7 minute read

Grand Unified Theory

Science EncyclopediaScience & Philosophy: Glucagon to Habitat

One of the major theoretical hurdles to a reachable synthesis of current theories of particles and force interactions into a grand unification theory (also known as Grand Unified Field Theory, Grand Unified Theory, or GUT) is the need to reconcile the evolving principles of quantum theory with the principles of general relativity advanced by German-American physicist Albert Einstein (1879-1955) nearly a century ago. The synthesis is made difficult because the unification of quantum mechanics (itself a unification of the laws of chemistry with atomic physics) with special relativity to form a complete quantum field theory consistent with observable data is itself not yet complete.

A grand unified theory of physics is not within the reach of our present technology and there are also theoretical obstacles to formulating a Grand unified theory.

A grand unified theory is a theory that will reconcile the electroweak force (the unified forces of electricity and magnetism) and the strong force (the force that binds quarks within the atomic nucleus together). A grand unified theory that could subsequently incorporate gravitational theory would, become the ultimate unified theory, often referred to by physicists as a "theory of everything" (TOE).

The technological barriers to a unified theory are a consequence of the tremendous energies required to verify the existence of the particles predicted by the theory. In essence, experimental physicists are called upon to recreate the conditions of the universe that existed during the first few millionths of a second of the Big bang - when the universe was tremendously hot, dense, and therefore energetic.

There are admittedly great difficulties and high mountains of inconsistency between quantum and relativity theory that may put such a "theory of everything" (TOE) far beyond our present grasp. Some scientists speculate that although a TOE is beyond our reach, we may be within reach of a grand unified theory (GUT) that, excepting quantum gravity, will unite the remaining fundamental forces.

Quantum theory was principally developed during the first half of the twentieth century through the independent work on various parts of the theory by German physicist Maxwell Planck (1858–1947), Danish physicist Niels Bohr (1885–1962), Austrian physicist Erwin Schrödinger (1887–1961), English physicist P.A.M. Dirac (1902–1984) and German physicist Werner Heisenberg (1901–1976). Quantum mechanics fully describes wave particle duality, and the phenomena of superposition in term of probabilities. Quantum field theory describes and encompasses virtual particles and renormalization.

In contrast, special relativity describes space-time geometry and the relativistic effects of different inertial reference frames (i.e., the relativity of describing motion) and general relativity describes the nature of gravity. General relativity fuses the dimensions of space and time. The motion of bodies under apparent gravitational force is explained by the assertion that, in the vicinity of mass, space-time curves. The more massive the body the greater is the curvature or "force of gravity."

Avoiding the mathematical complexities, a fair simplification of the fundamental incompatibility between quantum theory and relativity theory may be found in the difference between the two theories with respect to the nature of the gravitational force. Quantum theory depicts a quantum field with a carrier particle for the gravitational force—that although not yet discovered—is termed a graviton. As a force carrier particle, the graviton is analogous to the photon that acts as the boson or carrie of the electromagnetism (i.e., light). In stark contrast, general relativity theory does away with the need for the graviton by depicting gravity as a consequence of the warping or bending of space-time by matter (or, more specifically, mass).

Although both quantum and relativity theories work extremely well in explaining the universe at the quantum and cosmic levels respectively, the theories themselves are fundamentally incompatible and hence the search for unification theories.

Such unifications are not trivial mathematical or rhetorical flourishes; they evidence an unswerving trail back towards the beginning of time and the creation of the universe in the big bang. What the electroweak unification reveals is that at higher levels of energy, (e.g., the energies associated with the big bang), the forces of electromagnetism and the weak force are really one in the same. It is only at the more modest present state of the universe, far cooler and less dense, that the forces take on the characteristic differences of electromagnetism and the weak force.

Experiments at high energy levels have revealed the existence of a number of new particles. According to modern field theory and the Standard model, particles are manifestations of field and particles interact (exert forces) through fields. Accordingly, for every particle (e.g., quarks and leptons—one form of a lepton is the electron) there must be an associate field. Forces between particles result from the exchange of particles that are termed virtual particles. Electromagnetism depends upon the exchange of photons (QED theory). The weak force depends upon the exchange of W+, W, and Zo particles. Eight different forms of gluons are exchanged in a gluon field to produce the strong force. Regardless, the energy requirements required to identify the particles associated with a unified field required by a grand unified theory are greater than present technologies can achieve. Most mathematical calculations involving quantum fields indicate that unification of the fields may require 1016 GeV. Some models allow the additional fusion of the gravitational force at 1018 GeV.

The higher energies needed are not simply a question of investing more time and money in building larger accelerators. Using our present technologies, the energy levels achievable by a particle accelerator are proportional to the size of the accelerator (specially the diameter of the accelerator). Alas, to archive the energy levels required to find the particles of a grand unified force would require an accelerator larger than our entire solar system.

Although a quantum explanation of gravity is not required by a grand unification theory that seeks only to reconcile electroweak and strong forces, it is important to acknowledge that the unification of force and particle theories embraced by the Standard model is not yet complete. Further, it may not be possible to rule out gravity and develop a unified theory of electroweak and strong forces that ignores gravity.



Feynman, Richard and Steven Weinberg Elementary Particles and the Laws of Physics. Cambridge, UK: Cambridge University Press, 1987.

Greene, Brian. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. New York: Vintage books, 2000.

Gribbin, John. Q is for Quantum: An Encyclopedia of Particle Physics. New York: The Free Press, 1998.

Hawking, Stephen. The Illustrated Brief History of Time, Updated and Expanded. New York: Bantam, 2001.

Klein, Etienne, et al. The Quest for Unity: The Adventure of Physics. Oxford, UK: Oxford University Press, 2000.

Mohapatra, Rabindra. Unification and Supersymmetry. Oxford, UK: Oxford University Press, 2002.


Weinberg, Steven. "A Unified Physics by 2050?" Scientific American. December, 1999.


Particle Data Group. Lawrence Berkeley National Laboratory. "The Particle Adventure: The Fundamentals of Matter and Force" [cited February, 5, 2003]. <http://particleadventure.org/particleadventure/>.

K. Lee Lerner


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Electroweak force

—A unification of the fundamental forces of electromagnetism (that light is carried by quantum packets termed photons manifested by alternating fields of electricity and magnetism) and the weak force.

Field theory

—A concept first advanced Scottish physicist James Clerk Maxwell (1831–1879) as part of his development of the theory of electromagnetism to explain the manifestation of force at a distance without an intervening medium to transmit the force. Einstein's general relativity theory is also a field theory of gravity.

Fundamental forces

—The forces of electromagnetism (light), weak force, strong force, and gravity. Aptly named, the strong force is the strongest force, but acts over only the distance of the atomic nucleus. In contrast, gravity is 1039 times weaker than the strong force and acts at infinite distances.

Gravitational force

—A force dependent upon mass and the distance between objects. The English physicist and mathematician Sir Isaac Newton (1642–1727) set out the classical theory of gravity in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). According to classical theory, gravitational force, always attractive between two objects, increases directly and proportionately with mass of the objects but is inversely proportional to the square of the distance between the objects. According to general relativity, gravity results from the bending of fused space-time. According to modern quantum theory, gravity is postulated to be carried by a vector particle termed a graviton.

Local gauge invariance

—In physics, a concept that asserts that all field equations ultimately contain symmetries in space and time. Gauge theories depend on difference in values as opposed to absolute values.

Strong force (or Strong interactions)

—A force that binds quarks together to form protons and neutrons and hold protons and neutrons—and to hold together the electrically repelling positively charged protons within the atomic nucleus.

Unified field theory

—In physics, a theory describing how a single set of particles and fields can become (or underlie) the observable fundamental forces of the electroweak force (electromagnetism and weak force unification) and the strong force.

Virtual particles

—A particle that is emitted and then reabsorbed by particles involved in a force interaction (e.g., the exchange of virtual photons between charged particles in involved in electromagnetic force interactions).

Weak force

—The force that causes transmutations of certain atomic particles. For example, weak force interactions in beta decay change neutrons and protons allowing Carbon-14 to decay into Nitrogen at a predictable rate useful in Carbon-14 dating.

Additional topics