The Mind-Bending Physics of String Theory

“Theories of everything” go back to the ancient Greeks and their idea that indivisible atoms are the building blocks of reality. This conjecture grew into modern atomic theory, which has been perfected by relativity theory and quantum theory. But there’s never been anything like string theory. Designed to describe the universe at the largest and smallest scales, string theory is the most ambitious and far-reaching theory of everything yet devised, leading to intriguing speculations, such as:

• Strings, Not Particles: In string theory, the fundamental units of matter and energy are inconceivably tiny strings, not point-like particles as in conventional theories. The advantage is a wide range of different modes that can represent diverse phenomena.
• Extra Dimensions: Strings can’t do their job without existing in a multitude of dimensions—10 according to the standard version of the theory. Why don’t we perceive these extra dimensions? They are “compactified” and thus invisible at our human scale.
• Multiple Universes: The versatility of string theory allows for different laws of physics that may have developed in separate regions of the rapidly expanding universe in the moments after the Big Bang. We might live in one isolated bubble of this multiverse.

For all of its amazing insights, string theory is, so far, unproven—and some critics claim that it is unprovable. Yet it has already led to breakthroughs in our understanding of black holes, the Big Bang, cosmic inflation, and dark energy, not to mention gravity and the other fundamental forces. In The Mind-Bending Physics of String Theory, noted string theorist Professor Shamit Kachru of the Stanford Institute for Theoretical Physics takes you deep into this fascinating subject in 12 lectures that are rich in mathematical concepts.

Spanning the multi-decade origin and evolution of string theory, Professor Kachru provides an up-to-date survey of modern theoretical physics and its astonishing power to uncover the most profound secrets of nature.

Solving the Mystery of Gravity

One of those profound secrets is the long-sought connection between gravity, electromagnetism, and the strong and weak nuclear forces. The last three are far stronger than gravity, but operate primarily in the subatomic region. They are perfectly described by quantum mechanics. Meanwhile, gravity is the outlier, involving enormous masses like stars and planets that extend their pull across vast reaches of space. Gravity is vanishingly weak at the quantum scale and can’t be explained by standard quantum theory.

String theory aims to unite these fundamental forces to forge a single, comprehensive theory that can simultaneously explain the physics of the subatomic and cosmic realms. Such a breakthrough would marry quantum mechanics and Einstein’s theory of gravity, known as general relativity—a match that has so far challenged the most brilliant minds in physics.

In The Mind-Bending Physics of String Theory, you investigate how string theory may be able to accomplish this and other feats. Some of the exotic concepts you explore include:

• Planck Length: Quantum mechanics and general relativity converge at the smallest possible distance, called the Planck length, which is more than a billion, billion times smaller than the diameter of a proton. This is the characteristic length of strings.
• Supersymmetry: One way to make the tricky math of string theory work out is to double the number of particles it has to explain! This may sound counterintuitive, but Dr. Kachru shows how a principle called supersymmetry makes the numbers compute like magic.
• Duality: Another problem is the multitude of string theories with markedly different parameters. However, on analysis, many turn out to be mathematically the same—a property called duality. Entirely new, more powerful string theories emerge from this process.

Venture across Time, Space, and Unseen Dimensions

As director of one of the world’s premier programs in theoretical physics, Dr. Kachru is in the vanguard of research on string theory. No comparable lecture series goes into the detail he presents in this course, profusely illustrated with animations and graphics. String theory is complicated, and he doesn’t pretend otherwise. However, even scientists simplify their subject to make calculations more manageable, as you learn through a stratagem called the “spherical cow.” This is a widespread practice in theoretical physics that removes complexities to streamline a problem—as if a farmer ignored the appendages on his herd of cattle to get a rough-and-ready estimate of the space needed for a barn.

Dr. Kachru uses supersymmetry as a spherical cow, discounting the fact that the symmetry between particle types cannot be perfect in the real world. Following this approach, he leads you through a breathtaking landscape, featuring:

• Branes: Short for “membranes,” these are surfaces on which strings can attach, giving rise to different particles and properties. Although represented in two dimensions, they can exist as p-branes in higher dimensions, where “p” is the number of dimensions.
• Compactification: Imagine a one-dimensional line, enlarged to reveal a hidden dimension, curled around like a tube. This extra dimension has been “compactified.” In string theory, the geometry of compactification can get quite complicated!
• Calabi–Yau Manifolds: The extra dimensions in 10-dimensional string theory (six plus the four familiar dimensions of space and time) can be compactified in a class of complex geometric shapes called Calabi–Yau manifolds, which lead to a host of subatomic phenomena.

Equipped with these concepts, you venture across time, space, and unseen dimensions. You explore the Big Bang, the dazzlingly brief era of cosmic inflation, the profound mystery of dark energy, the enigma of black holes, the conundrum of quantum gravity, the unification of the four fundamental forces, and much more—everything really, for The Mind-Bending Physics of String Theory explores a set of ideas that may be, at long last, the theory of everything.