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Where do all the forces of nature come from? All the forces that we see in nature today have been discovered really to be aspects of four basic forces of nature

  • Electromagnetism
  • Weak nuclear force
  • Strong nuclear force
  • Gravity

One may ask, why are there four basic forces in nature? Why not 3, or 5? Why not an infinite number of different forces – or why not just one? After 200 years of study, physicists have marshaled an amazing array of evidence which shows that three of these basic forces indeed are apparently just different aspects of one greater force.


The technique by which we have unified the first three of these forces has produced what is known as a Grand Unified Theory (GUT).  For the last 70 years physicists have been exploring models which be able to also unify the fourth force, gravity, with the first three. Should this be possible, it would be termed a Theory of Everything (TOE).

There may be no a priori reason why the correct description of nature has to be a unified field theory. However, this goal has led to a great deal of progress in modern theoretical physics and continues to motivate research.


From Wikipedia:

A GUT is a model in particle physics in which at high energy, the three gauge interactions of the Standard Model which define the electromagnetic, weak, and strong interactions or forces, are merged into one single force. This unified interaction is characterized by one larger gauge symmetry and thus several force carriers, but one unified coupling constant.

If Grand Unification is realized in nature, there is the possibility of a grand unification epoch in the early universe in which the fundamental forces are not yet distinct.

Unifying gravity with the other three interactions would provide a theory of everything (TOE), rather than a GUT. Nevertheless, GUTs are often seen as an intermediate step towards a TOE.

The novel particles predicted by GUT models are expected to have masses around the GUT scale,  a few orders of magnitude below the Planck scale—and so will be well beyond the reach of any foreseen particle collider experiments. Therefore, the particles predicted by GUT models will be unable to be observed directly. Instead the effects of grand unification might be detected through indirect observations such as proton decay, electric dipole moments of elementary particles, or the properties of neutrinos.[1] Some GUTs, such as the Pati-Salam model, predict the existence of magnetic monopoles.



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Learning Standards

AP Physics Curriculum Framework
Essential Knowledge 1.D.1: Objects classically thought of as particles can exhibit properties of waves.
a. This wavelike behavior of particles has been observed, e.g., in a double-slit experiment using elementary particles.
b. The classical models of objects do not describe their wave nature. These models break down when observing objects in small dimensions.

Learning Objective 1.D.1.1:
The student is able to explain why classical mechanics cannot describe all properties of objects by articulating the reasons that classical mechanics must be refined and an alternative explanation developed when classical particles display wave properties.

Essential Knowledge 1.D.2: Certain phenomena classically thought of as waves can exhibit properties of particles.
a. The classical models of waves do not describe the nature of a photon.
b. Momentum and energy of a photon can be related to its frequency and wavelength.

Content Connection: This essential knowledge does not produce a specific learning objective but serves as a foundation for other learning objectives in the course.


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