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Name: Midhat
Status: student
Age: 17
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002


Question:
Why do electrons and other particles spin. Why do all planets spin


Replies:
Midhat,

As for the particles, I do not know why they spin. We do not even know if they really do spin. We know they are magnetic. We know that a moving electric charge makes a magnetic field. We also know that electric charge moving in a circle makes a magnetic field exactly like that of the various particles. Saying the particles spin is an easy way to explain their magnetism. It also agrees with the different kinds of collisions that can happen. If the particles do really spin, nobody knows why.

As for planets spinning, that is a whole different thing. Try to throw a baseball without having it spin at all. Try to throw a Frisbee without any spin. Try to hit a golf ball with no spin at all. Unless an object has special wings like a glider, it is almost impossible to send an object flying without having it spin. A planet is usually a whole bunch of matter that has crashed together. After all the crashing is over and done with, after the planet is all "glued together", there is nothing to keep it still. It is very likely to be spinning and very likely to be orbiting. Different planets spin at different rates because there is no law about it. I imagine somewhere in the universe is a planet that does not spin. None of the nine in our solar system qualify.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


The "spin" of electrons and other atomic and sub-atomic particles is not the same concept as the "spin" of a planet except that both involve angular momentum. The "why" of electron spin is not really the way to verbalize the phenomenon. The behavior of electrons in a magnetic field (Zeeman effect), the spectrum of hydrogen (and other atoms), and other behaviors require the introduction of a new quantum number. This falls out as a natural consequence of Doric's theory of the electron and Feynman's quantum electrodynamic description (see "Lectures on Physics" by R. Feynman for a discussion that is readable). So spin is an observation that "is", and the "why" is that is the way nature behaves.

Vince Calder


The "spins" you refer to have very different. Planets and most astronomical bodies spin about some axis or revolution. In the case of the moon, this spinning may become "locked in" thus the observation that the moon shows the same face to the earth. This is caused by gravitational drag. The web site: http://home2.planetinternet.be/ballaux gives details.

Electron "spin" is a different animal. Beginning in the early 20th century the study of electronic spectrum of the hydrogen atom (the simplest of all atoms) began in earnest, at higher and higher resolution. The high resolution spectra revealed multiple lines, where the theory at the time predicted that only a single line should appear. The "reasons" for the multiplets involved corrections for the finite size of the nucleus, rather than a "point charge" and corrections for Einstein's special theory of relativity. The quantum mechanical calculation incorporating these corrections (and some others) correctly predicted the multiple lines. The electron behaves "as though it were spinning about an axis". The term "spin" was applied as a metaphor, because the physicists of that era were steeped in the jargon of Newtonian classical mechanics. The book by P.A.M. Dirac treated the quantum mechanical in detail. What the "spin" means is that there are certain solutions to the wave function for the hydrogen atom that required the introduction of a "new" quantum number, the "spin" quantum number. This does not mean that the electron is physically spinning about its own axis of rotation; that choice of terminology is metaphorical. The experiments of Stern and Gerlach also confirmed the existence of "spin". It is also interesting that Dirac's treatment of the electron also predicted the existence of an "anti-electron" -- we now call it a "positron" which is a twin of the electron with positive charge rather than negative charge. The correctness of Dirac's treatment is strongly validated by this prediction, because it was some time before the anti-electron was observed experimentally. There are several other "corrections" applied to the hydrogen atom. You can find these discussed on a web site I am sure but I have not done the search.

Vince Calder



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