Dynamochora — the Physical domain
Terminology
"Physics" is derived from the Greek physis, meaning "nature", and that root is therefore better to denote the natural sciences in general rather than this more limited discipline. The universality it suggests is at odds with my clarification goals in this work.
The diversity of covered phenomena makes the identification of a single, central object quite elusive. A robust proposal is the concept of force, or that of power. Although strictly speaking, the concept of force is more present in Newtonian mechanics, and that of power in thermodynamics, analogs exist in every single theory, and the flow of energy or momentum they involve is central to the Lagrangian approach that henceforth constitutes the lingua franca of physical formalism. Hence the choice to use as a root for this domain the Greek dyna- or dynamo-
Dynatoms (physical objects)
It may look like a peculiar goal to ringfence the objects of the "physical domain". Have we not spoken for many pages of the "physical realm", one of three realms alongside the mental and the abstract? Indeed, it is true that physics, as it came to be known, was always distinguished in the minds of practitioners by its universality and generality. An object can be inanimate and outside of biology’s purview. A substance can be chemically inert. But nothing in the physical realm can be "physically inert". A quark, a rock, a human being, the earth or the whole universe: none of them can escape the laws of physics.
Is that all? Not quite. We have also explained above that each domain and each discipline would be associated to stages of the Scala Naturae. Physics (the dynamochora)) takes as its object the first four levels: fundamental quanta, elementary particles, atoms, elements and (small) molecules on the micro side, and the physical universe, together with its internal high-scale structures, on the macro side.
That is familiar terrain for modern physics. After all, for the past century, its most spectacular progress has been in understanding reality at extremely small or extremely large scales. So it’s pretty intuitive to map the physical "stages" onto modern theories: molecules on molecular physics, elements on nuclear physics, atoms on atomic physics, particles on the Standard Model of particle physics, fundamental quanta on possible theories "beyond the Standard model", and the universe on relativistic cosmology.
Yet, as we’ve just said, physics is not a specialized science of very small, very large or very fast things. The key is that we are considering ordinary objects rather than physical objects per se. For instance this gives us the ability to consider a human being as a living being, as a chemical compound or as a physical object. In the latter case, we would only consider it to have physical properties like mass, temperature, electrical charge, etc.
In that sense, we could say that, although a whole human being is not to be analyzed by microphysics, to consider it as only a physical object and to "do physics" about it only requires us to mobilize properties that emerge at or before the molecule stage, and nothing above. Let’s take a specific physical theory as example. If I need to use Galileo’s law of gravity to predict my free fall while bungee jumping, all that matters about "me" as a physical object is my mass, its distribution, and my shape. A mannequin with similar characteristics would not be treated differently by the theory. That is understandable, since atoms and molecules do have a mass, a position, a momentum: at common speed and energy levels, these properties are fully emerged at the atomic stage. For the purpose of physical theories, I can therefore acceptably be considered as "a bunch of atoms". If I were instead considered as "a group of living cells", we would already be talking of biology, not physics.
That principle is implicit in classical physics, although it is never laid out very rigorously. Any student just beginning to study Newton’s dynamics will be told about the movements of "material points". Mathematically, that is simply a quintuplet \((x,y,z,t,m)\) that denotes the presence of a mass at a given position and a given time. Of course, there is no such thing as a material point with zero volume and infinite density. But all students understand that the point represents something like a billiard ball, or in other contexts, a planet. A modern student would also know that the point might well be an atom. After all, an atom might be the closest thing to a material point in classical physics, since it is about the smallest object for which a classical approximation stands. To model a "point-like" planet Earth in elementary studies of orbital mechanics, we can then mentally picture a very heavy atom, right at the centre of the Earth.
That is a remarkable fact that cannot be taken for granted. The Earth, or a billiard ball, have plenty of internal structural properties beyond their mass and position. In another universe, the approximation that is classical physics may well rely on the elasticity, viscosity, refraction index or chemical solubility of objects to compute their trajectory. But it is not the case in ours. The form of classical equations give rises to Gauss’s theorem, which allows us to be completely blind to a body’s volume. As long as we "look" from outside its boundaries and it has spherical symmetry, all that counts is its mass and the position of its center. Arise, "material point"!
Hopefully it is now clearer why the physical domain can be considered to cover ordinary objects up to atoms and molecules: this lets us include in that domain studies of not only their transparent physical analogues (a single atom, a free elementary particle...), but also collections, as long as their behavior is considered from the perspective of those same physical properties attributable to the single objects. That is enough to build a concept of the "material point" or "rigid body" of classical mechanics, the "charged particle" of electrostatics or the "charged current" of electrokinetics and magnetostatics. Other objects of classical physics like the light or the gravitational field are now demonstrably grounded directly in "elementary" objects (respectively, the photon and spacetime), while thermodynamic systems’ proper characteristics emerge from the collective mechanical behavior of atoms or simpler particles.
Dynamoplases (physical events)
We can apply the same rationale to get a sense of physical events: those events that transform the physical properties of a physical body.
For general mechanics they would include motion or propagation (change of position), collision or scattering (change of momentum), annihilation (that one is specific to relativistic quantum mechanics). In relation to gravity we can evoke free fall, orbital capture, or gravitational collapse.In electromagnetic theories one could talk of electrical charging or of induction. Optical events include reflection, refraction, interference, diffraction, polarization. Thermodynamic events include heat transfer, phase transitions, compression or expansion. Atomic physics, which is concerned with electronic layers, oversees excitation and ionization, while nuclear physics, which only concerns atoms’ nuclei, consider radioactive decay, nuclear fission and nuclear fission.
Dynamonoia (physical concepts)
We could well have perused physical theories, past and present, and come up with scores and scores of additional physical objects and events. We could also explore the vast expanses of that field of research and reap thousands of physical concepts. But if we have to pick one, and only one, "big idea" to represent this domain as its dynamonoia, it should be action.
Let us not get stopped in the minutiae of this choice. The goal here is not to sanctify a given physical quantity among others, whose exact formulation differs from theory to theory, nor to elevate an principle ("least action" or "stationary action"), as incredibly fruitful as it is.
Action is a nodal point within a network of important ideas that form the background of any form of physical knowledge, in particular variation (action is grounded in some for of parametrization; there can be physics without time but not without a concept of change), symmetry (the symmetries of the action determine those of the theory), geometry (even as our experience of space has become more and more remote from the actual foundations of physics, geometric principles have risen anew in generalized context; to minimize the action is often expressed as "drawing a straight line" in some exotic phase space).
Dynamosophy (physical philosophy)
The applied philosophy of the physical domain should not be equated with philosophy of physics, which is largely a philosophy of science. Of course, the relevant objects, events and concepts have largely been conjured by scientists using scientific methods, which makes the distinction porous. But we must carefully follow our distinctions.
Dynamosophy would encompass the ontology, metaphysics, semantics and value theory of the dynamochora. The metaphysics of time, space, of quantum systems, and a plethora of other objects is a common theme. The epistemics of physical phenomena (what can be known about them and how to justify that knowledge) certainly lies at the border of philosophy of science. The aesthetics of physical objects or phenomena is a sub-field of the broader aesthetics of nature (which is distinct from the aesthetics of physical theories, since those are not physical objects).
Dynamology (physical science)
Our focus on specific ordinary objects up to molecules as well as collections thereof restrics the scope of dynalogy, as compared to the common usage of "physics", which has grown further and further to become a nearly all-encompassing physical science, on par with the old "natural philosophy".
In describing that scope, we need to be careful about two category mistakes. The first is the frequent identification of a sub-discipline with a theory, e.g. to say that quantum field theory is to elementary particles what solid mechanics is to large solids. Solid mechanics is a generic term that has been applied to various theories over time, while quantum field theory is a theory, that is unfathomably good at describing elementary particles, but might still be replaced. The second mistake is the lack of procedural clarity in how to delineate the scope: is physics a science of certain things? or a science that uses certain methods? Why is the change of internal structure of a material through its magnetization usually considered to be physics, while the change of its internal structure through ionization is normally part of chemistry? Why is the decay of a uranium nucleus into a thorium nucleus part of physics, while the combustion of glucose into carbon dioxyde and water is chemistry? Negligent approaches to that question lead to fuzzy boundaries with chemistry; that may not be a huge issue for the teacher or learner, but is one to the classificatory-minded. Our distinction between dynalogy and hylelogy aims to remedy these ailments.
Sub-disciplines
Level-specific sub-disciplines can be directly described as the study of specific entities in nature.
Particle physics has been thought of as the study of fundamental particles. Of course, we don’t know what is really fundamental. Around the time that term was first used, it was applied to nucleons (protons and neutrons), electrons and photons. Conventionally, particle physics now apply to those objects and their analogs at the same level of complexity, as well as anything "smaller". In terms of settled physics today, leptons, neutrinos, EW bosons and Higgs fields are described in the Standard Model of Electroweak interactions, while quark (and their hadrons aggregates) and gluons are tackled by Quantum Chromodynamics (QCD)
Nuclear physics, at the next level of organization, is concerned with the formation and breakdown of atomic nuclei, through the aggregation of hadrons bound by the residual strong force. Predecessors to the Standard Model like the Yukawa theory of nuclear force, or the Fermi theory of radioactive decay, can still be used as approximations and feed into modern-day nuclear models.
Atomic physics uses either non-relativistic quantum mechanics combined with classical electromagnetism, or the relativistic quantum field theory of electromagnetism (QED), to study the internal structure of atoms and their interaction with radiation like light.
Molecular physics obviously has large overlaps with chemical physics and physical chemistry, but also includes specific studies like molecular dynamics.
Universal physics (often called cosmology, which I avoid here to be consistent with my rebranding of astronomy with that term) is the part of physics that directly studies the universe or its large-scale components. It relies mostly today on General Relativity and cosmological extensions like the Lambda-CDM model.
Level-agnostic sub-disciplines study physical objects of arbitrary sizes, from the perspective of properties that are fully emerged at molecular, atomic or lower levels. Unsurprisingly, we can break them down into one very general practice (mechanics), and two more that examine the effects of the fundamental interactions that manifest at macroscopic scale. In other sections we have described the four fundamental interactions, but also noted the nuclear forces’ extremely short range at our energy level (the strong force is already neutralized by the existence of protons and neutrons; the weak force is limited in range by the mass of its mediator bosons: even macroscopic effects like radioactivity, once they’ve been identified as distinct from those of electromagnetic radiations, have only been described from the fundamental level as the decay of subatomic particles).
Mechanics is the general science of motion and force as it relates to matter. Those concepts used to be split between kinematics and statics before being unified into the Newtonian dynamics. The adjunction of additional quantities like energy and work, of new techniques grouped under the category of analytical mechanics (lagrangians, hamiltonians...), and the transition from point-like particles to spatially extended, rigid solids led to the durable synthesis of classical mechanics. The principles of special and general relativity, or equivalently, new descriptions of spacetime, led to its replacement by relativistic mechanics at high energy levels. Meanwhile, an even fuller break occurred with the development of quantum mechanics. Since both quantum and relativistic principles become important at high energy, their combination (involving only special relativity) was explored in detail and developed into quantum field theory. It is crucial to acknowledge two things:
Relativistic and quantum mechanics have specific domains of application but their domain of validity is much wider. Current quantum mechanics might not be valid at levels of energy above the ones we’ve been able to observe in particle accelerators, but it is at all other levels. A single electron or photon might be archetypes of "quantum objects", but a rock, a star or a mouse can also be described as wavefunctions. They can equally be described as relativistic objects in 4D space, of course. But obviously, most practical applications will solely use classical mechanics to represent them. This doesn’t make classical physics "wrong", or "just an approximation": it can contribute physical facts, about physical properties, of physical objects. There will always be a small error, within the implicit uncertainty associated with Newtonian dynamics under regular conditions. It would be true of any theory.
There is also a tendency to set in stone the relation between different versions of a theory and their most common mathematical expressions. Quantum theory nowadays describes complex operators and wavefunctions in Hilbert space. Newtonian dynamics describes n-uplet points in a vector space called phase space (the position and momentum of massive points). It is sometimes said that because quantum theory is more fundamental and its validity domain more extensive, the world is really described by complex vectors in Hilbert space, which supposedly also explains many a "strange" feature of the quantum world. That is a fallacy: the capacity of mathematical theories to be re-formulated in new instances calling entirely different images to the human mind is boundless. In fact, there is a formulation of classical mechanics due to Koopman and von Neumann that is entirely based on square-integrable, complex wavefunctions in Hilbert space, and uses a host of mathematical techniques developed for quantum mechanics. Like in classical mechanics though , there is no uncertainty about measurements and interferences do not occur in a double-slit experiments. That is not due to the "nature" that our mentality attributes to its objects, but instead to intrinsic mathematical simplifications. In a world where Newton would have somehow couched his own theory in that language, there would be no inclination to explain the lesser validity range of classical mechanic through the "dull" nature of its mathematical framework. We would simply be presented with two very similar theories mathematically speaking, with one being made "more right" than the others by the unction of empirical experience only.
Gravity is concerned with the movement of massive bodies subjected to the Earth’s gravity (in Galileo’s original formulation), to a gravitational force (in Newton’s theory), or evolving in a curved spacetime (in Einstein’s general relativity theory). In classical physics, gravity is conceptually distinct from general dynamics (which connect forces with acceleration, without ever specifying what forces ought to look like). However the practical connection was impossible to avoid due to the complete identification of inertial mass with gravitational mass: most macroscopic objects are electrically and magnetically neutral, but none (except, in the classical worldview, for light) are "gravitationally neutral". With general relativity, the identification is justified, and what had been perceived as a "gravitational interaction" is explained by spacetime geometry, thus entirely subsuming the macroscopic "theory of gravity" into a general mechanics.
Electromagnetism & Optics designate a group of sub-disciplines that study electrically or magnetically charged bodies, electric currents, and light. Respectively, those sub-disciplines have been called electrostatics, magnetostatics, electrokinetics and optics. Electro- and magnetostatics were early on seen as somewhat subsumed into mechanics since charged bodies were still endowed with mechanical properties. Electrostatics and electrokinetics merged into electrodynamics when the interaction between charges and currents was better understood. Together with magnetostatics, it was then unified into electromagnetic theory following the discovery of magnetic induction. The perfect, classical expression of electromagnetism was in Maxwell’s equations which explained all these phenomena via the dynamics of an electromagnetic vector field. Meanwhile, light, the only visible type of radiation, was described in successive paradigms as a geometric "line", a beam of corpuscles, and a wave. In Maxwell’s theory, the free electromagnetic field, an alternating wave, was soon identified with light, thus subsuming optics into an electromagnetic theory of light and helping to explain newly discovered radiations like X-Ray, infrared or ultraviolet, in one fell swoop. Like classical mechanics, that theory is still in use for a wide range of energy levels, and the go-to modelization for practical purposes. Yet, the most precise description of these phenomena by far is provided by Quantum Electrodynamics (QED) (a post-symmetry-breakup product of the Standard Model), which describes the electromagnetic field as a photon wavefunction, and the effect of electromagnetic interactions as caused by photon exchange between charged particles (arguably, the theory of electromagnetism at that degree of precision is now entirely part of mechanics, since that interaction can be represented as the relativistic quantum-mechanical behavior of elementary particles).
Statistical sub-disciplines explain the behavior of systems with a very large number of components. All other sub-disciplines are typically expressed in terms of a single object, or a few objects interacting, but can be expanded to cover "many-body" problems. Statistical physics, however, consider ensembles with arbitrary large number of components. Given that the dimension of the phase space under consideration can then quickly tend towards infinity, the same techniques cannot be applied, and emergent concepts appear, such as partition functions, state function (like entropy, free energy...), etc.
All of the above theories can be expanded into a statistical version. The original kinetic theory of gases, due to Maxwell, Boltzmann and Gibbs, has been tremendously successful at deriving pre-existing results in thermodynamics, therefore achieving its rigorous reduction (in certain cases) to mechanics. It also provided the statistical blueprint for futures theories.
Classical statistical mechanics start from the initial kinetic theory, which studies a low-density gas with a large number of identical, but indistinguishable molecules, that only interact with the system’s borders or with each other via binary, elastic shocks. From that simple physical concept sprang the Maxwell-Boltzmann distribution, which describes the probability distribution of micro-states in phase space. That approach can be used for many other case studies in classical mechanics, such as denser gases, inelastic shocks, or interacting particles (gases in a gravitational field, or a classical plasma of charged particles).
Relativistic versions of statistical mechanics take into account the relevant symmetries, which obviously results in velocity distributions being capped by the speed of light, and other effects.
Quantum statistical mechanics turn out fundamentally different due to the indistinguishability of the particles. The outcome is also determined by the particles’ spins, with fermions following Fermi-Dirac statistics, while bosons follow Bose-Einstein statistics. Like classical and relativistic distributions, those can be refined for different theoretical backgrounds or more and more general cases.
Dynamotechny (physical technique)
We have seen in previous discussions of the techne level that applied science is closely linked to the concepts of engineering and technology. Dynamurgy, the techne slice of the physical domain, or more colloquially applied physics, is of course the very core of engineering "proper" and the breeding ground for the oldest human inventions.
The progress of humanity’s material civilization is inseparable from the harnessing of energy. Energy is conserved, but humans can act by transforming one form into another, or by altering its expression.
The concept of energy as it is used in applied physics is closer to its understanding in classical physics than to a modern theoretical formulation. It is marked by the classical divisions between mechanics, gravity, electromagnetism, optics and thermodynamics, as well as between kinetic and potential forms of energy.
Mechanical energy
Kinetic mechanical energy represents the energy of a body in motion in a referential: it is the work required to achieve that velocity. For instance, in the simple 1D classical case: \[E_k= \int \mathbf{F} \cdot \mathbf{ds}= \int \frac{d\mathbf{p}}{dt} \cdot \mathbf{ds} =m\int (\mathbf{\dot{v}} \cdot \mathbf{v}) dt = \frac{mv^2}{2}\]
Potential mechanical energy represents the energy stored in a certain configuration of a system: changes in potential energy are equal to the work performed by the forces in that mechanical system. In an idealized spring model where the pull and push forces are directly proportional to the distance from equilibrium: \(\mathbf(F)=-k\mathbf(x)\), variations in potential energy are simply expressed as: \[\Delta E_p= \int \mathbf{F} \cdot \mathbf{ds}= - k \int \mathbf{x} \cdot \mathbf{ds} =k\int x \cdot dx = k (\Delta x)^2\] In fact, elasticity emerges from the material characteristics of bodies (and therefore lies outside the strict scope of the physical domain). Since the tendency of molecules in a material to attract each other or push against each other is an electrostatic influence, that form of energy transfer is ultimately electrostatic in nature, but its capacity to convert displacement into momentum at a macroscopic level makes it fall into the mechanical category. By evoking elasticity, we’re anticipating on our next domain, since the porosity between the dynamochora (physics) and hylechora (chemistry + materials) is higher for applied matters.
Potential gravitational energy Gravitational energy is a form of potential energy that corresponds to the gravitational field surrounding a body. On the surface of a planet, we may use the Galilean approximation of a uniform field, while more generally in classical physics, the Newtonian formulation can be used to derive variations in potential energy.
Electromagnetic energy
Electrical potential energy can manifest in electrostatics as a close analog to the gravitational case, or in electrokinetics as the energy from a circuit’s voltage (\(\frac{1}{2}CV^2\)), which obviously derives from it.
Magnetic energy represents in a concrete electric circuit the amount of work done to generate an electric current (\(\frac{1}{2}LI^2\)), and is considered as an electrical analog to kinetic energy.
Radiant energy is the energy of the electromagnetic field perturbation in EM waves, which make up all the classically known forms of radiation.
Thermal energy is proportional to a body’s temperature, corresponding to the intuitive idea that a body one spends energy bring to a higher temperature is absorbing that energy. In the statistical picture, that internal thermodynamic energy is understood as the microscopic kinetic energy from internal agitation. Heat is a form of non-mechanical energy transfer between thermal systems that is now identified with radiant energy (heat at daily-life temperatures is generally invisible infrared radiation, but thermal exchanges can generate EM waves at every frequency).
Nuclear energy refers to the form of radiative energy, discovered more recently than the ones mentioned above, from radioactive decay. It was soon unveiled that gamma decay could be identified with the generation of a high-energy EM waves, while alpha and beta decays consist in the emission of fast helium nuclei and electrons, respectively.
Binding energies are forms of potential energies derived from the above that reflect the energy transferred from or to a system when the structure of its microscopic components is altered. For instance, ionization energy is the electrostatic potential energy that corresponds to an electron’s bound to the nucleus. Nuclear binding energy can be released in a nuclear fusion process.
From the perspective of human civilization, there is a number of pre-existing energy stores: radiant (solar), thermal (geo-heat), mechanical (wind, waves, tides...), plus that which can be unbound from the structure of inert materials (fossil fuels, biomass, uranium...) or living beings (human and animal labor). At the other end of the chain, humans strive for radiant energy (light), thermal energy (hot or cold) or mechanical energy (moving things, raising things). One primary goal of applied physics is then to understand and enable the transduction of energy forms into each other. Let’s mention a few examples:
A turbine converts the thermal energy of steam into mechanical energy.
A generator converts kinetic energy to electrical energy.
A dam converts potential mechanical energy to electrical energy.
An electric motor converts electrical energy to kinetic energy.
An electric heater converts electrical energy to heat.
An electric bulb converts electrical energy to light.
A solar panel converts radiant energy to electrical energy.
etc.
Obviously, applied physics is not just about the transduction between different energy forms.
From prehistoric tools to modern-times electronic-circuitry, engineering has consisted in the design of systems to harness the flow of energy. We will illustrate that by using the bond graph formalism, an abstract generalization from electric circuitry that can describe universal systems, albeit in an idealized, linearized fashion. Let’s take the example of a mechanical system.
Power, the rate of energy flowing into elements of the system is the product of two power conjugates: \(\mathcal{P}=\mathbf{F}\cdot\mathbf{v}\).
The force \(\mathbf{F}\) is the effort variable, which "pushes" energy through the system.
The velocity \(\mathbf{v}\) is the flow variable, which represents the rate at which the energy flows through the system.
The power conjugates are the time derivatives of momentum (\(\mathbf{F}=\dot{\mathbf{p}}\)) and displacement (\(\mathbf{v}=\dot{\mathbf{x}}\)). The generalized displacement and momentum are the state variables of the system, so if we assume only linear relationships between \(\mathbf{F}\), \(\mathbf{v}\), \(\mathbf{p}\) and \(\mathbf{x}\), a system can be described via a first-degree differential matrix equation.
In the bond graph formalism, energy flows freely between elements, which are characterized by the number of ports (connections) to the network:
1-port elements:
Effort source (Energy-supplying):
An external input of force, for instance: the gravity field on Earth, or a constant-force spring.Flow source (Energy-supplying):
An external input of velocity, for instance: a current, a wind, a motor.Resistance (Energy-dissipating):
Resistance elements impose a constitutive relation between effort and flow (or equivalently, momentum and displacement). In the case of a mechanical system the relation between force and velocity is characteristic of an idealized friction: \(\mathbf{F}=-c\mathbf{v}\).Rigidity (Energy-storing):
Rigidity elements impose a constitutive relation between effort and displacement. In the case of a mechanical system the relation between force and displacement is characteristic of an idealized spring: \(\mathbf{F}=k\mathbf{x}\). The spring stores potential energy.Inertia (Energy-storing):
Inertia elements impose a constitutive relation between flow and momentum. In the case of a mechanical system the relation between velocity and momentum is just the integral form of Newton’s law: \(\mathbf{p}=m\mathbf{v}\). The inertia of massive bodies can be seen as a store of kinetic energy.
2-port elements (Energy-preserving): 2-port elements can change the value of effort and flow, but need to preserve power. If one such elements has two ports with state variables \((F_1,v_1)\) and \((F_2,v_2)\), then \(F_1 v_1=F_2 v_2\).
Transformers connect the efforts between the two ports, and the flows between the two ports: \(F_1=mF_2\) , \(mv_1=v_2\). A simple example is that of a lever, like a shovel or a wheelbarrow.
Gyrators connect the effort and flow of opposite ports: \(F_1=nv_2\) , \(nv_1=F_2\). An example example in the mechanical domain is a gyroscope. Another one in an electromechanic circuit is the ideal electric motor, which connects a torque (rotary mechanical effort) to an electric intensity (electrical flow) and an angular velocity (rotary mechanical flow) to a voltage (electrical effort).
N-port elements (Energy-preserving): N-port elements are nodes in the circuit that connect 3 or more elements. To enforce power preservations, two ideal cases emerge:
Effort junctions connect branches with equal forces. Power conservation then imposes a relation on velocities.
Flow junctions connect branches with equal velocity. Power conservation then imposes a relation on forces.
Let’s recapitulate the equivalence between generalized bond graph nomenclature and that of mechanical and electrical systems. \[\begin{array}{|c c c |c c|c c|} \hline &&\text{Constitutive relation}& \text{Mechanical system}& & \text{Electrical system}& \\ \hline \text{Effort} &e&& \text{Force}& \mathbf{F} &\text{Voltage} & V \\ \hline \text{Flow} & f& &\text{Velocity}&\mathbf{v} & \text{Intensity}& I \\ \hline \text{Momentum}&p &p=\int e \,dt& \text{Momentum}&\mathbf{p} & \text{Flux}& \Phi \\ \hline \text{Displacement}&q &q=\int f \, dt & \text{Displacement}& \mathbf{x} & \text{Charge}&q \\ \hline \text{Rigidity}&K & e=Kq &\text{Spring stiffness}&k &\text{Elastance}& 1/C \\ \hline \text{Resistance} &R &e=Rf & \text{Damper parameter}&c &\text{Resistance}& R \\ \hline \text{Inertia} &I & p=If &\text{Mass}& m &\text{Inductance}& L \\ \hline \text{Power} & \mathcal{P}& \mathcal{P}=ef&\text{Mechanical power}&\mathbf{F}\cdot\mathbf{v} & \text{Electrical Power}&VI \\ \hline \end{array}\]
As will become more and more familiar, as we move from episteme to techne, to delineate sub-disciplines becomes harder and the border with other domains itself becomes porous. The fields of applied physics, engineering science or technology is often divided according to the ultimate application of the knowledge at hand, as in automotive engineering, or power station engineering. That is a good solution for matters of professional training, but a poor one for our purposes, since it is very context-dependent and can hardly be generalized. It could also be divided to closely match the sub-discipline of dynalogy (theoretical physics). For that, I would remark that the level-specific vs level-agnostic distinction is less sensible. Knowledge of atomic physics, nuclear physics or molecular physics is a significant input of modern applied physics for sure. But the nature of the energy being ultimately harnessed to enable the targeted applications is more central. For example, laser science is entirely predicated on atomic physics rather than classical optics, yet its ultimate applications make it more lumpable with the rest of optical and electrical engineering, rather than in a hypothetic "atomic engineering" with mechanical and thermal applications of atomic knowledge.
Our classification will then follow the forms of energy outlined above (a list which itself follows broadly the categories of macroscopic classical physics):
Mechanical engineering pertains to the transformations of mechanical energy. This includes the "core" of the discipline, the study and design of mechanisms that apply forces and control movements to perform certain actions. by way of these mechanisms, hominids and humans have been able to act on their environment and shape it to their needs.
Gravitational engineering is not a common category. Yet, it is a legitimate distinction: gravitational potential energy at the surface of the earth can be expressed via a simple approximation (proportional to altitude), and the corresponding energy exchanges are wholly enmeshed within mechanical engineering. In outer space, the control of bodies’ movements in a gravitational field is more complex and energy exchanges with that field become predominant. At this stage of our civilization’s developments, such "engineering" is mostly represented by astronautics, the sub-discipline used to devise the voyage of spacecrafts or probes.
Electrical engineering is an umbrella term that covers the harnessing of all forms of electromagnetic and radiant energy (electrical engineering, electronic engineering, optical engineering, etc.). That energy domain is not rife with direct applications (those which see humans enjoy that energy as an end upon itself). Simple ones include the generation of radiant energy for humans to enjoy visible light at will (electric bulbs), or of lower-frequency radiation to induce heat transfers (electric heating). That being said, electric circuitry and EM waves emissions are the absolute bedrock of our current civilization as an intermediary between other forms of energy.
The major asset of electromagnetic and radiant energy is that it is fast. Electromagnetic energy is transmitted through a medium like air, or through conductors like metals, at a speed that is below, but very close to the speed of light. Compare that with mechanical systems that are limited by constraints like friction or inertia.
Electromagnetic transmission is also extremely efficient: although it does experience energy loss or signal distortion, those are tiny compared to what mechanical gears or pulleys may experience. Combined with speed, this means that aggregate losses over very long distances are minimal.
Electrical systems can operate over a wide range of power levels, while mechanical systems are typically designed to sustain a narrow band only.
All in all, we can say that electrical systems are extremely scalable: Babbage was able to build a simple mechanical computer. With physical constraints as they are, it is unimaginable for a mechanical engine to replicate the operation of a modern computer, or to transmit information all over the earth as it is now done.
Thermal engineering deals specifically with the design of heat transfer or temperature control systems. Such systems can be intermediaries in more complex endeavors, or final applications, since human beings naturally seek to control the thermal features of their environment, for their own selves of course, or for food conservation and preparation, for instance.
Nuclear engineering simply aims to harness nuclear energy. In the civil domain, there is no direct application, since direct exposure to nuclear energy is harmful to human beings; however it can be readily converted into thermal, then electric energy.
Cross-disciplines
Cross-disciplines follow a pattern that will become familiar in the physical sciences. To cross dynalogy with other disciplines from the natural sciences is roughly to consider the "physics of X" where X is the type of objects that’s normally at the forefront of the other discipline. For example biophysics or medical physics consider our body as physical objects: the ductility of our bones, or the characteristics of electrical activity in our brains. They "back up" compared to a discipline (biology) that considers its objects of study as living objects, to one that considers the same as per their physical properties only (which explains why the methods of physics can be used). We’ll call it grounded cross-disciplinarity.
This won’t work with the mathematics, or the human sciences since their objects (minds, social institutions, musical pieces, algebraic structures...) are not instantiations of physical kinds. Quantum physics has very little to say about the Supreme Court or a sonata by Corelli! A dangerous mistake that’s now ingrained in the terminology is to confuse applications of physics with the application of mathematical methods used in physics... "Sociophysics" or the likes will denote the use of methods akin to those of statistical mechanics to explain social behavior. But these are mathematical methods that happen to be famously used in physics, which certainly was an inspiration to those who first applied them to social entities; but they do not stem from physical science itself.
The correct crossing between natural and non-natural sciences is an application to the actual physical objects from the crossed domain: artifacts, edifices, personas. We will call this induced cross-disciplinarity. Importantly, the crossing between two natural sciences can be (in certain cases) grounded, as we’ve seen, but it can also be induced.
Arithmodynalogy: the application of physical knowledge to the study and design of mathematical artifacts (for instance, the use of mechanical engineering, electrical engineering, electronics, quantum theory to design computer architectures).
Cosmodynalogy has encompassed gravitational physics since Newton. Today it represents the broader realm of astrophysics, which does not restrain itself to the orbits of astral bodies but explores the physical processes responsible for their whole lifecycle.
Hyledynalogy: chemical physics is a sort of reductionist enterprise that "zooms in" single instances of chemical reactions to explain their characteristics at the level of molecular physics; physical chemistry is more generally a way to denote the part of chemistry that relates to thermodynamic or optical properties.
Geodynalogy: geophysics considers planets as complex physical systems with internal structures, mechanical properties as well as thermic or magnetic ones.
Ecodynalogy: ecophysics apply physics to ecosystems, being at the same time a restriction of geophysics to that smaller scale, and an overlap with biophysics, as living populations mesh with the geological features of the planetary surface.
Biodynalogy: biophysics has many sub-disciplines itself, echoing the many levels of organization of life, from molecular biophysics (e.g. the physical structure of DNA) to cellular biophysics (the mechanics of division processes) to the biomechanics, thermodynamics or electrical properties of larger entities like bones, organs or whole organisms. It also includes medical physics, which studies the physical properties of living bodies to develop imaging or analytic technology.
It is worth interrupting here and notice that for brevity we’ve skipped over most induced crossings between natural sciences and physics,with the exception of medical physics, a rich field in its own right. The others represent the physical study and engineering of equipment or machines connected to other domain, such as spacecraft or satellites (cosmo-), textile, metalworking or glassmaking equipment (hyle-), mining equipment (geo-), agricultural equipment (bio- and eco-).
Topodynalogy: geography is the science of location and place from a human perspective. Physical geography is concerned with the physical features of those locations, but to seek a connection with physical sciences is very quickly to cross the line into geology, for which geophysics would be the relevant cross-discipline. The "physics of geography" should rather be concerned with the use of physics towards geospatial aims (maps, localization,...), to develop means of transportation, or to study the physical features of human dwellings (air, heat or water flows in human environments).
Chremodynalogy covers physical applications to economics Let’s re-iterate, financial or econometric theories that "look like" statistical mechanics are not physics, they are part of Chremoarithmology! Physics can be applied to the economic domain with regards to the physical aspects of warehousing, commerce, and other concrete instances of the economic chain of value.
Phylodynalogy: the physical imprint of the social domain is not distinctive. Sociology deals largely in institutions; their concrete instantiations like social symbols (decorations, badges, ensign), social buildings, or social activities (firefighting, social care...) are ruled by physics like all things concrete, but are not distinctive enough to delineate a clear cross-discipline.
Dikaidynalogy: physics may not apply directly to legal theory as an abstract endeavor, but is used for concrete legal investigations and reasoning, forensics being the best-known and most spectacular case. It constitues an application of physics to the legal domain.
Polidynalogy: a lot of politology could attract the same comment as sociology, in its dealing with physics: being mostly concerned with institutions and fairly benign concrete objects (government buildings, administrative records...), the crossing might not be fruitful. But the political domain also includes a vast military and policing wing, whose physical projection in the world has, sadly, been of serious significance. Applications of physics to weaponry and other military structures and installations is paramount for practitioners.
Chronodynalogy: physics applied to history include horology (the study of time measurement techniques and apparatus), the science of conservation and restoration of historical artifacts, and the application of physics to various auxiliary disciplines focusing on different types of artifacts (ancient coins, books, sigils, flags, etc.).
Psychodynalogy: mental states and human minds are generally connected to physics by way of biology. Yet, there is a field of neurophysics considering high-level electromagnetic or quantum parameters as possible (partial) grounds for mental activity, or more simply as its consequences. The psychological domain is also represented in a multitude of artifacts used for furnishing, maintenance, clothing, personal care, which can all be studied as physical objects.
Glossodynalogy: the study of forms of linguistic expression like writing, can be aided by some physical sub-disciplines, as can the study of recording, printing, or general communication equipment.
Tropodynalogy: the cultural domain is rich in concrete realizations of its abstract structures, be they in religion, culture, entertainment or recreation, all subject to physical applications. One emblematic case is the physical study of sporting activities.
Theorodynalogy: scientific investigations often require experimental conditions in a laboratory context, and complex instrumentation to conduct experiments or detect their outcomes. Instrumentation physics or laboratory engineering are natural applications.
Hermeneodynalogy: physics can be applied to the literary domain only in an ancillary way, for instance as it relates to the science of books as physical objects, or to the acoustics of public reading.
Kalodynalogy: the intersection with the artistic domain is of much more import. Visual arts are...visual, and the relation to optics has always been extremely deep; painters and sculptors have often be at the forefront to study lightning and color: their perception and effect, but also the physical aspect of their combination. The arts have also led to the development of artifacts to create or present artworks. Physical applications abound when it comes to the study of paint pigments, of photographic and cinematographic equipment, of tools and technical equipment, etc. Performing arts also have a strong imprint in the physical domain, since dancing and other physical performances can be the object of physical studies.
Dynamurgy (physical activities)
Dynamurgy, the set of human activities anchored to the physical domain, apply the knowledge from dynamotechny (the part of engineering anchored to that same domain). Broadly speaking, they represent vast swathes of the following activities, though they do not by any means cover them exhaustively, since they are shared with other poiesis domains.
Manufacturing, the physical transformation of materials, substances or components into products, to be fed as inputs into further manufacturing processes, or to be used by humans. This extends from simple craftsmanship in prehistoric times to the making of modern microchips
Construction, the erection of buildings, which could be seen as part of a manufacturing process (making a building and its trimmings), but is generally treated separately due to the ample overlap into the geological domain, as we’ll see.
Energy production, by which we actually mean the transduction and transmission of natural energy sources to forms and locations required by human life, also has a dynamurgic component (in a gas power plant, the combustion reaction is of a chemical rather than physical nature, but the conversion of heat into electricity and its transmission relates to physical consideration).
Dynamergons (physical artifacts)
The artifacts that are involved in activities of the physical domain can generically be called machines.
In the mechanical realm, machines are composed of structural elements like frame members, bearings, axles, splines, fasteners, seals... which impose constraints on motion, and of mechanisms. When we introduced the bond graph formalism, we explained that power is conserved, but it could dissipate (friction) or be stored (springs). Besides, power conjugates can be made to vary while keeping power constant: by converting velocity into force, a mechanism can generate a mechanical advantage. Complex mechanisms are compounds (kinematic chains) of simpler ones. Efforts to classify and analyze machines have led from the Renaissance era to identifying the six most basic blocks, known as simple machines. All have been known for millenia:
The wedge, exemplified by an axe, has been around since prehistoric times.
The inclined plane is probably as ancient, and is thought to have been used to move megaliths or build early structures.
The wheel and axle mechanism is though to date back as far as 5,000 years ago.
The lever appeared in the form of a simple balance scale, and would be used later in cranes to move large stone block and build structures like the Giza pyramid.
The pulley dates back to the 2nd millenium in Mesopotomia.
The screw was the last simple machine to be invented, in Assyria.
Later on, it was realized that even this short list of 6 machines could be reduced to 2 basic devices: a rotating body (lever, pulley, wheel) and a sliding block on a surface (screw, incline plane, wedge). The difference in mechanisms then really stems from structural elements: the joints (whose four types are: revolute joint, prismatic joint, cam joint and gear joint).
In the electrical realm, the analogous object is a circuit element, which we can even more directly match to the bond graph concepts: the five linear elements are resistors, inductors, conductors, transformers and gyrators.
Dynamodomes (physical edifices)
Buildings where humans engage in manufacturing, in energy production or in construction are variously called factories, plants, mills, yards, workshops, building sites...
Dynamanthropes (physical personas)
The physical facet of human beings is simply represented by their body, the purely physical projection of their being. That body falls, conducts electricity, vibrates with sounds, floats in water, like object that would share its physical properties.
Dynamopraxy (physical experience)
The human experience (segment of the praxis plane) that corresponds the physical domain is the interaction (creation or usage) of artifacts. To interact physically with the world is the wont of dynamanthropes, but the connection with dynamurgy and dynamergons can only be established for man-made physical objects.