Multiple Possible Universes
and Extra Dimensions

Parallel Universes and the Theory of Everything

Multiple Possible Universes and Extra Dimensions
Parallel Universes and the Theory of Everything

A parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own.

A specific group of parallel universes is called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute physical reality.

The multiverse is the hypothetical set of multiple possible universes (including our universe) that together comprise everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them.

The term was coined in 1895 by the American philosopher and psychologist William James.

The various universes within the multiverse are sometimes called parallel universes.

The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered.

Multiverses have been hypothes
ized in cosmology, physics, astronomy, philosophy, transpersonal psychology and fiction, particularly in science fiction and fantasy.

In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternative realities", and "alternative timelines", among others.

A multiverse of a somewhat different kind has been envisaged within the multi-dimensional extension of string theory known as M-theory, also known as Membrane Theory.

In M-theory our universe and others are created by collisions between p-branes in a space with 11 and 26 dimensions (the number of dimensions depends on the chirality of the observer); each universe takes the form of a D-brane.

Objects in each universe are essentially confined to the D-brane of their universe, but may be able to interact with other universes via gravity, a force which is not restricted to D-branes. This is unlike the universes in the "quantum multiverse", but both concepts can operate at the same time.

In string theory, D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named.

D-branes were discovered by Dai, Leigh and Polchinski, and independently by Horava in 1989. In 1995, Polchinski identified D-branes with black p-brane solutions of supergravity, a discovery that triggered the Second Superstring Revolution and led to both holographic and M-theory dualities.

D-branes are typically classified by their dimension, which is indicated by a number written after the D. A D0-brane is a single point, a D1-brane is a line (sometimes called a "D-string"), a D2-brane is a plane, and a D25-brane fills the highest-dimensional space considered in bosonic string theory. There are also instantonic D(-1)-branes, which are localized in both space and time.

An intriguing feature of string theory is that it involves the prediction of extra dimensions.

The number of dimensions is not fixed by any consistency criterion, but flat spacetime solutions do exist in the so-called "critical dimension".

Cosmological solutions exist in a wider variety of dimensionalities, and these different dimensions are related by dynamical transitions.

The dimensions are more precisely different values of the "effective central charge", a count of degrees of freedom which reduces to dimensionality in weakly curved regimes.

Beginning with a brief consideration of classical
physics, which concentrates on the major conflicts in physics, Greene establishes a historical context for string theory as a necessary means of integrating the probabilistic world of the standard model of particle physics and the deterministic Newtonian physics of the macroscopic world.

Greene discusses the essential problem facing modern physics: unification of Albert Einstein's theory of General Relativity and Quantum Mechanics.

Greene suggests that string theory is the solution to these two conflicting approaches.

Greene frequently uses analogies and thought experiments to provide a means for the layman to come to terms with the theory which has the potential to create a unified theory of physics.