Tomorrow evening at 8 o'clock some 4,000 people will begin filling the Masonic Auditorium here. At the exact same time in Moscow - 7 o'clock Friday morning - more than 3,000 people will gather in a similar auditorium. A half-hour later (barring technical difficulties) the two audiences will see each other's faces and hear each other's voices on the giant screens in the respective halls.
This ''space bridge'' will be the sixth live videoconference to link the United States and the Soviet Union. The elaborate $100,000 production is being staged by the Soviet government and a US peace group called World Beyond War. It's the latest in an increasing number of efforts to use the power of satellite telecommunications to foster world peace.
''It is a technology whose time has come,'' summarizes Carl Sagan of Cornell University, host of ''Cosmos,'' the popular science series of the Public Broadcasting Service (PBS).
Dr. Sagan organized a conference more than a year ago on nuclear winter, the theory that a nuclear war would dramatically and detrimentally alter the world's climate. At part of this event, a tele-conference was arranged so that American and Soviet scientists could directly discuss the technical issues involved.
''As long as you don't know your adversary, it is all too easy to demonize him. But, with a degree of direct contact, you learn that the others are not all evil or eight feet tall, but people just as we are. This puts everyone in the right frame of mind to acknowledge that we both share the same fragile planet and must make a genuine effort to get along,'' he argues.
Richard Rathbun was in the audience during the nuclear winter teleconference. An architect and former Peace Corps volunteer, Mr. Rathbun and his wife are coordinators of the World Beyond War project. ''We found (the teleconference) a very powerful and moving experience,'' Rathbun recalls.
World Beyond War is an offshoot of Creative Initiative, a nonprofit educational foundation based in Palo Alto, Calif. A tight-knit environmental organization, the foundation first received national attention through its opposition to the construction of commercial nuclear reactors in California. The group, which has an annual budget of $1.4 million, is concentrated in the West Coast but claims 20,000 active members in 17 states.
In 1983, World Beyond War began presenting an annual award to honor those who have made contributions to the cause of world peace. (Last year, the prize went to the US Catholic bishops for their pastoral letter, ''The Challenge of Peace: God's Promise and Our Response.'') In June, when the International Physicians for the Prevention of Nuclear War (IPPNW) was chosen as the recipient, ''we knew we had the perfect situation for a space bridge,'' Rathbun explains.
The IPPNW was cofounded by Bernard Lown of Harvard University and Evgeni I. Chazov, the Soviet Union's deputy minister of health. Mr. Chazov's high position in the Soviet bureaucracy helped gain essential government participation.
''From the very first, the Soviets have seemed totally receptive. Our feeling (while visiting Moscow) was that this was cleared at the highest levels,'' confirms Edmund Kyser, who is overseeing the telecommunications portion of tomorrow's ceremony.
Members of World Beyond War say telecommunications of this sort can play a vital role in achieving nuclear disarmament and world peace. ''When you cut your finger, it bleeds a while; then it heals. But psychological hurts may never heal. And, let's face it, our two governments have been insulting each other for decades. This (ceremony) is a little bit of the healing process,'' Rathbun explains.
Jack Niles, a futurist and telecommunications expert at the University of Southern California, says he believes these alternate channels of communication are extremely valuable:
''To the extent that these efforts expand communications they are positive. Videoconferencing is a wonderful tool for promoting understanding. If you read even a little history, you soon realize that lack of communication has been the greatest single cause of war.''
Yet Professor Niles adds a word of caution: Don't expect a few teleconferences to make much difference in the Soviet Union's policies. During the last 40 years, there have been repeated cases when apparent Soviet enthusiasm for increased communication with the West heightened Western expectations. So far none has led to substantive progress.
To ''rust away the Iron Curtain'' will require the application of significant and consistent pressure for decades without major mistakes, the telecommunication expert suggests. So far this amount of patience and consistency has proved to be beyond the capability of US political institutions, Niles says.
According to Rathbun, his group has been very aware of the possibility that efforts such as theirs might be used by the Soviets for propaganda purposes.
''Our position is that we do not want to do anything that will embarrass either government. There is a tacit understanding that you are using each other, but it is for something which is bigger than any system. After all, survival is bigger than politics,'' he says. They have been the initiators in this project and the Soviets have not made demands the group felt obliged to veto, he adds.
Both World Beyond War members and Dr. Sagan agree the Soviet concern about the risk of nuclear war is sincere. Evidence of Soviet sincerity is seen the prominence given to programming on the consequences of nuclear war. The nuclear winter teleconference was shown uncut on prime-time Soviet television to an audience of 100 million. So, too, was a later teleconference featuring Drs. Lown and Chazov of the IPPNW where Lown challenged the effectiveness of civil defense shelters, a longstanding Soviet government policy. In the US, these same programs were relegated to late-night slots by the handful of PBS stations that carried them. The Soviet Gosteleradio plans to give the World Beyond War ceremony equally prominent treatment, Mr. Kyser says.
The peace group's production has snowballed into a much bigger event than had been planned. Besides the San Francisco audience, the presentation will be transmitted, or ''downlinked,'' to another 3,000 people in 12 US cities including Seattle, Boston, Portland, Ore., and Austin, Texas. PBS stations in San Francisco and Denver and in Wisconsin will be carrying the event live, as will cable networks in several West Coast cities.
Since the nuclear winter conference, the number of US-USSR teleconferences being set up has been increasing rapidly, Sagan observes. It is a trend that is likely to continue. Next month, the Turner network in Atlanta will air two programs by the British Broadcasting Corporation on nuclear war, followed by a New York-Moscow link featuring two American and two Soviet scientists discussing ways to promote disarmament. And serious discussions have been going on in Congress about establishing a live telecommunications channel between Capitol Hill and the Kremlin.