Something unreal is happening at the Pentagon. The rest of Washington, having cheered on the first Reagan defense budget, is starting to look at -- and to gulp at -- the administration's plans for a continuing large arms buildup. Even as this happens, however, Secretary of Defense Caspar Weinberger is turning his attention to a potential new great leap forward that would dwarf the one under way.

Already defense spending is set on a course of at least 7 percent annual real growth over the next five years. But, according to The Post's George Wilson, Weinberger has asked the armed services to indicate what kind of an industrial base would be needed to double or triple defense spending in an emergency, or to devote half the country's GNP (currently almost $3 trillion) to defense in a single year.

When, a bit dazed, I bounced these numbers off one close Weinberger, aide, he advised relaxation: it's just contingency planning. He said the new administration had found the existing industrial base insufficient to produce the extra hardware it believes urgent, but that important and proportionate steps were already being taken. For instance, the Pentagon has just gotten Congress to approve multi-year defense procurement to give contractors more confidence to plan ahead.

But I think there's more to it. We are coming into the presence of a new way of thinking into the presence of a new way of thinking about defense, the idea -- unfamiliar since World War II -- of a national-security commitment so unending and all-consuming as to subdue other economic and social priorities as though we were at war.

The idea of an "insufficient industrial base" jostles people brought up on President Eisenhower's parting indictment of the "military-industrial complex." To such people, the problem has been to tame the tiger, not let it run. If there was a shortage of this item or that, it was bound to be small or transitory, if not simply invented, and the way to ease it was to put the workforce on overtime or to make do without. The principal shortcomings of our defnese plan were assumed to be in the realm of inefficiency, mismanagement and profligacy rather than in insufficiency.

Now that the first budgets are out of the way, however, the notion of strenghtening the industrial base is emerging as a critical part of the Reagan program of "rearming America." Its intellectual patron is Undersecretary of Defense for Policy Fred C. Ikle, the carefully spoken defense intellectual who was Richard Nixon's arms control director. Its fullest statement is an essay Ikle published last year in a collection called "National Security in the 1980s: From Weakness to Strength." The sponsor was the San Francisco-based Instutite for Contemporary Studies, an outfit financed by, among others, Richard M. Scaife.

Even if the United States accepted a 9 percent annual compound rate of military investment through the 1980s, Ikle states, the Soviet Union would likely continue to gain in relative military power. This arithmetic led him to examine the kind of crises, short of nuclear, that might provoke the political decision to expand defense spending "massively" -- say, by three to nine times. He criticized the two "extreme assumptions" that have crippled such planning: either it has been assumed our great economic strength could be marshalled quickly if there were need, or planners have felt war would come and end so quickly that economic mobilization would be too late to matter.

Ikle's conclusion is to get ready, really ready, for a long non-nuclear war: prepare the necessary economic controls and the lifting of environmental restrictions and the like, update standby preparations for production facilities, build stockpiles, etc.

Well, this is a season for pessimistic scenarios. And perhaps there is benefit in demonstrating to the Soviets that we are not flinching across any part of the spectrum of potential threats. If things are as bad as Ikle feels it prudent to plan for, however, ought we not ask if there are other ways to prepare? Some might suggest a first strike; others would support a more affordable arms buildup combined with a more vigorous search for diplomatic solutions; still others might prefer to be "better dead than red." I go for Option B.

There is in this administration a longing not for perfect security, but for a degree of security that it seems to me is beyond achieving in our tumultuous 20th century world and whose very pursuit could become destablizing in various ways, economic, strategic and so on. In this atmosphere, the new ideas on industrial moblization have progressed from think tanks to Pentagon paper. We will see whether they survive the political wars.