Herman Kahn believes things are going to be just fine. Inflation will keep falling. Investment will rise. US technology will dissipate the threat of an energy crisis, reduce pollution, feed the world, and vastly improve the national quality of life. Basic family values will reassert themselves and gross national product will double by the year 2002. In short, a boom - economic, political, and social - will transform the United States during the last years of the 20th century.
''The main point is that there are strong indications that both boom and revitalization are on the way, that we are already doing many of the things that need to be done. There are some problems which we should fix; if we don't fix them the boom may come anyway but is less sure. . . . I believe that even with all the problems we now have, mankind in general and the United States in particular are still likely to have a great future,'' concludes Mr. Kahn, chairman of the Hudson Institute think tank, in his new book, ''The Coming Boom.''
Mr. Kahn, a bearded, rotund adviser to presidents, builds his predictions on the thesis that Ronald Reagan may represent a turning point in US politics. President Reagan has a chance to forge a tight coalition of economic, defense, and social conservatives whose rise to political power could sweep away the malaise of the 1970s and establish an ''ideology of progress'' in America. This new optimism will then provide a receptive atmosphere for the government policies and technical changes which will power the coming boom, Kahn says.
The fight against inflation will continue and be won, Kahn predicts. Unconventional financing methods will provide stable sources of funds for reindustrialization. A resurgence in US defense spending should head off a very real Soviet threat.
In short order, synfuels and other unconventional energy sources will provide vast sources of non-OPEC energy. Biotechnologies will work wonders in agriculture, medicine, and engineering. Electronic chips will help automate homes and factories, making possible massive information services that will boost white-collar productivity.
The economy will forge ahead, with ''growth averaging about 3 percent or more over the next two decades - i.e., the GNP about doubling between 1982 and 2002, '' Kahn writes.
This outlook is, needless to say, highly optimistic. It's at polar odds with many well-publicized studies of the future, such as the Club of Rome's ''Limits to Growth,'' and the ''Global 2000'' report ordered by President Carter.
As quoted in Kahn's book, ''Global 2000'' concludes that ''barring revolutionary advances in technology, life for most people on earth will be more precarious in 2000 than it is now - unless the nations of the world act decisively to alter current trends.''
Is Kahn a bewhiskered Pollyanna? Are his critics nothing but short-sighted hand-wringers?
It is crucial in judging these questions to follow the chains of reasoning back to their basic assumptions. ''Global 2000'' says the world's future is gloomy - ''barring revolutionary advances in technology,'' and ''unless the nations of the world act.'' It is certainly a possibility that there will be wonderful new technologies, and that the nations of the world will act.
Kahn, similarly, admits that ''bad luck and bad management are always possible.'' ''Social limits'' might slow economic growth; inflation might still diminish the coming boom. The anti-Reagan forces of ''neo-liberal symbolists'' might once again seize political power. Any of these events are, at the very least, plausible.
Kahn says his conclusions are simply decisions based on not-proven but good-enough-for-our-purposes information.
''Even if the contexts and scenarios turn out to be imperfect, they can still provide a valuable frame of reference for looking beyond current assumptions,'' he writes.
Predictions of coming booms or coming nightmares can thus be seen as talking points, ways of deciding what events might follow from particular economic and political changes.
As Kahn says, ''History has a habit of being richer, more imaginative, and more perverse than any analyst who is concerned about credibility and plausiblity ever could be.''