Quantum Gravity QUOTES

1 " I don't know if the young Einstein had encountered the Paradiso during his intellectual wanderings in Italy, and whether or not the vivid imagination of the Italian poet may have had a direct influence on his intuition that the universe might be both finite and without boundary. Whether or not such influence occurred, I believe that this example demonstrates how great science and great poetry are both visionary, and may even arrive at the same intuitions. Our culture is foolish to keep science and poetry separated: they are two tools to open our eyes to the complexity and beauty of the world. "

― Carlo Rovelli , Quantum Gravity

2 " Dirac's quantum mechanics thus allows us to do two things. First, to calculate which values a physical variable may assume. This is called "calculation of the spectrum of a variable"; it captures the granular nature of things. When an object (atom, electromagnetic field, molecule, pendulum, stone, star, and the like) interacts with something else, the values computed are those that its variables can assume in the interaction (relationism). The second thing that Dirac's quantum mechanics allows us to do is to compute the probability that this or that value of a variable appears at next interaction. This is called "calculation of an amplitude of transition." Probability expresses the third feature of the theory: indeterminacy, the fact that it does not give unique predictions, only probabilistic ones.

This is Dirac's quantum mechanics: a recipe for calculating the probability that one or another value in the spectrum appears during an interaction. That's it. What happens between one interaction and the next is not mentioned in the theory. It does not exist. "

― Carlo Rovelli , Quantum Gravity

3 " The probability of finding an electron or any other particle at one point or another can be imagined as a diffuse cloud, denser where the probability of seeing the particle is stronger. Sometimes it is useful to visualize this cloud as if it were a real thing. For instance, the cloud that represents an electron around its nucleus indicates where it is more likely that the electron appears if we look at it. If you have encountered them at school, these are the atomic orbitals. "

― Carlo Rovelli , Quantum Gravity

4 " If we see a child playing on the beach, it is only because between him and ourselves there is this lake of vibrating lines that transport his image to us. Is the world not marvelous? "

― Carlo Rovelli , Quantum Gravity

5 " To view Schrodinger's wave as something real is to give it too much weight-it doesn't help us to understand the theory; on the contrary, it leads to greater confusion. Except for special cases, the Schrodinger's wave is not in physical space, and this divests it of all its intuitive character. But the main reason why Schrodinger's wave is a bad image of reality is the fact that when a particle collides with something else, it is always at a point: it is never spread out in space like a wave. If we conceive an electron as a wave, we get in trouble explaining how this wave instantly concentrates to a point at each collision. Schrodinger's wave is not a useful representation of reality: it is an aid to calculation that permits us to predict with some degree of precision where the electron will reappear. The reality of the electron is not a wave: it is how it manifests itself in interactions, like the man who appeared in the pools of lamplight, while the young Heisenberg wandered pensively in the Copenhagen night. "

― Carlo Rovelli , Quantum Gravity

6 " In the Andromeda Galaxy, the duration of this "extended present" is (with respect to us) two million years. Everything that happens in Andromeda during these two million years is neither past nor future with respect to ourselves. If a friendly advanced Andromeda civilization decided to send a fleet of spacecraft to visit us, it would make no sense to ask whether the fleet has already left "now," or not yet. The only meaningful question is when do we receive the first signal from the fleet: from that moment on, not earlier, the departure of the fleet is in our past. "

― Carlo Rovelli , Quantum Gravity

7 " Light is thus nothing more than a rapid vibration of the spiderweb of Faraday's lines, which ripple like the surface of a lake as the wind blows. It isn't true that we "do not see" Faraday lines. To "see" is to perceive light, and light is the movement of the Faraday lines. "

― Carlo Rovelli , Quantum Gravity

8 " The fact that space and time are intimately connected, as in figure 3.2, implies a subtle restructuring of Newton's mechanics, which Einstein rapidly completes in 1905 and 1906. A first result of this restructuring is that as space and time fuse together in a single concept of spacetime, so the electric field and the magnetic fields fuse together in the same way, merging into a single entity that today we call the "electromagnetic field." The complicated equations written by Maxwell for the two fields become simple when written in this new language. "

― Carlo Rovelli , Quantum Gravity

9 " But Einstein realizes that energy and mass are two facets of the same entity, just as the electric and magnetic fields are two facets of the same field, and as space and time are two facets of the one thing, spacetime. This implies that mass, by itself, is not conserved; and energy-as it was conceived at the time-is not independently conserved, either. One may be transformed into the other: only one single law of conservation exists, not two. What is conserved is the sum of mass and energy, not each separately. Processes must exist that transform energy into mass, or mass into energy. "

― Carlo Rovelli , Quantum Gravity

10 " Our entire current technology is founded on the use of a physical thing-electromagnetic waves-that was not discovered empirically: it was predicted by Maxwell, simply by searching for the mathematical description accounting for the intuition Faraday got from bobbins and needles. This is the outstanding power of theoretical physics. "

― Carlo Rovelli , Quantum Gravity

11 " Scientific thinking explores and redraws the world, gradually offering us better and better images of it, teaching us to think in ever more effective ways. Science is a continual exploration of ways of thinking. Its strength is its visionary capacity to demolish preconceived ideas, to reveal new regions of reality, and to construct new and more effective images of the world. This adventure rests upon the entirety of past knowledge, but at its heart is change. The world is boundless and iridescent; we want to go and see it. We are immersed in its mystery and in its beauty, and over the horizon there is unexplored territory. The incompleteness and the uncertainty of our knowledge, our precariousness, suspended over the abyss of the immensity of what we don't know, does not render life meaningless: it makes it interesting and precious. "

― Carlo Rovelli , Quantum Gravity

12 " Our intuitive idea of the present, the ensemble of all events happening "now" in the universe, is an effect of our blindness: our inability to recognize small temporal intervals. An illegitimate extrapolation from our parochial experience. "

― Carlo Rovelli , Quantum Gravity

13 " The story goes that in the dimly lit old halls of Kracow University, an austere professor of physics came out of his study waving around Einstein's article, screaming, "The new Archimedes is born! "

― Carlo Rovelli , Quantum Gravity

14 " And it's here that Einstein's extraordinary stroke of genius occurs, one of the greatest flights in the history of human thinking: what if the gravitational field turned out actually to be Newton's mysterious space? What if Newton's space was nothing more than the gravitational field? This extremely simple, beautiful, brilliant idea is the theory of general relativity. "

― Carlo Rovelli , Quantum Gravity

15 " Planets circle around the sun, and things fall, because space around them is curved. "

― Carlo Rovelli , Quantum Gravity

16 " The notions of fields and particles, separated by Faraday and Maxwell, end up merging in quantum mechanics.

The way this happens in the theory is elegant: the equation of Dirac determines the values a variable can take. Applied to the energy of Faraday's lines, they tell us that this energy can take on only certain values and not others. Since the energy of the electromagnetic field can take on only certain values, the field behaves line a set of packets of energy. These are precisely the quanta of energy introduced by Planck and Einstein thirty years later. The circle closes, and the story is complete. The equations of the theory, written by Dirac, account for the granular nature of light, which Planck and Einstein had intuited. "

― Carlo Rovelli , Quantum Gravity

17 " The electromagnetic waves are vibrations of Faraday's lines, but also, at small scale, swarms of photons. When they interact with something else, as in the photoelectric effect, they manifest themselves as particles: on our eyes, light rains in separate droplets, in single photons. Photons are the "quanta" of the electromagnetic field.

But the electrons and all the other particles of which the world is made are equally "quanta" of a field! A "quantum field" similar to Faraday and Maxwell's, subject to granularity and to quantum probability. Dirac writes the equations for the field of the electrons and of the other elementary particles. The sharp distinction between fields and particles introduced by Faraday vanishes. "

― Carlo Rovelli , Quantum Gravity