Actual and Virtual Energy: From Aristotle to Rankine and Back Again

Paris, France - June 14 2021: Foucault pendulum moving in the Pantheon de Paris. It was conceived by Leon Foucault in 1851 as an experiment to demonstrate the Earth's rotation.

When energy is ‘potential’ where or when is it? If potential energy is not also already in some sense ‘actual,’ existing in the world somewhere as energy, for example, as a lump of coal that might be burnt, then does it only come into existence when it becomes actualized, as Aristotle might write? A quick review of the historical roots of the idea of energy, as well as the differences between kinetic and potential energy, may help us answer this question.

The origin of the scientific concept of ‘energy’ is traceable to the experiments conducted in the early 19th century by physicist Joule who proved that heat can be converted into motive power. This validated the claims of mathematician and philosopher Leibniz that something called ‘living force’ (vis viva) is conserved when, for example, heat is used to boil water. The first use of ‘energy’ in the more or less modern scientific sense was in Young’s 1807 ‘Course of Lectures on Natural Philosophy and the Mechanical Arts.’

Here, Young uses ‘energy’ to specifically refer to Leibniz’s vis viva. However, since vis viva’s mathematical formula was slightly different to Leibniz’s and because ‘energy’ seemed to be that very substance which put ‘force’ into action (it also meant something very specific in classical mechanics), ‘energy’ came to mean that quantity which conserved across physical transformations; for instance, how the energy in coal (chemical energy) when burnt transforms into the energy of heat (thermal energy) and can then boil water (kinetic energy) which, through expansion of the water, can then push a piston (mechanical energy).

Through all these transformations of energy, the same amount of energy is conserved. As Rankine writes, ‘There can be no doubt that the word ‘energy’ is especially suited to mean something like ‘the power of performing work’ for not only does the meaning to be expressed harmonise perfectly with the etymology of ἐνέργεια, but the word ‘energy’ has never been used in precise scientific writings in a different sense; and thus the risk of ambiguity is avoided.’

Other than energy’s philosophical roots in Aristotle’s energeia, which means something like ‘in activity’ or simply ‘reality,’ energy, prior to the 19th century, had meant ‘strength,’ ‘vigour’ and ‘force of expression.’ In Thomson and Tait’s 1862 article ‘Energy,’ published in the evangelical Scottish periodical Good Words (the first attempt made to introduce the new scientific sense of the word to the general public), the authors apologise in their opening paragraph to any reader that was expecting an article dedicated to ‘manly sports.’

Although energy can take many forms, transforming from chemical (coal) to thermal (a burning furnace), from thermal to electromagnetic (electricity), its modality, is split into ‘potential’ energy and ‘kinetic’ energy. ‘Potential energy’ and ‘actual energy’ predated ‘potential’ and ‘kinetic’ and were first introduced by Rankine in 1853. Despite the trope that Aristotle’s energeia (ἐνέργεια) has no connection to ‘energy,’ Rankine’s definitions of ‘potential’ and ‘actual’ energy in fact draw on the well-known philosophical dichotomy between potential and actual reality in Aristotle.

What is interesting in Rankine’s original classification of energy as ‘potential’ and ‘actual,’ is that ‘actual energy’ is understood as ‘a measurable, transferable, and transformable affection of a substance, the presence of which causes the substance to tend to change its state in one or more respects.’ Actual energy, is placed prior to ‘potential energy’ since it is understood as cause of ‘potential energy,’ which, for Rankine, ‘is measured by the amount of a change in the condition of a substance, and that of the tendency or force whereby that change is produced.’

An example of potential energy, using Rankine’s definitions, would be the lifting of a weight up some height (1 metre) against gravity, and the potential that this weight now has to fall (this is now the measurement for 1 joule of energy, the standard unit of energy used). Different to kinetic energy, which is the energy of movement, actual energy for Rankine would be something similar to what is now called ‘internal energy,’ the total energy that a body or system is possesses, for example the internal energy of a heated gas is the sum of all the moving gas particles added together.

It becomes potential energy only once this energy enacts change. If, for example, heated gas (that has a temperature higher than 100°C) is put in contact with water at room temperature, only then does the water boil! Rankine’s ‘actual energy’ differs only in relation to the work it performs (the boiling of the water). Actual energy is placed prior to potential energy for Rankine because the water can boil only if the gas is already hot, a different order from the more widespread model where potentiality, which could be defined as meaning something that is not yet in existence but could be, is usually placed prior to actuality, for example, a potential brick is not yet an actual brick. Its existence in the clay is still only potential; this clay could become a brick.

Considering Joule’s equivalence between heat and mechanical work and Carnot’s observation regarding the unidirectional flow of heat (heat only moved from hot to cold, what comes to be known as Carnot’s principle and then ‘entropy’), we could say that the actual energy of a system can potentially become mechanical work only once two different systems with two different ‘actual’ energies are put into relation.

In the example above, the hot gas and its actual energy is put into relation (placed next to) the cold water and its actual energy (the energy of all the moving water particles). The mechanical work of boiling was possible and becomes actual only once the water is placed next to the hot gas. While the water was not yet boiling, the water and gas already exist, they did not come into existence only once they were placed into relation. In other words, the energies of the gas and the water were already actual. This is why, as Rankine points out, Carnot had named such a modality ‘virtuelle’ and not potential.

Virtuality, instead of meaning not yet actual, describes the reality of potential relations or configurations: the not yet placed into relation of two actual energies. In order to respond to the question asked in the opening paragraph about the when and where of potential energy, we might say that energy is only ever actual; it is always in existence or ‘in activity.’ This is why Rankine writes that energy ‘harmonises’ with Aristotle’s ἐνέργεια.

However, actual energies can become potential energies and these potential energies can be put to work. To complete the picture, Aristotle’s energeia, whose meaning, as stated above, is ‘in activity,’ should be distinguished from its conceptual sister entelechia (ἐντελέχεια). Entelechia is activity with a purpose, a teleology, a goal. Using the example of boiling water, we could say that the cold water and the hot gas are actual, their relation is virtual, the heating of the water is potential, and the water boiling is the entelechy of this potential. It is the completion of a particular purpose. Without purpose, energy is only ever actual and or virtual.

Joel White