One of the essentials for an alien civilization to climb the technology ladder and be able to confidently contemplate star travel is energy, high-density, efficient energy. We on Earth know of some sources of energy, and there may be more yet undiscovered, lying around like nuclear power was a hundred years ago, unknown, unanticipated and not much looked for. Nuclear power was right in front of us all the time, in the form of solar fusion, but we had insufficient knowledge to appreciate what was right in front of our eyes. It took some Earth-bound investigation to gain a clue that it existed before we recognized what we had been looking at since the first humans.
If there is an energy source yet unknown and unanticipated, that would be wonderful for us and for any alien civilization, as technology is a river that flows in one direction only. Every planet would discover it, barring civilizational collapse or some catastrophe. But since it is unanticipated, there is no way to plot a trajectory for an alien civilization using it. We are left hypothesizing that what we know about is all there is. Our current knowledge of physics is fairly complete, and perhaps good enough so we can extrapolate with some surety that there are no other sources of energy we could take advantage of.
There are two sides to this energy limitation. One is that we have to find sources of energy that the universe has left around waiting for us to access. In other words, no matter how much we know about energy, if there is no free energy that we can use to turn into sustenance or propulsion, for two examples, we are limited and confined to our little planet. The other side is energy storage. Suppose we have enough energy, but it is bulky or inefficient to use. Then we need to use our knowledge of energy to devise some means of carrying it around.
That was done for us for local, terrestrial transportation, by the finding of fossil fuels, which, by utilizing the chemical bond between carbon atoms, primarily, we can find liquids to put in tanks and gases to pump through pipes which make energy use efficient. For interplanetary flight, we can still use chemical bonds, some more compact ones, for propulsion, which allows us to get to our nearest local planets fairly efficiently, at least for exploration purposes. There are some orbits that are efficient to use, so even for more extensive visiting of nearby planets we can use chemical power, although nuclear would be probably be nicer. And nuclear is needed for long distance flights to the rest of the solar system.
Fusion, the sun's power source, has been the Holy Grail for energy for as long as it has been known, which still less than a century. To make it work, we have to continue experimenting to get to a proof of principle, which simply means we can do engineering around this power source to make it release its energy in a mass way. One way of figuring out efficiency is the ratio of energy produced for useful purposes versus the energy needed to produce the plant, the fuel, to dispose of waste produces including radioactive ones, and to dismantle the planet when its lifetime is over. This is a very hard number to compute, but it provides at least the concept of what we are shooting for. The energy ratio changes as resources are consumed on the planet, as the costs of extracting scarce ones jumps up and this makes the whole ratio drop.
There are three questions that we would like to have answered in the next century or two, regarding fusion. One is, can a plant be made to produce more useful power than it consumes on a operational basis. A second is, can a plant be made to produce more useful energy than the total energy used to make it, by some factor like five or ten? The third is, how big does a plant answering the first question or the second question have to be? Without positive answers to the first two, there will be no fusion power in asymptotic technology for general planet-wide use. Without a satisfactory answer for the third, there will be no fusion power for the exploration of interstellar space. The obvious fall-out from this is: what would an alien civilization do with a negative result?
Negative results take much more work to obtain with confidence than positive ones, as there are for a long period, ideas for how to convert a negative tentative result into a positive one, and until these ideas are all explored, one way or another, the negative result is still in abeyance. But let's just suppose that an alien civilization has spent a thousand years doing diligent research on fusion, and it simply doesn't work. No one can figure out how to get the second question answered positively, and it doesn't make much difference then if the first one is answered positively or not.
What would they do?
This affects every aspect of society, especially their long-term planning. An alien civilization passing asymptotic technology does long-term planning, taking into account all the options that they have before themselves. What do they do for energy? Does it destroy any hope they have of star travel, and so will a meme they might have for it be thwarted? Does the lack of fusion mean that alien civilizations die off quickly, and that is why we don't see any of them? Does the lack of fusion mean that there is no possibility of star travel, at least to more than a passing star, and this is the reason that we don't see any of them? There is another possibility that affects our quest to understand why no aliens have been visible to us and that is a negative answer to the third question: size. If fusion works on the home planet, and the alien civilization can revel in an abundance of power, but there is no way to make a ship big enough to have a power plant, this could be the reason no aliens visit Earth, or anywhere else outside of their home solar system.
Thus some discussion is in order on the longevity of a non-fusion alien civilization, and on the possibility of non-fusion star travel. While it is idyllic to think that fusion will power alien civilizations, the alternative should be thought through, just for completeness.
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