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.