What’s a grand unified theory?

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The Grand Unified Theory (GUT) is the search for a concise and simple theory in physics as of 2011 that describes the interaction between the four fundamental forces of nature: electromagnetism, the weak and strong nuclear forces, and gravity. Since any grand unified theory attempts to bring together current understanding of physical forces, it is often referred to as research in classical unified field theories. There are many different unified field theories to explain the behavior of matter and energy, however string theory is seen as the best prospect as of 2011 for having a chance to tie the effects of gravity to the other three fundamental forces.

Particle physics currently uses a theory known as the Standard Model to describe the interaction of physical forces in nature. The current form of the Standard Model as of 2011 was established in the 1970s and represents the interaction of three of the four fundamental forces: electromagnetism and the weak and strong nuclear forces. With the development of the Standard Model, it was revealed that electromagnetism and the weak nuclear force were actually aspects of the same force at high energy levels. This built on earlier discoveries in the late 19th century by British physicist James Maxwell, who combined the forces of magnetism and electricity as two aspects of the same force: electromagnetism. The Grand Unified Theory, therefore, can be considered an attempt to merge the Standard Model with the effects of gravity.

Because the development of a simple grand unified theory may not account for all the interactions of matter and energy in space, often the final theory that unifies all forces is called the Theory of Everything (TEP). Three major arenas of study in physics would need to be merged into one to produce a workable theory of everything, and these include the Standard Model, general relativity, and quantum mechanics. A key proposition for this involves string theory research, which describes a ground state of matter in which everything in the physical universe is ultimately composed of vibrating rings of energy, each the size of a Planck length, or 10-33 centimeters. These strings are hypothesized to be the direct cause of the effects of all four fundamental forces in nature, the existence of matter and energy, as well as space and time. One of the weaknesses of string theory, however, is that it is extremely complex and requires interactions in ten dimensions of space, whereas an ultimate grand unified theory is imagined as a simple equation no more than about 1 inch (2.5 centimeters) in length when written in normal handwriting size.

Perhaps the biggest challenge in arriving at an elegant form of a grand unified theory as of 2011, however, is to work the physical cosmology of gravity into equations. Gravity is the least understood of all physical forces and may not be a force at all. Some research suggests that gravity is simply an effect of the warping of space by matter, as Einstein’s findings suggest, because an origin for the source of gravity has never been found. All other forces in nature have fundamental particles attached to them, so physics assumed that there was a graviton particle to explain gravity. Until evidence is obtained to explain how gravity works at both the macroscopic and microscopic levels, a Grand Unified Theory that unifies all physical forces in nature will remain elusive.

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