Is eternal movement possible or not

Perpetual motion machine

Lexicon> Letter P> Perpetual motion machine

Definition: a machine that runs continuously without energy supply (moves forever)

English: perpetual motion machine

Category: physical basics

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 05.12.2013; last change: 02/21/2021


Various machines, such as electric motors, can stay in motion permanently, but are dependent on a constant supply of external energy. A machine that without Energy supply could move forever as would Perpetual motion machine designated. (The only temporary function, e.g. using a built-in battery, which is exhausted over time, would not apply.) Especially if it could still deliver usable energy at the same time, that would obviously be a very attractive achievement: You would have to only build enough or large enough of these machines to be able to cover mankind's energy needs over the long term. Unfortunately, according to the current state of science, it is unfortunately not to be expected that a perpetual motion machine, however built, could actually work, because otherwise this would contradict fundamental laws of nature, which, according to all available knowledge, seem impossible to violate. Allegedly functioning perpetual motion machines have often been presented - as plans (thought experiments) without actually built machines - but they have never withstood a scientific test. Either a hidden source of energy (or source of exergy) has been discovered, or it just doesn't work.

There have already been very many different approaches for developing a perpetual motion machine, which differ greatly in terms of certain basic properties. They are particularly differentiated according to which of the main principles of thermodynamics they would violate if they worked. One speaks of a perpetual motion machine of the first kind if it violates the first law of thermodynamics, and accordingly of the second or third kind (see below).

First kind of perpetual motion machine

The first law of thermodynamics essentially formulates the basic principle of energy conservation: The entire energy of a closed (isolated from the environment) system remains unchanged in all processes. So energy can neither be created out of nothing or disappear without a trace. Only energy can be converted from one form into one or more other forms.

A type 1 perpetual motion machine (see Figure 1) would therefore be able to continuously deliver energy (e.g. in the form of mechanical work) without absorbing energy in any other form. Various historical approaches for a perpetual motion machine were of this nature. For example, a water cycle should be created in which water transported downwards gives off energy, but can be brought back to the starting point with less than this amount of energy, so that there is still usable energy left over. Similar attempts have also been made many times based on solid objects. The well-known laws of theoretical mechanics (e.g. in the form of Hamiltonian mechanics) strictly rule out such a thing - no matter in which way one may try. In individual cases, however, it can by no means be trivial to uncover the respective error that causes the implementation of a proposed perpetual motion machine to fail.

Perpetual motion machine of the second kind

The second law of thermodynamics brings additional restrictions for possible physical processes, especially for the conversion of heat into mechanical energy. According to modern understanding, this is based on the principle that the entire entropy of a closed system can never decrease. But that would be the case, z. For example, if a machine in an environment with the same temperature everywhere would absorb heat from this environment and convert it into mechanical energy (Figure 2): Anergy is more or less entropy-rich energy, while exergy is entropy-free energy. Such a machine would effectively not necessarily generate energy (like a perpetual motion machine of the first type), but it would convert low-value energy (anergy) into exergy. For example, this would allow a submarine whose propulsion merely extracts heat from the surrounding water, with the propulsion energy being fed back into the water as heat at the end, e.g. B. by friction. Similarly, a refrigeration machine that even generates electricity from the heat it absorbs, a perpetual motion machine of the second kind, would be impossible.

A perpetual motion machine of the second kind would not generate any energy, but would convert low-value energy into exergy. That would be extremely useful, but based on current knowledge, it is most likely impossible.

A similar variant would be a machine that generates electrical energy directly from heat without needing a temperature gradient and giving off waste heat.

The same law would also be violated if heat were transferred from a medium with a lower temperature to a medium with a higher temperature without the need for additional exergy (e.g. as the drive energy of a heat pump). If this were successful, it would be possible to heat a house with the help of ambient heat alone, even on a dark, cold night, for example. Such a machine could also be connected to a heat engine, which would then also deliver mechanical energy.

Incidentally, a heat engine (Figure 3) with an efficiency above Carnot efficiency would be equivalent to a perpetual motion machine of the second type, or could easily be added to it, e.g. B. with a heat pump that returns the waste heat to the heat reservoir of the higher temperature.

Perpetual motion machine 3rd art

There is comparatively little material in connection with the perpetual motion machine of the 3rd kind. If one relates it to the 3rd law of thermodynamics, it would presuppose a reservoir at absolute zero of the temperature, which according to this law cannot exist.

In the literature, however, there is also the definition that a 3rd type perpetual motion machine would only move forever, but without being able to give off energy. After all, this is not completely impossible - at least feasible for a long time if a system is very well separated from the rest of the world so that it can hardly lose any energy there, and if the stored mechanical energy is not transferred to “inner degrees of freedom”, i.e. in Heat can be converted. For example, a top would in principle be able to rotate forever in a vacuum, far away from all potentially interfering objects, once it was pushed: it could not release energy to the outside, and the conversion of its rotational energy into heat would be impossible because of the conservation of the angular momentum.

A perpetual motion machine of this type is in principle possible, at least approximately; however, such a thing is not useful in terms of energy supply.

Clever tricks based on known physics, or the search for new physics?

A further distinction can be made according to whether the conceived functionality of a perpetual motion machine should be based on well-known physical laws (for example on the known laws of mechanics or thermodynamics), or whether “new physics” is postulated.

Historically there have been many approaches that wanted to use known physical processes in a clever way to generate energy. Since it has been possible for some time to prove mathematically with certainty that z. If, for example, the conservation of energy and other restrictions for all physically possible processes follow from the fundamental laws of mechanics and thermodynamics, such approaches are certainly doomed to failure. Corresponding attempts indicate ignorance of well-known relationships today.

The search for new physics, which under certain circumstances could also allow a perpetual motion machine, is not unscientific per se. However, pseudoscience is very common in this area.

On the other hand, it would in principle be quite possible that z. For example, there are forces or interactions that are still unknown today, for which restrictions such as conservation of energy do not apply. Since fundamental things, especially in the field of cosmology as well as particle physics, are so far not understood, it is at least conceivable that future physical theories will limit the scope of the law of energy conservation. There have already been observations, for example in connection with phenomena of radioactivity, in which possible explanations are being considered that would have ruled out the conservation of energy for certain processes. However, such approaches have never been able to gain acceptance and are generally met with great skepticism in science - also because very general theoretical considerations suggest the general validity of the law of conservation of energy.

Likewise, processes would at least be conceivable in which energy would be supplied to a machine in an undetectable way, for example in the form of an unknown and invisible form of radiation or through unknown microscopic processes that can tap an unknown energy source. This variant would actually only be seemingly a perpetual motion machine: a machine with an overlooked energy supply, which therefore only apparently violates the law of conservation of energy. (A nuclear reactor would be such a machine for those who do not know anything about the nuclear processes in it and also have the wrong impression that the reactor could supply heat indefinitely.) Such machines could be clearly distinguished from a machine with more actual Violation of conservation of energy (or, alternatively, a decrease in total entropy) based on new physics.

How are you supposed to use forces that you don't even know? Speculative theories about it are in principle not disreputable, but there is a lot of useless pseudoscience in this direction.

In principle, it should be quite difficult to purposefully exploit forces that one does not know; So one would have to be guided to some extent by speculative theories in this search. There are such in particular in the esoteric area, where converters for “free energy”, “vacuum energy” and the like are often mentioned in an extremely vague way (which by the way indicates the search for an apparent perpetual motion machine, with current conservation of energy for a possibly expanded energy concept). The search for “new physics”, for example in the form of new forces, is by no means to be criticized; this is also part of science. However, it does not make sense to use the typical methods of pseudoscience to create confusion - for example by mixing different meanings of terms such as “free energy” or by claiming observations that can never be convincingly demonstrated. Scientifically convincing evidence that a perpetual motion machine of the first or second type could be possible has not yet become known despite many efforts over centuries.

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See also: conservation of energy, energy, exergy, entropy, thermodynamics, main principles of thermodynamics
as well as other articles in the physical basics category