A Wealth of Motions: Six Generations in the History of the Physics of Motion


Since Galileo launched his trajectory, there have been six broad generations that have traced the continuing development of concepts of motion. These are: 1) Universal Motion; 2) Phase Space; 3) Space-Time; 4) Geometric Dynamics; 5) Quantum Coherence; and 6) Complex Systems. These six generations were not all sequential, many evolving in parallel over the centuries, borrowing from each other, and there surely are other ways one could divide up the story of dynamics. But these six generations capture the grand concepts and the crucial paradigm shifts that are Galileo’s legacy, taking us from Galileo’s trajectory to the broad expanses across which physicists practice physics today.

Universal Motion emerged as a new concept when Isaac Newton proposed his theory of universal gravitation by which the force that causes apples to drop from trees is the same force that keeps the Moon in motion around the Earth, and the Earth in motion around the Sun. This was a bold step because even in Newton’s day, some still believed that celestial objects obeyed different laws. For instance, it was only through the work of Edmund Halley, a contemporary and friend of Newton’s, that comets were understood to travel in elliptical orbits obeying the same laws as the planets. Universal Motion included ideas of momentum from the start, while concepts of energy and potential, which fill out this first generation, took nearly a century to develop in the hands of many others, like Leibniz and Euler and the Bernoullis. This first generation was concluded by the masterwork of the Italian-French mathematician Joseph-Louis Lagrange, who also planted the seed of the second generation.

The second generation, culminating in the powerful and useful Phase Space, also took more than a century to mature. It began when Lagrange divorced dynamics from geometry, establishing generalized coordinates as surrogates to directions in space. Ironically, by discarding geometry, Lagrange laid the foundation for generalized spaces, because generalized coordinates could be anything, coming in any units and in any number, each coordinate having its companion velocity, doubling the dimension for every freedom. The Austrian physicist Ludwig Boltzmann expanded the number of dimensions to the scale of Avogadro’s number of particles, and he discovered the conservation of phase space volume, an invariance of phase space that stays the same even as 1023 atoms (Avogadro’s number) in ideal gases follow their random trajectories. The idea of phase space set the stage for statistical mechanics and for a new probabilistic viewpoint of mechanics that would extend into chaotic motions.

The French mathematician Henri Poincaré got a glimpse of chaotic motion in 1890 as he rushed to correct an embarrassing mistake in his manuscript that had just won a major international prize. The mistake was mathematical, but the consequences were profoundly physical, beginning the long road to a theory of chaos that simmered, without boiling, for nearly seventy years until computers became common lab equipment. Edward Lorenz of MIT, working on models of the atmosphere in the late 1960s, used one of the earliest scientific computers to expose the beauty and the complexity of chaotic systems. He discovered that the computer simulations were exponentially sensitive to the initial conditions, and the joke became that a butterfly flapping its wings in China could cause hurricanes in the Atlantic. In his computer simulations, Lorenz discovered what today is known as the Lorenz butterfly, an example of something called a “strange attractor”. But the term chaos is a bit of a misnomer, because chaos theory is primarily about finding what things are shared in common, or are invariant, among seemingly random-acting systems.

The third generation in concepts of motion, Space-Time, is indelibly linked with Einstein’s special theory of relativity, but Einstein was not its originator. Space-time was the brain child of the gifted but short-lived Prussian mathematician Hermann Minkowski, who had been attracted from Königsberg to the mathematical powerhouse at the University in Göttingen, Germany around the turn of the 20th Century by David Hilbert. Minkowski was an expert in invariant theory, and when Einstein published his special theory of relativity in 1905 to explain the Lorentz transformations, Minkowski recognized a subtle structure buried inside the theory. This structure was related to Riemann’s metric theory of geometry, but it had the radical feature that time appeared as one of the geometric dimensions. This was a drastic departure from all former theories of motion that had always separated space and time: trajectories had been points in space that traced out a continuous curve as a function of time. But in Minkowski’s mind, trajectories were invariant curves, and although their mathematical representation changed with changing point of view (relative motion of observers), the trajectories existed in a separate unchanging reality, not mere functions of time, but eternal. He called these trajectories world lines. They were static structures in a geometry that is today called Minkowski space. Einstein at first was highly antagonistic to this new view, but he relented, and later he so completely adopted space-time in his general theory that today Minkowski is almost forgotten, his echo heard softly in expressions of the Minkowski metric that is the background to Einstein’s warped geometry that bends light and captures errant space craft.

The fourth generation in the development of concepts of motion, Geometric Dynamics, began when an ambitious French physicist with delusions of grandeur, the historically ambiguous Pierre Louis Maupertuis, returned from a scientific boondoggle to Lapland where he measured the flatness of the Earth in defense of Newtonian physics over Cartesian. Skyrocketed to fame by the success of the expedition, he began his second act by proposing the Principle of Least Action, a principle by which all motion seeks to be most efficient by taking a geometric path that minimizes a physical quantity called action. In this principle, Maupertuis saw both a universal law that could explain all of physical motion, as well as a path for himself to gain eternal fame in the company of Galileo and Newton. Unfortunately, his high hopes were dashed through personal conceit and nasty intrigue, and most physicists today don’t even recognize his name. But the idea of least action struck a deep chord that reverberates throughout physics. It is the first and fundamental example of a minimum principle, of which there are many. For instance, minimum potential energy identifies points of system equilibrium, and paths of minimum distances are geodesic paths. In dynamics, minimization of the difference between potential and kinetic energies identifies the dynamical paths of trajectories, and minimization of distance through space-time warped by mass and energy density identifies the paths of falling objects.

Maupertuis’ fundamentally important idea was picked up by Euler and Lagrange, expanding it through the language of differential geometry. This was the language of Bernhard Riemann, a gifted and shy German mathematician whose mathematical language was adopted by physicists to describe motion as a geodesic, the shortest path like a great-circle route on the Earth, in an abstract dynamical space defined by kinetic energy and potentials. In this view, it is the geometry of the abstract dynamical space that imposes Galileo’s simple parabolic form on freely falling objects. Einstein took this viewpoint farther than any before him, showing how mass and energy warped space and how free objects near gravitating bodies move along geodesic curves defined by the shape of space. This brought trajectories to a new level of abstraction, as space itself became the cause of motion. Prior to general relativity, motion occurred in space. Afterwards, motion was caused by space. In this sense, gravity is not a force, but is like a path down which everything falls.

The fifth generation of concepts of motion, Quantum Coherence, increased abstraction yet again in the comprehension of trajectories, ushering in difficult concepts like wave-particle duality and quantum interference. Quantum interference underlies many of the counter-intuitive properties of quantum systems, including the possibility for quantum systems to be in two or more states at the same time, and for quantum computers to crack unbreakable codes. But this new perspective came with a cost, introducing fundamental uncertainties that are locked in a battle of trade-offs as one measurement becomes more certain and others becomes more uncertain.

Einstein distrusted Heisenberg’s uncertainty principle, not that he disagreed with its veracity, but he felt it was more a statement of ignorance than a statement of fundamental unknowability. In support of Einstein, Schrödinger devised a thought experiment that was meant to be a reduction to absurdity in which a cat is placed in a box with a vial of poison that would be broken if a quantum particle decays. The cruel fate of Schrödinger’s cat, who might or might not be poisoned, hinges on whether or not someone opens the lid and looks inside. Once the box is opened, there is one world in which the cat is alive and another world in which the cat is dead. These two worlds spring into existence when the box is opened—a bizarre state of affairs from the point of view of a pragmatist. This is where Richard Feynman jumped into the fray and redefined the idea of a trajectory in a radically new way by showing that a quantum trajectory is not a single path, like Galileo’s parabola, but the combined effect of the quantum particle taking all possible paths simultaneously. Feynman established this new view of quantum trajectories in his thesis dissertation under the direction of John Archibald Wheeler at Princeton. By adapting Maupertuis’ Principle of Least Action to quantum mechanics, Feynman showed how every particle takes every possible path—simultaneously—every path interfering in such as way that only the path with the most constructive interference is observed. In the quantum view, the deterministic trajectory of the cannon ball evaporates into a cloud of probable trajectories.

In our current complex times, the sixth generation in the evolution of concepts of motion explores Complex Systems. Lorenz’s Butterfly has more to it than butterflies, because Life is the greatest complex system of our experience and our existence. We are the end result of a cascade of self-organizing events that began half a billion years after Earth coalesced out of the nebula, leading to the emergence of consciousness only about 100,000 years ago—a fact that lets us sit here now and wonder about it all. That we are conscious is perhaps no accident. Once the first amino acids coagulated in a muddy pool, we have been marching steadily uphill, up a high mountain peak in a fitness landscape. Every advantage a species gained over its environment and over its competitors exerted a type of pressure on all the other species in the ecosystem that caused them to gain their own advantage.

The modern field of evolutionary dynamics spans a wide range of scales across a wide range of abstractions. It treats genes and mutations on DNA in much the same way it treats the slow drift of languages and the emergence of new dialects. It treats games and social interactions the same way it does the evolution of cancer. Evolutionary dynamics is the direct descendant of chaos theory that turned butterflies into hurricanes, but the topics it treats are special to us as evolved species, and as potential victims of disease. The theory has evolved its own visualizations, such as the branches in the tree of life and the high mountain tops in fitness landscapes separated by deep valleys. Evolutionary dynamics draws, in a fundamental way, on dynamic processes in high dimensions, without which it would be impossible to explain how something as complex as human beings could have arisen from random mutations.

These six generations in the development of dynamics are not likely to stop, and future generations may arise as physicists pursue the eternal quest for the truth behind the structure of reality.

Top 10 Books to Read on the History of Dynamics

Here are my picks for the top 10 books on the history of dynamics. These books have captivated me for years and have been an unending source of inspiration and information as I have pursued my own interests in the history of physics. The emphasis is on dynamics, rather than quantum and particle physics, although these traditional topics of “modern physics” have inherited many of the approaches of classical mechanics.

(1) Diacu, F. and P. Holmes (1996). Celestial encounters: The origins of chaos and stability. Princeton, N.J., Princeton Univ. Press.

Diacu and Holmes have written a clear, accessible and information-rich general history of the role that the solar system played in the development of dynamical theory, especially issues of the stability of the solar system.

(2) Pais, A. (2005) Subtle is the Lord: The Science and the Life of Albert Einstein: Oxford.

Pais has produced a masterpiece with his inside view of the historical development of Einstein’s ideas, for both special and general relativity. Through Pais’ story telling, it is possible to follow each turn in Einstein’s thinking as he proposed some of the most mind-bending ideas of physics.

(3) Thorne, K. S. (1994). Black holes and time warps : Einstein’s outrageous legacy. New York, W.W. Norton.

This book is an exuberant journey through the history of general relativity seen through the eyes of the recent Nobel Prize winner Kip Thorne. The book is full of details, many of them personal recollections as GR went from its early days through the “golden age” with John Wheeler located at the center of the motion.

(4) Schweber, S. S. (1994). QED and the men who made it: Dyson, Feynman, Schwinger, and Tomonaga. Princeton, Princeton University Press.

Schweber has produced a master work in the same genre as Pais, describing the development of QED in such moment-by-moment detail that you feel you are living the history itself. The description of Feynman’s stumble into the world of the “grown ups” at the Shelter Island and Pocono Conferences is priceless.

(5) Bacaer, N. (2011). A Short History of Mathematical Population Dynamics, Springer.

This compact little book is one of my favorites in terms of conciseness and completeness. It tracks a history that is little known inside physics, but which has taken on out-sized importance in the new era of complex systems where evolutionary dynamics describes diverse systems from neural networks to genetic algorithms.

(6) Gleick, J. (1987). Chaos: Making a New Science, Viking.

Gleick’s book is an absolute classic. This was one of my first introductions into the history of modern physics when I read it at the end of my post-doc position at Bell Labs in 1989. It has been a role model for my own dive into the history of physics.

(7) Cassidy, David C. (2010). Beyond Uncertainty : Heisenberg, Quantum Physics, and The Bomb. New York, NY, Bellevue Literary Press.

Cassidy’s sequel to his first book on Heisenberg (Uncertainty) is in the same master genre as Pais and Schweber. Reading page by page allows you to live the history yourself as Heisenberg struggled to escape from an overbearing father (and a disastrous doctoral defense) to make his mark on the world of physics.

(8) Jammer, M. (1989), The conceptual development of quantum mechanics. Tomash Publishers Woodbury, N.Y., American Institute of Physics.

Although dry and a dense read, this book is definitive. If you ever want to understand step-by-step how quantum mechanics evolved from the early thinking of Bohr to the advanced transformations of Dirac and Jordan, this is the book you want as a reference. It is endlessly deep and detailed.

(9) Crowe, M. J. (2007), Mechanics from Aristotle to Einstein: Green Lion Press.

This book is filled with lots of myth-busting about the early days of physics. It’s amazing that what we call “Newtonian Physics” was mostly not invented by Newton himself, but by others … even by his nemesis Leibniz!

(10) Coopersmith, J. (2010), Energy, the Subtle Concept: The Discovery of Feynman’s Blocks from Leibniz to Einstein: Oxford, Oxford University Press.

Coopersmith shows how the history of concepts of work and energy is surprisingly obscure. Newton himself made no mention of energy, and it took nearly 100 years for a clear picture of energy to emerge, despite its central role in dynamical systems.


There are many wonderful review articles in review journals. A few of my favorites are:

Aubin A. and Dahan Dalmedico, D. (2002). “Writing the History of Dynamical Systems and Chaos: Longue Durée and Revolution, Disciplines and Cultures”. Historia Mathematica, 29, 273-339.

Ginoux, J. M. and C. Letellier (2012). “Van der Pol and the history of relaxation oscillations: Toward the emergence of a concept.” Chaos 22(2).

Gutzwiller, M. (1998), Moon-Earth-Sun: The oldest three-body problem, Reviews of Modern Physics, vol. 70, No. 2

Jenkins, A. (2013). “Self-oscillation.” Physics Reports-Review Section of Physics Letters 525(2): 167-222.

Morgan, G. J. (1998). “Emile Zuckerkandl, Linus Pauling, and the molecular evolutionary clock, 1959-1965.” Journal of the History of Biology 31(2): 155-178.



Galileo Unbound: The Physics and History of Dynamics

Welcome to Galileo Unbound: The History and Physics of Dynamics. This is the Blog site where you can find the historical background and the physical concepts behind many of the current trends in the physics of complex systems. It is written at the level of college undergraduates in fields of study like science or engineering. Advanced high school students should be able to find little gems here, too.

The topics here will fall under two headings that mirror my two recent books: Introduction to Modern Dyanamics (Oxford University Press, 2015) and Galileo Unbound (Oxford University Press, 2018). The first is a college junior-level physics textbook describing the mathematical details of modern dynamics. The second is a general interest book on the historical development of the same ideas. The physical concepts in both books will be expanded upon in this Blog at a general level of understanding. I hope you enjoy the broad range of topics that will appear here.

Good company in a journey makes the way seem shorter. — Izaak Walton