In the world of physics, a "thought experiment" is the most exotic form of experiment - one that requires no equipment save one's brain cells, plus the courage to imagine the unimaginable.

A recent example is work by three physicists at the California Institute of Technology in Pasadena, who pose a seemingly preposterous question: is it possible to build a machine that travels through time?Their tentative answer: maybe.

It might work if an advanced civilization could figure out how to travel through so-called "wormholes," which are infinitesimally tiny tunnels between different parts of the space-time continuum, according to an article in the Sept. 26 Physical Review Letters by noted astrophysicist Kip Thorne and his colleagues, Michael Morris and Ulvi Yurtsever.

But their time machine wouldn't resemble the elegant vehicles of science fiction that journey back and forth to the days of the dinosaurs, or the crowning of Henry VIII.

In any case, they don't plan to imitate the hero of H.G. Wells' famed novel "The Time Machine" by building a time-traveling device. Their sole goal, Thorne says, is to better understand laws of physics by speculating how far they could be pushed.

In other words, "what we're trying to do is to understand fundamental physics by thinking about a situation that sounds like science fiction," he said in a phone interview.

Indeed, their speculations are in an honored tradition of great thinkers who have enhanced knowledge by thinking, in the spirit of the Red Queen, of "at least six impossible things before breakfast."

A simple example occurred in the 14th century.

At that time humans assumed the sun "rose" and "set" because it orbited the Earth every day. But the philosopher-economist Nicole Oresme pointed out that, in theory, the Earth might be rotating, creating the illusion of a moving sun.

To those who argued humans would be thrown off a moving Earth, he posed the following thought experiment: Imagine a sailor on a boat that is slowly spinning in the breeze. The sailor sees a distant tower. As the boat rotates, the sailor could, if he didn't know any better, imagine the tower to be moving around the boat, rather than vice versa; yet the sailor isn't thrown from the rotating boat.

Oresme's thought experiment anticipated at least two key discoveries of later centuries - Nicolaus Copernicus' demonstration that the Earth orbited the sun (not vice versa) and Galileo Galilei's concept of inertial motion.

Other famous thought experiments include those of:

- Albert Einstein, the great physicist who made relativity a household word in the early 20th century. His theories of relativity (the General Theory and the Special Theory) changed the way physicists think about space and time.

- Erwin Schrodinger, the German physicist who was a pioneer of quantum mechanics, the theory that underlies our modern understanding of matter, radiation and the atom. Quantum mechanics also defies common sense because it implicates the observer in determining the outcome of physical events - for example, the path taken by photons or particles of light.

To help elucidate its mysteries, Schrodinger offered an eerie thought experiment involving a cat trapped in a box. In the quantum world, whether the cat survives or is killed by poison gas hinges partly on whether an outside observer is watching. This thought experiment is now known as "Schrodinger's cat."

In the recent article, Thorne and his colleagues base their concept of a time machine on the generally accepted idea that the universe is permeated by "quantum foam" - a sea of "wormholes."

A wormhole is a kind of tunnel that connects two parts of the space-time continuum, the underlying fabric of reality.

As Thorne puts it, "The handle in a coffee cup is like a wormhole that leads you from one part of the cup to another."

Also in theory, each wormhole is a place where time literally doesn't exist - there's no past, present or future.

And again in theory, each wormhole is incredibly small, measuring 10 to the minus 30th centimeter. The trick to building a time machine, they say, is to enlarge a wormhole so that someone could slip through it into another part of space and time.

How? They speculate about ways a future society could generate powerful gravitational or electrical fields that might widen a wormhole and keep it open long enough for time travel.

But "whether wormholes can be created and maintained entails deep, still-undefined issues," they concede.

Einstein mentioned the possibility of time travel, but dismissed it as inconceivable. The reason, he said, was that time travel posed a seemingly inescapable paradox: if someone could travel back in time, then in theory he could meet himself. So why wouldn't he "recall" having met himself earlier in his life?

By discussing time travel, "one might fear you could get into paradoxes that physics could never resolve," Thorne acknowledged.

"Many people's attitude is that there had better be something in physics that absolutely forbids you from building time machines. That's the attitude of many of my colleagues."

But, he said, "We rather strongly believe the laws of physics will insist that everything is self-consistent, that you cannot go back in time and meet yourself, that you cannot change history.

"If the laws of physics do permit backward time travel, then the laws of physics do not permit the changing of history."