Relations between the superpowers have followed Korean Air Lines Flight 007 in a downward nose dive, and it is not yet clear when they are going to level off.

This is one of the few assessments to be shared by both sides as the East-West war of words over the downing of the South Korean airliner by the Soviet Union enters its fourth week. Soviet and U.S. officials now say that it will take many months to get their dealings back on a stable course, let alone to improve relations.

The plane incident and its aftermath have effectively destroyed the first stirrings of detente to emerge since the crackdown in Poland in December 1981. They also have encouraged each side's worst opinions of the other.

Americans are confirmed in their belief that the Russians are barbarians who place a very low value on human life and cannot admit a mistake.

The Soviet analysis, meanwhile, is that President Reagan has used the tragedy to step up his efforts to gain worldwide military superiority. Soviet officials charge that the U.S administration has mounted a colossal propaganda offensive to help push MX missile plans through Congress and persuade European public opinion to accept the deployment of U.S. medium-range nuclear missiles targeted on Soviet cities.

Fortunately, there is a kind of self-correcting mechanism built into the U.S.-Soviet relationship that prevents a total rupture. Each side knows that the other has the capacity to blow the world apart. The signals may be bad, but both Moscow and Washington have said they will continue arms control talks in Geneva.

The question arises, however, whether these negotiations can be separated from the overall political climate. Western diplomats here acknowledge that the effect of the plane incident is to strengthen the position of hawks in the Reagan administration who do not want to do any business with the Soviets. It is also likely to become more difficult to sell an arms agreement to Congress.

"This incident is going to affect East-West relations here for a long time to come, mainly because of the way the Soviets are handling it," said one western ambassador here. "They're giving us nothing with which we could argue for a change in the U.S. approach to the problem."

It is more difficult to gauge the impact of the plane incident on the internal policy debate in the Soviet Union since the political process here is largely hidden from view. One has to make deductions on the basis of offhand remarks by Soviet officials and journalists, the Kremlin's propaganda line and knowledge of how the Soviet system works.

One senior Soviet analyst on East-West relations predicted privately that there could be a hardening of attitudes in the Soviet military establishment to the idea of an arms agreement with the United States.

"You have to understand our military people. For years they've felt as if we're lagging behind the United States in the military field, and it's only recently that we've acknowledged that we have parity," he said. "It's difficult for them to change from trying to catch up with America to agreeing that both sides should cut back."

It has been noticeable that military leaders such as Marshal Nikolai Ogarkov and his deputies have taken the lead in presenting the Kremlin's point of view during the controversy over the plane episode. The sudden spate of statements by senior military officials contrasts with their reticence just a few years ago on controversial issues.

Western diplomats here say pushing the military to the front of the stage could in part be a tactical ploy by the Kremlin to distance the civilian leadership from the decision to shoot down the plane. Hints about the need to corral the military into accepting any arms agreement can also be viewed as a subtle way of encouraging the United States to make concessions, in the same way that American negotiators sometimes find it useful to conjure up an image of a recalcitrant Congress.

Whatever the shades of opinion within the Kremlin, there is one point on which the entire leadership seems to agree: the need not to show any weakness when under political pressure from the United States.

The Kremlin has made it clear that it is prepared to continue trading allegations for as long as Washington wants. A recent article in Pravda "exposing" the use by the United States of civilian airliners as spy planes concluded with the blunt warning: "If these facts are not enough, others may be found."

At times Soviet propagandists seem more concerned with demonstrating the futility of charges and countercharges than really persuading the rest of the world to accept their version of the incident.

Needless to say, each side publicly insists that the initiative for a cease-fire in this war of words must come from the other.

In view of this acrimonious background, both U.S. and Soviet officials are now expressing greater pessimism about the chances of success at the arms negotiations in Geneva. It is widely accepted that a meeting between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko is necessary to break the deadlock in the strategic, or long-range, missile talks, and there seems little prospect of another such meeting taking place in the near future.

"If you leave negotiations like these to the experts, nothing is ever going to get settled," a Soviet official said. "What they need is the political impetus from the top to get them going."

U.S. officials counter by saying that the Soviet Union is trying to frighten Western Europeans by raising the prospect of nuclear confrontation. They seriously doubt whether Moscow is prepared to accept a compromise in the talks on medium-range nuclear missiles, also at Geneva, that would allow some level of U.S. deployment in Europe.

Soviet policy makers seem certain that Reagan will be reelected if he decides to run, and their assumption is that he will run. But they calculate that the dispute over the plane will eventually blow over, just as time has taken the edge off Soviet actions in Czechoslovakia, Afghanistan and Poland.

Most American analysts accept that there will be a swing back to improved relations between Moscow and Washington, but they refuse to predict when this might happen.

"Incidents like this are going to continue to happen, and we're going to continue to be adversaries because that is in the nature of our different political systems," a U.S. official here commented. "The trick is not to pretend that we are becoming friends but just to establish a more stable relationship between two powers, each of which has the ability to blow the whole world apart."