In the classical world, a charged, spinning object has magnetic properties that are very much like those exhibited by these elementary particles. Physicists love analogies, so they described the elementary particles too in terms of their 'spin.

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Instead we have learned simply to accept the observed fact that the electron is deflected by magnetic fields. If one insists on the image of a spinning object, then real paradoxes arise; unlike a tossed softball, for instance, the spin of an electron never changes, and it has only two possible orientations.

In addition, the very notion that electrons and protons are solid 'objects' that can 'rotate' in space is itself difficult to sustain, given what we know about the rules of quantum mechanics. The term 'spin,' however, still remains. Bachmann of Birmingham-Southern College adds some historical background and other details: "Starting in the s, Otto Stern and Walther Gerlach of the University of Hamburg in Germany conducted a series of important atomic beam experiments.

Knowing that all moving charges produce magnetic fields, they proposed to measure the magnetic fields produced by the electrons orbiting nuclei in atoms. Much to their surprise, however, the two physicists found that electrons themselves act as if they are spinning very rapidly, producing tiny magnetic fields independent of those from their orbital motions. Soon the terminology 'spin' was used to describe this apparent rotation of subatomic particles.

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It is analogous to the spin of a planet in that it gives a particle angular momentum and a tiny magnetic field called a magnetic moment. Based on the known sizes of subatomic particles, however, the surfaces of charged particles would have to be moving faster than the speed of light in order to produce the measured magnetic moments. Furthermore, spin is quantized, meaning that only certain discrete spins are allowed.

This situation creates all sorts of complications that make spin one of the more challenging aspects of quantum mechanics. Spin is likewise an essential consideration in all interactions among subatomic particles, whether in high-energy particle beams, low-temperature fluids or the tenuous flow of particles from the sun known as the solar wind.

Indeed, many if not most physical processes, ranging from the smallest nuclear scales to the largest astrophysical distances, depend greatly on interactions of subatomic particles and the spins of those particles. Stenger, professor of physics at the University of Hawaii at Manoa, offers another, more technical perspective: "Spin is the total angular momentum, or intrinsic angular momentum, of a body. The spins of elementary particles are analogous to the spins of macroscopic bodies.

In fact, the spin of a planet is the sum of the spins and the orbital angular momenta of all its elementary particles. So are the spins of other composite objects such as atoms, atomic nuclei and protons which are made of quarks. In quantum mechanics, angular momenta are discrete, quantized in units of Planck's constant divided by 4 pi.

The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory.

## Spin: The Quantum Property That Should Have Been Impossible

Planck's constant: Planck's constant is a fundamental parameter in quantum mechanics. It determines the size of the discrete units or energy, mass, spin, etc. Its value is 1. Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field.

This principle is generalized by the principle of equivalence. String theory is an example of a theory of quantum gravity. Quarks exist in six varieties up, down, charm, strange, top, bottom and three "colors" red, green, blue. Riemannian geometry: mathematical framework for describing curved shapes of any dimension.

## User:Riccardo Guida/Proposed/Quantum field theory: origins - Scholarpedia

Plays a central role in Einstein's description of spacetime in general relativity. Schroedinger equation: equation governing the evolution of probability waves in quantum mechanics. Can be viewed as the "fabric" out of which the universe is fashioned; it constitutes the dynamical arena within which the events of the universe take place.

Effectively the union of quantum chromodynamics and the elecroweak theory. String theory harmoniously unites quantum mechanics and general relativity, the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.

Entails a doubling of the known elementary particle species.