That's exactly my point: if nanotech performs as advertised by its starriest-eyed advocates, then interstellar colonization can be done with small payloads and energy is cheap enough that they can be launched easily. That is a very big "if," and not one we can shrug off or assume in advance as the underlying principle of all our models.
What if nanotech turns out to have many of the same limits as its closest natural analogue, biological cells? Biotech is great for doing chemistry, but not so great for assembling industrial machinery (like large solar arrays) in a hostile environment.
As for the "nuclear cars and basement reactors" being out of the picture because of politics and not engineering, that's... really quite impressively not true, I think. Fission reactors create neutrons that slip through most materials like a ghost and can riddle you with radiation unless you stand far away or have excellent shielding. Radioactive thermal generators require synthetic or refined isotopes that are expensive by nature because they have to be [i]made[/i], atom by atom... and they're still quite radioactive if they're hot enough to be a useful power source.
The real problem isn't the atomic power source itself, it's the shielding you need to keep it from giving you cancer. There's no easy way to miniaturize that, because neutron capture cross-sections play no favorites and can't be tinkered with.
This stuff is not a toy, and there are very good reasons of engineering why it never made the leap from industrial equipment to household use, except in the smallest and most trivial scales (such as americium in smoke detectors). It's not just about politics.
The way you put it does seem to disparage biologists, yes. The biologists are doing work that is qualitatively different from what physicists do, and that produces results the physicists never will (without the aforementioned thousand tons of computronium, at least). In a very real sense, biologists are exploring an entirely different ideaspace from the one the physicists live in. No amount of investigation into physics in isolation would have given us the theory of evolution, for instance.
And weirdly, I'm not a biologist; I'm an apprentice physicist. I still recognize that they're doing something I'm not, rather than something that I might get around to by just doing enough physics to make their results obvious.
This is profoundly misleading. Physicists already have a good handle on how the things biological systems are made of work, but it's a moot point because trying to explain the details of how living things operate in terms of subatomic particles is a waste of time. Unless you've got a thousand tons of computronium tucked away in your back pocket, you're never going to be able to produce useful results in biology purely by using the results of physics.
Therefore, the actual study of biology is largely separate from physics, except for the very indirect route of quantum physics => molecular chemistry => biochemistry => biology. Most of the research in the field has little to do with those paths, and each step in the indirect chain is another level of abstraction that allows you to ignore more of the details of how the physics itself works.
I wouldn't have assigned much of a prior probability to either of those common sociobiological beliefs, myself. It would hardly surprise me if they were both complete nonsense.
So what do you mean when you say that these beliefs are "standard" or "widely held?" Obviously, I am not a representative sample of the population, so I may have no opinion on a widely held belief. But I'm not aware of strong evidence that these beliefs are widely held, or at any rate are more widely held than the evidence would warrant.
Or, with tongue firmly in cheek, I claim that I'm presenting counterevidence for the common belief that [insert proposition here] is a common belief...
The catch is that complex models are also usually very wrong. Most possible models of reality are wrong, because there are an infinite legion of models and only one reality. And if you try too hard to create a perfectly nuanced and detailed model, because you fear your bias in favor of simple mathematical models, there's a risk. You can fall prey to the opposing bias: the temptation to add an epicycle to your model instead of rethinking your premises. As one of the wiser teachers of one of my wiser teachers said, you can always come up with a function that fits 100 data points perfectly... if you use a 99th-order polynomial.
Naturally, this does not mean that the data are accurately described by a 99th-order polynomial, or that the polynomial has any predictive power worth giving a second glance. Tacking on more complexity and free parameters doesn't guarantee a good theory any more than abstracting them out does.
From a social psych standpoint, it's very interesting: why do people come up with something, then fail to use it in ways that we would consider obvious and beneficial?
I think a lot of it is hidden infrastructure we don't see, both mental and physical. People need tools to build things, and tools to come up with new ideas: the rules of logic and mathematics may describe the universe, but they are themselves mental tools. Go back to Hellenic civilization and you find a lot of the raw materials for the Industrial Revolution, what was missing? There are a lot of answers to that question: "cheap slaves messing up the economy," "no precision machining capability," "no mass consumption of timber, coal, and iron in quantities that force the adoption of industrial methods," and so on. They all boil down to "something subtle was missing, so that intelligent people didn't come up with the trick."
I speculate that one of the most important missing pieces was the habit of looking at everything as a source of potential new tricks for changing the world.
I know of no confirmed historical evidence of wheelbarrows being used until around the time of the Peloponnesian War in Greece, and as I understand it they subsequently vanished in the Greco-Roman world for roughly 1600 years until being reintroduced in the Middle Ages. Likewise, wheelbarrows are not evident in Chinese history until the first or second century AD.
So wheelbarrows are an application of wheels, but they're a much later application of the technology, one that did not arise historically for two to four millennia after the invention of the two or four-wheeled animal-drawn cart.
If we use a broader definition of wheelbarrow as "hand cart," we have older evidence stretching back at least to the ancient Indus Valley some time in the second or third millennium BC.
But if we stick only to inventions we have historical evidence of, there's still a gap of thousands of years between the invention of the wheel and the invention of the hand cart throughout Eurasia. The fact that Montezuma's Aztecs made no use of the wheelbarrow, rickshaw, or hand cart is hardly more remarkable than the fact that Charlemagne's Franks didn't, either.
To make this calculation in a MWI multiverse, you still have to place a zero (or extremely small negative) value on all the branches where you die and take most or all of your species with you. You don't experience them, so they don't matter, right? That's a specialized form of a general question which amounts to "does the universe go away when I'm not looking at it?"
If one can make rational decisions about a universe that doesn't contain oneself in it (and life insurance policies, high-level decorations for valor, and the like suggest this is possible), then outcomes we aren't aware of have to have some nonzero significance, for better or for worse.
As for "question in its own right," I think you misunderstood what I was getting at. If advanced civilizations are probable and all or nearly all of them try to go Omega, and they've all (in our experience, on this worldline) failed, it suggests that the probability must be extremely low, or that the power benefits to be had from going Omega are low enough that we cannot detect them over galaxy-scale distances.
In the first case, the odds of dissenters not drinking the "Omegoid" Kool-Aid increase: the number of people who will accept a multiverse that kills them in 9 branches and makes them gods in the 10th is probably somewhat larger than the number who will accept one that kills them in 999999999 branches and makes them gods in the 10^9th. So you'd expect dissenter cultures to survive the general self-destruction of the civilization and carry on with their existence by mundane means (or trying to find a way to improve the reliability of the Omega process)
In the second case (Omega civilizations are not detectable at galactic-scale distances), I would be wary of claiming that the benefits of going Omega are obvious. In which case, again, you'll get more dissenters.
A machine-phase civilization might still find (3a) or (3b) an issue depending on whether nanotech pans out. We think it will, but we don't really know, and a lot of technologies turn out to be profoundly less capable than the optimists expect them to be in their infancy. Science fiction authors in the '40s and '50s were predicting that atomic power sources would be strongly miniaturized (amusingly, more so than computing devices); that never happened and it looks like the minimum size for a reasonably safe nuclear reactor really is a large piece of industrial machinery.
If nanotech does what its greatest enthusiasts expect, then the minimum size of industrial base you need to create a new technological civilization in a completely undeveloped solar system is low (I don't know, probably in the 10-1000 ton range), in which case the payload for your starship is low enough that you might be able to convince people to help you build and launch it. Extremely capable nanotech also helps on the launch end by making the task of organizing the industrial resources to build the ship easier.
But if nanotech doesn't operate at that level, if you actually need to carry machine tools and stockpiles of exotic materials unlikely to be found in asteroid belts and so on... things could be expensive enough that at any point in a civilization's history it can think of something more interesting to do with the resources required to build an interstellar colony ship. Again, if the construction cost of the ship is an order of magnitude greater than the gross planetary product, it won't get built, especially if very few people actually want to ride it.
Also, could you define "singleton" for me, please?
Your aliens are assigning zero weight to their own death, as opposed to a negative weight. While this may be logical, I can certainly imagine a broadly rational intelligent species that doesn't do it.
Consider the problems with doing so. Suppose that Omega offers to give a friend of yours a wonderful life if you let him zap you out of existence. A wonderful life for a friend of yours clearly has a positive weight, but I'd expect you to say "no," because you are assigning a negative weight to death. If you assign a zero weight to an outcome involving your own death, you'd go for it, wouldn't you?
I think a more reasonable weighting vector would say "cessation of existence has a negative value, even if I have no subjective experience of it." It might still be worth it if the probability ratio of "superman to dead" is good enough, but I don't think every rational being would count all the universes without them in it as having zero value.
Moreover, many rational beings might choose to instead work on the procedure that will make them into supermen, hoping to reduce the probability of an extinction event. After all, if becoming a superman with probability 0.0001% is good, how much better to become one with probability 0.1%, or 10%, or even (oh unattainable of unattainables) 1!
Finally, your additional motivation raises a question in its own right: why haven't we encountered an Omega Civilization yet? If intelligence is common enough that an explanation for our not being able to find it is required, it is highly unlikely that any Omega Civilizations exist in our galaxy. For being an Omega Civilization to be tempting enough to justify the risks we're talking about, I'd say that it would have to raise your civilization to the point of being a significant powerhouse on an interstellar or galactic scale. In which case it should be far easier for mundane civilizations to detect evidence of an Omega Civilization than to detect ordinary civilizations that lack the resources to do things like juggle Dyson spheres and warp the fabric of reality to their whims.
The only explanation of this is that the probability of some civilization within range of us (either in range to reach us, or to be detected by us) having gone Omega in the history of the universe is low. But if that's true, then the odds are also low enough that I'd expect to see more dissenters from advanced civilizations trying to ascend, who then proceed to try and do things the old-fashioned way.