Debunking Perpetual Motion

Joshua Yoon
December 7, 2017

Submitted as coursework for PH240, Stanford University, Fall 2016


Fig. 1: Overbalanced wheel which was one of the first recorded physical attempts to create a perpetual motion machine. (Source: Wikimedia Commons)

Albert Einstein, the man behind General Relatively and the photelectric effect, once purportedly said "imagination is more important than knowledge." For many this was a source of inspiration for insights that led to major discoveries over the last hundred years, but for others it was evident they took this a bit too far. Various scholars during the Middle Ages were fascinated with the idea of a perpetual motion machine possessing an uncanny ability to perform work indefinitely without any additional work. Their drawing display their early attempts towards obtaining an unlimited source of energy which would be sustainable and used for performing simple tasks. [1] Naturally, it became evident that as time passed on, these "inventions" were not what they had set themselves out to accomplish and largely became known no more than intellectual curiosities by the scientific community. However, with every proposed perpetual motion machine comes a challenge posed for scientists to disprove its ability to perform as originally described. [2] Here we highlight a few important examples of perpetual motion devices which continue to inspire us to think critically of its flaws and ultimate fallacies.

Overbalanced Wheel

Let's first take a concrete example to illustrate what a machine operating under perpetual motion would potentially look like. Many early examples utilized using a wheel that would continue to spin indefinitely. One of the earliest devices on record is known as Bhaskara's wheel or the "overbalanced wheel" where it utilized the fact that weights were not equidistant from its axis of rotation and in effect would take advantage of the imbalance of torque. [3] More mass is kept continually on one side of the axle while there is also greater torque as well and as each weight shifts to a larger radius, there is an impulse which is designed to sustain rotation. However, there are moments when in fact the torque will be balanced, which means that perpetual motion is unattainable since we can't guarantee that the axis is frictionless.

Maxwell's Demon

Fig. 2: Maxwell's Demon thought experiment which generated a lot of interest in how information is related to energy. (Source: Wikimedia Commons)

Rather than building a perpetual machine, James Clerk Maxwell came up with a thought experiment titled "Maxwell's Demon". [4] In this experiment, there is an enclosed box partitioned into two smaller rooms by a wall in the middle. In the beginning, there are gas molecules roaming around with different velocities and are equally distributed. There is a "demon" in the middle that opens the door in the middle, allowing gas molecules to pass through when it so desires. The "demon" chooses to let the gas molecules that are speedy into one side while the slow moving ones are guided to the other side. At some point, one side of the box is hotter than the other, which means that the overall entropy of the system has decreased. The 2nd law of thermodynamics states that the total entropy can never decrease over time for an isolated system. Maxwell's Demon, however, seems to have violated that principle and it seems as if it's now possible to extract energy from this setup. Various people have pointed out that in order for this to happen, the "demon" must be able to measure, store, and erase information. When it undergoes these steps, then the overall entropy of the demon must have increased, which is larger than the amount of entropy that has decreased in the box.


Based on a few examples out of many, it is clear that there are ultimately flaws in how they operate and our initial understanding of the system. But it's clear that some good can come out of very outlandish but creative scenarios. They force us to think about the very fundamental ideas of physics and challenge our own intuitions of the world. So, are people who pursue the idea of perpetual motion then foolish? Are they in for a wild-goose chase? Or is there something they know that we don't? Curiosity can drive some to do wonderful things, but it can also produce madness from others. Either way, let us admire our ability to dream of things that never were and let our imagination take us to new places never been.

© Joshua Yoon. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] D. Tsaousis, "Perpetual motion machine," J. Eng. Sci. Tech. Rev. 1, 53, (2008).

[2] C. Wadlow, "Patents For Perpetual Motion Machines," J. Intellect. Prop. Law Practice 2, 136 (2007).

[3] A. Jenkins, "Self-oscillation," Physics Reports 525, 167 (2013).

[4] J. D. Collier, "Two Faces of Maxwell's Demon Reveal the Nature of Irreversibility," Stud. Hist. Phil. Sci. 21, 257 (1990).