Ch. 1 notes, part1
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Q.M. is a new
(and absolutely necessary) way of predicting the
behavior of microscopic objects.
It is based on several radical, and generally also counter-intuitive, ideas:
1) Many aspects of the world are essentially probabilistic, not deterministic.
2) Some aspects of the world are essentially discontinuous
Those who are not shocked when they first come across quantum theory cannot
possibly have understood it."
Humans have divided physics into a few artificial categories, called theories, such as
classical mechanics (non-relativistic and relativistic)
quantum mechanics (non-relativistic)
general relativity (theory of gravity)
thermodynamics and statistical mechanics
quantum electrodynamics and quantum chromodynamics (relativistic versions of
Each of these theories can be taught without much reference to the others. (Whether any
theory can be
that way is another question.) This is a bad way to teach and view
physics, of course, since we live in a single universe that must obey one set of rules. Really
smart students look for the connections between apparently different topics. We can only
really learn a concept by seeing it in context, that is, by answering the question: how does
this new concept fit in with other, previously learned, concepts?
Each of these theories, non-relativistic classical mechanics for instance, must rest on a set of
. Laws or Postulates are statements that are
presented without proof; they cannot be proven; we believe them to be true because they
(E.g. Newton's 2
, is a postulate; it
cannot be proven from more fundamental relations. We believe it is true because it has been
abundantly verified by experiment. )
Actually, Newton's 2
Law has a limited
regime of validity
. If you consider objects going
very fast (approaching the speed of light) or very small (microscopic, atomic), then this "law"
begins to make predictions that conflict with experiment. However, within its regime of
validity, classical mechanics is quite correct; it works so well that we can use it to predict the
time of a solar eclipse to the nearest second, hundreds of year in advance. It works so well,
that we can send a probe to Pluto and have it arrive right on target, right on schedule, 8 years
after launch. Classical mechanics is not wrong; it is just incomplete. If you stay within its
well-prescribed limits, it is correct.
© University of Colorado, Michael Dubson (mods by S. Pollock )