The Bulletin of the Atomic Scientists celebrated the signing of the Intermediate-range Nuclear Forces treaty last year by turning "the clock back to six minutes to midnight," a step back from the brink of apocalypse. The symbolic clock, the anti-nuclear publication said, had previously been even closer to nuclear destruction: at three minutes to midnight. But on the eve of the fifth meeting between President Reagan and Soviet leader Mikhail S. Gorbachev, experts differ on whether the INF pact actually made the world a safer place to live. President-elect George Bush, who will participate in the Reagan-Gorbachev meeting, hopes to expand on the INF treaty as he eventually engages Gorbachev in negotiations.