By agreeing to reopen arms control talks, the United States and the Soviet Union have all but conceded that they intend to do what rival strategists never publicly admit: to use the visible evidence of negotiation -- on television and in print -- as a substitute for any near hope of a substantive agreement.

The two superpowers said they have "agreed to enter into new negotiations" on the "whole range of questions concerning nuclear and outer space arms." It was that portion of their joint declaration on Thanksgiving day that captured the headlines.

But the Kremlin and the Reagan administration were exceptionally frank in acknowledging that they have yet to decide not only how they will proceed, but just what they will be negotiating about.

That was clearly stated in the formal, two-paragraph statement announcing the Jan. 7-8 Geneva meeting between Secretary of State George P. Shultz and Foreign Minister Andrei A. Gromyko. Their assigned task -- "to reach a common understanding as to the subject and objectives of such negotiations" -- registers the great gulf between the two nations on all the basic questions of controlling intercontinental and European-based nuclear weapons, and the military use of space.

What the two nations have decided is that it is advantageous to their rival self-interests to be engaged in negotiations, although neither now will even offer a forecast about what may emerge from the process, or when.

At the outset of the venture, therefore, many critics are quick to dismiss it as simply elaborate atmospherics that can lead only to illusory expectations, as have so many other encounters between the superpowers. But whether that proves true this time, the two nations have their own competitive purposes for launching it, beyond the avowed purpose of reaching mutually beneficial accords.

In the United States and in many other nations, the agreement to resume nuclear arms control negotiations, which were aborted a year ago by the Soviet Union, is being widely hailed as a diplomatic coup for the Reagan administration. The Soviet Union, however, obviously has concluded that its interests are being served by joining the process, and it has at least as many reasons for believing that.

For its part, the Reagan administration gains international credit for reviving the dormant arms control negotiations, by using an "umbrella" concept as a restarting point to encompass all of the various talks.

Domestically, the administration expects to gain more tangible benefits: not only greater public approval for its foreign policy as a whole, but voting support in Congress to relieve pressure on its defense spending program, especially on weaponry directly tied to arms control sensitivities. Congress is holding hostage the production of MX strategic missiles and the schedule for testing antisatellite weapons, awaiting progress on arms control.

The administration at the same time will need all the support it can muster to muffle the lack of consensus among its policy-makers on basic strategy for dealing with the Soviet Union. That deep split, which the administration apparently has decided to live with by leaving its top officials in the State and Defense departments and the White House in their present posts, can plague any attempt to convert arms control negotiations into negotiable accords.

As for the Soviet Union, it extricates itself from the corner it painted itself into last November by breaking off the Geneva negotiations on European-based intermediate-range nuclear missiles. The unattainable Soviet demand for U.S. withdrawal of newly deployed Pershing II missiles and cruise missiles also contributed to halting the companion U.S.-Soviet talks on limiting intercontinental strategic missiles.

Exactly how the Soviet demand for removing the Pershing and cruise missiles will be finessed is yet to be resolved. But the stage has been partially set by labeling the projected round of "umbrella" negotiations in Geneva as "new negotiations."

The administration also is holding out the prospect of considering "mutual restraint" on deploying further new nuclear missiles in Europe. Blocking more Pershing and cruise missiles is a major Soviet goal.

Furthermore, by opening the new Geneva talks to negotiations on space weapons, the administration moves into at least potential bargaining range for what has become the prime Soviet objective, an attempt to forestall Reagan's Strategic Defense Initiative -- the development of a "Star Wars" defense. That plan, which could revolutionize the concept of basing nuclear deterrence on offensive weapons, will not be abandoned by tradeoff, the president has said, but the possibility of some form of moratorium on it has been deliberately left open.

Collectively, therefore, it is not surprising that the Soviet Union, which has experienced three convulsive changes of leadership within two years and increasing strains on its resources, finds it advantageous to make a new beginning on arms control negotiations.

In launching that process, neither nation has forsworn its intense competition against the other's interests elsewhere. Before Shultz and Gromyko meet in Geneva, the North Atlantic Treaty Organization will hold its annual round of December meetings of defense and foreign ministers in Europe.

The western leaders are bound to welcome the new U.S.-Soviet negotiating venture. But U.S. policy-makers also know that will bring the United States under renewed pressure from concurrent Soviet attempts to exploit divisions in the allied alignment, an inescapable price for superpower negotiations, whether symbolic or substantive.