The conundrum of perpetual motion
perpetual motion

April 6, 2021

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It is no surprise that the 19th  century was a time of scientific breakthroughs and wondrous inventions. However, for the people of Philadelphia, this meant that for only one dollar they could admire the most amazing machine in the world: a machine that stayed in motion forever without any external source of energy. The inventor of this device was a gentleman called Charles Redheffer, and his invention attracted a lot of spectators from all over the state. His machine stayed in motion due to two weights which, using gravity, pushed a large horizontal gear away from the shaft of the machine. Underneath the large horizontal gear, Redheffer had placed a smaller gear, both gears were able to rotate independently from each other, but were interlocked using a third gear. Therefore, the motion of the large horizontal gear powered the smaller gear. The result was a gravity-driven machine that kept on turning forever, leaving the audience bewildered and amazed. It truly seemed like the impossible quest for endless motion had come to an end. Unfortunately,  even though Redheffer might have been a cunning man who had fooled his audience, he could not outwit the universe, and his machine was eventually exposed for what it truly was: a hoax.

Charles Redheffer’s perpetual motion machine

But Redheffer hasn’t been the only one trying to create a device that stays in motion forever. For ages scientists and inventors have been trying to create a so-called perpetual motion machine. The first sketch of such a mechanism dates back to the 12th century and was made by an Indian mathematician named Bhaskara. He sketched a wheel with curved reservoirs which would be partially filled with mercury. The idea was that due to the shape of the reservoirs one side of the wheel would be perpetually heavier than the other side, leading the wheel to roll endlessly. However, this first attempt at a perpetual motion machine failed, since the flowing mercury would shift the center of mass downwards causing the wheel to stop turning and simply swing back and forth like a pendulum until eventually, it would just stop moving.

Bhaskara’s wheel

In fact, a truly perpetual motion machine has never been designed and to our knowledge never will be. The simple reason for this is that it is impossible. To our knowledge, a perpetual motion machine would violate the laws of thermodynamics, the branch of physics that describes the relationship between different forms of energy. It is actually common to classify perpetual motion designs using the specific law that they break. Most designs of perpetual motion machines from the middle ages to the nineteenth century break the first law of thermodynamics, which states that energy can neither be created nor destroyed. Hence, the total energy present in a process will always stay the same, it only ever changes in form.

There have been many alterations to the overbalanced wheel from Bhaskara, using water, balls, or even pins. However, none of them was able to solve the problem of the downwards moving center of mass. Therefore, many other attempts to self-powering machines have been made. Physicist and philosopher Robert Boyle examined for example the possibility of a self-watering pot, while other scientists looked at mechanisms such as self-blowing fans or self-pumping water pumps. Another popular method was to use magnets. A magnet at the top of the ramp would attract a ball at the bottom, but right before the ball reaches the top it would fall into a hole leading back to the bottom of the ramp, starting a vicious circle. However, all these ideas do not work since the force that drives the moving parts to defy gravity (for example a magnet), would prevent the moving parts from falling down again. For all these machines to keep on moving, additional energy is needed to nudge the device beyond its stopping point and therefore breaking the first law of thermodynamics.

John Wilkins’ ramp

The first law of thermodynamics also immediately rules out any useful perpetual motion machine. The dream that we might one day draw energy from such devices will always stay exactly that, a dream. Of course, the fact that a perpetual motion machine won’t be useful doesn’t mean we should immediately stop in our pursuit of perpetual motion. There are still a lot of possibilities left to explore. Unfortunately, the laws of physics – again – put a stop to such optimistic thinking. In the case that a perpetual motion machine doesn’t violate the first law of thermodynamics, it will almost surely violate the second law of thermodynamics. The second law states that the entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. Entropy is essentially the measure of disorder and randomness in a system. The second law of thermodynamics therefore essentially entails that energy tends to spread out, and hence there is almost always some energy in a perpetual motion machine that is wasted.  Take for example the orbital motion from the planets. The orbital motion from the earth around the sun seems to resemble perpetual motion. However, when planets rotate around their common center of gravity, they emit gravitational waves and these waves slowly drain energy from the orbital motion. However, planets may appear to be perpetual motion systems, because the time needed before any changes in the orbital motion are observable is enormous.

Hence, to prevent energy from leaving the process a perpetual motion machine should satisfy a few requirements. Firstly,  the machine should not have any rubbing parts, because the friction between these parts will generate heat, and will therefore leak energy out of the system. Secondly, the machine should be operated in a vacuum, since even the friction between the machine and air will result in energy losses, even though these are very small. Finally, any perpetual motion machine may also not make any sound, since this is also a form of energy. In the end, with the knowledge that we have today, building a perpetual motion system is simply impossible. However, this doesn’t mean the quest for such a machine is pointless. Take Redheffer for example, he was still able to earn thousands of dollars from his so-called perpetual motion machine and even after he was exposed,  continued his scam in New York. The simple reason for this is that people were, and always will be, curious and intrigued by the notion of perpetual motion, and the search for a machine that might fulfill this wish for never-ending motion will always keep scientists and inventors busy. 

This article is written by Fenna Beentjes

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