UNDER THE MICROSCOPE: The pithy remark that there is no such thing as a free lunch is a comment on charitable social work, but it also applies in the scientific realm. One of the oldest dreams of the inventor, and one that continues to fascinate many, is the concept of the perpetual motion machine, writes Dr William Reville
In essence, a perpetual motion machine is a device that, once started, will operate for ever, with no need of energy from the outside world. No such device has ever been built. Science declares one cannot be built, because the concept violates the laws of thermodynamics and a working model is therefore impossible. This does not keep inventors from trying - and patent offices regularly receive applications to patent new designs.
Recorded attempts to achieve perpetual motion go back at least 1,500 years. A London doctor, Robert Fludd, proposed a waterwheel in 1618 that would operate without a river. Fludd's system, once started, would go on operating for eternity, at least according to his theory. Once you poured water into the system it would turn a wheel, which would power a pump, which would pump water back over the wheel, which would power the pump, and so on.
Developments in physics in the 19th century put paid to the concept of perpetual motion machines. Among the more important were the introduction of the physical concept of energy by Thomas Young, in 1807; the analysis of steam-engine efficiency by Sadi Carnot, in 1824; the establishment of the mechanical equivalent of heat by James Joule and Julius Meyer, in 1843; the first law of thermodynamics, stated by Hermann Helmholtz in 1847; and the second law of thermodynamics, stated by Rudolf Clausius in 1850. The laws of thermodynamics explain why Fludd's device will not work, but those laws were unknown in 1618.
Perpetual motion machines tend to fall into two categories: those that violate the first law of thermodynamics and those that violate the second. They are correspondingly called machines of the first kind and machines of the second kind.
A perpetual motion machine of the first kind generates more energy than it consumes, which means it can create energy out of nothing. The first law of thermodynamics, which is also known as the law of conservation of energy, says energy can be neither created nor destroyed. If the amount of energy in a system remains constant, then the energy that leaves the system must equal the energy that enters it.
The second law of thermodynamics states that physical and chemical processes proceed in such a direction that the randomness of the universe increases to the maximum possible. The spontaneous flow of heat from a hotter to a colder body is an example of this. Furthermore, energy cannot be converted into work with 100 per cent efficiency. Not all of the mechanical energy applied to an electrical generator is converted to electricity, for example, because of energy lost in friction, which is dissipated as heat.
This explains why Fludd's perpetual-motion waterwheel could never work. The friction generated by the wheel and pump would turn into heat energy and leak away into the environment, draining the limited total energy of the device and quickly causing it to wind down and stop.
You may well say that I am making too much of perpetual motion, that perpetual motion is not impossible. Do the planets not orbit the sun in perpetual motion, do electrons not orbit the nucleus of the atom in perpetual motion? It is true that these motions seem perpetual, but even here the second law of thermodynamics views the universe as a whole as incapable of perpetual motion.
Most proposals for perpetual motion machines are of the first kind; that is, their designs allow them to create energy continuously as they operate, in violation of the first law of thermodynamics. Those of the second kind do not claim to create energy, but they usually try to tap into the existing thermal energy of a heat reservoir and put it to good use.
It might be pointed out, for example, that there is an enormous amount of heat in our oceans. Could some machine power its way through the ocean by allowing water through its front, extracting heat from it, then using the heat to power movement, allowing the cold water to flow out of the rear?
In order to get work out of thermal energy, however, you must tap into it as it flows from a hotter to a colder region. The water in our oceans is in thermal equilibrium - there is no flow between hotter and colder regions that can be harnessed for work.
The first English patent for a perpetual motion machine was granted in 1635. By 1903, 600 patents had been granted for them. Patent applications are assessed according to four criteria: novelty, usefulness, "non-obviousness" and enablement (that is, the patent must reveal how to construct the device). A patent is not a certification that a device will work, and undoubtedly patents for perpetual motion machines will continue to issue occasionally from the constant stream of applications submitted by starry-eyed inventors.
William Reville is associate professor of biochemistry and director of microscopy at University College Cork