Have you heard the joke about the elderly rabbi who tries to settle a bitter dispute between two men? The rabbi listens to one man’s case and pronounces him right. Then he hears the second man’s case, and concludes the second man is right. At this point his eavesdropping wife steps in and points out that both men can’t possibly be right. To which the rabbi replies, “And you are right as well!”
That conundrum lies at the heart of two new books: Christophe Galfard’s “The Universe in Your Hand,” and Carlo Rovelli’s “Seven Brief Lessons on Physics.” Rovelli uses the case of the indecisive rabbi to illustrate the dilemma faced by theoretical physicists in the 21st century, except in this case what is under dispute are two competing “rule books” for reality: Einstein’s general theory of relativity, and quantum mechanics. Each functions perfectly well within its specific realm: Quantum mechanics governs the subatomic world of the very small, while general relativity describes how the world works at very large scales. But neither offers a complete description of how the world works.
Galfard is a protégé of Stephen Hawking’s, co-authoring a young adult book with Hawking and his daughter, Lucy, in 2007 (“George’s Secret Key to the Universe”). Those Y.A. roots show in “The Universe in Your Hand.” There’s a lot to be said in defense of plain, simple language, but in this case it proves a mixed bag. The earlier chapters read more like draft scripts for the television series “Cosmos,” covering very familiar ground (the sun, the moon, our solar system, stars and galaxies) without doing much to make the material seem fresh.
More problematic is Galfard’s frequent use of the second person — no doubt to provide a stronger sense of immediacy for the reader — which wears thin rather quickly and adds a whiff of condescension to the overall tone. He also tends to repeat himself a great deal; for Galfard, if a point is worth making, it’s worth restating at least twice more. The book could easily be trimmed by a third by eliminating some of those redundancies.
That chatty plain-spoken approach pays off, however, once Galfard digs into the headier realms of special relativity, quantum mechanics, black hole physics and string theory. As befits a Hawking protégé, he’s quite skilled at clever analogies. For instance, the excitation of atoms is “a bit like children being offered sweets at a party,” and the sweets that the children prefer are analogous to which kinds of light an atom will absorb, seen in the absorption lines of atomic spectra. And he deftly sums up why distances must contract and time must dilate under the rules of relativity: Something has to give in order for the speed of light to remain constant regardless of the viewpoint of the observer.
Where Galfard really shines is in his crystal-clear explanation of quantum field theory — a welcome inclusion for a popular physics book. Most stick with the intuitive description of matter being made of atoms, and atoms being made of elementary particles, with those particles being composed of quarks. But in reality, the world is made up of fields. Particles are just what we see as a manifestation of those fields. Case in point: The electromagnetic field is “a sea of force out of which virtual particles of light can pop at any moment.”
Galfard even dares to venture where many popular science writers fear to tread with a careful breakdown of how physicists deal with infinities. If we wish to calculate the probability of two electrons bouncing off each other, for example, we can use a classical equation describing how billiard balls scatter as a first approximation. Physicists would typically then make successive small tweaks to arrive at the correct answer for two electrons. But in quantum field theory, such tweaks give an answer of infinity. This is clearly wrong, since any probability must be less than one.
There is a mathematical trick to get the correct answer, essentially akin to sweeping the infinities under the rug and pretending they don’t exist. As Galfard puts it, “One does not need to know about atoms to compare apples on a market stand.” And it works. Once that is done, the predictions of quantum field theory match experimental results to an accuracy of one part in a billion.
But this doesn’t work when it comes to gravity. As Galfard explains, the essence of quantum field theory is that the elementary particles associated with any given field are made of the field itself. With gravity, that “field” is space-time. So space-time is fundamentally different in general relativity and quantum field theory. Finding a way to reconcile the two has thus far eluded physicists.
It happens that Carlo Rovelli is one of the founders of loop quantum gravity and one of the leading candidates for achieving that reconciliation, along with string theory. So naturally it informs his outlook in “Seven Brief Lessons.” This slim volume expands on his very short essays for the Italian newspaper Il Sole 24 Ore on the biggest physics breakthroughs of the 20th century and beyond: general relativity, quantum mechanics, the cosmos, elementary particles, quantum gravity, probability and black hole thermodynamics, and our own humble place in all of this.
In clear, elegant prose, Rovelli guides the reader through a whirlwind tour of some of the biggest ideas in physics. His passion for his chosen field is evident on every page. For him, general relativity is on a par with such masterpieces of human genius as Mozart’s Requiem, Shakespeare’s “King Lear” and the Sistine Chapel. In the opening essay, he recalls a summer at the seaside in Calabria when he was still a student. He watched the water’s surface ripple and sway, as space-time curves in response to matter and energy, and understood for the very first time the elegant simplicity of Einstein’s equations — and also its revolutionary implications. “Within this equation there is a teeming universe,” he writes.
Rovelli never once mentions string theory in his essay devoted to quantum gravity, but he has plenty to say about loop quantum gravity, which is far less familiar to general readers. Loop quantum gravity dispenses entirely with continuous space-time, describing it instead as being made up of billions upon billions of grains, or loops, that Rovelli likens to “atoms of space.” The equations of loop quantum gravity determine how these atoms evolve, and it’s the connections between these loops that give the theory its name. In Rovelli’s worldview, space and matter continually interact with each other: “Space is created by the linking of these individual quanta of gravity,” he writes. “Every process dances independently with its neighbors, to its own rhythm.”
This notion of interconnection is a recurrent theme for Rovelli, for whom reality is “only interaction.” He sees it not only in the interplay between space-time and matter, but also in the probabilistic nature of thermodynamics, and in how time’s apparent flow arises from the “intimate connection between time and heat,” with implications for memory and consciousness. He even sees it in the different languages we use to describe our complex world and our place in it. These also “intersect, intertwine and reciprocally enhance one another, like the processes themselves.”
Despite the similarity in subject matter, these two books target different audiences. With its breezy conversational style, “The Universe in Your Hand” is well suited for the general reader with little to no prior knowledge of science, particularly the earlier chapters. Rovelli’s “Seven Brief Lessons” has a deeper philosophical bent — it’s a rare science book that cites Lucretius — and should appeal to readers with a similar sensibility. One can easily imagine perusing these essays while comfortably ensconced in an overstuffed chair by the fire, a snifter of cognac in hand.
Given that there are so many popular science books available that cover this very well-trodden ground, the real question is, why should readers buy these books rather than any of the others? Ultimately, both Galfard and Rovelli succeed in putting their own unique stamp on the material. The reader will come away from either book with a deeper understanding of how modern physics has brought us closer to an ultimate understanding of reality.
Jennifer Ouellette’s most recent book is “Me, Myself, and Why: Searching for the Science of Self.”
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