The Physics of Modern Dynamics (with Python Programs)

It is surprising how much of modern dynamics boils down to an extremely simple formula

This innocuous-looking equation carries such riddles, such surprises, such unintuitive behavior that it can become the object of study for life.  This equation is called a vector flow equation, and it can be used to capture the essential physics of economies, neurons, ecosystems, networks, and even orbits of photons around black holes.  This equation is to modern dynamics what F = ma was to classical mechanics.  It is the starting point for understanding complex systems.

The Magic of Phase Space

The apparent simplicity of the “flow equation” masks the complexity it contains.  It is a vector equation because each “dimension” is a variable of a complex system.  Many systems of interest may have only a few variables, but ecosystems and economies and social networks may have hundreds or thousands of variables.  Expressed in component format, the flow equation is

where the superscript spans the number of variables.  But even this masks all that can happen with such an equation. Each of the functions fa can be entirely different from each other, and can be any type of function, whether polynomial, rational, algebraic, transcendental or composite, although they must be single-valued.  They are generally nonlinear, and the limitless ways that functions can be nonlinear is where the richness of the flow equation comes from.

The vector flow equation is an ordinary differential equation (ODE) that can be solved for specific trajectories as initial value problems.  A single set of initial conditions defines a unique trajectory.  For instance, the trajectory for a 4-dimensional example is described as the column vector

which is the single-parameter position vector to a point in phase space, also called state space.  The point sweeps through successive configurations as a function of its single parameter—time.  This trajectory is also called an orbit.  In classical mechanics, the focus has tended to be on the behavior of specific orbits that arise from a specific set of initial conditions.  This is the classic “rock thrown from a cliff” problem of introductory physics courses.  However, in modern dynamics, the focus shifts away from individual trajectories to encompass the set of all possible trajectories.

Why is Modern Dynamics part of Physics?

If finding the solutions to the “x-dot equals f” vector flow equation is all there is to do, then this would just be a math problem—the solution of ODE’s.  There are plenty of gems for mathematicians to look for, and there is an entire of field of study in mathematics called “dynamical systems“, but this would not be “physics”.  Physics as a profession is separate and distinct from mathematics, although the two are sometimes confused.  Physics uses mathematics as its language and as its toolbox, but physics is not mathematics.  Physics is done best when it is done qualitatively—this means with scribbles done on napkins in restaurants or on the back of envelopes while waiting in line. Physics is about recognizing relationships and patterns. Physics is about identifying the limits to scaling properties where the physics changes when scales change. Physics is about the mapping of the simplest possible mathematics onto behavior in the physical world, and recognizing when the simplest possible mathematics is a universal that applies broadly to diverse systems that seem different, but that share the same underlying principles.

So, granted solving ODE’s is not physics, there is still a tremendous amount of good physics that can be done by solving ODE’s. ODE solvers become the modern physicist’s experimental workbench, providing data output from numerical experiments that can test the dependence on parameters in ways that real-world experiments might not be able to access. Physical intuition can be built based on such simulations as the engaged physicist begins to “understand” how the system behaves, able to explain what will happen as the values of parameters are changed.

In the follow sections, three examples of modern dynamics are introduced with a preliminary study, including Python code. These examples are: Galactic dynamics, synchronized networks and ecosystems. Despite their very different natures, their description using dynamical flows share features in common and illustrate the beauty and depth of behavior that can be explored with simple equations.

Galactic Dynamics

One example of the power and beauty of the vector flow equation and its set of all solutions in phase space is called the Henon-Heiles model of the motion of a star within a galaxy.  Of course, this is a terribly complicated problem that involves tens of billions of stars, but if you average over the gravitational potential of all the other stars, and throw in a couple of conservation laws, the resulting potential can look surprisingly simple.  The motion in the plane of this galactic potential takes two configuration coordinates (x, y) with two associated momenta (px, py) for a total of four dimensions.  The flow equations in four-dimensional phase space are simply

where the terms in the light blue box describe a two-dimensional simple harmonic oscillator (SHO), which is a linear oscillator, modified by the terms in the magenta box that represent the nonlinear galactic potential.  The orbits of this Hamiltonian system are chaotic, and because there is no dissipation in the model, a single orbit will continue forever within certain ranges of phase space governed by energy conservation, but never quite repeating.

Hamilton4D.py

(Python code on GitHub.)

```#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Hamilton4D.py
Created on Wed Apr 18 06:03:32 2018

@author: nolte

Derived from:
D. D. Nolte, Introduction to Modern Dynamics: Chaos, Networks, Space and Time, 2nd ed. (Oxford,2019)
"""

import numpy as np
import matplotlib as mpl
from mpl_toolkits.mplot3d import Axes3D
from scipy import integrate
from matplotlib import pyplot as plt
from matplotlib import cm
import time
import os

plt.close('all')

# model_case 1 = Heiles
# model_case 2 = Crescent
print(' ')
print('Hamilton4D.py')
print('Case: 1 = Heiles')
print('Case: 2 = Crescent')
model_case = int(input('Enter the Model Case (1-2)'))

if model_case == 1:
E = 1       # Heiles: 1, 0.3411   Crescent: 0.05, 1
epsE = 0.3411   # 3411
def flow_deriv(x_y_z_w,tspan):
x, y, z, w = x_y_z_w
a = z
b = w
c = -x - epsE*(2*x*y)
d = -y - epsE*(x**2 - y**2)
return[a,b,c,d]
else:
E = .1       #   Crescent: 0.1, 1
epsE = 1
def flow_deriv(x_y_z_w,tspan):
x, y, z, w = x_y_z_w
a = z
b = w
c = -(epsE*(y-2*x**2)*(-4*x) + x)
d = -(y-epsE*2*x**2)
return[a,b,c,d]

prms = np.sqrt(E)
pmax = np.sqrt(2*E)

# Potential Function
if model_case == 1:
V = np.zeros(shape=(100,100))
for xloop in range(100):
x = -2 + 4*xloop/100
for yloop in range(100):
y = -2 + 4*yloop/100
V[yloop,xloop] = 0.5*x**2 + 0.5*y**2 + epsE*(x**2*y - 0.33333*y**3)
else:
V = np.zeros(shape=(100,100))
for xloop in range(100):
x = -2 + 4*xloop/100
for yloop in range(100):
y = -2 + 4*yloop/100
V[yloop,xloop] = 0.5*x**2 + 0.5*y**2 + epsE*(2*x**4 - 2*x**2*y)

fig = plt.figure(1)
contr = plt.contourf(V,100, cmap=cm.coolwarm, vmin = 0, vmax = 10)
fig.colorbar(contr, shrink=0.5, aspect=5)
fig = plt.show()

repnum = 250
mulnum = 64/repnum

np.random.seed(1)
for reploop  in range(repnum):
px1 = 2*(np.random.random((1))-0.499)*pmax
py1 = np.sign(np.random.random((1))-0.499)*np.real(np.sqrt(2*(E-px1**2/2)))
xp1 = 0
yp1 = 0

x_y_z_w0 = [xp1, yp1, px1, py1]

tspan = np.linspace(1,1000,10000)
x_t = integrate.odeint(flow_deriv, x_y_z_w0, tspan)
siztmp = np.shape(x_t)
siz = siztmp[0]

if reploop % 50 == 0:
plt.figure(2)
lines = plt.plot(x_t[:,0],x_t[:,1])
plt.setp(lines, linewidth=0.5)
plt.show()
time.sleep(0.1)
#os.system("pause")

y1 = x_t[:,0]
y2 = x_t[:,1]
y3 = x_t[:,2]
y4 = x_t[:,3]

py = np.zeros(shape=(2*repnum,))
yvar = np.zeros(shape=(2*repnum,))
cnt = -1
last = y1[1]
for loop in range(2,siz):
if (last < 0)and(y1[loop] > 0):
cnt = cnt+1
del1 = -y1[loop-1]/(y1[loop] - y1[loop-1])
py[cnt] = y4[loop-1] + del1*(y4[loop]-y4[loop-1])
yvar[cnt] = y2[loop-1] + del1*(y2[loop]-y2[loop-1])
last = y1[loop]
else:
last = y1[loop]

plt.figure(3)
lines = plt.plot(yvar,py,'o',ms=1)
plt.show()

if model_case == 1:
plt.savefig('Heiles')
else:
plt.savefig('Crescent')

```

Networks, Synchronization and Emergence

A central paradigm of nonlinear science is the emergence of patterns and organized behavior from seemingly random interactions among underlying constituents.  Emergent phenomena are among the most awe inspiring topics in science.  Crystals are emergent, forming slowly from solutions of reagents.  Life is emergent, arising out of the chaotic soup of organic molecules on Earth (or on some distant planet).  Intelligence is emergent, and so is consciousness, arising from the interactions among billions of neurons.  Ecosystems are emergent, based on competition and symbiosis among species.  Economies are emergent, based on the transfer of goods and money spanning scales from the local bodega to the global economy.

One of the common underlying properties of emergence is the existence of networks of interactions.  Networks and network science are topics of great current interest driven by the rise of the World Wide Web and social networks.  But networks are ubiquitous and have long been the topic of research into complex and nonlinear systems.  Networks provide a scaffold for understanding many of the emergent systems.  It allows one to think of isolated elements, like molecules or neurons, that interact with many others, like the neighbors in a crystal or distant synaptic connections.

From the point of view of modern dynamics, the state of a node can be a variable or a “dimension” and the interactions among links define the functions of the vector flow equation.  Emergence is then something that “emerges” from the dynamical flow as many elements interact through complex networks to produce simple or emergent patterns.

Synchronization is a form of emergence that happens when lots of independent oscillators, each vibrating at their own personal frequency, are coupled together to push and pull on each other, entraining all the individual frequencies into one common global oscillation of the entire system.  Synchronization plays an important role in the solar system, explaining why the Moon always shows one face to the Earth, why Saturn’s rings have gaps, and why asteroids are mainly kept away from colliding with the Earth.  Synchronization plays an even more important function in biology where it coordinates the beating of the heart and the functioning of the brain.

One of the most dramatic examples of synchronization is the Kuramoto synchronization phase transition. This occurs when a large set of individual oscillators with differing natural frequencies interact with each other through a weak nonlinear coupling.  For small coupling, all the individual nodes oscillate at their own frequency.  But as the coupling increases, there is a sudden coalescence of all the frequencies into a single common frequency.  This mechanical phase transition, called the Kuramoto transition, has many of the properties of a thermodynamic phase transition, including a solution that utilizes mean field theory.

The simulation of 20 Poncaré phase oscillators with global coupling is shown in Fig. 4 as a function of increasing coupling coefficient g. The original individual frequencies are spread randomly. The oscillators with similar frequencies are the first to synchronize, forming small clumps that then synchronize with other clumps of oscillators, until all oscillators are entrained to a single compromise frequency. The Kuramoto phase transition is not sharp in this case because the value of N = 20 is too small. If the simulation is run for 200 oscillators, there is a sudden transition from unsynchronized to synchronized oscillation at a threshold value of g.

The Kuramoto phase transition is one of the most important fundamental examples of modern dynamics because it illustrates many facets of nonlinear dynamics in a very simple way. It highlights the importance of nonlinearity, the simplification of phase oscillators, the use of mean field theory, the underlying structure of the network, and the example of a mechanical analog to a thermodynamic phase transition. It also has analytical solutions because of its simplicity, while still capturing the intrinsic complexity of nonlinear systems.

Kuramoto.py

(Python code on GitHub.)

```#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sat May 11 08:56:41 2019

@author: nolte

D. D. Nolte, Introduction to Modern Dynamics: Chaos, Networks, Space and Time, 2nd ed. (Oxford,2019)
"""

# https://www.python-course.eu/networkx.php
# https://networkx.github.io/documentation/stable/tutorial.html
# https://networkx.github.io/documentation/stable/reference/functions.html

import numpy as np
from scipy import integrate
from matplotlib import pyplot as plt
import networkx as nx
from UserFunction import linfit
import time

tstart = time.time()

plt.close('all')

Nfac = 25   # 25
N = 50      # 50
width = 0.2

# model_case 1 = complete graph (Kuramoto transition)
# model_case 2 = Erdos-Renyi
model_case = int(input('Input Model Case (1-2)'))
if model_case == 1:
facoef = 0.2
nodecouple = nx.complete_graph(N)
elif model_case == 2:
facoef = 5
nodecouple = nx.erdos_renyi_graph(N,0.1)

# function: omegout, yout = coupleN(G)
def coupleN(G):

# function: yd = flow_deriv(x_y)
def flow_deriv(y,t0):

yp = np.zeros(shape=(N,))
for omloop  in range(N):
temp = omega[omloop]
g = G.node[omloop]['coupling'][cloop]

temp = temp + g*np.sin(y[cindex]-y[omloop])

yp[omloop] = temp

yd = np.zeros(shape=(N,))
for omloop in range(N):
yd[omloop] = yp[omloop]

return yd
# end of function flow_deriv(x_y)

mnomega = 1.0

for nodeloop in range(N):
omega[nodeloop] = G.node[nodeloop]['element']

x_y_z = omega

# Settle-down Solve for the trajectories
tsettle = 100
t = np.linspace(0, tsettle, tsettle)
x_t = integrate.odeint(flow_deriv, x_y_z, t)
x0 = x_t[tsettle-1,0:N]

t = np.linspace(1,1000,1000)
y = integrate.odeint(flow_deriv, x0, t)
siztmp = np.shape(y)
sy = siztmp[0]

# Fit the frequency
m = np.zeros(shape = (N,))
w = np.zeros(shape = (N,))
mtmp = np.zeros(shape=(4,))
btmp = np.zeros(shape=(4,))
for omloop in range(N):

if np.remainder(sy,4) == 0:
mtmp[0],btmp[0] = linfit(t[0:sy//2],y[0:sy//2,omloop]);
mtmp[1],btmp[1] = linfit(t[sy//2+1:sy],y[sy//2+1:sy,omloop]);
mtmp[2],btmp[2] = linfit(t[sy//4+1:3*sy//4],y[sy//4+1:3*sy//4,omloop]);
mtmp[3],btmp[3] = linfit(t,y[:,omloop]);
else:
sytmp = 4*np.floor(sy/4);
mtmp[0],btmp[0] = linfit(t[0:sytmp//2],y[0:sytmp//2,omloop]);
mtmp[1],btmp[1] = linfit(t[sytmp//2+1:sytmp],y[sytmp//2+1:sytmp,omloop]);
mtmp[2],btmp[2] = linfit(t[sytmp//4+1:3*sytmp/4],y[sytmp//4+1:3*sytmp//4,omloop]);
mtmp[3],btmp[3] = linfit(t[0:sytmp],y[0:sytmp,omloop]);

#m[omloop] = np.median(mtmp)
m[omloop] = np.mean(mtmp)

w[omloop] = mnomega + m[omloop]

omegout = m
yout = y

return omegout, yout
# end of function: omegout, yout = coupleN(G)

omega = np.zeros(shape=(N,))
omegatemp = width*(np.random.rand(N)-1)
meanomega = np.mean(omegatemp)
omega = omegatemp - meanomega
sto = np.std(omega)

lnk = np.zeros(shape = (N,), dtype=int)
for loop in range(N):
nodecouple.node[loop]['element'] = omega[loop]
lnk[loop] = np.size(list(nx.neighbors(nodecouple,loop)))

avgdegree = np.mean(lnk)
mnomega = 1

facval = np.zeros(shape = (Nfac,))
yy = np.zeros(shape=(Nfac,N))
xx = np.zeros(shape=(Nfac,))
for facloop in range(Nfac):
print(facloop)

fac = facoef*(16*facloop/(Nfac))*(1/(N-1))*sto/mnomega
for nodeloop in range(N):
nodecouple.node[nodeloop]['coupling'] = np.zeros(shape=(lnk[nodeloop],))

facval[facloop] = fac*avgdegree

omegout, yout = coupleN(nodecouple)                           # Here is the subfunction call for the flow

for omloop in range(N):
yy[facloop,omloop] = omegout[omloop]

xx[facloop] = facval[facloop]

plt.figure(1)
lines = plt.plot(xx,yy)
plt.setp(lines, linewidth=0.5)
plt.show()

elapsed_time = time.time() - tstart
print('elapsed time = ',format(elapsed_time,'.2f'),'secs')

```

The Web of Life

Ecosystems are among the most complex systems on Earth.  The complex interactions among hundreds or thousands of species may lead to steady homeostasis in some cases, to growth and collapse in other cases, and to oscillations or chaos in yet others.  But the definition of species can be broad and abstract, referring to businesses and markets in economic ecosystems, or to cliches and acquaintances in social ecosystems, among many other examples.  These systems are governed by the laws of evolutionary dynamics that include fitness and survival as well as adaptation.

The dimensionality of the dynamical spaces for these systems extends to hundreds or thousands of dimensions—far too complex to visualize when thinking in four dimensions is already challenging.  Yet there are shared principles and common behaviors that emerge even here.  Many of these can be illustrated in a simple three-dimensional system that is represented by a triangular simplex that can be easily visualized, and then generalized back to ultra-high dimensions once they are understood.

A simplex is a closed (N-1)-dimensional geometric figure that describes a zero-sum game (game theory is an integral part of evolutionary dynamics) among N competing species.  For instance, a two-simplex is a triangle that captures the dynamics among three species.  Each vertex of the triangle represents the situation when the entire ecosystem is composed of a single species.  Anywhere inside the triangle represents the situation when all three species are present and interacting.

A classic model of interacting species is the replicator equation. It allows for a fitness-based proliferation and for trade-offs among the individual species. The replicator dynamics equations are shown in Fig. 5.

The population dynamics on the 2D simplex are shown in Fig. 6 for several different pay-off matrices. The matrix values are shown in color and help interpret the trajectories. For instance the simplex on the upper-right shows a fixed point center. This reflects the antisymmetric character of the pay-off matrix around the diagonal. The stable spiral on the lower-left has a nearly asymmetric pay-off matrix, but with unequal off-diagonal magnitudes. The other two cases show central saddle points with stable fixed points on the boundary. A very large variety of behaviors are possible for this very simple system. The Python program is shown in Trirep.py.

Trirep.py

(Python code on GitHub.)

```#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
trirep.py
Created on Thu May  9 16:23:30 2019

@author: nolte

Derived from:
D. D. Nolte, Introduction to Modern Dynamics: Chaos, Networks, Space and Time, 2nd ed. (Oxford,2019)
"""

import numpy as np
from scipy import integrate
from matplotlib import pyplot as plt

plt.close('all')

def tripartite(x,y,z):

sm = x + y + z
xp = x/sm
yp = y/sm

f = np.sqrt(3)/2

y0 = f*xp
x0 = -0.5*xp - yp + 1;

plt.figure(2)
lines = plt.plot(x0,y0)
plt.setp(lines, linewidth=0.5)
plt.plot([0, 1],[0, 0],'k',linewidth=1)
plt.plot([0, 0.5],[0, f],'k',linewidth=1)
plt.plot([1, 0.5],[0, f],'k',linewidth=1)
plt.show()

def solve_flow(y,tspan):
def flow_deriv(y, t0):
#"""Compute the time-derivative ."""

f = np.zeros(shape=(N,))
for iloop in range(N):
ftemp = 0
for jloop in range(N):
ftemp = ftemp + A[iloop,jloop]*y[jloop]
f[iloop] = ftemp

phitemp = phi0          # Can adjust this from 0 to 1 to stabilize (but Nth population is no longer independent)
for loop in range(N):
phitemp = phitemp + f[loop]*y[loop]
phi = phitemp

yd = np.zeros(shape=(N,))
for loop in range(N-1):
yd[loop] = y[loop]*(f[loop] - phi);

if np.abs(phi0) < 0.01:             # average fitness maintained at zero
yd[N-1] = y[N-1]*(f[N-1]-phi);
else:                                     # non-zero average fitness
ydtemp = 0
for loop in range(N-1):
ydtemp = ydtemp - yd[loop]
yd[N-1] = ydtemp

return yd

# Solve for the trajectories
t = np.linspace(0, tspan, 701)
x_t = integrate.odeint(flow_deriv,y,t)
return t, x_t

# model_case 1 = zero diagonal
# model_case 2 = zero trace
# model_case 3 = asymmetric (zero trace)
print(' ')
print('trirep.py')
print('Case: 1 = antisymm zero diagonal')
print('Case: 2 = antisymm zero trace')
print('Case: 3 = random')
model_case = int(input('Enter the Model Case (1-3)'))

N = 3
asymm = 3      # 1 = zero diag (replicator eqn)   2 = zero trace (autocatylitic model)  3 = random (but zero trace)
phi0 = 0.001            # average fitness (positive number) damps oscillations
T = 100;

if model_case == 1:
Atemp = np.zeros(shape=(N,N))
for yloop in range(N):
for xloop in range(yloop+1,N):
Atemp[yloop,xloop] = 2*(0.5 - np.random.random(1))
Atemp[xloop,yloop] = -Atemp[yloop,xloop]

if model_case == 2:
Atemp = np.zeros(shape=(N,N))
for yloop in range(N):
for xloop in range(yloop+1,N):
Atemp[yloop,xloop] = 2*(0.5 - np.random.random(1))
Atemp[xloop,yloop] = -Atemp[yloop,xloop]
Atemp[yloop,yloop] = 2*(0.5 - np.random.random(1))
tr = np.trace(Atemp)
A = Atemp
for yloop in range(N):
A[yloop,yloop] = Atemp[yloop,yloop] - tr/N

else:
Atemp = np.zeros(shape=(N,N))
for yloop in range(N):
for xloop in range(N):
Atemp[yloop,xloop] = 2*(0.5 - np.random.random(1))

tr = np.trace(Atemp)
A = Atemp
for yloop in range(N):
A[yloop,yloop] = Atemp[yloop,yloop] - tr/N

plt.figure(3)
im = plt.matshow(A,3,cmap=plt.cm.get_cmap('seismic'))  # hsv, seismic, bwr
cbar = im.figure.colorbar(im)

M = 20
delt = 1/M
ep = 0.01;

tempx = np.zeros(shape = (3,))
for xloop in range(M):
tempx[0] = delt*(xloop)+ep;
for yloop in range(M-xloop):
tempx[1] = delt*yloop+ep
tempx[2] = 1 - tempx[0] - tempx[1]

x0 = tempx/np.sum(tempx);          # initial populations

tspan = 70
t, x_t = solve_flow(x0,tspan)

y1 = x_t[:,0]
y2 = x_t[:,1]
y3 = x_t[:,2]

plt.figure(1)
lines = plt.plot(t,y1,t,y2,t,y3)
plt.setp(lines, linewidth=0.5)
plt.show()
plt.ylabel('X Position')
plt.xlabel('Time')

tripartite(y1,y2,y3)
```

Topics in Modern Dynamics

These three examples are just the tip of the iceberg. The topics in modern dynamics are almost numberless. Any system that changes in time is a potential object of study in modern dynamics. Here is a list of a few topics that spring to mind.

Bibliography

D. D. Nolte, Introduction to Modern Dynamics: Chaos, Networks, Space and Time, 2nd Ed. (Oxford University Press, 2019) (The physics and the derivations of the equations for the examples in this blog can be found here.)

D. D. Nolte, Galileo Unbound: A Path Across Life, the Universe and Everything (Oxford University Press, 2018) (The historical origins of the examples in this blog can be found here.)

How Number Theory Protects You from the Chaos of the Cosmos

We are exceedingly fortunate that the Earth lies in the Goldilocks zone.  This zone is the range of orbital radii of a planet around its sun for which water can exist in a liquid state.  Water is the universal solvent, and it may be a prerequisite for the evolution of life.  If we were too close to the sun, water would evaporate as steam.  And if we are too far, then it would be locked in perpetual ice.  As it is, the Earth has had wild swings in its surface temperature.  There was once a time, more than 650 million years ago, when the entire Earth’s surface froze over.  Fortunately, the liquid oceans remained liquid, and life that already existed on Earth was able to persist long enough to get to the Cambrian explosion.  Conversely, Venus may once have had liquid oceans and maybe even nascent life, but too much carbon dioxide turned the planet into an oven and boiled away its water (a fate that may await our own Earth if we aren’t careful).  What has saved us so far is the stability of our orbit, our steady distance from the Sun that keeps our water liquid and life flourishing.  Yet it did not have to be this way.

The regions of regular motion associated with irrational numbers act as if they were a barrier, restricting the range of chaotic orbits and protecting other nearby orbits from the chaos.

Our solar system is a many-body problem.  It consists of three large gravitating bodies (Sun, Jupiter, Saturn) and several minor ones (such as Earth).   Jupiter does influence our orbit, and if it were only a few times more massive than it actually is, then our orbit would become chaotic, varying in distance from the sun in unpredictable ways.  And if Jupiter were only about 20 times bigger than is actually is, there is a possibility that it would perturb the Earth’s orbit so strongly that it could eject the Earth from the solar system entirely, sending us flying through interstellar space, where we would slowly cool until we became a permanent ice ball.  What can protect us from this terrifying fate?  What keeps our orbit stable despite the fact that we inhabit a many-body solar system?  The answer is number theory!

The Most Irrational Number

What is the most irrational number you can think of?

Is it: pi = 3.1415926535897932384626433 ?

Or Euler’s constant: e = 2.7182818284590452353602874 ?

How about: sqrt(3) = 1.73205080756887729352744634 ?

These are all perfectly good irrational numbers.  But how do you choose the “most irrational” number?  The answer is fairly simple.  The most irrational number is the one that is least well approximated by a ratio of integers.  For instance, it is possible to get close to pi through the ratio 22/7 = 3.1428 which differs from pi by only 4 parts in ten thousand.  Or Euler’s constant 87/32 = 2.7188 differs from e by only 2 parts in ten thousand.  Yet 87 and 32 are much bigger than 22 and 7, so it may be said that e is more irrational than pi, because it takes ratios of larger integers to get a good approximation.  So is there a “most irrational” number?  The answer is yes.  The Golden Ratio.

The Golden ratio can be defined in many ways, but its most common expression is given by

It is the hardest number to approximate with a ratio of small integers.  For instance, to get a number that is as close as one part in ten thousand to the golden mean takes the ratio 89/55.  This result may seem obscure, but there is a systematic way to find the ratios of integers that approximate an irrational number. This is known as a convergent from continued fractions.

Continued fractions were invented by John Wallis in 1695, introduced in his book Opera Mathematica.  The continued fraction for pi is

An alternate form of displaying this continued fraction is with the expression

The irrational character of pi is captured by the seemingly random integers in this string. However, there can be regular structure in irrational numbers. For instance, a different continued fraction for pi is

that has a surprisingly simple repeating pattern.

The continued fraction for the golden mean has an especially simple repeating form

or

This continued fraction has the slowest convergence for its continued fraction of any other number. Hence, the Golden Ratio can be considered, using this criterion, to be the most irrational number.

If the Golden Ratio is the most irrational number, how does that save us from the chaos of the cosmos? The answer to this question is KAM!

Kolmogorov, Arnold and Moser: (KAM) Theory

KAM is an acronym made from the first initials of three towering mathematicians of the 20th century: Andrey Kolmogorov (1903 – 1987), his student Vladimir Arnold (1937 – 2010), and Jürgen Moser (1928 – 1999).

In 1954, Kolmogorov, considered to be the greatest living mathematician at that time, was invited to give the plenary lecture at a mathematics conference. To the surprise of the conference organizers, he chose to talk on what seemed like a very mundane topic: the question of the stability of the solar system. This had been the topic which Poincaré had attempted to solve in 1890 when he first stumbled on chaotic dynamics. The question had remained open, but the general consensus was that the many-body nature of the solar system made it intrinsically unstable, even for only three bodies.

Against all expectations, Kolmogorov proposed that despite the general chaotic behavior of the three–body problem, there could be “islands of stability” which were protected from chaos, allowing some orbits to remain regular even while other nearby orbits were highly chaotic. He even outlined an approach to a proof of his conjecture, though he had not carried it through to completion.

The proof of Kolmogorov’s conjecture was supplied over the next 10 years through the work of the German mathematician Jürgen Moser and by Kolmogorov’s former student Vladimir Arnold. The proof hinged on the successive ratios of integers that approximate irrational numbers. With this work KAM showed that indeed some orbits are actually protected from neighboring chaos by relying on the irrationality of the ratio of orbital periods.

Resonant Ratios

Let’s go back to the simple model of our solar system that consists of only three bodies: the Sun, Jupiter and Earth. The period of Jupiter’s orbit is 11.86 years, but instead, if it were exactly 12 years, then its period would be in a 12:1 ratio with the Earth’s period. This ratio of integers is called a “resonance”, although in this case it is fairly mismatched. But if this ratio were a ratio of small integers like 5:3, then it means that Jupiter would travel around the sun 5 times in 15 years while the Earth went around 3 times. And every 15 years, the two planets would align. This kind of resonance with ratios of small integers creates a strong gravitational perturbation that alters the orbit of the smaller planet. If the perturbation is strong enough, it could disrupt the Earth’s orbit, creating a chaotic path that might ultimately eject the Earth completely from the solar system.

What KAM discovered is that as the resonance ratio becomes a ratio of large integers, like 87:32, then the planets have a hard time aligning, and the perturbation remains small. A surprising part of this theory is that a nearby orbital ratio might be 5:2 = 1.5, which is only a little different than 87:32 = 1.7. Yet the 5:2 resonance can produce strong chaos, while the 87:32 resonance is almost immune. This way, it is possible to have both chaotic orbits and regular orbits coexisting in the same dynamical system. An irrational orbital ratio protects the regular orbits from chaos. The next question is, how irrational does the orbital ratio need to be to guarantee safety?

You probably already guessed the answer to this question–the answer must be the Golden Ratio. If this is indeed the most irrational number, then it cannot be approximated very well with ratios of small integers, and this is indeed the case. In a three-body system, the most stable orbital ratio would be a ratio of 1.618034. But the more general question of what is “irrational enough” for an orbit to be stable against a given perturbation is much harder to answer. This is the field of Diophantine Analysis, which addresses other questions as well, such as Fermat’s Last Theorem.

KAM Twist Map

The dynamics of three-body systems are hard to visualize directly, so there are tricks that help bring the problem into perspective. The first trick, invented by Henri Poincaré, is called the first return map (or the Poincaré section). This is a way of reducing the dimensionality of the problem by one dimension. But for three bodies, even if they are all in a plane, this still can be complicated. Another trick, called the restricted three-body problem, is to assume that there are two large masses and a third small mass. This way, the dynamics of the two-body system is unaffected by the small mass, so all we need to do is focus on the dynamics of the small body. This brings the dynamics down to two dimensions (the position and momentum of the third body), which is very convenient for visualization, but the dynamics still need solutions to differential equations. So the final trick is to replace the differential equations with simple difference equations that are solved iteratively.

A simple discrete iterative map that captures the essential behavior of the three-body problem begins with action-angle variables that are coupled through a perturbation. Variations on this model have several names: the Twist Map, the Chirikov Map and the Standard Map. The essential mapping is

where J is an action variable (like angular momentum) paired with the angle variable. Initial conditions for the action and the angle are selected, and then all later values are obtained by iteration. The perturbation parameter is given by ε. If ε = 0 then all orbits are perfectly regular and circular. But as the perturbation increases, the open orbits split up into chains of closed (periodic) orbits. As the perturbation increases further, chaotic behavior emerges. The situation for ε = 0.9 is shown in the figure below. There are many regular periodic orbits as well as open orbits. Yet there are simultaneously regions of chaotic behavior. This figure shows an intermediate case where regular orbits can coexist with chaotic ones. The key is the orbital period ratio. For orbital ratios that are sufficiently irrational, the orbits remain open and regular. Bur for orbital ratios that are ratios of small integers, the perturbation is strong enough to drive the dynamics into chaos.

Python Code

(Python code on GitHub.)

```#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Wed Oct. 2, 2019
@author: nolte
"""
import numpy as np
from scipy import integrate
from matplotlib import pyplot as plt
plt.close('all')

eps = 0.9

np.random.seed(2)
plt.figure(1)
for eloop in range(0,50):

rlast = np.pi*(1.5*np.random.random()-0.5)
thlast = 2*np.pi*np.random.random()

orbit = np.int(200*(rlast+np.pi/2))
rplot = np.zeros(shape=(orbit,))
thetaplot = np.zeros(shape=(orbit,))
x = np.zeros(shape=(orbit,))
y = np.zeros(shape=(orbit,))
for loop in range(0,orbit):
rnew = rlast + eps*np.sin(thlast)
thnew = np.mod(thlast+rnew,2*np.pi)

rplot[loop] = rnew
thetaplot[loop] = np.mod(thnew-np.pi,2*np.pi) - np.pi

rlast = rnew
thlast = thnew

x[loop] = (rnew+np.pi+0.25)*np.cos(thnew)
y[loop] = (rnew+np.pi+0.25)*np.sin(thnew)

plt.plot(x,y,'o',ms=1)

plt.savefig('StandMapTwist')
```

The twist map for three values of ε are shown in the figure below. For ε = 0.2, most orbits are open, with one elliptic point and its associated hyperbolic point. At ε = 0.9 the periodic elliptic point is still stable, but the hyperbolic point has generated a region of chaotic orbits. There is still a remnant open orbit that is associated with an orbital period ratio at the Golden Ratio. However, by ε = 0.97, even this most stable orbit has broken up into a chain of closed orbits as the chaotic regions expand.

Safety in Numbers

In our solar system, governed by gravitational attractions, the square of the orbital period increases as the cube of the average radius (Kepler’s third law). Consider the restricted three-body problem of the Sun and Jupiter with the Earth as the third body. If we analyze the stability of the Earth’s orbit as a function of distance from the Sun, the orbital ratio relative to Jupiter would change smoothly. Near our current position, it would be in a 12:1 resonance, but as we moved farther from the Sun, this ratio would decrease. When the orbital period ratio is sufficiently irrational, then the orbit would be immune to Jupiter’s pull. But as the orbital ratio approaches ratios of integers, the effect gets larger. Close enough to Jupiter there would be a succession of radii that had regular motion separated by regions of chaotic motion. The regions of regular motion associated with irrational numbers act as if they were a barrier, restricting the range of chaotic orbits and protecting more distant orbits from the chaos. In this way numbers, rational versus irrational, protect us from the chaos of our own solar system.

A dramatic demonstration of the orbital resonance effect can be seen with the asteroid belt. The many small bodies act as probes of the orbital resonances. The repetitive tug of Jupiter opens gaps in the distribution of asteroid radii, with major gaps, called Kirkwood Gaps, opening at orbital ratios of 3:1, 5:2, 7:3 and 2:1. These gaps are the radii where chaotic behavior occurs, while the regions in between are stable. Most asteroids spend most of their time in the stable regions, because chaotic motion tends to sweep them out of the regions of resonance. This mechanism for the Kirkwood gaps is the same physics that produces gaps in the rings of Saturn at resonances with the many moons of Saturn.

For a detailed history of the development of KAM theory, see Chapter 9 Butterflies to Hurricanes in Galileo Unbound (Oxford University Press, 2018).

For a more detailed mathematical description of the KAM theory, see Chapter 5, Hamiltonian Chaos, in Introduction to Modern Dynamics, 2nd edition (Oxford University Press, 2019).

Dumas, H. S., The KAM Story: A friendly introduction to the content, history and significance of Classical Kolmogorov-Arnold-Moser Theory. World Scientific: 2014.

Arnold, V. I., From superpositions to KAM theory. Vladimir Igorevich Arnold. Selected Papers 1997, PHASIS, 60, 727–740.

Read all the stories behind the history of chaos theory, in Galileo Unbound from Oxford Press:

The 1960’s are known as a time of cultural revolution, but perhaps less known was the revolution that occurred in the science of dynamics.  Three towering figures of that revolution were Stephen Smale (1930 – ) at Berkeley, Andrey Kolmogorov (1903 – 1987) in Moscow and his student Vladimir Arnold (1937 – 2010).  Arnold was only 20 years old in 1957 when he solved Hilbert’s thirteenth problem (that any continuous function of several variables can be constructed with a finite number of two-variable functions).  Only a few years later his work on the problem of small denominators in dynamical systems provided the finishing touches on the long elusive explanation of the stability of the solar system (the problem for which Poincaré won the King Oscar Prize in mathematics in 1889 when he discovered chaotic dynamics ).  This theory is known as KAM-theory, using the first initials of the names of Kolmogorov, Arnold and Moser [1].  Building on his breakthrough in celestial mechanics, Arnold’s work through the 1960’s remade the theory of Hamiltonian systems, creating a shift in perspective that has permanently altered how physicists look at dynamical systems.

Hamiltonian Physics on a Torus

Traditionally, Hamiltonian physics is associated with systems of inertial objects that conserve the sum of kinetic and potential energy, in other words, conservative non-dissipative systems.  But a modern view (after Arnold) of Hamiltonian systems sees them as hyperdimensional mathematical mappings that conserve volume.  The space that these mappings inhabit is phase space, and the conservation of phase-space volume is known as Liouville’s Theorem [2].  The geometry of phase space is called symplectic geometry, and the universal position that symplectic geometry now holds in the physics of Hamiltonian mechanics is largely due to Arnold’s textbook Mathematical Methods of Classical Mechanics (1974, English translation 1978) [3]. Arnold’s famous quote from that text is “Hamiltonian mechanics is geometry in phase space”.

One of the striking aspects of this textbook is the reduction of phase-space geometry to the geometry of a hyperdimensional torus for a large number of Hamiltonian systems.  If there are as many conserved quantities as there are degrees of freedom in a Hamiltonian system, then the system is called “integrable” (because you can integrated the equations of motion to find a constant of the motion). Then it is possible to map the physics onto a hyperdimensional torus through the transformation of dynamical coordinates into what are known as “action-angle” coordinates [4].  Each independent angle has an associated action that is conserved during the motion of the system.  The periodicity of the dynamical angle coordinate makes it possible to identify it with the angular coordinate of a multi-dimensional torus.  Therefore, every integrable Hamiltonian system can be mapped to motion on a multi-dimensional torus (one dimension for each degree of freedom of the system).

Actually, integrable Hamiltonian systems are among the most boring dynamical systems you can imagine. They literally just go in circles (around the torus). But as soon as you add a small perturbation that cannot be integrated they produce some of the most complex and beautiful patterns of all dynamical systems. It was Arnold’s focus on motions on a torus, and perturbations that shift the dynamics off the torus, that led him to propose a simple mapping that captured the essence of Hamiltonian chaos.

The Arnold Cat Map

Motion on a two-dimensional torus is defined by two angles, and trajectories on a two-dimensional torus are simple helixes. If the periodicities of the motion in the two angles have an integer ratio, the helix repeats itself. However, if the ratio of periods (also known as the winding number) is irrational, then the helix never repeats and passes arbitrarily closely to any point on the surface of the torus. This last case leads to an “ergodic” system, which is a term introduced by Boltzmann to describe a physical system whose trajectory fills phase space. The behavior of a helix for rational or irrational winding number is not terribly interesting. It’s just an orbit going in circles like an integrable Hamiltonian system. The helix can never even cross itself.

However, if you could add a new dimension to the torus (or add a new degree of freedom to the dynamical system), then the helix could pass over or under itself by moving into the new dimension. By weaving around itself, a trajectory can become chaotic, and the set of many trajectories can become as mixed up as a bowl of spaghetti. This can be a little hard to visualize, especially in higher dimensions, but Arnold thought of a very simple mathematical mapping that captures the essential motion on a torus, preserving volume as required for a Hamiltonian system, but with the ability for regions to become all mixed up, just like trajectories in a nonintegrable Hamiltonian system.

A unit square is isomorphic to a two-dimensional torus. This means that there is a one-to-one mapping of each point on the unit square to each point on the surface of a torus. Imagine taking a sheet of paper and forming a tube out of it. One of the dimensions of the sheet of paper is now an angle coordinate that is cyclic, going around the circumference of the tube. Now if the sheet of paper is flexible (like it is made of thin rubber) you can bend the tube around and connect the top of the tube with the bottom, like a bicycle inner tube. The other dimension of the sheet of paper is now also an angle coordinate that is cyclic. In this way a flat sheet is converted (with some bending) into a torus.

Arnold’s key idea was to create a transformation that takes the torus into itself, preserving volume, yet including the ability for regions to pass around each other. Arnold accomplished this with the simple map

where the modulus 1 takes the unit square into itself. This transformation can also be expressed as a matrix

followed by taking modulus 1. The transformation matrix is called a Floquet matrix, and the determinant of the matrix is equal to unity, which ensures that volume is conserved.

Arnold decided to illustrate this mapping by using a crude image of the face of a cat (See Fig. 1). Successive applications of the transformation stretch and shear the cat, which is then folded back into the unit square. The stretching and folding preserve the volume, but the image becomes all mixed up, just like mixing in a chaotic Hamiltonian system, or like an immiscible dye in water that is stirred.

Recurrence

When the transformation matrix is applied to continuous values, it produces a continuous range of transformed values that become thinner and thinner until the unit square is uniformly mixed. However, if the unit square is discrete, made up of pixels, then something very different happens (see Fig. 3). The image of the cat in this case is composed of a 50×50 array of pixels. For early iterations, the image becomes stretched and mixed, but at iteration 50 there are 4 low-resolution upside-down versions of the cat, and at iteration 75 the cat fully reforms, but is upside-down. Continuing on, the cat eventually reappears fully reformed and upright at iteration 150. Therefore, the discrete case displays a recurrence and the mapping is periodic. Calculating the period of the cat map on lattices can lead to interesting patterns, especially if the lattice is composed of prime numbers [6].

The Cat Map and the Golden Mean

The golden mean, or the golden ratio, 1.618033988749895 is never far away when working with Hamiltonian systems. Because the golden mean is the “most irrational” of all irrational numbers, it plays an essential role in KAM theory on the stability of the solar system. In the case of Arnold’s cat map, it pops up its head in several ways. For instance, the transformation matrix has eigenvalues

with the remarkable property that

which guarantees conservation of area.

Selected V. I. Arnold Publications

Arnold, V. I. “FUNCTIONS OF 3 VARIABLES.” Doklady Akademii Nauk Sssr 114(4): 679-681. (1957)

Arnold, V. I. “GENERATION OF QUASI-PERIODIC MOTION FROM A FAMILY OF PERIODIC MOTIONS.” Doklady Akademii Nauk Sssr 138(1): 13-&. (1961)

Arnold, V. I. “STABILITY OF EQUILIBRIUM POSITION OF A HAMILTONIAN SYSTEM OF ORDINARY DIFFERENTIAL EQUATIONS IN GENERAL ELLIPTIC CASE.” Doklady Akademii Nauk Sssr 137(2): 255-&. (1961)

Arnold, V. I. “BEHAVIOUR OF AN ADIABATIC INVARIANT WHEN HAMILTONS FUNCTION IS UNDERGOING A SLOW PERIODIC VARIATION.” Doklady Akademii Nauk Sssr 142(4): 758-&. (1962)

Arnold, V. I. “CLASSICAL THEORY OF PERTURBATIONS AND PROBLEM OF STABILITY OF PLANETARY SYSTEMS.” Doklady Akademii Nauk Sssr 145(3): 487-&. (1962)

Arnold, V. I. “BEHAVIOUR OF AN ADIABATIC INVARIANT WHEN HAMILTONS FUNCTION IS UNDERGOING A SLOW PERIODIC VARIATION.” Doklady Akademii Nauk Sssr 142(4): 758-&. (1962)

Arnold, V. I. and Y. G. Sinai. “SMALL PERTURBATIONS OF AUTHOMORPHISMS OF A TORE.” Doklady Akademii Nauk Sssr 144(4): 695-&. (1962)

Arnold, V. I. “Small denominators and problems of the stability of motion in classical and celestial mechanics (in Russian).” Usp. Mat. Nauk. 18: 91-192. (1963)

Arnold, V. I. and A. L. Krylov. “UNIFORM DISTRIBUTION OF POINTS ON A SPHERE AND SOME ERGODIC PROPERTIES OF SOLUTIONS TO LINEAR ORDINARY DIFFERENTIAL EQUATIONS IN COMPLEX REGION.” Doklady Akademii Nauk Sssr 148(1): 9-&. (1963)

Arnold, V. I. “INSTABILITY OF DYNAMICAL SYSTEMS WITH MANY DEGREES OF FREEDOM.” Doklady Akademii Nauk Sssr 156(1): 9-&. (1964)

Arnold, V. “SUR UNE PROPRIETE TOPOLOGIQUE DES APPLICATIONS GLOBALEMENT CANONIQUES DE LA MECANIQUE CLASSIQUE.” Comptes Rendus Hebdomadaires Des Seances De L Academie Des Sciences 261(19): 3719-&. (1965)

Arnold, V. I. “APPLICABILITY CONDITIONS AND ERROR ESTIMATION BY AVERAGING FOR SYSTEMS WHICH GO THROUGH RESONANCES IN COURSE OF EVOLUTION.” Doklady Akademii Nauk Sssr 161(1): 9-&. (1965)

Bibliography

[1] Dumas, H. S. The KAM Story: A friendly introduction to the content, history and significance of Classical Kolmogorov-Arnold-Moser Theory, World Scientific. (2014)

[2] See Chapter 6, “The Tangled Tale of Phase Space” in Galileo Unbound (D. D. Nolte, Oxford University Press, 2018)

[3] V. I. Arnold, Mathematical Methods of Classical Mechanics (Nauk 1974, English translation Springer 1978)

[4] See Chapter 3, “Hamiltonian Dynamics and Phase Space” in Introduction to Modern Dynamics, 2nd ed. (D. D. Nolte, Oxford University Press, 2019)

[5] V. I. Arnold and A. Avez, Ergodic Problems of Classical Mechanics (Benjamin, 1968)

[6] Gaspari, G. “THE ARNOLD CAT MAP ON PRIME LATTICES.” Physica D-Nonlinear Phenomena 73(4): 352-372. (1994)

Top 10 Topics of Modern Dynamics

“Modern physics” in the undergraduate physics curriculum has been monopolized, on the one hand, by quantum mechanics, nuclear physics, particle physics and astrophysics. “Classical mechanics”, on the other hand, has been monopolized by Lagrangians and Hamiltonians.  While these are all admittedly interesting, the topics of modern dynamics that monopolize the time and actions of most physics-degree holders, as they work in high-tech start-ups, established technology companies, or on Wall Street, are not to be found.  These are the topics of nonlinear dynamics, chaos theory, complex networks, finance, evolutionary dynamics and neural networks, among others.

There is a growing awareness that the undergraduate physics curriculum needs to be reinvigorated to make a physics degree relevant to the modern workplace.  To that end, I am listing my top 10 topics of modern dynamics that can form the foundation of a revamped upper-division (junior level) mechanics course.  Virtually all of these topics were once reserved for graduate-student-level courses, but all can be introduced to undergraduates in simple and intuitive ways without the need for advanced math.

1) Phase Space

The key change in perspective for modern dynamics that differentiates it from classical dynamics is the emphasis on the set of all possible trajectories that fill a “space” rather than emphasizing single trajectories defined by given initial conditions.  Rather than study the motion of one rock thrown from a cliff top, modern dynamics studies an infinity of rocks thrown from every possible point and with every possible velocity.  The space that contains this infinity of trajectories is known as phase space (or more generally state space).  The equation of motion in state space becomes the dynamical flow, replacing Newton’s second law as the central mathematical structure of physics.  Modern dynamics studies the properties of phase space rather than the properties of single trajectories, and makes rigorous and unique conclusions about classes of possible motions.  This emphasis on classes of behavior is more general and more universal and more powerful, while also providing a fundamental “visual language” with which to describe the complex physics of complex systems.

2) Metric Space

The Cartesian coordinate plane that we were all taught in high school tends to dominate our thinking, biasing us towards linear flat geometries.  Yet most dynamics do not take place in such simple Cartesian spaces.  A case in point, virtually every real-world dynamics problem has constraints that confine the motion to a surface.  Furthermore, the number of degrees of freedom of a dynamical system usually exceed our common 3-space, expanding to hundreds or even to thousands of dimensions.  The surfaces of constraint are hypersurfaces of high dimensions (known as manifolds) and are almost certainly not flat hyperplanes. This daunting prospect of high-dimensional warped spaces can be surprisingly simplified through the concept of Bernhard Riemann’s “metric space”.  Understanding the geometry of a metric space can be as simple as applying Pythagoras’ Theorem to sets of coordinates.  For instance, the metric tensor can be taught and used without requiring students to know anything of tensor calculus.  At the same time, it provides a useful tool for understanding dynamical patterns in phase space as well as orbits around black holes.

3) Invariants

Introductory physics classes emphasize the conservation of energy, linear momentum and angular momentum as if they are special cases.  Yet there is a grand structure that yields a universal set of conservation laws: integrable Hamiltonian systems.  An integrable system is one for which there are as many invariants of motion as there are degrees of freedom.  Amazingly, these conservation laws can all be captured by a single procedure known as (canonical) transformation to action-angle coordinates.  When expressed in action-angle form, these Hamiltonians take on extremely simple expressions.  They are also the starting point for the study of perturbations when small nonintegrable terms are added to the Hamiltonian.  As the perturbations grow, this provides one doorway to the emergence of chaos.

4) Chaos theory

“Chaos theory” is the more popular title for what is generally called “nonlinear dynamics”.  Nonlinear dynamics takes place in state space when the dynamical flow equations have terms that algebraically are products of variables.  One important distinction between chaos theory and nonlinear dynamics is the occurrence of unpredictability that can emerge in the dynamics when the number of variables is equal to three or higher.  The equations, and the resulting dynamics, are still deterministic, but the trajectories are incredibly sensitive to initial conditions (SIC).  In addition, the dynamical trajectories can relax to a submanifold of the original state space known as a strange attractor that typically is a fractal structure.

5) Synchronization

One of the central paradigms of nonlinear dynamics is the autonomous oscillator.  Unlike the harmonic oscillator that eventually decays due to friction, autonomous oscillators are steady-state oscillators that convert steady energy input into oscillatory behavior.  A prime example is the pendulum clock that converts the steady weight of a hanging mass into a sustained oscillation.  When two autonomous oscillators (that normally oscillator at slightly different frequencies) are coupled weakly together, they can synchronize to the same frequency.   This effect was discovered by Christiaan Huygens when he observed two pendulum clocks hanging next to each other on a wall synchronize the swings of their pendula.  Synchronization is a central paradigm in modern dynamics for several reasons.  First, it demonstrates the emergence of order when a collective behavior emerges from a collection of individual systems (this phenomenon of emergence is one of the fundamental principles of complex system science).  Second, synchronized systems include such critical systems as the beating heart and the thinking brain.  Third, synchronization becomes a useful tool to explore coupled systems that have a large number of linked subsystems, as in networks of nodes.

6) Network Dynamics

Networks have become one of the driving forces of our modern interconnected society.  The structure of networks, the dynamics of nodes in networks, and the dynamic growth of networks are all coming into focus as we live our lives in multiple interconnected webs.  Dynamics on networks include problems like diffusion and the spread of infection and connect with topics of percolation theory and critical phenomenon.  Nonlinear dynamics on networks provide key opportunities and examples to study complex interacting systems.

7) Neural Networks

Perhaps the most enigmatic network is the network of neurons in the brain.  The emergence of intelligence and of sentience is one of the greatest scientific questions.  At a much simpler level, the nonlinear dynamics of small numbers of neurons display the properties of autonomous oscillators and synchronization, while larger sets of neurons become interconnected into dynamic networks.  The dynamics of neurons and of neural networks is a  key topic in modern dynamics.  Not only can the physics of the networks be studied, but neural networks become tools for studying other complex systems.

8) Evolutionary Dynamics

The emergence of life and the evolution of species stands as another of the greatest scientific questions of our day.  Although this topic traditionally is studied by the biological sciences (and mathematical biology), physics has a surprising lot to say on the topic.  The dynamics of evolution can be captured in the same types of nonlinear flows that live in state space.  For instance, population dynamics can be described as a large ensemble of interacting individuals that are born, flourish and die dependent on their environment and on their complicated interactions with other members in their ecosystem.  These types of problems have state spaces of extremely high dimension far beyond what we can visualize.  Yet the emergence of structure and of patterns from the complex dynamics helps to reduce the complexity, as do conceptual metaphors like evolutionary fitness landscapes.

9) Economic Dynamics

A non-negligible fraction of both undergraduate and graduate physics degree holders end up on Wall Street or in related industries.  This is partly because physicists are numerically fluent while also possessing sound intuition.  Therefore, economic dynamics is a potentially valuable addition to the modern dynamics curriculum and easily expressed using the concepts of dynamical flows and state space.  Both microeconomics (business competition, business cycles) and macroeconomics (investment and savings, liquidity and money, inflation, unemployment) can be described and analyzed using mathematical flows that are the central toolkit of modern dynamics.

10) Relativity

Special relativity is a common topic in the current upper-division physics curriculum, while general relativity is viewed as too difficult to expose undergraduates to.  This is mostly an artificial division, because Einstein’s “happiest thought” occurred when he realized that an observer in free fall is in a force-free (inertial) frame.  The equivalence principle, that states that a frame in uniform acceleration is indistinguishable from a stationary frame in a uniform gravitational field, opens a wide door that connects special relativity to general relativity.  In an undergraduate course on modern dynamics, the metric tensor (described above) is introduced in simple terms, providing the foundation to develop Minkowski spacetime, and the next natural extension is to warped spacetime—all at the simple level of linear algebra combined with partial differentiation.  General relativity ties in many of the principles of the modern dynamics curriculum (dynamical flows, state space, metric space, invariants, nonlinear dynamics), and the students can simulate orbits around black holes with ease.  I have been teaching General Relativity to undergraduates for over ten years now, and it is a highlight of the course.

Introduction to Modern Dynamics

For further reading and more details, these top 10 topics of modern dynamics are defined and explored in the undergraduate physics textbook “Introduction to Modern Dynamics: Chaos, Networks, Space and Time” published by Oxford University Press (Second Edition: 2019).  This textbook is designed for use in a two-semester junior-level mechanics course.  It introduces the topics of modern dynamics, while still presenting traditional materials that the students need for their physics GREs.

Galileo Unbound

Book Outline Topics

• Chapter 1: Flight of the Swallows
• Introduction to motion and trajectories
• Chapter 2: A New Scientist
• Galileo’s Biography
• Chapter 3: Galileo’s Trajectory
• His study of the science of motion
• Publication of Two New Sciences
• Chapter 4: On the Shoulders of Giants
• Newton’s Principia
• The Principle of Least Action: Maupertuis, Euler, and Voltaire
• Lagrange and his new dynamics
• Chapter 5: Geometry on my Mind
• Differential geometry of Gauss and Riemann
• Vector spaces rom Grassmann to Hilbert
• Fractals: Cantor, Weierstrass, Hausdorff
• Chapter 6: The Tangled Tale of Phase Space
• Liouville and Jacobi
• Entropy and Chaos: Clausius, Boltzmann and Poincare
• Phase Space: Gibbs and Ehrenfest
• Chapter 7: The Lens of Gravity
• Einstein and the warping of light
• Oppenheimer versus Wheeler
• The Golden Age of General Relativity
• Chapter 8: On the Quantum Footpath
• Heisenberg’s matrix mechanics
• Schrödinger’s wave mechanics
• Bohr’s complementarity
• Einstein and entanglement
• Feynman and the path-integral formulation of quantum
• Chapter 9: From Butterflies to Hurricanes
• KAM theory of stability of the solar system
• Steven Smale’s horseshoe
• Lorenz’ butterfly: strange attractor
• Feigenbaum and chaos
• Chapter 10: Darwin in the Clockworks
• Charles Darwin and the origin of species
• Fibonnacci’s bees
• Economic dynamics
• Mendel and the landscapes of life
• Evolutionary dynamics
• Linus Pauling’s molecular clock and Dawkins meme
• Chapter 11: The Measure of Life
• Huygens, von Helmholtz and Rayleigh oscillators
• Neurodynamics
• Euler and the Seven Bridges of Königsberg
• Network theory: Strogatz and Barabasi

In June of 1633 Galileo was found guilty of heresy and sentenced to house arrest for what remained of his life. He was a renaissance Prometheus, bound for giving knowledge to humanity. With little to do, and allowed few visitors, he at last had the uninterrupted time to finish his life’s labor. When Two New Sciences was published in 1638, it contained the seeds of the science of motion that would mature into a grand and abstract vision that permeates all science today. In this way, Galileo was unbound, not by Hercules, but by his own hand as he penned the introduction to his work:

. . . what I consider more important, there have been opened up to this vast and most excellent science, of which my work is merely the beginning, ways and means by which other minds more acute than mine will explore its remote corners.

Galileo Galilei (1638) Two New Sciences

Galileo Unbound (Oxford University Press, 2018) explores the continuous thread from Galileo’s discovery of the parabolic trajectory to modern dynamics and complex systems. It is a history of expanding dimension and increasing abstraction, until today we speak of entangled quantum particles moving among many worlds, and we envision our lives as trajectories through spaces of thousands of dimensions. Remarkably, common themes persist that predict the evolution of species as readily as the orbits of planets. Galileo laid the foundation upon which Newton built a theory of dynamics that could capture the trajectory of the moon through space using the same physics that controlled the flight of a cannon ball. Late in the nineteenth-century, concepts of motion expanded into multiple dimensions, and in the 20th century geometry became the cause of motion rather than the result when Einstein envisioned the fabric of space-time warped by mass and energy, causing light rays to bend past the Sun. Possibly more radical was Feynman’s dilemma of quantum particles taking all paths at once—setting the stage for the modern fields of quantum field theory and quantum computing. Yet as concepts of motion have evolved, one thing has remained constant—the need to track ever more complex changes and to capture their essence—to find patterns in the chaos as we try to predict and control our world. Today’s ideas of motion go far beyond the parabolic trajectory, but even Galileo might recognize the common thread that winds through all these motions, drawing them together into a unified view that gives us the power to see, at least a little, through the mists shrouding the future.