This is an opinion piece, mainly written in 1989. I think many people will disagree, and I hope this will lead to discussion. The numbers are guesses, but I have read enough of the literature to understand and to be able to dispute many less optimistic calculations. [However, I have to regard Frank Tipler as a wishful thinker.]
It's hard to see how humanity can survive more than 10^50 years, give or take a factor of 2 in the exponent. Even that might require drastic concentration of intelligence in an artificial system that would carry on what our descendants might regard as the essence of human-descended life.
If we want humans like the present ones, living on planets like the present ones, illuminated by suns like the present ones, then 10^25 is perhaps the limit.
Giving up the planets but preserving human form may give 10^30 years.
Let us suppose that the scientific picture relevant to human survival remains the same for billions of years. Our descendants will probably take the same view of the eventual doom of the species that a person takes of his own death. Death is a misfortune to be put off as long as possible, but in the meantime there are many interesting, worthwhile, and pleasant things to do.
The present solar system is good for a few times 10^9 years, but new stars, not too different from our own may be forming up to 10^20 years from now.
The human population of the solar system can grow quite a lot for those of our descendants willing to live in space. For example, the energy output of the sun would support 10^26 people using it with present day efficiency.
Maybe we would find some other reason not to let our population grow that large, but remember that there are likely to be many human societies, more diverse than society is now, and some of them may have reasons to grow quite large.
I should admit favoring this diversity and am repelled by Utopias of any kind postulating uniformity and control of emigration. Indeed the one thing that is likely to motivate humanity to last a long time is diversity of human society.
With the technologies based on present day science with which I am confident, journeys to nearby stars will take high hundreds to low thousands of years, thus requiring multi-generation journeys with present human biology. I'm sure some of us will undertake them even if there is faster alternative, and humanity will occupy the galaxy in a few hundred thousand years if we aren't limited by encountering other civilizations. The tribes that do the travelling will be aided by continuous transmissions from home and needn't fall behind intellectually more than the distance enforced by the speed of light.
Laser powered space travel systems permitting substantial fractions of the velocity of light also seem plausible. These have been proposed by Robert Forward.
However, most likely biology will give us much longer lives within a few hundred years, and this will completely change the psychology of long voyages as well as other aspects of human psychology and sociology.
Very likely it will also be possible to send robots ahead of us to transform other stellar systems to our liking, but maybe we'll all prefer to reserve the adventures for humans. That's a long shot because of the afore-mentioned multiplicity of societies.
The settlement of the solar system is somewhat overdue to my impatient mind. If we in the U.S. had set our minds to it, we could have colonized an asteroid by now. If this had been done, we would begin to know how people born there feel about life in space, when they would be inclined to declare their independence, and how fast they could or would expand from their own resources. Future historians, however, will consider the delay that annoys me as not worth mentioning.
Fortunately, the U.S. is not the only country in the world, and the world-wide assumption that new ventures are first undertaken in the U.S. or the Soviet Union will die off in the next 10 or 20 years, and there will be 20 countries capable of vigorous space exploration and colonization. Some of them will do it, and the U.S. will follow suit. Indeed the first steps could be done for amounts of money corresponding to that major universities raise in major fund drives, i.e. it could be done with U.S. or Japanese private donated money. This money might be easier to get than investments.
As for the earth itself, I think it will progress about as it has done since World War II, with more countries drawn into democratic capitalist expanding economies. An enormously destructive major war is still possible but increasingly unlikely. Humanity would even survive such a war, and would reach the same point in a few hundred years that it would reach without it.
Many ecological dooms are being urged on us, but I believe that what real substance there is in the warnings will be met with essentially minor changes in our way of life. If we had a world government, it would be possible for an ideological fad like the belief in environmental doom to be warded off by symbolic sacrifices to conquer the world. Fortunately, there seems to be sufficient variety of countries for them not all to go crazy at once.
There's plenty of nuclear energy for a few billion years, and other substances can be concentrated with the use of energy when concentrated ores run out. Methods of expanding food supply exist for any likely population expansion in the next 200 years, and that gives our smart descendants plenty of time to figure out what to do. The specific proposals of the 1990s will be regarded the way we regard projections made in the middle of the nineteenth century.
A main reason for discussing the very long term future is to establish that our civilization is in its extreme infancy relative to its expected life.
When humanity expands beyond the earth, it will become clear that the earth should be regarded as a garden rather than as a wilderness with humanity one among many species. We will arrange the earth as we find most convenient and aesthetic. Fortunately, the earth is large and we prize variety. Some parts will be wild, but most will be tamed in a much greater variety of ways than we can presently imagine.
However, even within the solar system, there will be other gardens. For example, both Mars and Venus can be moved into the earth's orbit and given earthlike atmospheres and climates. Some people already have figured out how these things can be done. If you want to figure out how to do it well, you'd better study mathematics, science and engineering.
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