Perpetual Motion Machines

The Second Law of Thermodynamics describes the general constraints upon the direction of heat transfer of a system and the maximum efficiency of a heat engine. It states that in any cyclic process the entropy, a state variable that is a measure of the statistical disorder of a system will either increase or remain the same. This can be presented in equation form as,

and T is the temperature, so that the change in entropy is always greater than or equal to zero. Since entropy must never decrease, the Second Law of Thermodynamics can be interpreted as an indicator of the direction of flow of time. That is to say, if we measure the entropy or disorder of a system at two distinct times, the time at which the system is in greater disorder is the later time.

There are many physical phenomena that are often misquoted as violating The Second Law of Thermodynamics, the idea that disorder must increase over time. Life on Earth, for example, is an obvious and frequently stated one. Evolution from tiny microbes (somewhat ordered) to complex human beings (very ordered) seems to be a clear example of order emerging from chaos. However, the Second Law of Thermodynamics applies to closed systems. This means that the entropy change of every process that went into making life on Earth must be taken into account. If one does this, one sees that the decrease in entropy that comes along with forming a human being is accompanied by an enormous increase in entropy from the radiation of energy from the sun (along with other increases and decreases). So that on balance the entropy, or disorder, of the Universe actually increases.

The First Law of Thermodynamics is that of "conservation of matter and energy," which states that matter and energy cannot be created or destroyed. Within an isolated system the total energy is constant. Any change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. The Laws of Thermodynamics thus place a limit on the efficiency of a heat engine. Efficiency is, simply put, a ratio of the useful output energy to total input energy. An efficiency of 1 means that all of the energy put into the system is spat out as useful energy, with no loss through waste processes. Efficiency is capped at 1 i.e. you cannot get more energy out of a closed system than you put in.

One of the greatest obstacles facing any object in motion that wishes to remain in motion is friction. Whenever two objects are in contact, there is friction between them. Overcoming this friction force requires giving up some kinetic energy. Thus over time as the object travels it loses all of its kinetic energy attempting to overcome friction and eventually it becomes motionless. If the object were to have infinite energy stores in order to pay this friction toll and maintain its path of motion unaltered, it would violate both the First and Second Laws of Thermodynamics. An entirely frictionless system could skirt around this bothersome friction hurdle. People have suggested that placing the perpetual motion machine inside a vacuum would create a frictionless system. Unfortunately, even vacuums aren’t ever ‘perfect’ vacuums and there is always some friction there that will stop the motion eventually.

The second big, and often subtly ignored problem, with the design of a perpetual motion machine, is that in order for it to be of any use, we must be able to extract work from it. It is no use to have set up a motion that continues forever if we cannot use it to power anything, for example a motor. The energy transfer of a machine is described by the heat that is put into the machine and the resulting work that it performs. This is given by the equation form of the First Law of Thermodynamics below,

Q is the heat transfer to the system from its surroundings, and -W is the work done by the system on its surroundings. If a perpetual motion machine were to exist, Q would be zero (no heat transfer into the system) and W would be agrowing positive number, resulting in an ever-increasing negative internal energy or an infinite extraction of internal energy which we know from the conservation of energy must be finite.  

Many attempts at developing perpetual motion machines have focussed on the use of magnets. Magnets can influence the motion of objects at some distance with seemingly no input of external energy. However, the mystic and sometimes counterintuitive properties of magnetics are also governed by the same physical laws that everything else is, including the First and Second Laws of Thermodynamics.

Almost always a magnetic perpetual motion machine consists of some variation of position and geometry of static and moving magnets. There are many examples of purported magnetic perpetual motion machines that can be found easily on the internet. Each one of them can be discredited by looking carefully at the system as a whole. If the starting motion is created with a nudge of the hand or a drop through gravity this is an input of energy which will slowly but surely dissipate. Any perpetual motion machine will grind to a halt with the addition of a load needed to extract work. Both the US and UK Government officially recognise the non-existence of perpetual motion machines by automatically rejecting patent petitions for devices claiming to demonstrate perpetual motion, with the UK patent office stating "articles or processes alleged to operate in a manner clearly contrary to well-established physical laws" are "regarded as not having industrial application".

There would be far-reaching and devastating consequences if the development of a perpetual motion machine and the necessary accompanying dissolution of the Laws of Thermodynamics happened. As alluded to before, the arrow of time would cease to have a direction with which to align itself, meaning that time may travel forward or backwards or perhaps in no direction at all. A perpetual motion machine that works would be capable of making energy from nothing, violating one of the most fundamental physical laws there is, the law of conservation of energy, unpinning most of what we know about the Universe today. Perhaps, in a rather fantastical imagining, the perpetual motion machine could be run backwards. If so, it would perpetually sap energy out of the Universe turning it into nothing until there was no energy left anywhere.