BEYOND the suburbs of science dwell all manner of amateur inventors, would-be Einsteins, saviours of the planet and other well-meaning crackpots. The one thing they invariably share is a fascination with science and a less than rigorous grounding in its fundamentals. A good number seem bent on bequeathing some marvel of engineering that can generate an endless supply of free energy—in short, a perpetual-motion machine.

Nowadays the United States Patent and Trademark Office (USPTO) refuses to review a perpetual-motion machine unless it is accompanied by a working prototype. In the past too many inventors had used patents to convince gullible investors that their machines had been “officially approved” by no less an authority than the USPTO. Other such tinkerers are sincere and often genuinely puzzled by the way their devices appear to work. As a rule, however, there's a belief that, somehow, the laws of physics do not apply to their particular contraption.

A year ago an Irish company called Steorn captured headlines around the world after running a whole-page advert in The Economist, claiming to have developed a machine that generated free energy. Recently Steorn's founder, Sean McCarthy, offered to demonstrate the device to the public. A ten-day event, starting on July 6th, was to be held at the Kinetica Museum in London. At the last minute, however, the demonstration was cancelled, owing to a technical defect caused (ostensibly) by heat from the lights used to illuminate the machine for the cameras. A new demonstration has yet to be scheduled.

The device, called “Orbo”, is said to comprise a rotor, with powerful magnets around its outer surface, which spins within a casing that has magnets on its inner surface. Mr McCarthy admits that, if Orbo really works, then it must defy the law of conservation of energy. This law, one of the cornerstones of science, says that energy can neither be created nor destroyed, but merely changed from one form into another—from, say, heat into light, sound or motion.

Many would-be builders of perpetual-motion machines see scientific laws, like the conservation of energy, as mere dogma waiting to be overthrown by radical freethinkers. But science had its reformation centuries ago. A select number of its indisputable axioms have been designated as laws. These are not man-made rules or opinions, but universal truths prised loose from nature by painstaking observation and experiment, and hammered on the anvil of experience by repeated testing. As such, they have survived every challenge thrown at them over the centuries.

Magnets, yes. Perpetual motion, no.

None more so than the first and second laws of thermodynamics: the science of machines and the physical processes associated with them. There are actually four laws of thermodynamics. For our purposes, the zeroth and the third laws are of more academic than practical interest. Understanding how the first and second laws operate is important because they underpin so much of our everyday world, and allow us to suss out quickly what's hocus and what's not.

The first law of thermodynamics—essentially a restatement of the conservation of energy—deals with energy, work and heat. A machine needs energy to work or else it just sits there doing nothing. If it is to be useful, it can do one of two things with the energy put into it: perform work or generate waste heat—or, in the real world, a bit of both. That means the energy input has to equal the output of work and waste heat. In short, the energy of the total system is conserved.

The second law of thermodynamics is more subtle still. It is built around the concept of entropy—a measure, if you will, of how energy gets dispersed. It can be thought of as nature's instinctive tendency towards disorder. In other words, it defines the irreversibility of processes involving heat and energy.

Thus, according to the second law, a process can occur only if it increases the total amount of entropy involved. It also means that heat cannot flow from something that's cold to something that's hot—that is to say, heat flows only along an irreversible one-way street. Most crucially, the second law makes it clear that heat cannot be converted completely into work. Some of the thermal energy has to be passed on to something at a lower temperature. The implication is that any device that needs energy to work can never be 100% efficient.

By their very definition, perpetual-motion machines are required to be at least 100% efficient. Thus, to continue running indefinitely, they violate either the first law (by promising something for nothing instead of merely conserving energy) or the second law (by implying that they convert all their energy into work). And that's nonsense. Remember: the first law says the best you can do is break even, while the second law adds, forget it, you can't even do that! Keep those two principles in mind, and you'll never go far wrong.