An Introduction to M-theory
In non-technical terms, M-theory presents an idea about the basic substance of the universe.
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 mechanics 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. Each theory is fundamentally based on vibrating, one-dimensional strings at approximately the length of theplanck length. Calculations have also shown that each theory requires more than the normal four spacetime dimensions. 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.
To the critics, however, these mathematical developments still don’t answer the nagging question: how do you test it? Since string theory is really a theory of creation, when all its beautiful symmetries were in their full glory, the only way to test it, the critics wail, is to re-create the Big Bang itself, which is impossible. But most string theorists think these criticisms are silly. They believe that the critics have missed the point. The key point is this: if the theory can be solved non-perturbatively using pure mathematics, then it should reduce down at low energies to a theory of ordinary protons, electrons, atoms, and molecules, for which there is ample experimental data. If we could completely solve the theory, we should be able to extract its low energy spectrum, which should match the familiar particles we see today in the Standard Model. Thus, the real problem is raw brain power: of only we were clever enough, we could write down M-theory, solve it, and settle everything.
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 to the universe as a whole, is used to claim that M-theory is the only candidate for a complete theory of the universe.
The evolution of this theory can be summarized as: Principle -> Symmetry -> Action -> Quantum Theory. According to Witten, the fundamental problem has been that string theory has been evolving backwards. As Witten says, “string theory is 21st century physics which fell into the 20th century by accident”. We were never “meant” to see this theory until the next century. Witten certainly believes we are on the right track, but we need a few more “revolutions” to finally solve the theory: “I think there are still a couple more superstring revolutions in our future, at least. If we can manage one more superstring revolution a decade, I think that we will do all right”.