In his classic book *The Dragons of Eden*, astronomer Carl Sagan tidily summarized the central challenge scientists face when they try to formulate grand new theories: “Remarkable claims require remarkable proof.” One of the most remarkable claims made in modern times comes from string theory, which holds that everything in the universe is composed of tiny vibrating strings of energy. In this view, every particle in your body, every speck of light that lets you read these words, and every packet of gravity that pushes you into your chair is just a variant of this one fundamental entity. Over the past three decades, string theory has increasingly captured the imagination of physicists. Hundreds of researchers around the world now hammer away at its equations every day, trying to make the different parts of the theory hang together. They, like me, consider it the greatest step forward in science since Albert Einstein and Max Planck introduced the key ideas of relativity and quantum mechanics about a century ago.

Yet string theory, like all scientific theories, eventually must face the harsh test Sagan described. So far, it cannot stand up. To be brutally honest, there is no proof whatsoever that string theory is correct.

How, then, can its advocates persist? Part of the answer lies in the theory’s breathtaking premise. The natural world abounds with a baffling variety of particles smaller than atoms and four seemingly independent forces: gravity, electromagnetism, and the strong and weak nuclear forces. By describing subatomic particles as vibrating strings, somewhat like taut rubber bands, string theory ties all these disparate parts into a single framework. Every type of particle—including the electrons that form part of ordinary matter and the photons that transmit the electromagnetic force—simply corresponds to a specific frequency of vibration of the string. Much as pulling on a rubber band changes its vibration frequency, altering a string’s mode of vibration transforms an electron into a neutrino, a quark, or another particle.

Strings have another enticing, even more esoteric property. As they vibrate, they force space and time to curl around them, giving rise to gravity in exactly the manner that Einstein described in his theory of relativity. String theory thus promises to merge the equations describing the action of the tiny world we cannot see—that of subatomic particles—with the equations describing gravity and the large-scale world we experience every day. Einstein spent the final three decades of his life searching for such a merger, which he likened to “reading the mind of God.” String theory may achieve what Einstein could not, a unified theory that explains how the universe works.

Throughout modern history, the discovery of each new unifying principle in physics has sparked stunning new practical insights. Isaac Newton’s laws of mechanics paved the way for steam engines and the industrial revolution. Michael Faraday and James Clerk Maxwell’s insight that electricity and magnetism are two aspects of the same force, electromagnetism, ultimately unleashed the age of electronics. Einstein’s realization that energy and matter are interchangeable helped usher in the nuclear age. We can only guess at the discoveries that might follow the confirmation of string theory.

Finally, the math behind string theory is extremely sophisticated and beautiful, and the equations have survived every mathematical challenge. People who have worked on string theory often walk away with a powerful, if unquantifiable, feeling that it smells like truth.

But any theory, no matter how grand, must be reproducible, and that is where testing string theory gets a little crazy. Each of the theory’s solutions represents an entire universe, so to test the theory fully, one would have to create a baby universe in a laboratory. State-of-the-art technology barely lets us escape the planet, never mind re-create another cosmos. So skeptics, who often admit the loveliness of the math, have long dismissed string theory as an untestable fantasy.

That could change soon. An array of new devices—including new atom smashers, gravity detectors, spaceborne satellites, and buried detectors—could provide significant evidence that would support string theory. The rub is that all this new evidence, no matter how compelling, will still provide only indirect proof.