WHILE THE STATES compete loudly and vigorously to be the site of the Superconducting Supercollider, there's one thing that you should know. Congress hasn't yet authorized the money to build it. The project's supporters, which include the administration, think it will be helpful to have the legislation moving along while a large number of congressmen are anxiously trying to grab the SSC for their states. But there's more at stake here than its location.

The key issue is financing. The SSC would be the largest scientific research instrument ever built, and probably the most expensive. It would be a circular tunnel 52 miles in circumference (the Washington Beltway, for comparison, is 64 miles) containing beams of protons steered by immensely powerful magnets. Because the magnets are to be made of superconductors, they would have to be refrigerated to very low temperatures. The idea is to achieve collisions among the protons at energy levels far higher than those achieved in any laboratory before. In the debris, physicists will learn much about the basic structure of matter and of the universe. It's a brilliant concept and holds much promise.

It will cost $4.4 billion in today's dollars, plus the inflation of the six years or so required to build it. Running it will cost another several hundred million dollars a year.

The Reagan administration swears that it intends the federal government to provide this money in addition to its present funding for basic scientific research. But that won't be the Reagan administration's decision. Nearly all of the money will be appropriated after the next president takes office. The final choices will be made, in any case, by Congress.

Congress ought to build the SSC only -- repeat, only -- if it is prepared to do it without cutting into the present support, barely adequate, for other scientific work. High-energy physics is an exciting field, but not the only exciting field in physics nor necessarily the most productive. Congress' first priority needs to be funds to support work in university laboratories, which not only advances knowledge but also contributes to the essential process of training the young scientists who are graduate students. The SSC, representing a tremendous investment in one narrow series of experiments, will make only a modest contribution there.

It's a matter of balance. If the SSC were to begin draining other, less spectacular research projects to meet its own extraordinary costs, the results would leave physics weaker. Before Congress votes money for construction, it needs to give very careful thought to the consequences for American science as a whole.