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Seven Brief Lessons on Physics

by Carlo Rovelli

Please visit my complete bookshelf. I took these notes to further my own learning.

[I. Brief Summary]

  • The seven lessons are about Einstein’s general theory of relativity, quantum mechanics, the architecture of the cosmos, elementary particles, quantum gravity, probability and the heat of black holes and, finally, how humans fit into this picture. We are stardust, the author reminds us, impossibly minor players in the pageant of the galaxies, and well on our way to becoming the agents of our own demise. The fearful aspects of "Seven Brief Lessons on Physics" arrive in its final chapter. The author is withering about humanity’s unwillingness to confront global warming.

[II. Big Ideas]

The Most Beautiful of Theories

- Eienstien's jewel, the general theory of relativity, is a masterpiece. The result of an elementary intuition that space and gravitational field are the same thing.
- The theory that elucidates how time does not pass indentically for everyone: two identical twins find that they are different in age if one of them has traveled at speed. 
- Einstein predicted that space time passes more quickly high up than below, nearer to Earth.
- In November 1915, he committed to print an article giving the complete solution: a new theory of gravity, which he called "The General Theory of Relativity."
- Newton had tried to explain the reason why things fall and the planets turn. He had imagined the existense of a "force" that draws all material bodies towards one another and called it "the force of gravity."
- How this force was was exerted between things distant from each other, without there being anything between them, was unknown. Newton imagined the bodies moved through space and that space is a great empty container. What this "space" was made of, this container of the world he invented, Newton could not say.
- Micahel Faraday and James Maxwell stumbled upon a key ingredient to Newton's cold world: the electromagnetic field. 
- The electromagentic field: this field is really entity that, diffused everywhere, carries radio waves, fills space, can vibrate and oscillate like the surface of a lake, and "transports" the electrical force. 
- Since his youth, Einstein had been fascinated by the electromagnetic field. He soon came to understand that gravity, like electricity, must be conveyed by a field as well: "a gravitational field" analogous to the "electical field" must exist. He aimed at understanding how "gravitational field" worked. 
- An extraordinary idea occured to him, a stroke of pure genius: the gravitational field is not diffused through space; the gravitational field is the space itself. This is the idea of the general theory of relativity. Newton's "space," through which things move, and the "gravitational field" are one and the same thing. 
- Space is no longer something distinct from matter——it is one the "material" components of the world. This was the moment of enlightenment. 
- When a large star has burned up all its combustible substance (hydrogen), it goes out. What remains is no longer supported by the heat of the combustion and collapses under its own weight, to a point where it bends space to such a degree that it plummets into an actual hole. These are famouse "black holes."
- Eienstien's equation shows that space cannot stand still; it must be expanding. This explains what we know "big bang."
- The theory contends that space moves like the surafce of the sea. The effects of these "gravitational waves" are observed in the sky on binary stars.
- In short summary, the theory describes a colorful and amazing world where universe explode, space collapses into bottomless holes, time sags and slows near a planet, and the unbounded extensions of interstellar space ripple and sway like the surface of the sea.

Quanta

- Quantum mechanics or quantum theory has led to modern applications like computers that has transformed our society.
- It is said the theory was born precisely in the year 1900. The German physicist Max Planck calculated the electric field in equilibrium in a hot box. He imagined that the energy of the field is distributed in quanta, that is, in packets or lumps of energy. Later, Einstein came to observation that those packets of energy were real.
- Einstein showed that light is made of packets: particles of light. We call them photons.
- If Planck is the father of the theory, Einstein is the parent who nurtured it. But as parents start to neglect their kids as they get old, Einstein followed the suit. 
- Dane Niels Bohr pioneered its development. Bohr understood that the energy of electrons in atoms can take on only certain values, like the energy of light, and crucially that electrons can only jump between one atomic orbit and another with determined energies, emitting or absorbing a photon when they jump. These are the famour quantum leaps.
- For example, periodic table, devised by Dmitri Mendeleev, which lists all the possible elementary substances of which universe is made, from hydrogen to uranium, are listed in particular structure because each element corresponds to one solution of the main equation of quantum mechanics. The whole chemistry emerges from this single equation of quantum mechanics.
- Werner Heinsberg was the first to write the equations of the new theory. He imagined that electrons do not always exist. They only exist when they are interacting/colliding with something else –– quantum leaps. The quantum leaps from one orbit to another are the only means they have of being real: an electorn is a srt of jumps from one interaction to another. When nothing disturbs it, it is not in any precise place. It is not in place at all.
- In quantum machnaics no object has a defintie position, except when colliding with something else.
- These interactive leaps with which object passes from one place to another do not occur in a predictable manner but largely at random. This is where law of probability enters. This goes to the heart of physics because everything is goverened in physics with firm laws. 
- This of course seemed absured to Einstein. The same Einstein who had shown the courage to think the unthinable –– time is not universal and that space is curved was now saying the world cannot be this strange. But he proposed Heinsberg for the Nobel Prize.
- Bohr patiently explained the new ideas to Einstein. Einstein objected. For years their dialogue continued and Einstein ultimately conceded to the new ideas and theory of quantum mechanics. 
- A century later the equation of quantum mechanics are used daily in widely varying fields –– by physicists, engineers, chemists, and biologists. 
- They are extremely useful in contemporary technology. Witout quantum mechanics there would be no transistors (modern computers).
- It remains a mystery. It cannot be described what happens to physical system but only shows how a physical system affects another physcial system.

The Architecture of the Cosmos

- Science becomes with a vision. Scientific thought is fed by the capacity to "see" things differently than they have previously been seen.
- Cosmos was conceptulaized for millennia as Earth below, the sky above. 
- Earth is a great stone that floats suspended in space, without falling. 
- Aristotle devised convincing scientific arguments to confirm the spherical nature of both Earth and the heavens around it where slestical objects run their course.
- The next leap was accomplished by Copernicus, inaugurating waht has come to be called the great scientific revolution: that our Earth is not at the center of the dance of the planets but that the sun is there instead.
- Soon the growth of our knowledge continued with the discovery of the Galaxy with the help of most powerful telescopes such as the Hubble telescope. 
- Space is not flat but curved. We have to imagine the texture of the universe, with its splashes of galaxies, being moved by waves similar to those of the sea, sometimes so agitated as to create the gaps that black holes. 

Particles

- Light is made up of photons, the particles of light intuited by Einstein. The things we see are made of atoms. Every atoms consists of nucleus surrounded by electrons. Every nucleus consists of tightly packed protons and neutrons. Both protons and neutrons are made up of even smaller particles called quarks.
- The force that glues quarks inside protons and neutrons is generated by particles called gluons. 
- Electrons, quarks, photons, and gluons are the components of space. They are the elementary particles studied in particle physics. 
- Few others are added, such as the neutrinos and the Higgs bosons. 
- The nature of these particles, and the way they move, is described by quantum mechanics. 
- Quantum mechanics and experiments with particles have taught us that the world is a continous and restless swarming of things.
- The Standard Model was finalized in 1970s, after a long series of experiments that confirmed all predictions. Its final confirmation occured in 2013 with the discovery of the Higgs boson. Many physicists have not taken the Standard Model entirely seriously. 
- Around every galaxy, astronomers observe a large of material that reveals its existence via the gravitational pull that is exerts upon stars and by the way it deflects light. It's clear that there is _something_ there, but we don't know yet. Nowadays it is called "dark matter."
- Evidence indicates that it is something _not_ described by the Standard Model; otherwise we would see it.
- It is hardly surprising that there are more things in heaven and earth than have been dreamed of in our philosophy or physics. 
- We did not suspect the existence of radio waves and neutrinos. 
- The Standard Model remains the best that we have. It's predictions have all been confirmed, and, apart from dark matter and gravity as described in the general theory of relativity as the curvature of space-time - it describes well every aspect of the perceived world. 

Grains of Space

- The twentieth century gave us the two gems: general relativity and quantum mechanics. 
- The study of gravitational waves, of black holes contradicts with the foundation of atomic physics, nucleur physics, the physics of elementary particles and the physics of condensed matter. 
- A university student attending lectures on general relativity in the morning and others on quantum mechanics in the afternoon might be forgiven for concluding that his professors are fools or have neglected to communicate with one another for atleast a century. In the morning the world is curved space where everything is continuous; in the afternoon it is a flat space where quanta of energy leap.
- A group of theoretical physicists scattered across the five continents trying to settle the issue. Their field of study is called "quantum gravity": its objective is to find a theory, that is, a set of equations but above all a coherent vision of the world with which to resolve the current schizophrenia. 
- Loop quantum gravity pursued by a loose band of researchers working to combine the theories of general relativity and quantum mechanics.
- The central result of loop quantum gravity is indeed that space is not continuous, that is not indefinitely divisible but made up of grains, or "atom of space."
- The theory describes these "atoms of space" in mathematical form and provides equations that determine their evolution. They are called "loops", or rings, becuase they are linked to one another, forming a network of relations that weaves the texture or space, like the rings of a finely woven immense chain mail. 
- The world described by the theory is thus further distanced from the one with which we are familiar. There is no longer space that "contains" the world, and there is no longer time "in which" event occurs. 
- The hypothetical final stage in the life of a star, where the quantum fluctuations of space-time balance the weight of matter, is what is known as a "Planck star." If the sun were to stop burning and to form a black hole, it would measure about one and a half-kilometers in diameter. Inside this black hole the sun's matter would continue to collapse, eventually becoming such a Planck star. Its dimension would then be similar to those of an atom. The entire matter of the sun condensed into the space of an atom: a Planck star.
- A Planck star is not stable: once compressed to the maximum, it rebounds and begins to expand again. This leads to explosion of the black hole. This process, as seen by a hypothetical observer sitting in the black hole on the Planck star, would be a rebound occuring at great speed. But time does not pass at the same speed for him as for those outside the black hole, for the same reason that in the mountains time passes faster than at sea level.
- A black hole if a rebounding star seen in extreme slow motion.
- With the loop theory, "big bang" may have actually been a "big bounce." Our world may have actually been born from a preceding universe that cotracted under its own weight until it was squeezed into a tiny space before "bouncing" out and beginning to re-expand, thus becoming the expanding universe that we observe around us. 
- Earth is not flat; it is not stationary.
- If we try to put together what we have learned in the twentieth century about the phsyical world, the clues point toward something profoundly different from our instinctive understanding of matter, space and time.
- Loop quantum gravity is an attempt to decipher these clues and to look a little farther into the distance. 

Probability, Time and the Heat of Black Holes

- "What is heat?"
- Until the mid-nineteenth century, physicists attempted to understand heat by thinking that it was a kind of fluid, called "caloric," or two fluids, one hot and one cold. The idea turned out to be wrong.
- A hot substance is where atoms move more quickly. They run, vibrate, bounce, and so on.
- Cold air is air in which atoms move slowly. 
- Heat, as we know, always moves from hot things to cold. A cold teaspoon placed in a cup of hot tea also becomes hot. Why does heat go from hot things to cold things and not vice versa?
- It is a crucial question because it related to the nature of time. 
- In every case in which heat exchange does not occur, we see that the future behaves exactly like the past. As soon as there is heat, however, the future is different from the past. 
- While there is no friction, a pendulum can swing forever. But if there is friction, then the pendulum heats its supports slightly, loses energy, and slows down. Friction produces heat. And immediately we are able to distinguish the future (towards which the pendulum slows) from the past. The difference between past and future exists only when there is heat.
- The fundamental phenomenon that distinguishes the future from the past is the fact that heat passes from things that are hotter to things that are colder. 
- So why does heat go from hot things to cold things? It is because of sheer chance as discovered by Boltzmann. This introduces the idea of probability.
- Heat does not move from hot things to cold things due to an absolute law: it does so only with a large degree of probability. 
- The reason for this is that it is statistically more probable that a quickly moving atom of the hot substance collides with a cold one and leaves it a little of its own energy, rather than vice versa. 
- The probability in play in the science of heat is in certain sense tied into our _ignorance_. It is easier to assign a lesser or greater degree of porbability to something. 
- The branch of science that clarifies these things is called "statistical physics." The probabilistic nature of heat and temperature, that is to say, thermodynamics.
- How the gravitational field behaves when it heats up is still an unsolved problem.
- We know what happens to a heated electromagnetic field: in an oven, for instance, there is hot electromagnetic radiation, which cooks a pie, and we know how to describe this. The electromagnetic waves vibrate, randomly sharing energym and we can imagine the whole as being like a gas photons that move fast and randomly like the molecules in a hot balloon. 
- But what is a hot gravitational field?
- The gravitational file is space itself, in effect space-time. What is a vibrating time? What exactly is the _flow_ of time?
- Physics describes the world by means of formulas that tell how things vary as a function of "time." But we can write formulas that tel us how things vary in relation to their "position," or how the taste of a risotto varies as a function of the "variable quantity of butter." 
- Time seems to "flow," whereas the quantity of butter or location in space does not "flow." Where does the difference come from?
- Another way of posing the problem is to ask oneself: what is the "present?"
- We say that only the things of the present exist: the past no longer exists and the future doesn't exist yet. But in physics there is nothing that corresponds to the notion of the "now."
- Compare "now" with "here." "Here" designates the place where a speaker is: for two different people "here" points to two different places. Consequently, "here" is a word the meaning of which depends on where it is spoken. 
- No one would dream of saying that things "here" exist, whereas things that are not "here" do no exist. So then why do we say that things are "now" exist and that everything else doesn't? 
- Physicists and philosophers have come to the conclusion that the idea of present that is common to the whole universe is an illusion and that the universal "flow" of time is a generalization that doesn't work.
- Time sits at the center of the tangle of problems raised by the intersection of gravity, qauntum mechanics and thermodynamics. A tangle of problems where we are still in dark.
- Using quantum mechanics, Stephen Hawkings successfully demonstrated that black holes are always "hot." They emit hit like a stove.

Ourselves

- What role doe we have as human beings who perceive, make decisions, laugh and cry? Do we also consist of quanta and particles? If so, where do we get that sense of individual existence and unique selfhood?
- One of the things we understand least about is ourselves.
- We are nodes in a network of exchanges. We are external observers. 
- We are made up of the same atoms and the same light signals as are exchanged between pine trees in the mountains and stars in the galaxies.
- As our knowledge has grown, we have learned that we are not at the center of universe. We are only small part of it. 
- In the awareness, that we can always be wrong, and therefore ready at any moment to change direction if a new track appears; but knowing also that if we are good enough we will get it right and will find what we are seeking. This is the nature of science. 
- The solution to the confusion lies elsewhere. When we say that we are free, and it's true that we can be, this means that how we can behave is determined by what happens within us, within the breain and not by the external factors. To be free doesn't mean that our behaviors is not determined by the laws of nature. It means that it is determined by the laws of nature acting in our brains. 
- Does this mean that when I make a decision it's "I" who decides? Yes, of course, because it would be absurd to ask whether "I" can do something different from what the whole complex of my neurons has decided. There is not an "I" and "the neurons in my brain." They are the same thing. An individual is a process: complex, tightly integrated. 
- We have a hundred billion neurons in our brains, as many as there are stars in a galaxy. "We" are the process formed by this entire intricacy, not just by the little of it of which we are conscious. 

[III. Quotes]

  • In his youth Albert Einstein spent a year loafing aimlessly. You don't get anywhere by not "wasting" time——something, unfortunately, that the parents of teenagers tend frequently to forget.
  • Ever since we discovered that Earth is round and reality is not as it appears to us: every time we glimpse a new aspect of it, it is a deeply emotional experience. Another veil has fallen.
  • A world of happening. Not things."
  • "Physics is not only a history of successes."
  • "Myths nourish science, and science nourishes myth."
  • "Our world may have actually been born from a preceding universe that contracted under its own weight until it was squeezed into a tiny space before ‘bouncing’ out and beginning to re-expand, thus becoming the expanding universe that we observe around us."
  • "I believe that our species will not last long. It does not seem to be made of the stuff that has allowed the turtle, for example, to continue to exist more or less unchanged for hundreds of millions of years, for hundreds of times longer, that is, than we have been in existence. We belong to a short-lived genus of species. All our cousins are already extinct. What’s more, we do damage."
  • "It's as if God had not designed reality with a line that was heavily scored but just dotted it with a faint outline."
  • "Our knowledge grows in real terms. It allows us to do new things that we had previously not even imagined."
  • "To the very last, the desire to challenge oneself and understand more. And the very last: doubt."
  • "A world of happening, not of things."
  • "Physics is not only a history of successes."
  • "Physics opens windows through which we see far into the distance."
  • "We realize that we are full of prejudices and that our intuitive image of the world is partial, parochical, inadequate."
  • "Some behavior is more probable, other behavior more improbable."
  • "People like us, who believe in physics, know that the distinction made between past, present and future is nothing than a persistent, stubborn illusion."
  • "It is not against nature to be curious: it is in our nature to be so."
  • "Nature is our home, and in nature we are at home."