because we have not yet built receivers yet. My argument is 
> "But someone would have by now." And unless we are in a narrow band of 
> outcomes where some civilizations have done this and are even now 
> expanding toward us but have not yet reached us or produced engineering 
> feats we can observe, they should have shown themselves by now.
> 
> My hunch is that alien civilizations may well exist, but are not 
> abundant (else we'd see the Galactic Federation already here, etc.), 
> and that neither time travel nor MWI travel/communication is possible.
> 
> --Tim May
> 



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