The Nobel Prize for physics was shared yesterday by two Americans and 84-year-old Soviet Pyotr Kapitsa, who defied Stalin in 1946 by refusing to work on the development of the hydrogen bomb.

Kapitsa won his half of the $165,000 prize for his fundamental work in magnetism and the physics of supercold temperatures. Removed from his job as head of the Institute of Physical Problems in Moscow for his defiance of Stalin, Kapitsa spent the next seven years until Stalin's death in 1953 teaching and writing papers on such things as the physics of hulahoops and how fast a person has to walk to walk on water.

Named cowinners of the physics prize were Arno Penzias, 45, and Robert Wilson, 42, who work at the Bell Telephone Laboratories in Holmdel, N.J. The two Americans won the award for their work in 1963 and 1964 identifying the fossil heat left behind by the "Big Bang" that created the universe more than 15 billion years ago.

'The discovery of Penzias and Wilson was a fundamental one," said Royal Swedish Academy of Sciences in announcing the awards. "It has made it possible to obtain information about cosmic processes that took place a very long time ago, at the time of the creation of the universe."

The academy also announced a sole winner of the Nobel Prize for chemistry, Peter Mitchell, 58, a British biochemist who conducts most of his research in a small laboratory built into an old farmhouse in Cornwall surreounded by 120 cows. Mitchell won for his explanation of how plants and animals converts nutrition into energy.

"It is very distracting working in one of the usual seats of learning," Mitchell said. "Being isolated down here it is easier to work more deliberately, and the success of our little institute goes to prove that small is beautiful."

Penzias and Wilson said they were surprised at their selection, which came from the discovery that the act of creation more than 15 billion years ago spread anough heat throughout the universe that some of it still lies in the space between the galaxies.

"There is a certain amount of disbelief in all this," Panzias said in a telephone interview. "You always think of Nobel Prize winners as older or smarter people, in any case people you don't know."

Penzias and Wilson have never met Kapitsa, but both said that nobody deserved the Nobel Prize more than the Russian. The bothe called Kapitsa an "extraordinary man."

An apprentice under Sir Ernest Rutherford at the Cavendish Laboratory in Cambridge University in England. Kapitsa spent 10 years in Britain, traveling back to the Soviet Union only in the summer months to visit his mother. In 1935, the Soviet officials refused him a promised exit visa on the grounds that his work was too important to be done in a foreign country.

The Soviets literally built a alboratory in Moscow for Kapitsa, which he built into a first-rate institution. Part of his citation from the Swedish Academy is that he showed an "amazing capacity to organize and lead work."

Kapitsa is best known for his work in low temperatures and the machinery he built to conduct experiments in the realm of 400 degrees below zero. The devices he built to maintain liquid helium at these temperatures is still called the "Kapitsa engine."

He will be remembered for many things," said R. V. Pound, of Harvard University, who knows Kapitsa, "but most of all he will be remembered for being an independent thinker in a country where independent thinking is not that easy."

The two Americans sharing the physics prize with Kapitsa won it for a discovery they made somewhat by accident. Using a small horn-shaped antenna in the New Jersey countryside, Penzias and Wilson set out in 1963 to find out all they could about the radiation striking the earth from space to incorporate their findings into the design of antennas of communicate with satellites.

"We did a good year's work and still didn't understand the radiation we saw between the stars." Penzias said. "It didn't vary from day to night or from season to season, and then we worked for another year in determining ther wasn't something wrong with out instrument. We finally determined it was the background radiation left over from the Big Bang."

The discovery did more than settle the question of how the universe was created. It stimulated interest in the physics of high-energy astronomy such as gamma ray astronomy and X-ray astronomy. Both these fields are at the forefront today of radio astronomy.

Penzias and Wilson published their findings in 1965 and have been working together on and off ever since.

Mitchell, the winner of the chemistry prize, was described by American chemists who knew him and his work as a man who revolutionized the field of bionergetics in 1961 by proposing a theory of how the oxidation of food in the body is turned into energy.

"He proposed that the driving force behind the conversation of food into energy was the generation of photos in the oxidation process," said Cornell University's Dr. Ephraim Racker. "He described how energy is generated in plants and animals by light, a proposal that is correct and which now allows us to understand numerous processes in the body that were never understood before.'