There is a Saudi Arabian joke that begins by asking, what is Riyadh's preferred price for a barrel of oil at the end of each of the next five years? The answer is $100 and $100 and $100 and $10 and $100. It usually takes people time to work out why this should be funny. The answer lies in the $10 price in the fourth year.

Oil producers, and not only Saudis, have learned that the best thing that can happen to a high oil price is that it should collapse every once in a while. An oil price collapse neatly bankrupts all those investors who bet their shirts on alternative energies like wind power and shale sands and biofuels. Governments of oil-importing nations abruptly cut off their subsidies they have been paying for alternative energies.

And then the oil price can bounce back up again, having wiped out the competition. This is what happened in the late 1970s, and again after the oil price collapse of 1985 (the one that derailed Mikhail Gorbachev's perestroika and helped bankrupt the Soviet Union). And it may well happened again in the coming years.

The question is whether the oil exporters can play that trick again, now that the North American and European and Japanese oil consumers are no longer the only giant markets. Much of the recent oil price rise can be traced directly to the soaring numbers of vehicles on Chinese and Indian roads, and that demand is likely to continue, keeping the price high even if the oil producers might like it to plunge from time to time.

But there is a new factor at work. The technology of battery power, and of biofuels, not to mention wind and wave power, looks close to reaching the critical mass that man's investment will continue even if the oil price collapses. The new A123 batteries developed in Massachusetts, a modification of the lithium-ion system, can deliver massive amounts of power, sufficient to accelerate a sports car and take multiple refills while being safe in crashes. General Motors is planning to install them in its new Volt electric cars and now Renault-Nissan wants them.

Wind power in urban settings is suddenly starting to look practicable, with the new generation vertical axis windmills produced by firms like Britain's Quietrevolution and America's PacWind technology. Google has now joined BP in investing in BrightSource Energy, a solar thermal company with an electricity generating contract with the giant Pacific Gas and Electric utility.

If the oil price drops to $25 a barrel next month, all these technologies and their investors will be in trouble. But the futures markets show the oil price staying high for months to come. The longer it stays high the closer comes the tipping point to alternative energy at which the Saudi joke stops being funny, even for major oil exporters.