ENERGY. - In general it means activity, action, power, force. In its technical meaning, the concept, specifically defined by science, appears only recently.
Aristotle in Met., IX, 8, 1050 a 22, determines the notion of ἐνέργεια in relation to ἐντελέχεια. Ἐνέργεια (ἐν ἔργῳ εἶναι) indicates activity or actualization, while ἐντελέχεια (ἐν τέλει ἔχεια) indicates the actuality or perfection that results from activity, that is, the form: τοῦτο ἐνέργεια λέγεται κατὰ τὸ ἔργον καὶ συντείνει πρὸς τὴν ἐντελέχειαν (Met., IX, 8, 1050 a 22; cf. St. Thomas in lume loc.). However (H. Bonitz, Index Aristotelicus, Berlin 1870, 253 b, 46 agg.) Aristotle generally uses ἐνέργεια and ἐντελέχεια as synonyms, to indicate act or perfection (Cf. W. D. Ross, Aristotle's metaphysics, II, Oxford 1948, p. 245). Ἐνέργεια, act (generally said also of motion, κίνησις; however in Met., IX, 6, 1048 b 18-36 κίνησις is opposed to ἐνέργεια, as imperfect act to immanent, perfect activity) is furthermore conceived by Aristotle in opposition to δύναμις, which is capacity, potency, in order to explain change (Phys., I, 8, 191 b 28; cf. Met., IX, 6 agg.): every passage occurs from δύναμις to ἐνέργεια: μεταβάλει πᾶν ἐκ τοῦ δυνάμει ὕντος τὸν τὸ ἐνέργεια ὄν. (op. cit., XII, 2, 1069 b 15). Ἐνέργεια however is in itself prior to δύναμις: φανερὸν ὅτι πρότερον ἐνέργεια δυνάμεως ἔστιν (op. cit., IX, 8, 1049 b 5). Since matter is ὕλην ἂ δυνάμεις (De An., II, 1, 412 a 9), indeed pure δύναμις (Met., VII, 3, 1029 a 20), it is opposed to ἐνέργεια (op. cit., VII, 13, 1038 b 6). In medieval Aristotelianism, ἐνέργεια corresponds to act, δύναμις to potency, ὕλη to matter.
The form of energy that most immediately strikes experience is the energy of motion, whereby a body in motion possesses a capacity to act on other bodies, striking them. Aristotle, according to the principle: "everything that moves is moved by another" (Phys., VIII, 4, 256 a 3), thinks that this capacity resides in the mover and, when this no longer touches the mobile body, in the air, which has received from the mover the virtuality of still pushing the body (cf. Phys., VIII, 10, 267 a 18 agg.). According to John Philoponus (Phys., IV, 8; ed. Vitelli, Berlin 1888, p. 636), a Christian commentator (6th cent.), this virtuality resides instead in the body itself, caused indeed by the mover in the initial push. In the Middle Ages John Buridan, basing himself also on experimental observations, took up John Philoponus's idea with his theory of impetus: that virtuality called "impetus," which resides in the mobile body and maintains its movement, and on which depends the activity it exerts upon encountering other bodies, is a quality that always remains unchanged, unless it is destroyed from outside, and therefore continues to move indefinitely (first sketch of the law of inertia). Buridan makes a remark on the quantitative magnitude of impetus: "Quanto plus est de materia, tanto illud corpus plus potest recipere de illo impetu et intensius, sicut etiam ferrum plus potest recipere de caliditate quam lignum vel aqua eiusdem quantitatis" (Phys., VIII, q. 12). Elsewhere: "Movens imprimit moto non solum motum sed communiter aliquem impetum... Et quanto est motus velocior, tanto etiam fit ille impetus intensior" (De caelo, III, q. 2). The magnitude of impetus is therefore in direct proportion to the quantity of matter and to the velocity of the mobile body.
Descartes attributes to every body in motion a quantity of motion, which is directly proportional to its mass and to its velocity (= mv; Princ. Phil., II, n. 36; ed. C. Adam and P. Tannery, Paris 1897-1910, VIII, p. 61); bodies act upon one another only by mechanical impact, by which they transmit to one another a determinate quantity of motion. The total quantity of motion, brought into act by the Creator at the beginning of the world, remains identical: "Je tiens qu'il y a une certaine quantité de mouvement dans toute la matière créée, qui n'augmente n'y diminue jamais; et que, lorsqu'un corps en fait mouvoir un autre, il perd autant de son mouvement qu'il luy en donne" (Correspondence, ed. cit., V, p. 135). This is the first step toward the law of the constancy of energy. Leibniz observes that Descartes has committed an error, error memorabilii
Cartesii (ed. L. Dutens, III, Geneva 1768, p. 180 agg.), when he affirms that the quantity of motion remains constant. What remains constant, for Leibniz, is the quantité de force: "Ainsi il ne se garde pas la même quantité de mouvement, mais il se garde la même quantité de force, qui doit être estimée par l'effet qu'elle peut produire" (ed. cit., III, p. 201). This quantity of force is called by Leibniz "living force" (ibid., p. 318), a name that has remained even today; he also demonstrates that "living force" is proportional to mv² (ibid., p. 181). Leibniz therefore affirms the law of the constancy of energy; however he knows only mechanical energy. This is understandable when one recalls that for Leibniz mechanism prevails in the physical world and every activity is by mechanical impact.
During the 18th century the conception prevailed of heat as a true substance, as a fluid that passes from one body to another. But at the beginning of the 19th century B. T. Rumford and H. Davis demonstrated the transformation of the energy of motion into heat. From this it followed that the law of the conservation of living force is not exact, since it can instead be transformed into heat. With this one arrives immediately at the modern principle of the conservation of energy. Already Leonardo da Vinci had realized that in machines the work of resistance equals that of the power. And Galileo had said clearly that with machines one cannot create energy. The principle of the conservation of energy in the modern sense is due to J. R. Mayer, L. A. Golding, J. P. Joule, H. von Helmholtz. The first to speak of it was J. R. Mayer, in a publication of 1842 (J. R. Mayer, Bemerkungen über die Kräfte der unbeliebten Natur, in Liebig's Annales der Chemie, 42 (1842)). He affirms that there exists in nature a single force, which always remains the same through the continuous changes of the organic and inorganic world; and he derives this principle from causality: "Le forze sono cause e per conseguenza si può loro applicare pienamente il principio: «causa aequat effectum»" (loc. cit., p. 233). From this he deduces that forces are "objects indestructible, changing, imponderable" (loc. cit., p. 234). J. P. Joule determined experimentally the numerical relation between mechanical energy and heat; he however, and likewise A. Golding, is convinced of the principle for a priori reasons. H. von Helmholtz too appeals to the principle of cause in order to establish the principle of the conservation of energy; in particular he demonstrates it on the basis of the impossibility of perpetual motion. Moreover, with his purely mechanical interpretation of the principle, he contributed to strengthening it.
In 1824 Sadi Carnot and later R. Clausius found the second principle of thermodynamics, by means of which one can define the entropy of a system (v. TERSIOLOGIA). It is demonstrated that within a thermally isolated system, in all irreversible transformations, that is, in fact, in all real transformations, entropy increases. This means that the thermal energy of the system degrades, that is, levels out. And since every energy can be transformed into heat, all the energy of the system in its transformations goes on leveling out. This leads to a general leveling out of energy, which tends to distribute itself uniformly throughout the universe in the form of heat. From the law of entropy it is deduced that the world evolves toward a state of maximum entropy, in which all energies will be transformed into heat and the temperatures of bodies will be equal. The statistical conception of entropy tempers this conclusion somewhat.
Turning to the purely physical field against the prevailing mechanism, G. Ostwald strayed into philosophy to the point of constituting, with the concept of energy, a metaphysical system: energeticism. Energeticism first of all eliminates matter: "Le caractère distinctif de l'énergétique est l'abandon du dualisme qui a regné jusqu'ici entre la matière et l'énergie; celle-ci prend la place du concept le plus général" (G. Ostwald, L'évolution d'une science, la chimie, Paris 1910, p. 313). "En analysant la matière, en déterminant les parties composantes, nous sommes arrivés à voir qu'elle constitue une notion superflue" (G. Ostwald, L'énergie, ibid. 1910, p. 171). Matter is in fact a complex of three properties: extension, weight and mass. These properties are three factors of energy, whose nature is indispensable in order that the things of this world may constitute an object of experience. Therefore every body is a complex of energy (G. Ostwald, Vorlesungen über Naturphilosophie, Leipzig 1905, pp. 170, 171, 180, 181). Only energy exists, which is the true, sole reality: "Die Energie ist die allgemeinste Substanz, denn sie ist das Vorhandene in Zeit und Raum, und sie ist das allgemeine Accident, denn sie ist das Unterschiedliche in Zeit und Raum" (G. Ostwald, op. cit., pp. 146-47). We know the world by means of sensations, which are differences of energy (cf. G. Ostwald, La déroute de l'atomisme contemporain, in Revue générale des sciences, 6 [1896], p. 950). Every phenomenon is a displacement of energy in space or a change of form of energy. "On entend par énergétique le développement de cette idée que tous les phénomènes de la nature doivent être conçus et représentés comme des opérations effectuées sur les diverses énergies" (G. Ostwald, L'énergie, p. 119). Energetic processes have their importance also in psychic, social and cultural life. Ostwald even arrives at formulating the "energetic imperative": "Vergunde keine Energie, verwerte sie!" (G. Ostwald, Der energetische Imperativ, Leipzig 1912, passim).
In physics, energy is in general that which can be produced by work or converted into work, work being the application of a force over a displacement (dL=1/2s). The energy of a body is its capacity to produce work: a compressed spring, a body in motion, a fuel, etc., possess energy. Energy can exist under very different forms: mechanical, electromagnetic, luminous, gravitational, elastic, chemical, etc. Any activity among bodies takes place by means of their energy. Energy is therefore in physics a concept with a very broad meaning.
In mechanics one considers kinetic and potential energy. Kinetic energy, also called vis viva, is that which a body in motion possesses and which it can develop by stopping, measured by the formula frac{1}{2} mv².
Potential or positional energy is that which a body possesses by virtue of being in a determinate position in a field of forces, inasmuch as, under the action of these forces, it generally has the possibility of performing work. In mechanics the principle holds that in a conservative field of forces the sum of the kinetic and potential energy of a body is constant. This is the principle of the conservation of energy in mechanics, and it indicates in substance that the work performed by the body depends on the extreme points of the trajectory. But this principle is never verified in practice; for example, a pendulum, once set in motion, should never stop again; this does not happen because of friction. At the beginning of the 19th century, with Joule's experiments, it became possible to link mechanical work L with heat Q by means of the relation JQ=L; J is the coefficient that links heat and work, the mechanical equivalent of heat; measuring work in kilogram-metres and heat in calorie-degrees, J=427; that is, one calorie, transforming into work, produces 427 kilogram-metres, and conversely the mechanical production of one calorie requires the loss of 427 kilogram-metres of work. It must be observed that it is always possible to transform work into heat; but, by the second law of thermodynamics, one cannot transform heat into work at will. Knowing the transformation of work into heat and vice versa, one is led to admit the possibility of the transformation of any form of energy into any other form, according to precise relations. Hence it is possible to extend the principle of the conservation of energy, valid in mechanics, to any other form of energy; for all phenomena, the energy of an isolated system, that is, not subject to external forces or actions, remains constant. This is the first law of thermodynamics and it has an immense importance in physics, like the analogous one of the conservation of matter or mass. The principle of the conservation of energy is based, more than on experience, on our theoretical conviction of the constancy of nature. Thus, for example, says H. Poincaré (La science et l'hypothèse, Paris 1902, p. 153), E. Meyerson (Identité et réalité, 4th ed., ibid. 1932, p. 234) and others.
With the theory of relativity the concept of energy received new precisions. Toward the end of the last century it had been observed that, when electrons and positive ions are in motion, to give them the same acceleration a force is required that is greater the faster their motion was, hence the greater their kinetic energy was, as if the increased kinetic energy had increased their mass. With the theory of relativity (v. RELATIVITÀ), A. Einstein found that as the velocity of a body increases, its mass also increases. And he calculated that the increase of mass corresponds to the increment of kinetic energy divided by the square of the velocity of light in a vacuum, according to the formula Δ m = frac{Δ z}{c²}. In consequence of this Einstein asked himself whether, just as kinetic energy has a mass, one should not think that the whole mass of the body was energy: matter would be an immense concentration of energy, according to the formula: E=mc². Not everyone received these views equally; many accepted Einstein's relation only for the mass of the kinetic energy, not for the total mass of the body.
After the theory of relativity the researches of modern atomic physics were carried out and it was found that a positive and a negative electron, meeting, give rise to a photon whose energy is equivalent to that of the two destroyed corpuscles. And it turns out that the inverse phenomenon is also possible. Einstein's formula is verified also in nuclear reactions, in which the energy that comes into play corresponds to the variation of mass of the reacting bodies, according to Einstein's formula. On the basis of these relativistic developments, the principle of the conservation of energy is not true, nor does that of the conservation of matter remain true. Therefore the two distinct principles must be absorbed into the more general principle of the conservation of momentum; because momentum belongs to matter as much as to energy, it must be conserved through its various forms.
As already for matter, so too with regard to energy one passed from the continuous to the discrete conception. Until M. Planck it was thought that energy entered into phenomena with continuous variations. In 1900 Planck, studying the distribution of energy in the spectrum of the "black body", supposed that energy was emitted and absorbed in a non-continuous way, according to exact multiples of an elementary quantity e called the "quantum", linked to the frequency y of the radiation by the formula z = hc, where h is Planck's constant (= 6.6 to 10⁻²⁷ in eq.s.). This constant is an action (energy multiplied by time) which Planck rightly said to be not a purely mathematical abstraction but a true physical reality. A few years later, in 1905, Einstein, explaining the photoelectric effect, supposed that radiant energy travels in space quantized into quanta of light or photons. Thus Planck's theory, according to which energy is absorbed and emitted in quanta, is integrated with Einstein's, which says that energy travels quantized in space.
This conception was subsequently confirmed and taken up again in quantum mechanics, which, observing that radiant e. — that is, photons — exhibits wave phenomena, for example, the diffraction and interference of light, and corpuscular phenomena, for example, the photoelectric cell and the Compton effect, deduces from this, in agreement with W. Heisenberg's uncertainty principle, that radiant e. has neither a purely corpuscular nor a purely wave-like character, but, according to N. Bohr's principle of complementarity, the corpuscular aspect and the wave aspect are "like two faces of an object, which cannot be observed together and yet both must be looked at one at a time, in order to describe the object completely" (L. De Broglie, I quanti e la fisica moderna, Italian trans. by U. Richard, Turin 1938, p. 228; cf. E. Persico, I fondamenti della meccanica atomica, Bologna 1945, p. 142 ff.).
As for the relationship between matter and e., bearing in mind the experimental results, it must be said that e. manifests a property similar to that of matter, called mass, by virtue of which it resists the push of a force. This does not at all imply that every mass must be a concentration of e.
From a metaphysical point of view, e. is the foundation of the activity of bodies, since every corporeal operation takes place by means of e. Every being, by its own intrinsic finality, has an essential tendency toward its own completion, a completion that takes place by means of operation, which puts it in communication with other realities. «Ἐκαστὸν ἔστιν, ὦν ἔστιν ἔργον, ἔκαστὸν τοῦ ἔργον (De caelo, II, 3, 286 a 8).» Aristoteles hic loquitur, dicens quod unumquodque quod habet propriam operationem, est propter suam operationem; quaelibet enim res appetit suam perfectionem sicut suum finem, operatio autem est ultima rei perfectio» (St. Thomas, De Caelo, II, lectio 4). This holds also for the body, from whose essence the operation springs. E., which founds the activity of bodies, is therefore rooted in the very essence of the body as an extension of it. E. is not matter, as energism maintained, but a value essential to the structure and behavior of matter.
E. is studied and measured by physics. The quantitative measurement of e. is possible because it is in the extended; the concrete extended, insofar as it is extended, is measurable and subject to the laws of geometry and mathematics. Because of it, e. too is measurable and subject to calculation (cf. P. Hoenen, Cosmologia, Rome 1945, p. 176 ff.).