The job of White House science adviser, diminished in importance and staff in recent years, has been taken over by a physicist with no prior experience in science politics.

With each new administration's redefinition of the job, the power and sensitivity of the office have gone up and down -- from a time when some early science advisers had easy access to the president and wielded great influence over military and technical matters, to the time when the office was abolished under President Nixon.

The new man coming in, George A. (Jay) Keyworth, 42, is a nuclear physicist who has directed the physics division at the Los Alamos National Laboratory for the past three years and has spent his career since college there. Until word of his nomination spread among scientists, he was almost unknown outside the narrow field of weapons research.

The last White House science adviser, Frank Press, had less influence on military matters than on domestic programs under President Carter. According to administration officials, Keyworth's influence will probably be the reverse of Press'.

The science establishment here and elsewhere has been nervous about the lack of science advice at the White House over the past five months, but after half-a-dozen scientists met Keyworth at a dinner recently, they were pleased.

"He is a bright, strong man, a quick study," said Philip Handler, president of the National Academy of Sciences. He said the Keyworth's lack of experience in science politics should not matter. "It will only matter if he feels unsure of himself. The rest of the science community will gather around him and be supportive while he engages in the early learning experiences."

The budget, size, and some say the clout of the science adviser's staff at the Office of Science adviser's staff at the Office of Science and Technology Policy have been reduced drastically under President Reagan -- Kekyworth is starting with a staff only about half the size of that of Press. Yet Keyworth says, "I was told I would have access to the president."

He said he would not be an advocate or lobbyist for science but would be an adviser whose background is in science and technology.

"The White House is a small, tight team. Science and technology underlie probably a majority of problems facing the government, so they want a science adviser as a member of the team," Keyworth said.

In the early years of the job, influence in defense matters was important. "You have to have someone in there to give balance and keep the military from extremes," says Jerome Wiesner, science adviser to President Kennedy. Once, Wiesner said, "when the military was promised 200 missiles, they came back and said they needed 3,000.

Wiesner said he could explain to Kennedy in technical terms why 3,000 was far too many. "I thought 400 was too many, but eventually [Defense Secretary Robert] McNamara came back with 900, which was better but more than we needed."

Others cited the Sputnik incident -- when the Soviet Union launched the first orbiting satellite and grabbed the lead in space in 1957 -- as a case when lack of a science adviser close to the president caused a policy failure.

The various agencies responsible for the space program had assured President Eisenhower that America was ahead in rockets and would put a satellite in orbit in 1956 or 1957. Outsiders tried to pass the message to Eisenhower that such a thing was impossible, but they had little effect, former science officials said.

William Baker, former head of Bell Laboratories and a member of the Reagan science task force, said that the science adviser's job is a personal matter for a president, in which the chief executive's confidence in and rapport with the adviser is most important.

Two defense conservatives, Dr. Edward Teller, who is called the father of the hydrogen bomb, and Harold Agnew of General Atomic Corp., are credited with helping Keyworth get the nomination.

Keyworth's background in defense and weapons research "will be a special asset," says Lewis Branscomb, chairman of the National Science Board and chief scientist at IBM. "The president needs someone who is knowledgeable and can ask the tough questions of the military to get the technology right. In the past, science advisers have savied the country billions of dollars by doing that."

Keyworth is frank and forceful, according to Walter Trella, a colleague at Los Alamos. George Cowan, another colleague at Los Alamos, says that Keyworth's scientific career began to get sidetracked early "into administration because he obviously had quite a talent for that. A good scientist is always tentative; a good administrator is always positive, even when he is wrong. People like Jay who can resolve that conflict and have the talents of both ar unusual."

As for his personal style, Cowan said, Keyworth is " . . . a Yalie. He comes from a sturdy New England Yankee background. I know that he spent all his spare money as an undergraduate racing his Porsche."

Keyworth was not the first candidate offered the job of science adviser. The job was first officially offered to Arthur Bueche, chief of research at General Electric, who turned it down for personal and finanacial reasons, according to White House officials.

Another candidate who could not take the job as it is defined under Reagan was Simon Ramo of the TRW corporation. A dozen other candidates were sounded out after Ramo and Bueche; among them was Keyworth, administration sources said.

Keyworth graduated from Yale in 1963, and took his doctorate in physics at Duke University in 1968.