The Road to Reality Study Notes

Each week The Road to Reality Book Club tackles a chapter of Sir Roger Penrose's Epic Tome. We use these meetings as an opportunity to write down the major points to be taken from our reading. Here we attempt to sum up what we believe Penrose was trying to convey and why. The hope is that these community-generated reading notes will benefit people in the future as they go on the same journey.

Other Resources

• The Portal Book Club - We have a weekly group that meets to talk about this book. Come join us in Discord!
• Chronological guide to concepts introduced in TRTR Google Doc
• Book Club Resources in Google Drive

Chapter 1 The Roots of Science

1.1 The quest for the forces that shape the world

Understanding natural processes has been a common pursuit since the dawn of humanity. After many millennia of chaos and frustration, it was discovered that the regular movement of celestial bodies, such as the sun and moon, could be described mathematically. It became apparent that mathematics unlocked deep truths about the universe. Many people in ancient times allowed their imaginations to be carried away by their fascination with the subject, leading to mystical associations with mathematical objects. One famous example from the ancient Greeks is the association between Platonic solids and the basic elementary states of matter.

1.2 Mathematical truth

There was a need to define a more rigorous method for differentiating truth claims. The Greek philosopher Thales of Miletus (c. 625-547 BC) and Pythagoras of Samos (c. 572-497 BC) are considered to be the first to introduce the concept of _mathematical proof_. Developing a rigorous mathematical framework was central to the development of science. Mathematical proof allowed for much stronger statements to be made about relationships between the arithmetic of numbers and the geometry of physical space.

A mathematical proof is essentially an argument in which one starts from a mathematical statement, which is taken to be true, and using only logical rules arrives at a new mathematical statement. If the mathematician hasn't broken any rules then the new statement is called a _theorem_. The most fundamental mathematical statements, from which all other proofs are built, are called _axioms_ and their validity is taken the be self-evident. Mathematicians trust that the axioms, on which their theorems depend, are actually _true_. The Greek philosopher Plato (c.429-347 BC) believed that mathematical proofs referred not to actual physical objects but to certain idealized entities. Physical manifestations of geometric objects could come close to the Platonic world of mathematical forms, but they were always approximations. To Plato the idealized mathematical world of forms was a place of absolute truth, but inaccessible from the physical world.

Chapter 2 An ancient theorem and a modern question

• summary

Chapter 3 Kinds of number in the physical world

3.2 The real number system

Natural Numbers:

‘Counting’ numbers from 1 to infinity.

Whole Numbers:

All counting numbers including 0, cannot be a fraction.

Integers:

All natural numbers and their negative counterparts and 0. If and are integers, then their sum , their difference , and their product are all integers (that is, the integers are closed under addition and multiplication), but their quotient may or may not be an integer, depending on whether can be divided by with no remainder.

Rational Numbers:

A number that can be expressed as the ratio a/b of two integers (or whole numbers) a and b, with b non-zero. The decimal expansion is alyas ultimately periodic, at a certain point the infinite sequence of digits consists of some finite sequence repeated indefinitely.

Irrational Numbers:

A number that cannot be expressed as the ratio of two integers. When an irrational number is expressed in decimal notation it never terminates nor repeats.

Quadratic Irrational Numbers

Arise in the solution of a general quadratic equation:

[math]\displaystyle{ Ax^{2} + Bx + C = 0 }[/math]

With A non-zero, the solutions being (derived from the quadratic formula):

[math]\displaystyle{ -\frac{B}{2A}\sqrt{\left(\frac{B}{2A}\right)^2}+\frac{C}{A}, \quad -\frac{B}{2A}\sqrt{\left(\frac{B}{2A}\right)^2}-\frac{C}{A} }[/math]

where, to keep within the realm of real numbers, be must have B2 greater than 4AC. When A, B, and C are integers or rational numbers, and where there is no rational solution to the equation, the solutions are quadratic irrationals.

Real Numbers:

A number in the set of all numbers above that falls on the real number line. It can have any value.

Algebraic Real Numbers:

Any number that is the solution to a polynomial with rational coefficients.

Transcendental Real Numbers

Any number that is not the solution to a polynomial with rational coefficients.

Chapter 4 Magical Complex Numbers

Penrose introduced the complex numbers, extending addition, subtraction, multiplication, and division of the reals to the system obtained by adjoining i, the square root of -1. Polynomial equations can be solved by complex numbers, this property is called algebraic closure and follows from the Fundamental Theorem of Algebra.

Complex numbers can be visualized graphically as a plane, where the horizontal coordinate gives the real coordinate of the number and the vertical coordinate gives the imaginary part. This helps understand the behavior of power series; for example, the power series $$1-x^2+x^4+\cdots$$ converges to the function $$1/(1+x²)$$ only when $$|x|<1$$, despite the fact that the function doesn't seem to have "singular" behavior anywhere on the real line. This is explained by singularities at $$x=i,-i$$.

Finally, the Mandelbrot set is defined as the set of all points $$c$$ in the complex plane so that repeated applications of the transformation mapping $$z$$ to $$z^2+c$$, starting with $$z=0$$, do not escape to infinity.

Chapter 5 Geometry of logarithms, powers, and roots

This is a first pass of main topics in this chapter. This should be expanded.

5.1 Geometry of complex algebra

What addition and multiplication look like geometrically on a complex plane.

• law of addition
• law of multiplication
• addition map
• multiplication map
  • what does multiply by i do? rotate

5.2 The idea of the complex logarithm

Relation between addition and multiplication when introducing exponents.

• $$b^{m+n} = b^m \times b^n$$

5.3 Multiple valuedness, natural logarithms

Different values can arrive at the same value. Rotation brings you back to the same place repeatedly.

• $$e^{i\theta}$$ is helpful notation for understanding rotating
• $$e^{i\theta} = cos \theta + i sin \theta$$
• (Worth looking into Taylor Series, which is related.)

Chapter 6 Real-number calculus

6.1 What makes an honest function?

• Differentiable, Analytic

6.2 Slopes of functions

• Derivative is the slope of the tangent line
• Finding the slope of the tangent line for every point

6.3 Higher derivatives; $$C^\infty$$-smooth functions

• Second derivatives
• Euler would require you to have functions that are $$C^\infty$$-smooth
• Not everything that is $$C^\infty$$-smooth is ok for Euler

6.4 The "Eulerian" notion of a function?

• Physics in trying to understand reality by approximating it.

6.5 The rules of differentiation

• Armed with these few rules (and loads and loads of practice), one can become an "expert at differentiation" without needing to hae much in the way of actual understanding of why the rules work!

6.6 Integration

• Fundamental theory of calculus shows integration and differentiation are inverse operations.
• If we integrated then differentiate, we get the same answer back. Non-commutative the other way.