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__Reality is Not What it Seems: The Journey to Quantum Gravity__ (2017)

** Carlo Rovelli (1956)**

Translated from the Italian by Simon Carnell and Erica Segre

280 pages

Theoretical physicist and author Carlo Rovelli warns readers already with the title of his book

__Reality is Not What it Seems__ to be prepared to leave our intuitive understanding of the physical world behind as he introduces us to the outlines and implications of current research into quantum gravity. As Rovelli mentions in the preface, this book is appearing in English

*after* his work

__Seven Brief Lessons on Physics__ (my review

here), though it had been written and published in Italian a couple of years before. He indicates that for those who have read

__Seven Brief Lessons__, this new book provides a more in-depth treatment of the topics; as described below, it also goes back to review the historical underpinnings of the millennia long path over which the developments have occurred.

It turns out that our daily interactions with and observations of the world provide little or no basis for understanding what happens at the level of elementary particles of the universe. In fact, they most likely mislead us, hindering our ability to accept physicists’ current theories and models.

The book’s subtitle,

__The Journey to Quantum Gravity__, hints at Rovelli’s approach to helping readers get past such obstacles of our intuition. Instead of simply diving into the topic of quantum gravity, or perhaps providing a little background by starting with, say, overviews of the theories of general relativity and quantum mechanics, Rovelli opens his story much earlier — very much earlier. He begins with the ancient Greeks, and their first conceptions of the idea of atoms.

By going so far back to begin his story, Rovelli aspires not simply to highlight the scientific roots that have, several millennia later, led to investigations into quantum gravity. He also looks to demonstrate that the larger goals of modern day researchers working on this cutting edge theory (and competing ones) lie within a framework and motivation first established in our distant past: the idea that we must strive to understand the world through observation and reason. Rovelli describes the transition to this early version of the scientific method made by the Greek philosopher Anaximander and those around him in the 5th century BCE, as they strived to move beyond using supernatural stories to explain physical phenomena. These philosophers argued that instead of venerating received ideas and descriptions of the world as unquestionable wisdom, philosophers must focus on using observation to build on and correct earlier ideas. Rovelli states that “from this moment onward, knowledge begins to grow at a vertiginous pace.” (17)

The first critical step occurred in the late 5th century BCE, when the philosopher Democritus and his teacher Leucippus proposed a structure of the world that has — remarkably — served as the basis for the development of our scientific understanding through to this day. The two philosophers formulated the idea that “the entire universe is made up of a boundless space in which innumerable atoms run … [and that] atoms are indivisible … elementary grains of reality, which cannot be further subdivided, and everything is made of them.” (20) In these lines, and a few additional paragraphs, Rovelli summarizes Democritus’ amazing leap forward from the superstitious beliefs of his time to a powerful vision “on which the knowledge of civilization would later be built.” (20)

In a fascinating review of the historical context of these early developments, Rovelli describes the many books Democritus wrote on an astonishing variety of topics, from science to philosophy to history to name but a few; he notes that these works had a dramatic influence on those who followed Democritus. Rovelli notes that for several centuries after Democritus — into the first centuries of the Common Era — philosophers and scientists steadily built upon his ideas. The extent of his impact becomes clear when Rovelli points out the shattering fact that we only know of Democritus’ work through the extensive discussions of it by his contemporaries and those that followed him: all of his works were lost in the vast and savage destruction wrought in the wake of the 4th century CE Roman declaration “that Christianity was to be the only and obligatory religion of the empire” (33). As a result of this heart-breaking loss notes Rovelli, “astronomy did not take any very significant step forward for more than a thousand years,” (45) in a Western world plunged into the darkness of the Middle Ages.

The West eventually rediscovered this Greek scientific heritage, which had been preserved in India and was eventually reintroduced to Europe through Persian and Arab scholars. (The surprising path of this knowledge back into Europe is discussed in works such as

__Moorish Spain__ by Richard Fletcher, and

__Ornament of the World__ by María Rosa Menocal.) This recovered knowledge reignited interest in astronomy and more broadly physics, Rovelli notes, influencing scientists such as Copernicus and Galileo. A next critical step, however, was made by Isaac Newton, who not only formalized what Democritus and those who had followed him proposed, but build a mathematical framework for it, describing how the force of gravity influences objects. In a schematic encapsulation that Rovelli then carries forward to explain developments in understanding through to the current research, he describes Newton’s view of the world as being made up of

*space*,

*time* and

*particles*.

Newton himself, according to Rovelli, recognized that while his theories describe many of the phenomena of the natural world, there are forces other than gravity at play; in the 1800’s, Michael Faraday and James Clark Maxell discovered and described mathematically one such force, that of electromagnetism. The critical insight was to “not think of forces acting directly between distant objects, [but rather] think that there exists an entity diffused throughout space that is modified by electric and magnetic bodies and that, in turn, acts upon … the bodies, … what is today called the

*field*.” (55) Thus, while Newton had described the existence of particles, and the concept of a force acting between them, Faraday and Maxwell describe a universe made up of both particles, and fields which exert forces on them.

This description of the physical world as being made up in part of fields began the movement of physics into descriptions of reality beyond our intuitive understanding of the world. Developments in the 20th century would only accelerate this movement, beginning with Albert Einstein’s publishing of his theory of special relativity. Rovelli points out two key features of Einstein’s theory.

One is that it combines space and time — which had been viewed until then as independent concepts — into a single

*spacetime*. Rovelli explains spacetime as implying that it makes no sense to think of “now” in a universal sense; at points away from an individual observer,

*now*, has a duration, which grows ever longer the greater the distance away — what Rovelli calls the “extended present.” He notes, for example, that: “In the Andromeda galaxy, the duration of this extended present is [with respect to an observer on Earth] two million years.” (72) Thus, space and time are tied intimately together.

A second key consequence that Einstein realized, and elaborated in his theory of special relativity was “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.” (74) Einstein goes on to calculate the relationship between mass and energy, the famous E=mc².

Though the theory of special relativity brought Einstein much renown, he realized according to Rovelli, that his theory “does not square with what was known about gravity” (77); he (and others) worked for years to incorporate gravity into the new models. Einstein comes to realize that the force of gravity exists as “a gravitational field [with descriptive] equations analogous to Maxwell’s” (78) for electricity. This leads Einstein, in his theory of general relativity, to propose just such a field description of gravity. But he then also adds an extraordinary concept: that the gravitation field is not simply present in space, but that

*it is space itself*. From this comes the picture of spacetime curved by a mass, such as the sun, and the Earth then not attracted to the sun by gravity, but rather rolling around the sun as it falls in a funnel-shaped curvature generated in spacetime by the sun.

The other “pillar[] of twentieth-century physics” (109) according to Rovelli, is quantum mechanics, which he describes as having been motivated by the work of Max Planck and Albert Einstein, who first characterized light as made up of packets of energy, called

*photons*. Rovelli notes that the details of the theory were then worked on over decades by a number of physicists; after giving an overview of the stages of development of quantum mechanics, he culminates with a summary of what he feels constitute the theory’s key conclusions. The first echo’s back to Democritus: that there is a fundamental granularity to nature, implying that a finite amount of information can exist in a system, a finite number of states. The second and third results are even more challenging to our deterministic intuition of the world: the future states of a system are indeterminate at the quantum level, meaning they can change randomly, and can only be predicted in a statistical sense, and also that events in a system only exist and occur relative to other events, that is, in the interactions between objects.

Rovelli points out that while quantum theory has been upheld in every experiment so far designed to test its descriptions of nature, “it remains shrouded in obscurity and incomprehensibility.” (109) He concludes his discussion on the theory with a brief description of the difficulties some physicists, including Einstein, have had in accepting its implications. He reminds us that quantum mechanics is a model of the universe, one that so far describes accurately the world as we have been able to test it, but one that, as with any other theory, may eventually be corrected by a different and better understanding of the world.

Having spent the first half of the book presenting an overview of the path physics has taken from its beginnings in ancient Greece to the accepted, modern-day theories, Rovelli pivots to the current, cutting edge in physics research: the work to find an answer to a critical missing piece in the existing theories of General Relativity and Quantum Mechanics that arises from the recognition that: “they cannot both be true, at least not in their present forms, because they appear to contradict each other.” (147)

Rovelli focuses on one proposed theory to address this dilemma, known as

*loop quantum gravity*, an area of study in which he is in fact one of the initial researchers. Over several chapters, he provides an overview of the theory and some of its consequences.

He begins by describing key outcomes in development of the theory. The first result runs counter to our (again deceptive) intuition, in this case our impression of space as an emptiness in which mass exists. Instead, the theory of quantum gravity postulates that “space is created by the interaction of individual quanta of gravity” (174), and so that space is not continuous, but rather made up of extremely — but not infinitely — small quanta. The result builds on a fascinating implication of Heisenberg’s uncertainty principle:
“The smaller the region where we try to locate a particle, the greater the velocity at which it escapes … [and so that particle] has a great deal of energy … [which] results in curving space so much that it collapses into a black hole. … [thus] quantum mechanics and general relativity, taken together, imply that there is a limit to the divisibility of space.” (152)

A second implication that has arisen out of the research on quantum gravity upends our understanding of time, and in particular the idea that there is an absolute passage of time for the universe. He points out that Einstein’s theories already abstracted time, by recognizing that it passes differently for observers at different locations, due to the relative locations or movements of observers. According to Rovelli, quantum gravity implies that at the quantum level time does not exist as an independent variable at all; thus events occur due to the interactions of fields, and are not aligned to an overarching guide of time.

Rovelli provides a way of thinking about this at our macro level view of the world. He describes a

*clock* as a mechanism that counts up events (for example, the swings of a pendulum), and when we use such a clock to measure the

*time* over which some process takes place (say the movement of an object), though we may represent the object’s movement as a function of time, in reality what we know is the distance the object moves as a function of the number of swings of the pendulum; that is, one series of events relative to another series of events.

*Time* is simply a construct that can be useful for our purposes at a macro scale of the universe, but it does not constitute an absolute and fundamental property of the universe.

Thus, Rovelli summarizes the theoretical direction of quantum gravity as forcing us to realize that
“The space and time that we perceive in large scale are blurred and approximate images of one of these quantum fields: the gravitational field.” (193)

In order to summarize the long road physics has taken from Newton’s formalization of the world understood already by the Greeks, through to the modern theory of quantum gravity, Rovelli includes a wonderfully concise diagram that he evolves over the course of the book into the final form shown below (193).

This schematic also reinforces the transformation from a view of the universe that aligns with our daily observation and experience, to an increasingly abstract and unintuitive understanding of the true nature of the cosmos.

In the final part of the book, Rovelli discusses several fascinating implications of the theory of quantum gravity, if it proves to be an accurate model of the universe.

Once such consequence alters the concept of the Big Bang — the idea that the universe began from a single point that exploded outwards — to what Rovelli refers to as the Big Bounce. The Big Bounce is based on ideas from quantum gravity that indicate that “the universe cannot be indefinitely squashed [as is] predicted by Einstein’s equations if we ignore quantum theory.” (207) Instead, according to quantum mechanics incorporating quantum gravity, as the volume into which the universe compresses grows ever smaller, the repulsion eventually grows large enough that a renewed explosive expansion would begin.

After a chapter on some of the empirical investigations being done to test the ideas of quantum gravity, Rovelli discusses the implications of the theory for our understanding of black holes. The key point again is that at extremely small volumes of space, quantum mechanics together with quantum gravity predict a different behavior than the theory of general relativity. One result is that black holes are not stable objects, but rather — similar to the universe as a whole — eventually constrict to a size at which the repulsion forces cause them to explode.

Rovelli then goes on to look at some of the broader implications of the fundamental quantization of space. One of these is the nonexistence of infinity, in many senses. Since the universe is no longer infinitely divisible, there is no such thing as an unaccountably infinite number of anything. We live, he writes, in “a vast cosmos, but a finite one.” (237) Even with his clear and lucid presentation, and accepting the idea that in the smallest sense there is quantization, and so finiteness, it can be a struggle to transform that understanding to the large scale of the universe, and wonder what lies beyond that finite extent. Again, I suppose, the failure of intuition to allow grasp new models of reality...

Rovelli follows with, in the second to last chapter, a discussion on the idea of

*information* and how physicists are thinking about the concept of information of a system given the theory of quantum gravity. Again the fundamental idea of quantization plays a critical role: “quantum mechanics can be understood as the discovery that information in nature is always

*finite*.” (245)

In the concluding chapter of this fascinating and thought-provoking work, Rovelli gives a clarion call to readers, one that serves both to acknowledge the challenge that modern physics presents to our intuitive views of the universe, as well as to encourage us to continue striving to broaden our understanding. He asks that we accept our own ignorance of the world, and rather than allow the frightening uncertainty of our ignorance to drive us into the arms of any person or group claiming to have all the answers, that we instead exalt in that ignorance and use it as motivation to continue pushing our knowledge forward: “To seek to look further, to go further, seems to me to be one of the splendid things that give sense to life.” (263)

**Other reviews / information:**

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**Other of my book reviews: FICTION Bookshelf and NON-FICTION Bookshelf**