Introduction to M-theory
In the early years of the 20th century, the atom – long believed to be the smallest building-block of matter – was proven to consist of even smaller components called protons, neutrons and electrons, which are known as subatomic particles. Beginning in the 1960s, other subatomic particles were discovered. In the 1970s, it was discovered that protons and neutrons (and other hadrons) are themselves made up of smaller particles called quarks. Quantum theory is the set of rules that describes the interactions of these particles.
In the 1980s, a new mathematical model of theoretical physics called string theory emerged. It showed how all the particles, and all of the forms of energy in the universe, could be constructed by hypothetical one-dimensional "strings", infinitesimal building-blocks that have only the dimension of length, but not height nor width. Further, string theory suggested that the universe is made up of multiple dimensions. Height, width, and length constitute three-dimensional space, and time gives a total of four observable dimensions; however, string theories initially supported the possibility of ten dimensions – the remaining six of which we cannot detect directly. This was later increased to 11 dimensions based on various interpretations of the ten dimensional theory that led to five partial theories as described below. Super-gravity theory also played a significant part in establishing the necessity of the 11th dimension.
These "strings" vibrate in multiple dimensions, and depending on how they vibrate, they might be seen in three-dimensional space as matter, light, or gravity. It is the vibration of the string which determines whether it appears to be matter or energy, and every form of matter or energy is the result of the vibration of strings.
String theory, as mentioned above, ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, there were five major string theories. The main differences between each theory were principally the number of dimensions in which the strings developed, and their characteristics (some were open loops, some were closed loops, etc.). Furthermore, all these theories appeared to be correct. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.
In 1994, a string theorist named Edward Witten of the Institute for Advanced Study and other important researchers considered that the five different versions of string theory might be describing the same thing seen from different perspectives. They proposed a unifying theory called "M-theory", in which the "M" is not specifically defined, but is generally understood to stand for "membrane". The words "matrix", "mother", "monster", "mystery", "magic" have also been claimed. M-theory brought all of the string theories together. It did this by asserting that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space.
M-theory is not complete, but the underlying structure of the mathematics has been established and is in agreement with all the string theories. Furthermore, it has passed many tests of internal mathematical consistency.
Until some way is found to observe the yet hypothetical higher dimensions, which are needed for consistency reasons, M-theory has a very difficult time making predictions that can be tested in a laboratory. Technologically, it may never be possible for it to be experimentally confirmed.
Some cosmologists are drawn to M-theory because of its mathematical elegance and relative simplicity. Physicist and author Michio Kaku has remarked that M-theory may present us with a "Theory of Everything" which is so concise that its underlying formula would fit on a T-shirt. Stephen Hawking originally believed that M-theory may be the ultimate theory but later suggested that the search for understanding of mathematics and physics will never be complete.
Stephen Hawking and Leonard Mlodinow, in the popular scientific book The Grand Design, take a philosophical position to support a view of the universe as a multiverse, and define it in the book as model-dependent realism which along with a sum-over-histories approach (see Path integral formulation of Quantum mechanics) to the universe as a whole, is used to claim that M-theory is the only candidate for a complete theory of the universe.
See also 
- M-theory (a technical, more complete description)
- String theory
- Superstring theory
- Theory of everything
- Quantum mechanics
- Kaku, Michio. "M-Theory: The Mother of all SuperStrings". Retrieved January 17, 2009.
- Brian Greene has written books explaining string theory and M-theory for the layperson in 1999, The Elegant Universe, ISBN 0-375-70811-1 and in 2004, The Fabric of the Cosmos, ISBN 0-375-41288-3.
- Musser, George (2008). The Complete Idiot's Guide to String Theory. Indianapolis: Alpha. p. 368. ISBN 978-1-59257-702-6.
- Smolin, Lee, The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (2006), Houghton Mifflin. ISBN 978-0-618-55105-7.
- Woit, Peter. Not Even Wrong: The Failure of String Theory & the Continuing Challenge to Unify the Laws of Physics, 2006. ISBN 0-224-07605-1 (Jonathan Cape), ISBN 0-465-09275-6 (Basic Books)
- The Elegant Universe - A Three-Hour miniseries with Brian Greene by NOVA (original PBS Broadcast Dates: October 28, 8-10 p.m. and November 4, 8-9 p.m., 2003). Various images, texts, videos and animations explaining string theory and M-theory.
- Superstringtheory.com - The "Official String Theory Web Site", created by Patricia Schwarz. Excellent references on string theory and M-theory for the layperson and expert.
- Basics of M-Theory by A. Miemiec and I. Schnakenburg is a lecture note on M-Theory published in Fortsch.Phys.54:5-72,2006.