A young, unknown physicist, George A. Keyworth, was named yesterday to be President Reagan's science adviser.

Keyworth, 42, has been director of the physics division at Los Alamos National Laboratory for three years, but was unknown outside the narrow field of weapons-related physics.

Though the budget and presumably the power of the White House Office of Science and Technology Policy have been reduced drastically under Reagan, Keyworth said yesterday, "I was told that I would have direct access to the president."

Keyworth said the job under Reagan would not be one of funneling the ideas and sentiments of science into the administration, or to initiate programs in science and technology.

"The desire of this administration is to have an adviser with scientific background . . . rather than a lobbyist or advocate for science," Keyworth said. "Science and technology underlie probably a majority of problems facing the government, so they want a science adviser as a member of a policy-making team.

"The key thing is for the science adviser to be aware of the priorities in the White House and to use his experience and abilities to help address those problems."

The two scientists given credit for helping to gain him the nomination are both considered conservative on defense -- Dr. Edward Teller, a physicist referred to as the father of the hydrogen bomb, and Dr. Harold Agnew, of General Atomic Corp. and former director of the Los Alamos laboratory.