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Light and Fields
Name: Matthew W.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: Thursday, June 06, 2002
Question:
If light does not have any electrons, than why does it
have magnetic properties, when magnetism is cause by the motion of
electrons in atoms?
Replies:
Matthew,
Actually, light has both electric and magnetic properties. There are two
things that can make an electric field: electric charge and a changing
magnetic field. There are two things that can make a magnetic field: a
moving electric charge and a changing electric field. Light is first
produced by a moving electric charge. The charge gives off some of its
energy as changing electric and magnetic fields. These changing fields then
transfer their energy to new changing electric and magnetic fields a little
further from the original charge. This continues, fields progressing at the
speed of light, until the fields crash into a charged particle. This is how
light can carry energy from one location to another. An example is light
from the Sun bringing energy to the Earth.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Having electrons is not a prerequisite for magnetic behavior. It is
true that SOME motions of electrons in atoms produce a magnetic field. There
are also some other motions of electrons in atoms that do not produce a
magnetic field. Light (and all electromagnetic radiation from x-rays to
microwaves) is composed of an alternating electric and magnetic field. James
Maxwell described classical electromagnetic behavior in terms of oscillating
electric and magnetic fields. Its predictions have been validated by
millions of experimental observations, so there is very little doubt about
its correctness on the macroscopic scale. The interaction of light and
electrons on the atomic scale is described by a theory with the daunting
name "quantum electrodynamics -- or QED for short, which takes into account
the theory of relativity and the laws of quantum mechanics. The predictions
of that theory have been shown to agree with experiment to within a few
parts in 10^12. This is in about the same ratio as the width of a human hair
to the distance between New York and Los Angeles, so that theory describing
the interaction of electromagnetic fields with electrons is consistent with
experimental measurements to an extraordinary accuracy. If the photon is
zero rest mass, travels at a constant speed in a vacuum (about 3x10^8
meters/sec), why does it have spin = 1, which in a "classical" analogy
would suggest that it is spinning?
No one knows, but that is the prediction of the theory and the result of
experimental measurements. The only thing left out of all this theory is how
does gravity behave on a quantum mechanical level. To my knowledge no one
has incorporated that interaction into the picture satisfactorily. You may
find a short book entitled: QED by Richard Feynman interesting reading. It
is an excellent presentation of the state of knowledge about all this stuff
without too much math, and even if you do not understand all of the math it
is still fascinating reading because he was such a remarkable teacher.
Vince Calder
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Update: June 2012
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