Six prominent American physicists have postponed indefinitely a planned trip to the Soviet Union to protest the imprisonment of dissident Soviet physicist Yuri Orlov.

The six physicists, all under the sponsorship of the Department of Energy, were due to arrive in Moscow two days ago to begin a three-week tour of nine nuclear physics research laboratories in seven Soviet cities. Deferral of their trip brings to 28 the number of American physicists to cancel or postpone trips to the Soviet Union since Yuri Orlov was sentenced to seven years in prison and five in exile.

"It is with the greatest regret that we have made this decision," Yale University's Dr. Alan Bromley, chairman of the six-member delegation, said in cables to the nine Soviet laboratories. "But it is our judgment that the concern of American scientists is so strong (over the Orlov sentencing) that little purpose will be served by our visit at this time."

Besides Bromley, th physicists voting to postpone their trip to the Soviet Union are Drs. Herman Feshbach of the Massachusetts Institute of Technology, Joseph Weneser of the Brookhaven National Laboratory, Gerald Garvey of the Argonne National Laboratory, Earle Hyde of the Lawrence Livermore Laboratory at Berkeley and O.L. Keller Jr. of the oak Ridge National Laboratory.

The postponement could easily become a cancellation, acknowledged the DOE's Dr. James Kane, who was coordinating the trip, especially if the upcoming trials of dissident Soviet scientists Alexander Ginsburg and Anatoly Scharansky go as badly for them as Orlov's trial did for him.

"We're using the word postponement," Kane said, "but it could turn out to be a cancellation."

The six physicists were to make up one of three U.S. delegations that visit the Soviet Union every other year under a nuclear physics exchange worked out between former president Nixon and Soviet President Leonid Brezhnev.

The delegation was set up to study the fundamental properties of matter, which it did in periodic visits to the nuclear particle accelerators in the Soviet Union where some of the leading Soviet high energy physicists work. Bromley and Kane described these visits as useful and productive.

"But this kind of cooperation can flourish only in an atmosphere of good will," Bromley said in his cables to the nine Soviet laboratories. "It is a fact that the atmosphere of good will has eroded as the result of events taking place in your country . . ."