In 1950, during a lunchtime conversation at Los Alamos, the physicist Enrico Fermi asked a deceptively simple question: “Where is everybody?” He was speaking about extraterrestrial life. Even then, scientists knew that the universe contained an extraordinary number of stars. Today, we know far more. Our own Milky Way contains hundreds of billions of stars, many with planets of their own. Beyond our galaxy lie hundreds of billions of other galaxies. The numbers are almost impossible to absorb.

If life emerged on Earth, why should it not have emerged elsewhere? If intelligence evolved here, why not elsewhere? Given the age of the universe, some civilizations, if they exist, could be millions of years older than our own. And yet, there is silence. No visitors. No confirmed signals. No evidence that anyone has reached us. This tension has become known as the Fermi Paradox. If intelligent life should be common, where is everybody?

Science fiction imagines alien civilizations crossing the stars with ease. But the universe revealed by science is less accommodating. The nearest star beyond our solar system is more than four light-years away. Most potentially habitable worlds are vastly farther. The speed of light imposes a limit that cannot be overcome simply by imagination or technological ambition. To send living beings across interstellar distances would require immense energy, extraordinary shielding, and journeys lasting decades, centuries, or more. The obstacle is not merely technology. The obstacle is physics. It may be that the universe contains many civilizations, each isolated on its own island of consciousness, separated by distances so vast that meaningful contact remains forever beyond reach. It is a humbling thought.

Modern science has repeatedly humbled humanity. Copernicus removed the Earth from the center of the cosmos. Darwin challenged assumptions about human uniqueness. Modern astronomy revealed that our galaxy is one among billions. We inhabit a small planet orbiting an ordinary........

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