In nuclear world, hope is most important, all experts agree. A world without nuclear explosives would not necessarily be free of war or superpower tension. And there would still be people who would know how to make these weapons.

In June 1946, the last serious attempt at world nuclear disarmament collapsed on the floor of the United Nations. An American plan for international control of nuclear activities was rejected by the Soviet Union; the Soviet counteroffer of a flat weapons ban went nowhere. J.Robert Oppenheimer was in despair. ``I am ready to go anywhere and do anything,'' said the famous atomic scientist, who had worked on the United States abolition plan. ``But I am bankrupt of further ideas.''

Forty years later, the superpowers have thousands more nuclear warheads. Ideas about how to get rid of them are still in short supply. Is there any way out of the nuclear predicament? What paths might lead to a nonnuclear world?

This assumes that a nonnuclear world is a desirable goal. Such a world would have its own dangers, as abolition of nuclear explosives would not abolish war or superpower tension. Men would still hold in their minds the knowledge of how to make nuclear weapons.

Many mainstream nuclear strategists dismiss thinking about disarmament as utopian in the extreme. But a little dose of utopian nuclear theorizing may be a good thing, says Harvard government Prof. Joseph Nye in his book, ``Nuclear Ethics.'' In the short run it involves no more risk than dreaming about winning the lottery; in the long run it might point the world toward a destination where things that now seem idealistic become possible.

Proposals made today for the abolition of nuclear weapons generally fall into one of two broad categories:

Technological. These plans rely in some measure on development of a defensive shield against nuclear weapons.

Political. Broad change in geopolitics could make nuclear weapons unneeded, under these theories.

The former category is the one most discussed by nuclear strategists today. Its best-known adherent is President Reagan, who bills the Strategic Defense Initiative (SDI) as a way of making nuclear weapons ``impotent and obsolete.''

In promoting SDI the President often gives the impression that he envisions a future with the US covered by a missile-proof astrodome, an achievement that might be called technically daunting. Aides offer a more sophisticated explanation of how SDI alone could drive nuclear weapons to zero: Defenses will simply become good enough so that both superpowers will channel their resources toward their shields and away from their nuclear swords.

Many analysts believe, however, that for nuclear defenses to be effective, they must be helped out by political restraints such as arms control pacts. Princeton University physics Prof. Freeman Dyson is one who takes this view. In his book, ``Weapons and Hope,'' he describes a mix of defenses and treaties he labels ``the defense-dominated future.''

An SDI-like shield may be a way to protect against the danger of the other guy cheating on a disarmament pact and hiding a small nuclear arsenal, according to Professor Dyson. He says such a relatively modest technical task is well within the realm of possibility.

If nuclear weapons are the seemingly all-powerful Goliaths of technology, then defensive interceptor rockets could be Davids - small, agile, and ultimately triumphant, Dyson says. ``After the technological supremacy of nuclear weapons has been challenged, their moral and political unattractiveness will stand revealed clearly to the eyes of the world,'' he writes.

Whether defenses will work well enough to become Davids, however, is still an open question. Except for Reagan loyalists and a scattering of other analysts, most people who think about nuclear weapons say the political paths to disarmament are more promising.

In the early years of the nuclear era, a particular approach often mentioned was disarmament through world government. This idea did not catch on. The thought of institutions that would accompany world government, like a world police force, was enough to give all but the most ardent idealists pause.

Today many peace activists talk about what might be called ``the snowball effect.'' In this theory, deep cuts in nuclear forces engender goodwill between the superpowers, which leads to further cuts, more goodwill, and so on, down to zero. Critics say that deep ideological differences separate the superpowers, and it is unrealistic to expect that just taking away the weapons will make the two countries trust each other enough to go so far.

Antinuclear author Jonathan Schell describes a more detailed approach in which the final step is labeled ``weaponless deterrence.'' He would have nuclear nations sign a disarmament pact and scrap their weapons, but keep mobile-warhead factories poised to resume production. Leaders would know that even if they cheated, hid a few weapons, and launched them at a rival, retaliation would eventually follow. Critics reply that these factories might become lightning rods tempting an adversary to make a preemptive strike.

The most mentioned nuclear-free political scenario is similar to ``the snowball effect,'' but much slower. It is one in which the relationship between the United States and the Soviet Union is gradually transformed, making nuclear weapons unnecessary. The Soviet Union would not have to become a capitalist country for this to occur; the two nations would not even have to become close friends. They would simply reach a state where neither felt threatened by the other.

Animosities between the superpowers rule out an easy or even an inevitable transformation. But the US and the Soviet Union share one important interest: the avoidance of nuclear war. And such changes of attitude do occur. Only a few years ago US leaders were calling Red China a great menace. France and West Germany were the most bitter rivals of modern European history until the end of World War II.

The great difficulty in launching off down the road toward abolition of nuclear weapons is that it is unmarked, and a misstep could have a terrible effect. Considering the state of the world, it is probably premature to expect much movement in this direction soon.

That does not mean nuclear experts and the average citizen should not dream of such a world, to make more likely interim moves to lessen reliance on nuclear weapons and lower the risk of nuclear war. If the imaginations of Western publics are filled with nothing but the thought of an eventual holocaust, apathy and political paralysis on the issue could be the result. It is useful to remember that nuclear experts of all ideological inclinations agree on one thing: the importance of hope.

Other stories in this series ran Nov. 18, 19, 20, 21.