THE STRIDES made recently at Princeton University in the effort to harness nuclear fusion are more of a milestone than a breakthrough in the search for a new way to meet the world's energy needs. The results of the experiments mean that the physicists have been right in their theories and calculations about how a fusion reaction could be created in a laboratory. That is a scientific achievement of major proportions. It is concrete evidence that mankind may be able to control and use the reaction that lights the sun and powers the hydrogen bomb.

Nevertheless, the operative word is still "may." These experiments do not mean the day is near when this source of unlimited energy can be counted upon to replace the fossil and nuclear fuels now in use. Nor do they mean that the time has come to focus on the particular process used at Princeton as the ultimate solution to the energy problem. There are too many problems remaining to be solved to permit such optimism.

The system being worked on in Princeton, and in several other places around the world, involves the creation of a magnetic "bottle" in which gaseous fuel can be contained at extremely high temperatures and densities while fusion occurs. If that bottle can be created and the fusion reaction inside it sustained, the output of energy as hydrogen atoms are forced together will be enormous. The experiments convince the experts that reaction will occur for the first time soon after they begin (in 1981) to use a device now under construction. Until two weeks ago, they said they "thought" this might happen then; now they are saying they are "confident" it will.

What they are confident of, however, is their ability to produce the fusion reaction in a laboratory. The steps from there to operating a fusion power station on an economic basis are huge. Even if all goes as well in the future as it has with the experiments, it is unlikely that the first power plant using a magnetic bottle as its key element will produce commercial power before the 21st century.

It is even possible that fusion power plants - if they come into being at all - will not use the principle that involves the magnetic bottle. There are alternative devices also under development that may produce the same result. Although work on the bottle system is more advanced and, right now, more promising than work on the alternatives, it poses more difficult technological and engineering problems at later stages. Thus the success at Princeton provides no reason to slacken research on those alternatives. One of them - and a possible candidate is a system using laser beams and pellets of fuel instead of magnetic fields and a gaseous fuel - may turn out to be more efficient or cheaper in the long run.

What the recent successes do mean, however, is that the scuffling for the federal research grants on which most basic energy research depends is likely to become more intense. The claim of Princeton and the other "bottle" groups on those grants has been strengthened. But the temptation must be resisted to divert funds from other fusion programs or even from research into solar and other nuclear energy programs. Some of those other systems will be needed to supply energy before a fusion system can come into operation even under the most optimistic timetable. And if the fusion systems never come to fruition - a possibility that still exists - solar or fission systems will have to meet the world's future needs.

One thing the government must now reconsider is whether the secrecy wrapped around the laser approach to fusion can be reduced. Because lasers have a use in weapons systems while magnetic bottles do not, almost all the research on the laser systems is being conducted in government laboratories. Yet it is the lack of secrecy and the large amount of international cooperation on the bottle approach that have brought success to the work at Princeton. The configuration of the machines in use there is Russian in origin. It seems possible that a broadening of the scientific base on which laser research is conducted might have a similar, stimulating effect.

Somewhere in this maze of science - lasers, bottles, solar satellites and so on - is a solution to the energy problem. The recent experiments do not pinpoint what it is. They underline the fact that it exists. That should encourage the government to be generous in its support of a variety of research programs aimed at the development of a source of clean and unlimited energy.