Unified field theory

Contents

Overview

In physics, unified field theory is an attempt to unify all the fundamental forces and the interactions between elementary particles into a single theoretical framework. The term was coined by Einstein who attempted to reconcile the general theory of relativity with electromagnetism in a single field theory. His quest proved elusive and a unified field theory, sometimes grandiosely referred to as the Theory of Everything (TOE, for short), has remained the holy grail for physicists, the long-sought theory which would explain the nature and behavior of all matter.

In physics, the forces between objects can be described as mediated by fields. Current theory says that at subatomic distances, these fields are replaced by quantum fields interacting according to the laws of quantum mechanics. Alternatively, using the particle-wave duality of quantum mechanics, fields can be described in terms of exchange particles that transfer momentum and energy between objects. Crudely speaking, objects interact as they emit and absorb exchange particles, in effect playing a subatomic game of "catch". The essential belief of a unified field theory is that the four fundamental forces (see below) as well as all matter are simply different manifestations of a single fundamental field.

A unified field theory aims to reconcile the four fundamental forces (or fields) of nature, namely:

Strong force: Force responsible for holding quarks together to form neutrons and protons, and holding neutrons and protons together to form nuclei. The exchange particles that mediate this force are gluons.
Electromagnetic force: It is the familiar force that acts on electrically charged particle. The photon is the exchange particle for this force.
Weak force: Responsible for radioactivity, it is a repulsive short-range interaction that acts on electrons, neutrinos and quarks. It is governed by the W boson.
Gravitational force: A long-range attractive force that acts on all particles. The exchange particles have been postulated and named gravitons.

History

Historically, the first unified field theory was developed by James Clerk Maxwell. In 1831, Michael Faraday made the observation that time-varying magnetic fields could induce electric currents. Until then, electricity and magnetism had been thought as unrelated phenomena. In 1864, Maxwell published his famous paper on a dynamical theory of the electromagnetic field. This was the first example of a theory that was able to encompass previous theories (namely electricity and magnetism) to provide a unifying theory of electromagnetism. However, today we know that the classical electrodynamics developed by Maxwell eventually breaks down near for quantum limit (for large momentum and energy transfer). A complete quantum description of the electromagnetic force was achieved in the 1940s, a theory known as quantum electrodynamics (QED). This theory represents the interactions of charged particles mediated by force carriers named photons. The theory is based on a space-time symmetry of the field called gauge (really phase) symmetry. The theory was so successful that the principle of continuous gauge symmetry was soon adopted for all forces.

In the 1967, two Americans Sheldon Glashow and Steven Weinberg and a Pakistani Abdus Salam proposed independently a theory unifying electromagnetism and the weak nuclear forces. They found that in seeking a quantum gauge field theory of the weak forces they were forced to introduce an additional force. They demonstrated that the gauge field from the weak interaction was structurally identical to the electromagnetic field. Quantum electrodynamics is then a consequence of a spontaneous symmetry breaking in a theory in which initially the weak and electromagnetic interactions are unified. This unified theory was governed by the exchange of four particles: the photon for electromagnetic interactions, and a neutral Z particle and two charged W particles for weak interaction. As a result of the spontaneous symmetry breaking the weak force becomes short range and the Z and W bosons acquire masses of the order of 90 <math> GeV/c^2 <math>. Their theory was given experimental support by the discovery, in 1983, of the Z and W bosons at CERN by Carlo Rubbia's team. For their insights, Glashow, Weinberg and Salam were awarded the Nobel Prize in Physics in 1979. Carlo Rubbia and Simon van der Meer received the Prize in 1984.

The next logical step towards the unification of the fundamental forces of nature was to include the strong interaction with the electroweak forces in a theory called the Grand Unified Theory (GUT). A quantum theory of the strong force had been developed in the 1970s under the name of Quantum Chromodynamics. The strong interaction acts between quarks via the exchange of particles called gluons. There are eight types of gluons, each carrying a color charge and an anti-color charge. Based on this theory, Sheldon Glashow and Howard Georgi proposed the first grand unified theory in 1974, which applied to energies above 1000 GeV. Since then there have been several proposals for GUTs, although none is currently universally accepted. A major problem for expermimental tests of such theories is the energy scale involved, which is well beyond the reach of current accelerators. However, there are some falsifiable predictions that have been made for low energy processes that do not involve accelerators. One of these predictions is that the proton is unstable and can decay. It is at present unknown if the proton can decay although experiments have determined a lower bound of <math> 10^{35} <math> years for its lifetime. It is therefore uncertain, at the present time, whether any GUT can provide an accurate description of matter.

Gravity has yet to be included in a theory of everything. Theoretical physicists have been so far incapable of formulating a consistent theory that combines general relativity and quantum mechanics. The two theories have proved to be incompatible and the quantization of gravity remains an outstanding problem in the field of physics. In recent years the quest for a unified field theory has largely focused on string theory. Much hope has been put on one of its offshoots known as M-theory (M. Kaku, B. Greene). Others theories that attempt to explain the quantization of gravity are twistor theory (R. Penrose and W. Rindler), Noncommutative geometry (A. Connes, J. Madore) and loop quantum gravity (L. Smolin, R. Gambini and J. Pullin).

Reductionism

There is much debate about the intrinsic value of searching for a possibly successful unified field theory. Besides the argument that there may not exist such a theory, some have argued that finding the final theory, that is the ultimate foundation of nature, will not unlock the mystery of the universe. This is the view that the understanding of the ultimate particles will not yield a complete knowledge of the behaviour of atoms and molecules or some higher level structure. Some physicists (e.g P.W. Anderson) have argued that large structures undergo collective behaviors which are not most usefully described in terms of the behavior of their constituents and therefore there is no reason to label the lower-level behaviors as more fundamental.

Quack theories

There are numerous "Unified Field Theories" that have been proposed by laypeople. Often couched in a cryptic language with numerous neologisms that are meant to impress or obscure meaning, these attempts are for the most part ill-conceived and devoid of merit. Such theories typically contain little in the way of falsifiable results or predictions; and, for the most part, have not been through a process equivalent to peer-review by scientists.

Professional physicists often described their work as a creative attempt with a straitjacket on. What is meant by this metaphor is that an eventual unified field theory must not only be consistent, but explain all the previously known aspects of gravity on a large scale, and of quantum mechanics on the subatomic level. Given the requirements of a successful unified field theory, that it must explain all previous known results in a single framework while making new and falsifiable predictions, it is no surprise that this is a difficult task. It is considered by most engaged in the area that the resolution is unlikely to be found by dilettantes. This holds doubly true of those who attempt to design a theory that also explains observations made during astral projection or other speculative "experimental techniques".

Contrary to what is sometimes claimed, the discovery of the unified field theory will not provide us with a cheap source of energy, nor fetch a beer in the fridge.