Notes from Seven Brief Lessons on Physics

Read this fascinating book Seven brief lessons on Physics by physicist Carlo Rovelli. It’s a thin book covering all the major theories of physics that came about in the twentieth century going over general theory of relativity (space time), quantum mechanics, elementary particles, heat and thermodynamics. It attempts to provide a coherent explanation of universe combining three distinct topics of “space,time, gravity”, “quantum mechanics” and “heat”. The best part is, it tries to convey intuition about these topics in a manner that can be understood by people who don’t have deep background in physics.

I read the book and while I was initially capturing rough notes, I cleaned them up and put them on medium. I would still highly recommend reading the book.

Here are the notes:

Chapter 1 — General Theory of Relativity

First chapter covers the “general theory of relativity”, which author compares to masterpieces that move us intensely like “Mozart’s requiem, Homer’s Odyssey”. This is the theory that fundamentally altered/revised Newton’s laws of motion around gravitational force and space.

Newton had explained, “force of gravity” as something that draws bodies towards each other. Secondly Newton explained space as a container, a fixed structure, through which all objects run until a force changes their path. What space itself was made of; Newton did not explain.

Einstein took a crack at explaining what is space, and in it he drew inspiration from “electrical field”. “Electrical field” carries radio waves, can vibrate and oscillate and “transports” the electrical force. Similar to the way electricity is conveyed by electrical field, gravity is conveyed by the “gravitational Field”. Einstein’s biggest discovery was his idea that this “gravitation field” is not diffused “through space”, rather gravitational field *is* the space. Space is not a fixed container, rather its dynamic shell (like a snail’s shell).

With this knowledge about Space being dynamic shell, the author explains why the earth moves around the sun? Sun is bending the space around itself, earth is rolling in a space that is inclined now (due to sun bending the space) like a marble that rolls in a funnel. There is no mysterious force generated at the center of the funnel, it’s just the curved nature of walls. Below figure shows the space, earth and sun

Fig: Source: Screenshot taken from Carlo Rovelli’s talk https://www.youtube.com/watch?v=s_PIEhPMU-k

Next, the author tackles what explains the curvature of space?

To describe the curvature of space, Gauss and his fellow Reiman came up with an equation that explained that “space curves where there is matter”. This equation describes “how space bends around a star (like sun)”. And because of this curvature, planets orbit around the star and also “light stops moving in a straight line”. In addition to space, time also curves. Time passes more quickly high up in the mountains than below on sea level.

Chapter 2 — Quantum mechanics

Quantum mechanics is completely different than “general theory of relativity”. While the “general theory of relativity” is an explanation of “gravity, space and time”, quantum mechanics tells us that “energy of electric field” is distributed in “quanta” (packets) and is not continuous. Max Planck used this concept to calculate “electric field in equilibrium in hot box.“

Einstein showed “light is made of packets: particles of light, called photons”. While Einstein explained the packet nature of light, Neil Bohr and his team further developed this theory. Bohr explained just like particle of light, the energy of electrons in atoms can only take certain values (i.e. electrons can only jump between one orbit and another with determined energies, emitting or absorbing a photon when they jump). These are the so called “quantum leaps”. In 1925, these equations of quantum theory appeared, replacing mechanics of Newton.

The whole of chemistry emerges from single equation of “quantum mechanics”. It explains why periodic table has the particular structure with their periods and elements having specific properties. This equation was designed by Heisenberg. According to him, electrons don’t “always exist”. They only exist when someone watches them, i,e there is some interaction. The quantum leaps are the only way to have them manifest as real. When there is no interaction, the electron is not in any place at all. In “quantum mechanics”, no object has a definite position, except when colliding. These interactive leaps, with which each object passes from one place to another do not occur in a predictable way, but are largely random. We can only predict the probability with which electron will reappear in certain place.

Chapter 3 — Architecture of Cosmos

In the third chapter, the author focuses on macro-cosmic structure of the universe. In the second half of the 20th century, scientists built on top of the two theories (quantum theory and general relativity) in 2 major areas

• Macro-cosmic structure of universe
• Microcosm of elementary particles

Author covers the timeline of how our understanding developed about earth with respect to other planets, solar system, galaxy and universe. Until Shakespeare's time, the earth was at the center of the universe. Copernicus was the first to explain earth is not at the center of the universe, rather its the sun. In the early 20th century, we realized that the solar system is just one among a vast number of others, and that the sun is no more than a star like others, one of a hundred billion stars in the galaxy.

In 1930, it was observed by measuring of nebulae(a small whitish cloud between the stars), that galaxy itself is a speck of dust in a huge cloud of galaxies that extends as far as the eye can see using most powerful telescopes. In the universe, there are thousands of billions of billions of planets such as earth. Each black dot taken from hubble telescope is a galaxy having hundred billion suns.

This whole universe with billions of stars and galaxies and 15 billion years in the making, emerged from extremely hot and dense small cloud. It began as a small ball and exploded to present dimensions.

Chapter 4 — Elementary particles

This chapter is about elementary particles. Within the universe, there is light and there are things. Light is made of photons, while things are made of atoms. Every atom has a nucleus surrounded by electrons. Protons and neutrons make up the nucleus. Protons and Neutrons are made of even smaller particles quarks. Forces that glue together these quarks in protons and neutrons are called gluons.

All these are elementary particles. There are a total of 10 in all; including neutrinos, higgs bosons. These elementary particles are like bricks in giant lego set.

Quantum mechanics describes the nature of these particles and the way they move. These particles don’t have pebble like quality, instead they are quanta of corresponding fields like photons are quanta of the electromagnetic field. They are elementary excitation of moving substratum, minuscule moving wavelets. As a result of quantum mechanics, they are never stable, they are a jump away from interactions. Quantum world is a continuous swarming of things, a continuous coming to light and disappearance of ephemeral entities.

Chapter 5 — Loop Quantum Gravity

In this chapter, the author combines quantum mechanics and theory of relativity (concept of gravity, space and time). He coins the name “Loop quantum gravity” for this theory. It says space (which is dynamic — can be compressed) is not continuous, rather is made of grains, “atoms of space” that are billion billion times smaller than atomic nuclei. The theory describes these atoms of space in mathematical form and provides equations that determine their evolution. These grains of space are what constitutes space. Space is created by linking these individual quanta of gravity.

At the minute scale of grains of space atoms, the dance of nature does not take place to the rhythm of single orchestrator across passage of time; rather every process dances independently with its neighbors. So, there is no longer “space” that contains the world or “time” in which events occur. There are only elementary processes wherein quanta of space and matter continually interact with one another. The illusion of space and time that continues around us is just a blurred vision of this swarming of elementary processes, just as a calm alpine lake is made of rapid dance of minuscule water molecules.

Above diagram (taken from Carlo Rovelli’s talk) illustrates view of quantum electromagnetic field and quantum gravitational space.

Black holes — are formed by collapsed stars. Crushed by its own weight, matter of these stars has collapsed upon itself, and disappeared from our view. In traditional Einstein’s theory of relativity this collapse is irreversible and will continue to shrink into what is known as a singularity, and no known force of nature can stop it. If, the theory of loop quantum gravity is correct, matter cannot collapse to infinitesimal point, as only finite chunks exist in space. So matter must have become increasingly dense, up to the point where quantum mechanics must have exerted counterbalancing pressure.

According to the author, Planck Star is the final stage in the life of the star, where quantum fluctuations of space time balance the weight of matter. If the sun was to stop burning and form a black hole, it would have a 1.5 km diameter. Inside this black hole, the sun’s matter would continue to collapse, eventually becoming a planck star, with dimensions similar to that of an atom. A planck star is not stable; once compressed to the maximum, it rebounds and starts expanding again, applying outbound pressure called quantum bounce, transforming black hole into white hole.

For an observer sitting inside black hole on the planck star, the rebound would be really fast, but for those outside the black hole the time would pass much more slowly.

As above slide taken from Carlo Rovelli’s talk shows, the time outside would be 10 billions years when time inside is 1 msecs!

Loop quantum can also explain the origin of the universe. Equation of loop quantum gravity allows us to go further back in reconstruction of history. When the universe is severely compressed, quantum theory generates a repulsive force, as a result big bang was a big bounce. So our world may have been a result from previous universe which would have contracted under its own weight, until it was squeezed into a tiny space before bouncing.

Chapter 6 — Tying it together with Heat — Probability, Time & Space, Gravitational and Electromagnetic field

In this chapter, the author provides intuition for “flow of time” by drawing correlation between heat and time. Hot substance is one in which atoms move more quickly compared to cold substance. Heat moves from hot things to cold things. Why? It’s just probability. Its statistically more probable that quickly moving atom of hot substance collides with cold one more frequently.

When heat exchange is negligible there is no difference between past and future. As soon as there is heat, the future is different from the past. When there is no friction, the pendulum can swing forever. But due to friction, pendulum heats its supports lightly, loses energy and slows down. And we are able to distinguish future from past.

Thermodynamics (study of heat) and Statistical physics (science of probability of different motions) were extended to electromagnetic and quantum phenomena. Heated electromagnetic field can be explained in the context of oven with electromagnetic waves vibrating, randomly sharing energy, the whole behaving like a gas of photons that move fast and randomly.

How do we explain heated gravitational field? When heat is added to gravitational field, time and space themselves must vibrate. What does time vibration mean? How do we describe flow of time?

From human intuition, “passage of time” comes naturally to us. For us time “runs”, “flows”. Our thoughts exists in “time”, the very structure of our language requires time. In Physics, we are able to describe formulas, as how things vary as a function of “time”. We can also explain how things vary in relation to “position” or how taste of risotto varies as a function of “amount of butter”. Time seems to flow, whereas quantity of butter or location does not “flow”.

Physicists conclude that the idea of “present” that is common to the whole universe is an illusion and that universal “flow” of time is a generalization that does not work. Special theory of relativity has shown that notion of “present” is subjective. At the same time, it’s hard for humans to ignore “flow of time”. As Einstein said “people like us who believe in physics, know that distinction made between past, present and future is nothing more than a persistent, stubborn illusion.”

If our notion of time, does not reflect a fundamental aspect of reality, why does it persist? According to the author, the answer lies in the intimate connection between time and heat. There is an observable difference between past and future when there is a flow of heat. So, the flow of time emerges from physics but not in the context of an exact description of things as they are, rather in context of statistics and thermodynamics.

For a super sensible human being, there would be no flowing of time, the universe would be a single block of past, present and future. But due to limitations of our consciousness, we perceive only a blurred vision of the world and live in time. All this does not sound very clear, time sits at the center of problems raised by intersection of gravity, quantum mechanics and thermodynamics.

While “quantum gravity” resolves the puzzle of two pieces, we still don’t have a theory that combines all three pieces of our fundamental knowledge in a coherent manner. Stephen Hawking has come closest to combining the three with his explanation of “heat of black holes”. The heat of black holes is like the Rosetta stone of physics encoded in three theories, waiting to unlock the true explanation of time.