String Theory

Ten Notable String Theorists

In This Topic Meeting the founders of string theory Rising to the challenge: A new generation of string theorists Branching out to make string theory popular among nonphysicists No new theory can develop without dedicated scientists working hard to refine and interpret it. Throughout this topic, you read about some of the pioneering work in […]

Edward Witten (String Theory)

Seen by many as the leading thinker of string theory, Witten introduced the concept of M-theory in 1995 as a way to consolidate the existing string theories into one comprehensive theory. Witten’s work in string theory also included the 1984 application of Calabi-Yau manifolds to explain the compactification of the extra dimensions. In 1951, Witten […]

Time Symmetry: Why Does Time Seem to Move Forward? (String Theory)

The space dimensions are interchangeable, but time is distinct because it seems to move in only one direction. This doesn’t have to be the case. In fact, the mathematical laws of physics work either way, even in a universe where time could run backwards. But time doesn’t run backwards at all, and a theory of […]

Dark Mystery No. 2: What Is Dark Energy (and Why Is It So Weak)? (String Theory)

There’s a lot of dark energy in the universe — about three times as much as visible matter and dark matter put together! This energy represents a repulsive force of gravity on large scales, pushing the edges of the universe apart. The abundance of dark energy by itself isn’t so much a problem; the real […]

Black Hole Information Paradox: What Happens to Missing Black Hole Matter? (String Theory)

The current thinking on the black hole information paradox is that there is a quantum system underlying the black hole, and that this quantum system never loses information, though the system can mix up the finer points in a complicated way. To reconcile this picture with Hawking’s calculations (see topic 14), the concept of complementarity […]

Quantum Interpretation: What Does Quantum Mechanics Mean? (String Theory)

Though quantum mechanics works to explain the results seen in laboratory experiments, there’s still not a single clear description of the physical principle that causes it to work the way it does. Though this is tied to the “collapse of the quantum wavefunction,” the exact physical meaning of the wavefunction, or of its collapse, remains […]

Dark Mystery No. 1: What Is Dark Matter (and Why Is There So Much)? (String Theory)

There appear to be two forms of matter in the universe: visible matter and dark matter. Scientists know dark matter exists because they can detect its gravitational effects, but they can’t currently observe dark matter directly. If extra matter weren’t there to hold galaxies together, the equations of general relativity show that they would fly […]

Fine-Tuning: Why Do Fundamental Constants Have the Values They Do? (String Theory)

Many of the fundamental constants in our universe seem precisely set in the range that allows life to form. This is one reason why some physicists have been turning toward the anthropic principle, because it so readily explains this fact. Physicists hope, however, that a theory of everything would explain the precision of these values […]

Baryon Asymmetry: Why Does Matter Exist? (String Theory)

After the big bang, raw energy was transformed into matter. If the energy of the early universe had cooled into equal amounts of matter and antimatter, these different forms of matter would have annihilated each other, leaving no matter in the universe. Instead, there was substantially more matter than antimatter, enough so that when all […]

Hierarchy Issues: Why Are There Gaps in Forces, Particles, and Energy Levels? (String Theory)

Most physicists, if they were to set out to create a universe, would have been a bit more conservative with their resources than the forces at work in our universe seem to have been. There are a wide range of force intensities, ranging from the incredibly weak gravitational force to the strong nuclear force that […]