Moscow — Late last month in a small apartment near the banks of the Volga, an intense physicist stopped eating. He seems to have given the Kremlin an uncharacteristic case of jitters.
Andrei Sakharov will almost surely lose his battle with Soviet authorities. They seem intent on refusing his latest demand: that a young Eurasian woman named Yelizaveta Alexeyeva be allowed to join Dr. Sakharov's stepson in the United States.
But the Kremlin does not want Andrei Sakharov to die, either; particularly not now, when the Soviet Union is seeking better relations with the West.
Dr. Sakharov and his wife, who joined him in the hunger strike, have been hospitalized for ''preventive care,'' the Soviet government newspaper Izvestia said Dec. 4. Miss Alexeyeva assumes they were taken to the hospital against their will and fears they are being force-fed. This cannot be confirmed.
The physicist was banished without trial to the ''closed'' city of Gorky, some 250 miles east of here, from which foreigners are barred, about two years ago.
Miss Alexeyeva, on her way to a train for Gorky Dec. 5, was bundled into a car by plainclothes men and advised to give up that idea. At this writing, neither she nor others of Dr. Sakharov's friends and relatives had any idea in which hospital he and his wife were.
The next move, presumably, is the Kremlin's.
Unfortunately for President Leonid Brezhnev, Andrei Sakharov is no ordinary physicist and no ordinary dissident. As a prodigy in the 1950s, he helped the Soviets develop a hydrogen bomb. At the age of 32, he became the youngest full member of the prestigious Academy of Sciences. The state decorated him with the Order of Lenin, the Stalin Prize, and the title, ''hero of socialist labor.''
Abroad, he has since become the best known of dissidents within the Soviet Union.
In 1975, Dr. Sakharov won the Nobel Peace Prize, but the Soviets refused to let him go pick it up.
Within the Soviet Union, Sakharov has commanded residual respect among some scientific colleagues, but his friends fear it is waning. One academic speaks privately of an abortive effort a few years ago to oust Sakharov from the Academy of Sciences:
One speaker began an esoteric discourse, then digressed with a personal assault on the dissident scientist. Pyotr Kapitsa, the venerable Soviet physicist, is said to have taken the floor and remarked, simply: ''This reminds me of another time an academy of sciences expelled one of its own.'' The reference to German scientists' disavowal of Albert Einstein was clear to all. The matter, the story goes, was dropped.
Presumably for all of these reasons, Dr. Sakharov is virtually the only major dissident here not sentenced to do time in a labor camp.
As Miss Alexeyeva talked with reporters Dec. 6, a crowd of Muscovites gathered not far away beside the capital's Ring Road. Their interest was the ornate musical clock on the wall of the Moscow Puppet Theater.
What about Sakharov, they were asked as they drifted away once the hour had clanged. ''I don't know anything about it,'' ventured a young man, with a knowing chuckle.
An older, well-dressed clock-watcher interrupted, seriously and not unkindly: ''I am an engineer. . . . My opinion is that the question of dissidents is less major than Sakharov and some others think.''