The world on a string – a primer

String theory is beautifully explained by Brian Greene in The Elegant Universe

Physics, the most fundamental branch of science, has two main theories – quantum mechanics and general relativity. Quantum mechanics explains the very small and light; atomic and subatomic levels. General relativity explains the very large and heavy; stars, galaxies and beyond.

Our everyday world is explained by Newtonian mechanics, whose principles can be derived from general relativity. But a major problem in physics is that quantum mechanics and general relativity are mutually incompatible, although the predictions made by each are unerringly accurate.

When certain cases are considered, such as the big bang, when the world was both incredibly small and incredibly massive, both quantum mechanics and general relativity must be invoked. Applying the equations of both theories to investigate the problem produces nonsensical results.

Nevertheless, it is extremely improbable that nature needs two sets of incompatible laws, one for the very large and another for the very small. String theory is physic's latest attempt to reconcile quantum mechanics and general relativity and is beautifully explained by Brian Greene in The Elegant Universe, Folio Society Edition, 2017.

Matter and force constitute the basic fabric of the physical world. The ancient Greeks guessed that matter is ultimately composed of tiny indivisible units called atoms. Science later demonstrated that atoms do exist, but they have sub-components – protons, neutrons and electrons. The electron has no sub-structure but protons and neutrons are composed of particles called quarks. Quarks come in two kinds – up-quarks and down-quarks. There is no evidence quarks have sub-components.

Everything we see in the universe is made of electrons, up-quarks and down-quarks. Also, a fourth fundamental particle, the neutrino, a ghostly almost mass-less entity, courses through the universe in vast numbers basically without interacting with other matter.

There are four fundamental forces in nature – the strong force, the weak force, electromagnetism and gravity. The strong and weak forces operate over extremely short distances and are only important inside atoms. The strong force holds protons and neutrons within the atom, the weak force is responsible for radioactivity. The electromagnetic force holds electrons in atoms but allows them to interact with electrons in other atoms to form molecules, the building blocks of matter.

Gravity

It is also responsible for most interactions we see in our environment. Gravity is a force through which all things with mass or energy are attracted towards one another. It is the weakest force but can operate over extremely long distances. Gravity keeps the planets orbiting around the sun and makes things fall to earth when we drop them.

Each force has an associated force particle that can be visualised as the smallest part of the force. The force particles of the strong force, the weak force, the electromagnetic force, and gravity are, respectively, gluons, weak gauge bosons, photons and gravitons.

If the properties of these fundamental particles and forces were only slightly different, our world could not exist. But no theory yet explains the four fundamental particles or the four forces.

There are compelling reasons to think there is a fundamental underlying reality to our world that, if understood, would explain everything. A Theory of Everything would explain the fundamental particles and forces of nature, and it would explain both the very small and the very large in one framework. Albert Einstein (1879-1955) spent the second half of his life searching for such a theory, without success.

This is where string theory comes on stage. It was postulated in the 1980s that the fundamental particles each consists of a tiny one-dimensional vibrating loop called a string. Replacing point-particle material constituents (electrons and quarks) with strings mathematically resolves the incompatibility between quantum mechanics and general relativity. Strings are the common basis for everything.

Just as violin strings produce different notes when they vibrate at different frequencies, string theory says that vibrations of these tiny loops produce the different realities that make up the entire natural world – the electron is a string vibrating one way, quarks are strings vibrating another way.

The mathematics underlying string theory are horrendously difficult and progress in developing string theory has been slow. But achievements have been realised, such as understanding some puzzling behaviour of black holes. It is to be hoped that the eventual complete elucidation of string theory will prove to be our Theory of Everything.

William Reville is an emeritus professor of biochemistry at UCC

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