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Elementary Particles

Force Carriers

gravitons, Gravitational Lens, Steven Weinberg, Weak Force, standard model of particle physics

Deeper web pages:

>  The Electromagnetic Force and Photons

>  The Strong Force and Gluons

>  The Gravitational Force and Gravitons

The gravitational force is probably the most familiar force, yet it is the only force not described by the standard model of particle physics. In 1915 German-born American physicist Albert Einstein developed a significant new approach to the concept of gravity: the general theory of relativity. While general relativity successfully described many phenomena, the theory was framed differently than were theories of particle physics, making relativity difficult to reconcile with particle physics. Through the end of the 20th century, all efforts to develop a theory of gravitation entirely consistent with particle physics failed.

Physicists call their goal of an overall theory a “theory of everything,” because it would explain all four known forces in the universe and how these forces affect particles. In such a theory, the particles that carry the gravitational force would be called gravitons. Gravitons should share many characteristics with photons because, like electromagnetism, gravitation is a long-range force that gets weaker with distance. Gravitons should be massless and have no electric charge or color charge. The graviton is the only force carrier not yet observed in an experiment.

Gravitation is the weakest of the four forces on the atomic scale, but it can become extremely powerful on a cosmic scale. For instance, the gravitational force between Earth and the Sun holds Earth in orbit. Gravity can have large effects, because, unlike the electromagnetic force, it is always attractive. Every particle in your body has some tiny gravitational attraction to the ground. The innumerable tiny attractions add up, which is why you do not float off into space. The negative charge on electrons, however, cancels out the positive charge on the protons in your body, leaving you electrically neutral.

Another unique feature of gravitation is its universality—every object is gravitationally attracted to every other object, even objects without mass. For example, the theory of relativity predicted that light should feel the gravitational force. Before Einstein, scientists thought that gravitational attraction depended only on mass. They thought that light, being massless, would not be attracted by gravitation. Relativity, however, holds that gravitational attraction depends on the energy of an object and that mass is just one possible form of energy. Einstein was proven correct in 1919, when astronomers observed that the gravitational attraction between light from distant stars and the Sun bends the path of the light around the Sun (Gravitational Lens).

The Weak Force and Vector Bosons

American physicists Sheldon Glashow and Steven Weinberg and Pakistani physicist Abdus Salam completed the first step toward finding a universal force in the 1960s with their standard model theory of particle physics. Using a branch of mathematics called group theory, they showed how the weak force and the electromagnetic force could be combined mathematically into a single electroweak force. The electromagnetic force seems much stronger than the weak force at low energies, but that disparity is due to the differences between the force carriers. At higher energies, the difference between the W and Z bosons of the weak force, which have mass, and the massless photons of the electromagnetic force becomes less significant, and the two forces become indistinguishable.



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