Carlo Rovelli, "Reality Is Not What It Seems"

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Carlo Rovelli, "Reality Is Not What It Seems: The Journey

To Quantum Gravity" (2014 in Italian, 2017 in English)

 

npr 서평

 

npr (National Public Radio) 서평도 "Rovelli의 책은 보석이다. 읽는 것이 즐겁다"라고 쓰고 있지만, 정말 그렇다. 이 책을 생전에 읽어볼 수 있은 게 참 다행이란 생각이다. 내가 사는 물리적 세계에 대한 '최신'의 물리학적 이해에 대해, 모르기보다는, 알고 싶은 때문이다. "This book gives an account of the current state of the search for our new image of the world, as I understand it today." Loop quantum gravity 이론 창안자의 하나인 저자  Carlo Rovelli가 책 서문에 쓴 말이다.

 

Quantum gravity 이론의 내용, 그 아이디어, 가정, 방법에 대해 대강, 아주 대강, 감은 잡게 해 준다.  그리고 재미있게 읽힌다. 그의 책 The Trouble with Physics 또한 재밌게 읽은, 이론물학자 Lee Smolin가 이 분야에서 어떤 기여를 했는지를 읽게 된 것도 좋았다.

 

작년에 읽은 Rovelli의 "There Are Places" (에세이집), "Seven Brief Lessons", "The Order of Times"도 다 즐겨 읽은 책들이다. 그의 "White Holes" (2023)도 읽기 시작했다. 도서관이 구입하기를 1순위로 기다려서 지난주에 빌렸다.

 

아래는 이 책에서:

 

(p. 193)

 

 

The general form of quantum theory compatible with special relativity is thus called "quantum field theory," and forms the basis of today's particle physics. Particles are quanta of a field, just as photons are quanta of light. All fields display a granular structure in their interactions.... [T]oday we have a theory called the "Standard Model" of elementary particles, which describes almost all we see, with the exception of gravity, in the context of quantum field theory. The Standard Model is completed by the 1970s. There are approximately fifteen fields, whose quanta are the elementary particles (electrons, quarks, muons, neutrinos, Higgs, and little else), plus a few fields similar to the electromagnetic one, which describe electromagnetic force and the other forces operating at a nuclear scale, whose quanta are similar to photons.... For more than thirty years, every single experiment of particle physics has done nothing but repeatedly confirm the Standard Model. (pp. 128-9)

 

We expect space to be granular. We expect "quanta of gravity," just as there are quanta of light, quanta of the electromagnetic field, and just as particles are quanta of quantum fields. But space is the gravitational field, and the quanta of the gravitational field are "quanta of space": the granular constituents of space. The central prediction of loop theory is therefore that space is not a continuum, ... it is formed of "atoms of space," a billion billion times smaller than the smallest of atomic nuclei. Loop theory describes this atomic and granular quantum structure of space in a precise mathematical form. It is obtained by applying the general equations of quantum mechanics written by Dirac to Einstein's gravitational field. (p. 168)

 

[Roger Penrose의 quanta of space: pp. 69-70, Carlo Rovelli, "White Holes" (2023)에서도]

 

The moment we take quantum mechanics into account, we recognize that time too must have those aspects of probabilistic indeterminacy, granularity, and relationality, which are common to all of reality. (p. 175)

 

Quantum events are no longer ordered by the passage of time, at the Planck scale. (p. 178)

 

We must learn to think of the world not as something that changes in time, but in some other way. Things change only in relation to one another. (p. 182)

 

[T]he world is made entirely from quantum fields (figure 7.8). These fields do not live in spacetime; they live, so to speak, one on top of the other: fields on fields. The space and time that we perceive in large scale are blurred and approximate image of one of these quantum fields: the gravitational field. Fields that live on themselves, without the need od spacetime to serve as a substratum, as a support, and which are capable by themselves of generating spacetime, are called "covariant quantum fields." ... The world, particles, light, energy, space, and time -- all of this is nothing but the manifestations of a single type of entity: covariant quantum fields. (pp. 192-3)

 

The quanta of gravity are the way in which space and time interact. The same mathematics coherently describes the quantum gravitational field as the other quantum fields. (p. 194)

 

The infinities that plague conventional quantum field theory, predicated on the notion of a continuous space, now vanish, because they were generated precisely by the assumption, physically incorrect, of the continuity of space. The singularities that render Einstein's equations absurd when the gravitational field becomes too strong also disappear: they are only the result of neglecting the quantization of the field. (p. 195)

 

The most studied alternative to the research direction recounted in this book is string theory. The majority of physicists who have worked on string theory ... expected that as soon as the new particle accelerator at CERN in Geneva began to function ... particles of a new kind never before observed, but anticipated by the theory, would immediately become evident: supersymmetric particles.... The loop theorists [of quantum gravity] were inclined to think that these particles might not exist. The supersymmetric particles were not observed. (p. 214)

 

The discovery of the Higgs boson [in 2013] is a rock-hard confirmation of the validity of the ideas behind the Standard Model of elementary particles, based on quantum mechanics. It is the verification of a prediction made thirty years previously. The Planck measurements [with the data made public in 2013] are solid confirmation of the standard cosmological model, based on general relativity with the cosmological constant. The detection of gravitational waves [announced in 2016] is a spectacular confirmation of general relativity.... What nature is conforming is simple: general relativity, quantum mechanics, and, within quantum mechanics, the Standard Model. (p. 215)

 

This is how Copernicus, Newton, Maxwell, and Einstein proceeded. They never try to "guess" a new theory; unlike, in my opinion, the way in which too many theoretical physicists are trying to do today. (p. 217)

 

All of this, I believe, indicates that in order to grasp the basic grammar of the world, we need to merge three basic ingredients, not just two: not just general relativity and quantum mechanics, but also the theory of heat, that is, statistical mechanics and thermodynamics, which we can also describe as "information theory." (p.248)

 

It is always heat and only heat that distinguishes the past from the future.... But heat is our way to name averages over many variables. The idea of thermal time reverses this observation. That is to say, instead of inquiring how time produces dissipation in heat, it asks how heat produces time. Thanks to Boltzman, we know that the notion of heat comes from the fact that we interact with averages. The idea of thermal time is that the notion of time, as well, comes from the fact we interact only with averages of many variables.... Time is an effect of our overlooking the physical microstates of things. Time is information we don't have. Time is our ignorance. (pp. 250-2)

 

And it is reliability that we need, not certainty. We don't have absolute certainty, and never will have it unless we accept blind belief. The most credible answers are the ones given by science, because science is the search for the most credible answers available, not for answers pretending to certainty. (p. 261)

 

It is a world that does not exist in space and does not develop in time. A world made up solely of interacting quantum fields, the swarming of which generates -- through a dense network of reciprocal interactions -- space, time, particles, waves, and light.... A world without infinity, where the infinitely small does not exist.... Quanta of space mingle with the foam of spacetime, and the structure of things is born from reciprocal information that weaves the correlations among the regions of the world. A world that we know how to describe with a set of equations. Perhaps to be corrected. (pp. 264-5)