One of the Administration's repeated arguments for pursuing SDI is the excuse given by any kid caught in a fight: somebody else started it. ''The Soviet Union,'' President Reagan said last fall, ''is about ten years ahead of us in developing a defensive system.'' As Richard Perle put it at the TIME conference, ''The Soviet SDI program preceded that of the U.S., and they've made a larger investment than we have.'' But in an analysis presented to the conference, Stephen Meyer, an associate professor of political science at M.I.T. who is a consultant for the Pentagon on Soviet technology, said that Moscow's program is far more limited in its aims than the U.S.'s, with its goal of building a space-based defense against missiles. ''It is true that the Soviets do have the largest SDI program in the world,'' Meyer said, ''but that effort is overwhelmingly an air-defense effort.'' It relies on radar systems, interceptor aircraft, surface-to-air missiles for anti-cruise-missile defense, all designed to protect targets close-in just before they are struck. Even the large radar being constructed at Krasnoyarsk in Siberia (which has become a focal point of the debate over whether Moscow is violating the ABM treaty) is not particularly impressive, according to Meyer. ''The systems do not move,'' he said, but rather are fixed in a certain direction. ''There is great uncertainty here about both the operational reliability of those systems and, more important, the computer- processing capability.'' Gerold Yonas, the chief scientist for SDI, noted of the Soviets that ''their laser weapon program, if you just look at the size of their facilities, is mind boggling.'' Exotic weapons systems--lasers, particle beams, orbiting accelerators and the like--are indeed on Moscow's research agenda, Meyer said. But he contended that they are 20 or 30 years away from becoming operational. In the Soviet Union, he said, the research phase, known as NIR, involves looking into basic scientific principles and ''is not linked to weapons programs at all.'' By one account, less than a third of these projects ever enter the design and engineering phase, known as OKR. Because plant construction begins long before prototypes and test models have been built, it is possible to determine when a project has progressed to the OKR phase. ''There is no such construction going on related to any of these exotic technologies at all,'' said Meyer. He pointed out that the Soviets have had serious difficulties making particle accelerators. No evidence suggests that they have successfully solved the problems generally besetting kinetic-energy weapons, which have rails that tend to warp after repeated firings, or gasdynamic and electrical-discharge lasers, technologies discarded by the U.S. a decade ago. Strategic defense requires far more than just kill mechanisms such as lasers, accelerators and particle beams. ''It requires sensing, tracking control and targeting,'' said Meyer. ''This is where perhaps the grossest of all Soviet weaknesses in technology industry lie.''