A perpetual motion machine is a hypothetical device designed to operate indefinitely without any external energy source. It would continuously perform work or produce energy, appealing to “free energy” enthusiasts. Established physics asserts that any machine claiming perpetual motion is fundamentally impossible. The failure of these designs is not a matter of engineering difficulty but an outright contradiction of the most fundamental laws governing the universe.
The Conservation of Energy
The impossibility of a perpetual motion machine is rooted in the law of energy conservation, which states that energy can neither be created nor destroyed. This principle explains why Perpetual Motion Machines of the First Kind (PMM1) cannot exist. A PMM1 attempts to produce more energy output than the energy it takes in, or to produce work from no input.
This concept directly violates the relationship between energy input and output in any closed system. For a machine to run, it must convert some form of energy—chemical, gravitational, electrical, or mechanical—into the mechanical work of its operation. A PMM1 would require a mechanism to spontaneously generate energy, which is a physical impossibility.
Any device claiming to run itself, such as a wheel using moving weights, must eventually stop. This is because the energy required to lift the weights equals the energy gained from their descent, resulting in zero net energy gain. Therefore, any machine designed to perform work must draw on an external source to replace the energy it expends, making continuous self-operation impossible.
The Impossibility of Perfect Efficiency
Even if a machine did not attempt to create energy, it would still fail due to the universal principle of entropy, described by the Second Law of Thermodynamics. This law explains why Perpetual Motion Machines of the Second Kind (PMM2) are impossible. A PMM2 would be a machine that is 100% efficient, converting all available energy into useful work without any loss.
In any real-world process, useful energy is inevitably converted into a less-useful, degraded form, most commonly heat dispersed into the environment. This degradation is known as an increase in entropy, which measures disorder or the unavailability of a system’s energy to do work. The second law mandates that the total entropy of an isolated system can never decrease, meaning every physical process is irreversible.
Manifestations of this entropy increase are seen in unavoidable energy sinks like friction and air resistance. When parts of a machine rub or move through the air, their mechanical energy converts into thermal energy (heat), which radiates away and cannot be recovered. Furthermore, 100% conversion of heat into work is impossible, as a fraction of thermal energy must always be rejected to a colder reservoir, even in heat engines. This constant energy loss means that any machine, no matter how well-engineered, will eventually slow down and stop.
Why the Idea Persists
The idea of a perpetual motion machine has persisted since the Middle Ages, with designs like the overbalanced wheel appearing in the 13th century. This centuries-long fascination stems from the powerful allure of “free energy” and the promise of endless, costless power. The concept represents a complete and self-sustaining system that appears to defy the limitations of resource consumption.
Many historical attempts and modern claims have been revealed as hoaxes or simple misunderstandings of physics. For instance, some devices appear to run for long periods by drawing power from unacknowledged external sources, such as changes in barometric pressure or ambient temperature. The ongoing appeal often exploits a lack of understanding regarding the strict physical laws that govern energy and efficiency.
The desire to harness seemingly ceaseless natural cycles, such as tides or planetary orbits, fuels the search for perpetual motion. However, these phenomena are not true perpetual motion machines; they are driven by external forces like the Sun’s energy or universal gravitation. The US Patent and Trademark Office has long refused to grant patents for perpetual motion machines unless a working model is provided, reflecting the scientific consensus that these devices cannot be built.