We were doing some family errands in northern New Jersey, driving on a road bearing east toward New York. The traffic seemed unusually slow. Then we saw the highway warning sign: Bridge Closed Seek Alternate. There is no alternate to the George Washington Bridge, we said. What's going on? Then we turned on the radio. The attached photo is from a highway overpass many miles away from the city. A thick plume of smoke fills the sky above the World Trade Center seen from New Jersey WTC burning before it fell

After an event of this magnitude the shock and disbelief are so great that a period of mental paralysis sets in. But when the paralysis lifts we ask how and why.

The amount of damage the United States -- and, by the domino effect, the world -- will suffer as a result of Tuesday's attacks is probably incalculable, both in economic and human terms. The injury caused by this kind of terrorism is so great that prevention is a necessary strategy; retribution is insufficient.

The question then becomes, what are the most cost effective ways to reduce the risk of terrorist acts? The costs must be measured not only economically, but in terms of their impact on our freedom.

One strategy that should be followed is to avoid the concentration of assets that made the World Trade Center such an irresistible terrorist target. We have the technological ability, through networking, to radically decentralize our economic infrastructure. This should be done, but it will take years to accomplish. Moreover, it involves the reversal of some current trends, such as the well-publicized insistence of venture capitalists that e-business startups locate in Silicon Valley, thereby creating another highly vulnerable node.

We must explore other areas of high vulnerability. Both terrorists and computer hackers seek situations of astounding leverage, whereby a relatively small effort can produce an enormous effect.

Perhaps a cost-effective measure to reduce airplane hijacking would be the redesign of aircraft to isolate the cockpit from the passenger cabin.

In terms of the human perpetrators, there is no effective way to prevent terrorists from obtaining access to means of destruction without constraining the personal liberties of law-abiding citizens. Probably one of the most effective long-term measures would be a system of national identity checking. This has been political anathema in the United States for a long time; yet it is widely practiced around the world. Moreover, non-governmental record-keeping has already eroded much of the privacy that politicians fear to invade. We are in the unenviable situation that only the underclass is unknown. More serious than the loss of privacy would be the threat against free association. Yet international terrorism takes advantage of free association to plan and execute crimes against humanity.

In the United States we have a bad habit of seeking short-term solutions to long-term problems. Even as we write this, many leaders are clamoring for prompt retribution. We hope that saner minds will understand that there is no effective retribution for these deeds. That does not mean we feel those responsible should not be apprehended and brought to justice.

Another immediate reaction has been for increased intelligence efforts. But Western intelligence is virtually useless in penetrating terrorist organizations that repudiate all Western values. If an intelligence effort has any chance of success, it is only an international anti-terrorist alliance; an alliance which is virtually world-wide. Will the wave of worldwide sympathy for the United States as victim produce significant changes in worldwide antiterrorist efforts? Time will tell.