A House committee has begun hearings to find out what's behind the Pentagon's plans to broaden the use of its lie detector (or polygraph) system. Defense officials testified that their aim was to check the veracity of Pentagon employees and combat espionage. Critics of the plan, however, said the real objective is to intimidate defense employees who might leak information.

But if the lie detector is as reliable as the Pentagon insists it is -- if it can distinguish truth from untruth, or even half-truths-- then why shouldn't the congressional committees start using polygraphs when holding hearings on defense questions?

It's not surprising that the legislators sometimes despair of ever getting at the true state of national security. Not long ago, Defense Secretary Caspar Weinberger went before the Senate Foreign Relations Committee to reinforce President Reagan's assertion that Russia's nuclear forces had become far superior to America's.

The chairman of the committee, Sen. Charles Percy (R-Ill.), noted that Weinberger previously had testified that the Soviets had only "begun" to build an edge of superiority. Sen. Christopher Dodd (D- Conn.) then read from Weinberger's annual report to Congress earlier this year, in which he said the United States would make every effort to prevent the Russians from "acquiring" such superiority. A patient Sen. Nancy Kassebaum (R-Kan.) was so frustrated that she shook her head and said, "Who can we believe?"

Weinberger has also been warning Congress that Russia is running short of oil, so it will soon be tempted to invade the Persian Gulf. At the same time, however, Weinberger's own Defense Intelligence Agency was telling Congress that Moscow's energy prospects are "highly favorable," and it is expected to be an exporter of oil for the foreseeable future.

One of the surprises of the Reagan administration is the conversion of Weinberger into a super-hawk, the ultimate hard- liner in a hard-line administration. He is for guns over butter, regardless of the cost: "It would be a tragic mistake" to cut military spending, he insists.

"For 10 years," he says, we have been force- feeding social programs and starving defense." He deplores substituting d,etente for the Cold War, and he warns: "The principal problem that we have in the world stems from the fact that we did cut defense in the past."

What would a polygraph make of these assertions if they were compared with Weinberger's defense of d,etente and the reduction of U.S. forces when he was ably serving as director of the budget under President Nixon?

In a 1972 symposium under the auspices of the American Enterprise Institute, Weinberger said that "even though threats continued to increase indefinitely, there is a limit to the amount that a nation can spend on security." He elaborated: "Some may feel it more important to invest money in education or health than to provide against what they consider remote contingencies in the national security field. The defense budget, in short, must be seen not only in terms of what we must defend ourselves against, but what we have to defend. The more we take from the common wealth for its defense, the smaller it becomes."

Weinberger noted that the Nixon administration had pursued not the current doctrine of "superiority" but of "strategic sufficiency," with emphasis on "maintaining a stable, credible balance." Further escalation in the strategic arms race, he explained, "would prove both expensive and ineffective in altering the strategic balance."

The initiation of SALT I talks under Nixon, he said, was an attempt to put this theory into practice. Moreover, he added, "the strategic arms treaty signed in 1972, whose purpose was to prevent a new round in the arms race, demonstrates the initial success of the new policy."

Citing the efforts of the Nixon administration to improve relations with the communist powers, Weinberger said, "It is important to realize that it is our diplomacy that has made possible the savings in the defense budget." Under Nixon and Weinberger, the U.S. armed forces were cut by 1,500,000 bodies.

Weinberger's justifiable conclusion was: "We simply could not have met, as we have, the desire of the public for lower taxes and more generous domestic programs if we had continued to expand defense programs at the rate of the previous administration."

Echoing the president, the secretary of defense now tells Congress the MX missile is just what the country needs, but the chairman of the Joint Chiefs admits that some of his fellow chiefs have grave misgivings about the weapon. Who is telling it like it is? Bring on the polygraph.

c1982, Newsday.