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### String Theory: AdS/CFT Correspondence

Maldacena’s AdS/CFT correspondence brought the holographic principle to center stage in string theory. Though presented in 1993, even Leonard Susskind says he thought it would be decades before there would [more…]

### String Theory and the Science Fiction of Time

Talking about time travel and string theory without mentioning science fiction would be an obvious omision. Here are some key science fiction novels and films related to the time travel concepts discussed [more…]

### String Theory and Three Dimensions of Space

One way to look at superstring theory is to realize that the directions a string can move can only be described with a basis of ten distinct vectors, so the theory describes a 10-dimensional vector space [more…]

### String Theory: Bending Space and Time

In relativity, gravity bends our four space-time dimensions, but in string theory more dimensions are bound up in other ways. Space-time is viewed as a smooth “fabric,” but that smooth fabric can be bent [more…]

### Are Extra Dimensions Really Necessary for String Theory?

Though string theory implies extra dimensions, that doesn’t mean that the extra dimensions need to exist as dimensions of space. Some work has been done to formulate a 4-dimensional string theory where [more…]

### String Theory's Alternative to Multiple Dimensions

There are different ways in which the extra dimensions required by string theory may possibly manifest and whether the extra dimensions are really necessary. In Gates’s approach, he essentially trades [more…]

### String Theory: Big Bangs and Crunches

String theory may offer some answers to what came before and what caused the big bang. In fact, under the big bang theory — formulated in a universe of quantum physics and relativity — the laws of physics [more…]

### String Theory: What Banged?

The original expansion of the universe is a major theoretical question for cosmologists, and many are applying the concepts of string theory in attempts to answer it. The big bang theory doesn’t offer [more…]

### String Theory Looks at Dark Matter

Two mysteries of our universe are the dark matter and dark energy. Dark matter is unseen matter that holds stars together in galaxies. String theory holds several possibilities for the existence of dark [more…]

### String Theory Looks at Dark Energy

Recent discoveries in string theory have allowed for dark energy to exist within string theory. Even more intriguing than dark matter is dark energy, which is a positive energy that seems to permeate the [more…]

### String Theory and The Evolution of the Universe

Other questions that scientists hope string theory can answer involve the way the universe changes over time. The brane world scenarios described earlier in this book offer some possibilities, as do the [more…]

### The Undiscovered Country: The Future of the Cosmos

Determining the solutions to string theory that apply to our universe might allow us to determine which future is most likely. In cosmology, the past and the future are linked together, and the explanation [more…]

### Exploring a Finely Tuned Universe

One major issue in cosmology for years has been the apparent fine-tuning seen in our universe. The universe seems specially crafted to allow life. One of the major explanations for this comes from string [more…]

### String Theory and Time Asymmetries

Time travel exists in physics because of possible solutions to Einstein’s general theory of relativity, mostly resulting in singularities. These singularities would be eliminated by string theory, so in [more…]

### String Theory: Moving through Space-Time

Understanding how time travel works within string theory would require a complete understanding of how the fabric of space-time behaves within the theory. So far, string theory hasn’t exactly figured that [more…]

### Is Time Travel Possible in String Theory?

The concept of time travel is often closely tied to infinities in the curvature of space-time, such as that within black holes. In fact, the discoveries of mathematically possible time travel were found [more…]

### String Theory: Travel through a Wormhole

In a solution known to string theory as an *Einstein-Rosen bridge* (shown in this figure and more commonly called a *wormhole*), two points in space-time could be connected by a shortened path. In some special [more…]

### String Theory: Problems with Wormholes

The problem with using wormholes to travel in space or time is that they are inherently unstable. When a particle enters a wormhole, it creates fluctuations that cause the structure to collapse in upon [more…]

### Crossing Cosmic Strings to Allow Time Travel

Cosmic strings are theoretical objects that predate string theory, but in recent years there’s been some speculation that they may actually be enlarged strings left over from the big bang, or possibly [more…]

### String Theory and the Scientific Method

String theory is at the cutting edge of science. It’s a mathematical theory of nature that, at present, makes few predictions that are testable. This begs the question of what it takes for a theory to [more…]

### The Need for Experimental Falsifiability in String Theory

Some scientists claim that because string theory currently fails the test of falsifiability, it’s not “real science.” Traditionally, an experiment can either confirm or refute a theory. An experimental [more…]

### String Theory: The Foundation of Theory is Mathematics

Science requires both theory and experiment to build explanations of what happens in the world. String theory has not yet made the leap from theory to experiment. [more…]

### String Theory: Objectivity and the Rule of Simplicity

String theory seems to violate the rule of simplicity. In science, one goal is to propose the fewest “entities” or rules needed to explain how something works. In many ways, the history of science is seen [more…]

### String Theory and Scientific Revolutions

The debates over string theory represent fundamental differences in how to view science. Many people have proposed ideas about what the goals of science should be. But over the years, science changes as [more…]

### String Theory: Science as Unification

String theory, if successful, might unify all fundamental physical forces of the universe down to one single equation. Science can be seen as a progressive series of [more…]