SUPERSTRINGS AND QUANTUM GRAVITY

To fully understand questions like where the universe came from, why the Big Bang occurred 13.7 billion years ago and what, if anything, existed before it, we need to better understand singularities like those in black holes and the singularity which marked the birth of the universe itself.

In order to achieve that, most scientists agree that a “quantum theory of gravity” (also known as "quantum gravity" or the “theory of everything”) is needed, which combines the General Theory of Relativity (our current best theory of the very large) and quantum theory (our current best theory of the very small). These may seem fundamentally incompatible concepts, but attempts are nevertheless continuing on several fronts to find a synthesis, perhaps the most promising (and currently the one receiving the most publicity) being that of string theory or superstring theory.

In the 1970s, the strongest candidate for a unified theory was probably “supergravity”, a field theory combining the principles of supersymmetry and general relativity. But, although the approach appeared promising, it soon became apparent that the calculations involved were so long and difficult that it may never be provable. Around 1984, however, there was a remarkable change of opinion in the world of theoretical physics in favour of string theory.

Superstring theory, shorthand for "supersymmetric string theory", views the basic building blocks of matter not as point-like particles but as tiny one-dimensional “strings”, which have length but no other dimension, like infinitely thin pieces of string or twine. Although there might seem to be an inconsistency between the idea of a universe composed of strings and the point-like particles we actually observe in experiments, this is because the strings are so tiny that we cannot resolve their shape, even with our best technology, so that they just appear to us as tiny featureless points, like the difference between a speck of dust seen with the naked eye and under a microscope.

(Click for a larger version) Artist's impression of the fundamental entities of superstring theory by Flavio Robles (Source: Berkeley Lab: http://www.lbl.gov/Publications/Currents/ Archive/June-16-2000.html)

A string may be open (i.e. have ends) or closed (i.e. joined up in loops), and the history of a string over time is represented by a two-dimensional strip (for open strings) or tube (for closed strings). Strings are composed of super-concentrated mass-energy which vibrate like a violin strings, with each distinct vibration mode corresponding to a fundamental particle (such as an electron or a photon, etc). The emission or absorption of one particle by another is represented by the dividing or joining together of strings, and the forces acting on particles correspond to other strings linking the particle strings in a complex “web”.

According to superstring theory, then, the universe is a kind of symphony and the laws of physics are its harmonies. The vibrations of strings, however, occur in a ten-dimensional world, with each one-dimensional point in our ordinary space actually consisting of a complicated geometrical structure in six dimensions, all wrapped up on the scale of the Planck length (the smallest distance or size about which anything can be known, equal to about 1.6 × 10^-35 metres, or about 10^-19 times smaller than a proton).

The speculation on incorporating additional dimensions into space-time goes back to the ideas of the Polish physicist Theodor Kalkuza in 1919 and, independently, the Swedish physicist Oscar Klein in 1926. They asked why it was not possible that electromagnetism could be unified with gravity in a notional five-dimensional universe, or that perhaps the electromagnetic force may relate to some curvature in a fifth dimension, just as gravity is due to curvature in four-dimensional space-time, as demonstrated by Einstein’s General Theory of Relativity.

General Relativity, which implicitly interprets gravity as curvature in four-dimensional space-time, is built in to the basic precepts of superstring theory in a way that may be consistent with quantum mechanics, and so it is hoped that the long-sought synthesis between gravity and quantum theory will naturally emerge. In fact, over ten dimensions (in which all but the four we are familiar with are “curled up” into tiny strings with diameters on the order of the Planck scale), it may even be possible that all the fundamental forces in nature can be accommodated into one “theory of everything”, known as quantum gravity.

Superstring theory is by no means the only candidate for a theory of quantum gravity which is being pursued, though. Other approaches include "loop quantum gravity" (in which space is represented by a network structure called a spin network, evolving over time in discrete steps), "causal dynamical triangulation" (a background independent approach which attempts to show how the space-time fabric itself evolves), "causal sets" (an approach which assumes that space-time is fundamentally discrete and that space-time events are related by a partial order) and even a recent one called “An Exceptionally Simple Theory of Everything”.

In fact, at least five different and competing superstring theories have developed, none of which are conclusive, however elegant. There is some evidence, though, that the inclusion of an eleventh dimension might reconcile these competing theories, as well as making it consistent with supergravity theory. With the additional dimension, the fundamental building block of the universe is no longer a string but a “membrane” or “brane”, leading to its designation as “membrane theory” or “M-Theory”.

Additionally, M-Theory and the incorporation of an eleventh dimension is also consistent with the existence of a multiverse, a convenient but ultimately unprovable solution to many of the more intransigent problems in theoretical physics. For example, if the membranes ripple, as it is supposed they do, then events like singularities (and the Big Bang itself) can be visualized as the result of collisions between rippling, wave-like membranes, with the initial Big Bang of our universe being just one of many in the constant encounters between membranes in parallel universes.