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Dynamic Equilibrium

We characterize Newtonian particles as being in some kind of steady balance with their environment. They are presumably interacting with countless photons, bouncing around a lot, and colliding with other particles. But despite much agitation, there is still a central tendency that might be called realistic motion, or perhaps naturalistic movement. Particles that depart too far from this balance may be called non-Newtonian, or even unphysical. To be more exact about this we define the kinetic and potential energy.

Kinetic Energy

Consider a material particle P, described by its rest mass  m and momentum  p. The kinetic energy of P is defined as

K \equiv \dfrac{\, p^{ 2}}{2m}

Since  m > 0 for material particles,  K is never negative. And in an inertial frame, momentum is proportional to the wavenumber  \kappa. So  K is proportional to  \kappa ^{2}. Then recall that the wavenumber depends only on the coefficients of dynamic quarks. So the kinetic energy depends strongly on P’s dynamic quark content. Dynamic quarks are objectified from somatic and visual sensations. So the kinetic energy depends strongly on these audio-visual sensations too.

Potential Energy

Let us also include the mechanical energy  E in the description of P. The difference between  E and the kinetic energy defines another number  \mathcal{U} called the potential energy

\mathcal{U} \equiv E - K

To evaluate  \mathcal{U}, recall that if P is in slow motion, then

    \begin{equation*} \begin{split}  E &\simeq mc^{2} \left(1 + \dfrac{p^{2}}{2m^{2}c^{2}} \right) \\ &= \rule{0px}{11px} mc^{2} \left(1 + \dfrac{K}{mc^{2}} \right) \\ &= \rule{0px}{11px} mc^{2} + K \end{split} \end{equation*}

So the potential energy is approximated by \mathcal{U} \simeq mc^{2}. Thus for slowly moving Newtonian particles, the potential energy depends strongly on the mass. Then remember that for heavy particles, a sensory interpretation of the mass relates mainly to baryonic quarks and thermal sensations. And so for Newtonian particles, the potential energy is mostly associated with thermal sensations too.

What is Dynamic Equilibrium?

A particle is in dynamic equilibrium when its kinetic and potential energies are equal to each other. At equilibrium \mathcal{U} = K and there is an equal sharing, or equipartition, of mechanical energy between kinetic and potential types. But the potential energy is defined above by E less K. So for a particle in dynamic equilibrium

E= \mathcal{U} + K = 2K

This account of dynamic equilibrium is succinct. And equipartition provides an important theoretical linkage to traditional notions of momentum. But it is not very helpful in the laboratory. Difficulties arise from using a hypothetical condition of perfect isolation to set the zero-value for energy measurements. Recall that our initial discussion of internal energy adopted the reference sensation of not seeing the Sun to grasp the notion of having no energy. So even in principle, there is no tangible reference standard for absolute-zero on the energy scale. And this is noticeable now that measurements can be made to a few parts in a trillion. Moreover, there are conflicts with theories of dispersion and gravitation which may deny the possibility of perfect isolation. Anyway, these difficulties are manageable because physical phenomena often occur within distinct energy regimes which have different ‘zeros’. In the laboratory we measure energy changes, that are related to each other by \Delta E = \Delta \mathcal{U} + \Delta K. Then a null-value standard for calibrated energy-difference measurements can be selected for experimental convenience. Results are reported using a slightly different version of the energy with a shifted origin

E = E^{\prime}  + \; \text{an arbitrary constant}

Then \Delta {E}^{\prime} = \Delta {E}. These energy-differences are more susceptible of precise laboratory observation than absolute values. But {E}^{\prime} \! \ne 2K and equipartition is inapt for shifted energies.

Sensory Interpretation


As noted above, the kinetic energy characterizes visual stimuli, whereas the potential energy depends more on thermal perception. So there must be a balanced experience of both thermal and visual sensation for events to be objectified as particles in dynamic equilibrium. This requirement for eyes-open visual sensation means that, for example, a dream about flying while asleep cannot meet equilibrium conditions. And neither can watching cartoons on TV, because television only transmits audio-visual sensations, not thermal sensations. So dynamic equilibrium is more like experiencing ordinary circumstances in classrooms and laboratories on Earth. Unlike many movies, dreams and hallucinations.

Dynamic equilibrium results from balanced energies, as suggested by the writhing dragons in this Indonesian weaving.
Tampan, Paminggir people. Lampung region of Sumatra, 19th century, 77 x 70 cm. Photograph by D Dunlop.