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### String Theory: Disagreements about the Anthropic Principle

At Stanford, Leonard Susskind and his colleagues seem to be embracing the anthropic principle. To hear (or read) Susskind on the subject, the string theory community is quickly jumping on board. It’s unclear [more…]

### Obstacles to Testing String Theory

String theory isn’t complete. There are a vast number of different string theory solutions — literally billions of billions of billions of billions of different possible variants of string theory, depending [more…]

### String Theory: Testing Supersymmetry

One major prediction of string theory is that a fundamental symmetry exists between bosons and fermions, called supersymmetry. For each boson there exists a related fermion, and for each fermion there [more…]

### String Theory: Testing Gravity from Extra Dimensions

The testing of gravity produces a number of ways to see if string theory predictions are true. When physicists test for gravity outside of our three dimensions, they [more…]

### How String Theory Could be Disproven

With any theory, it’s typically easier to disprove it than to prove it, although one criticism of string theory is that it may have become so versatile that it can’t be disproved. This article assumes [more…]

### Could Proton Decay Spell Disaster for String Theory?

If one of the older attempts at unification of forces (called *grand unification* *theories* or GUTs) proves successful, it would have profound implications for string theory. One of the most elegant GUTs [more…]

### Evidence of String Theory: Gamma Ray Bursts

Among the various phenomena in the universe, two types produce large amounts of energy and may provide some insight into string theory: *gamma ray bursts* [more…]

### Evidence of String Theory: Cosmic Rays

Cosmic rays could offer evidence of the validity of string theory. Scientists are unlikely to see some improbable events in laboratories on Earth, at least without a lot of work, so sometimes they look [more…]

### Support for String Theory: Exploring the Universe

The problem with conducting experiments in string theory is that it requires massive amounts of energy to reach the level where the Standard Model and general relativity break down. There are different [more…]

### Evidence for String Theory: Particle Accelerators

For high-energy particle physics, particle accelerators help string theorists proceed with experiments in apparatuses that they control. A *particle accelerator* [more…]

### String Theory: What Are Dimensions?

Any point in a mathematical space can be defined by a set of coordinates, and the number of coordinates required to define that point is the number of dimensions the space possesses. In a 3-dimensional [more…]

### String Theory: The History of 2-Dimensional Space

One of the most fascinating aspects of string theory is the requirement of extra dimensions to make the theory work. String theory requires nine space dimensions, while M-theory seems to require ten space [more…]

### String Theory: Three Dimensions of Space

To understand string theory, you need to understand the meaning of the extra dimensions required. First, look at the concept of dimensions in a very general way, and the different approaches mathematicians [more…]

### String Theory and the History of Non-Euclidean Geometry

Before string theory introduced the concept of extra dimensions, the fascination with strange warping of space in the 1800s was perhaps nowhere as clear as in the creation of [more…]

### More Dimensions Make String Theory Work

For most interpretations, superstring theory requires a large number of extra space dimensions to be mathematically consistent: M-theory requires ten space dimensions. With the introduction of branes as [more…]

### Perturbation Theory: String Theory’s Method of Approximation

The equations of string theory are incredibly complex, so they often can only be solved through a mathematical method of approximation called perturbation theory. This method is used in quantum mechanics [more…]

### String Theory and Branes: Stretching Out a String

In a sense, the introduction of M-theory marks the end of “string theory,” because it ceases to be a theory that contains only fundamental strings. M-theory also contains multidimensional membranes, called [more…]

### String Theory and the Discovery of D-Branes

While analyzing the equations of string theory, some physicists realized that the ends of open strings didn’t just hover out in empty space. Instead, it was as if the end of the open string was attached [more…]

### String Theory: Particles from P-Branes

In the mid-1990s, Andrew Strominger performed work on another type of brane, called *p-branes,* which were solutions to Einstein’s general relativity field equations. The [more…]

### String Theory: Branes are Required by M-Theory

Joe Polchinski's work proved that D-branes weren’t just a hypothetical construct allowed by string theory, but they were essential to any version of M-theory. Strongly motivated by Edward Witten’s proposal [more…]

### String Theory: Two Types of Branes Are One

At energy levels where predictions from string theory and general relativity match up, D-branes and *p*-branes are equivalent. Though Polchinski was aware of Strominger’s work on [more…]

### String Theory: Branes Explain Black Holes

One of the major theoretical insights that string theory has offered is the ability to understand some black hole physics. These are directly related to work on [more…]

### String Theory: Brane Worlds

Ever since the inception of string theory, one of the major conceptual hurdles has been the addition of extra dimensions. With the introduction of all of these new objects, string theorists have begun [more…]

### Matrix Theory as a Potential M-Theory

Most string theorists believe that matrix theory is equivalent to M-theory. A year after the proposal of M-theory, Leonard Susskind introduced a suggestion for what the “M” could stand for. [more…]

### String Theory: Insight from the Holographic Principle

Another key insight into string theory comes from the *holographic principle,* which relates a theory in space to a theory defined only on the boundary of that space. The holographic principle isn’t strictly [more…]