When theoretical physicist Rovelli’s *Seven Lessons in Physics* became an international bestseller in 2015, his Italian publisher proceeded to translate this, his previous book, which turns out to be an admirable addition to a popular genre: explaining what scientists know about the universe and their struggles to learn more.

Most authors in this subject begin with the Greeks and make their ways through Newton, Galileo, and Maxwell to the glories of Einstein’s relativity and the founders of quantum mechanics. In good hands, this is a smooth ride, and Rovelli—the head of the Quantum Gravity group at Aix-Marseille University and one of the founders of the loop quantum gravity theory—is good. Then the story gets tougher. Relativity and quantum mechanics are incompatible, and physicists today are trying to combine them to produce a single, satisfying theory of everything. This requires complex ideas that dissect everything, space-time included, to a very, very tiny state where their quintessence emerges. Rovelli delivers a respectful nod to string theory, but he belongs to the rival school of quantum loop gravity, the central feature of which is that space itself is quantized. “The central prediction of loop theory is…that space is not a continuum,” writes the author, “it is not divisible ad infinitum, it is formed of ‘atoms of space,’ a billion billion times smaller than the smallest of atomic nuclei.” Got it? For some readers, the narrative will be a slog. Science buffs will admire Rovelli’s lucid writing, but at some point, many will realize that they no longer understand.

Cutting-edge theoretical physics for a popular audience that obeys the rules (little math, plenty of drawings), but it’s not for the faint of heart.