Imagine a world without oil. No gasoline, no plastic, no medicine: a crisis of biblical proportions. But for some geologists, the coming oil crisis will begin not when the last drop has been consumed, but when world oil production reaches its peak and starts declining. When this occurs, prices will skyrocket, unless demand somehow miraculously declines as well.   Energy experts are hard at work trying to pinpoint when the peak might occur. As usual, the experts disagree. For the Energy Information Administration (EIA), ever the optimists, oil production will peak sometime between 2021 and 2112. Geologist Colin Campbell points to as early as 200S-yes, this year. MIT economist Morris Adelman falls somewhere in-between, contending that for the next 25 to 50 years, the oil available to the market is for al1 intents and purposes infinite.” The difference is crucial, for if the Colin Campbell of the world are correct, there will be insufficient time for an orderly transition to alternative energy sources.   The pessimists may have a better case. Many claim that the so-called oil optimists may only be tempering their estimates to avoid throwing world markets into a panic. Moreover, another factor that critics believe sheds doubt on projections such as those by the EIA is that many countries exaggerate their reserves. This means that the amount of remaining oil is probably far less than the estimates suggest. Also, pessimists point to another incidence of peak and subsequent decline: the discovery of new oil fields peaked in the 1960s and has been dropping ever since.   The coming decline in oil production, whether it occurs sooner or later, could cause a worldwide depression. At the very least, it will heighten existing tensions among oil importers. China and Japan are already in strong competition for Siberian oil, while the United States, Russia, and Iran are waging a diplomatic tug-of-war over oil-rich Kazakhstan and Azerbaijan. Those part of the world that are already a hotbed of political and social unrest-the Middle East, for example-face even greater instability as nations jostle for access to their oil reserves. We need alterative energy, and we need it now.