I return five weeks abroad to find myself once again facing - in darkness and doubt - the central puzzle of the Carter administration. Nothing very bad has happened either here or abroad since mid-March.

But the president's performance rating has slipped to an all-time low in both the Harris (60-percent negative) and Gallup (46 percent negative) polls. How come?

The answer does not lie, as Carter and the aides are pleased to believe, in the "revolutionary new" approaches thaey have taken to difficult problems supposedly ignored by previous administrations of Presidents Nixon and Ford both grappled earnestly with the truly vexing issues of foreign policy: detente with Russia, peace in the Middle East, secure defense, and more equity between the rich and the poor.

Carter may have the first "comprehensive" energy program, but the biggest achievement to date was the conservation imposed by setting floors on miles per gallon in the 1975 energy bill. If Carter paid more attention to jobs than Nixon and Ford , they were a good deal more serious about inflation.

I am a friend and admirer of Bob Strauss, but the only reason to expand his duties from trade to inflation is that he has the sales patter and glad hand required for selling a program that is essentially empty. The Federal Reserve Board has reached that same conclusion, and is now enough for even a Republican administration.

Chops and changes in policy, to be sure, do work against the president's reputation. He didn't have to come out publicly for deployment of the neutron warhead. Having done so, the decision to renege made him look weak and indecisive. Similarly with the easing of disarmament demands first served on the Soviet Union, and the shift toward more support for the dollar in foreign markets, and a dozen other issues.

My own view is that most of the changes are for the better. But he may have lost control on one central issue - the Middle East - and I can see where those who expected much of the president find him hesitant and indecisive.

But that only says that the fault lies as much in the expectations of the electorate as in the actions of the administration. Undoubtedly the most illusory fo the expectations - one cultivated by Carter in the campaign - was that his would be a different administration - open, direct, honest, aboveboard and straight. That boast now looks as tarnished as the claim that Wheaties is the breakfast of champions. Since the Lance affair, if not before, most of the electorate has simply regarded Carter as just another politico.

Among the more politically active, there remains hopes and expectations which repeatedly find public expression. The observation of Sun Day this week expresses a widespread hope for solar energy.

Throughout the county - from Maine to Washington, literally - there are demonstrations against nuclear power. Another current example is the protest movement, expressed in college demonstrations and in some picketing, against American investments in South Africa.

Behind those well-meaning actions, however, there are only illusions. Solar energy cannot make an important contribution to this country's needs until the end of the century. Nuclear energy will be required in big doses, and the problem is not to ban it but to make it safe. As to Africa, The United States will be lucky if it can prevent a civil war in Rhodesia and head off further Soviet and Cuban penetration in Ethiopia. Cutting down American investment in South Africa would have, as almost its sole consequence, making the unhappy lives of the blacks there a little more unhappy.

What all this says to me is that Carter is not going to solve his difficulties by a few fixes in White House staff operations. His problem is a far deeper issue in the climate of opinion - the spirit of the times. Interest has been pulled from public to private affairs, and there is no majority - certainly not one ready to make sacrifices - for the polices the administration keeps serving up week after week, day after day and almost hour after hour.

Nothing I can see on the horizon seems likely to change that prospect. I doubt there will be either a war or a depression, or anyting else very bad. But it is hard to see how the president can regain control over events, or even the momentum that would make a second term a sure thing - the more so as he seems to have the presidency in the first place, the faculty for brilliantly reading the public mood.