A summary of "Broca’s Brain:
On Science, Space, and Man" by Carl Sagan

Broca’s Brain is a collection of essays by American astrophysicist and science writer Carl Sagan, published in 1979. Its chapters grew out of articles written between 1974 and 1979 for The Atlantic Monthly, The New Republic, Scientific American, Playboy, and other publications. The book covers a wide range of topics — from the history of science and the nature of the human brain to pseudoscience, space exploration, and the nature of religious experience — and is united by a common question: can we understand the universe in which we live?

A Visit to the Skulls and Brains

The first chapter opens with a description of Sagan’s visit to the Musée de l’Homme in Paris. Wandering through the storerooms, he passes shelves of hominid skulls, jars of preserved human embryos, and cylindrical jars containing human heads brought from New Caledonia and other colonies — a man with a red mustache, a little girl with coral earrings, three children in a single bottle. In the far corner, he discovers shelves of human brains preserved in formalin.

One of the vessels bears the inscription "P. Broca." Paul Broca (1824–1880) was a surgeon, neurologist, and anthropologist who founded the Anthropological Society in Paris, which, from 1859, operated under the watchful eye of a plainclothes police agent. Broca discovered a small region in the left frontal lobe of the cortex — Broca’s area, responsible for articulate speech — which became one of the first indications of the functional specialization of the hemispheres. Physical anthropologist Ralph Holloway of Columbia University finds traces of Broca’s area in the skulls of Homo habilis — two-million-year-old tool-making ancestors of humans.

Holding the darkened jar containing Broca’s brain, Sagan ponders whether personality can be preserved in neural connections, and what prejudices of their era does every scientist, even the most progressive, carry? Broca supported Darwin and fought clericalism, yet he considered men superior to women and whites superior to Blacks. The question is: what "self-evident truths" of our time will prove unforgivably narrow-minded for the next generation?

A grain of salt and the laws of nature

The second chapter answers the question posed in the title: "Can humans comprehend the Universe?" Sagan takes one microgram of table salt — about 10 ^atoms of sodium and chlorine. The brain stores no more than 10 ^bits of information, a hundred times less than is needed to describe even this tiny grain. In the ordinary sense, the Universe is unknowable.

But salt is a crystal with a regular structure. If we know the law of its structure, a few dozen bits describe the arrangement of all the atoms. This is what makes science possible: nature obeys laws, and the compact mathematical formulations of these laws accommodate an infinite variety of phenomena. Einstein’s special theory of relativity shows that when moving at speeds close to the speed of light, the mass of a body increases to infinity, the thickness in the direction of motion tends to zero, and time almost stops — common sense is of no help here. Quantum mechanics forbids molecules from rotating at arbitrary angles: only strictly defined positions are permitted. The more stringent the constraints imposed by nature on matter and energy, the more knowledge humans gain.

Albert Einstein

The third chapter is dedicated to Einstein — the book was published in Ulm in 1879, marking the centenary of his birth. Sagan describes a childhood in which teachers failed to recognize his talent: at Munich’s Luitpold Gymnasium, the boy was advised to leave school, told he would "never amount to anything." At 12, Einstein was transfixed by the clarity of Euclidean geometry; at 15, he traveled through northern Italy, escaping school. He entered the Zurich Federal Institute of Technology only on his second attempt and graduated only thanks to the notes of his friend Marcel Grossmann.

In 1902, Grossman Sr. arranged for him to work at the Bern Patent Office. In 1905, in the 26-year-old clerk’s "spare time," four papers were published in Annalen der Physik: an explanation of the photoelectric effect (partially for which he was awarded the Nobel Prize in 1921), a description of Brownian motion, the special theory of relativity, and the equation E = mc², which expresses the equivalence of mass and energy. The complete conversion of one gram of matter into energy would produce an explosion equivalent to approximately a thousand tons of TNT.

Einstein published his general theory of relativity in 1915: gravity is not a force, but the curvature of space by masses. In 1919, British expeditions to Brazil and the island of Principe recorded the deflection of starlight near the Sun during a total eclipse — exactly as predicted by the theory. When Hitler became chancellor in 1933, Einstein left Germany.

Pseudoscience: Velikovsky and other "paradoxalists"

Much of the book is devoted to examining what Sagan calls "paradoxalism" — pretentious, unproven explanations for well-studied phenomena. A separate chapter examines Immanuel Velikovsky’s theory from Worlds in Collision: he claimed that Venus was ejected from Jupiter as a comet and made several close encounters with Earth, causing the biblical catastrophes. Sagan consistently demonstrates, using calculations, the physical inconsistency of this idea: to break away from Jupiter, a body the mass of Venus would require an energy of approximately 10 ^ergs — that’s all the energy the Sun radiates in an entire year. Such a body would completely melt or disintegrate upon breakaway.

Another section is devoted to a certain Norman Bloom, who claimed to be a messenger of God and sent letters to scientists and governments. Sagan analyzes Bloom’s rhetoric as a paradigmatic case of pathological conviction amid pseudoscientific language.

Space exploration and the search for life

In the chapters on astronomy, Sagan describes expeditions to Mars (the 1976 Viking probes and their search for signs of life in Martian soil) and Venus. He also examines the history of rocketry and the role of Robert Goddard, the inventor of the first liquid-fueled rocket, who was ridiculed in the press in the 1920s, including a New York Times editorial that claimed jet propulsion was impossible in a vacuum.

In his chapters on the search for extraterrestrial intelligence (SETI), Sagan examines the Drake equation, estimates the likelihood of intelligent radio signals, and describes the operation of the 305-meter Arecibo radio telescope in Puerto Rico, which sent a coded transmission toward the globular cluster M13 in 1974.

Science, Religion, and Ultimate Questions

In the final chapters, Sagan addresses the phenomenon of religious experience. He analyzes the neurological mechanisms of sensations described by people who have had near-death experiences — a tunnel with a light at the end, encounters with deceased relatives, a sense of peace. Sagan suggests that these images may be memories of one’s own birth, imprinted in the brain: passage through the birth canal leaves traces in neural structures long before conscious memory is formed. This is a hypothesis, he admits, but it is testable.

In his final essay, he formulates his position on religion: if God exists, then it is He who endowed us with reason and curiosity; if not, then reason and curiosity are the only tools for survival. In both cases, the pursuit of knowledge is justified and irrevocable.