| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
Hi --_______________________________________________
The confluence of two recent threads got me thinking.
Students always ask, why should we learn modern physics
(relativity, atoms, and quantum mechanics) when the world
around us is well described by classical physics?
A time-honored (and correct, albeit incomplete) answer goes
like this:
At the end of the 19th century, classical physics suffered
from some well-known paradoxes that rather loudly begged
for explanation. These are cases where theories that were
well established as successful over a wide range failed to
work in certain cases, and it wasn't obvious how to repair
them within a classical framework. Examples:
a) ultraviolet catastrophe or lack thereof
b) photoelectric effect
c) ether drift or lack thereof
d) Gibbs paradox
e) spectroscopy and other atomic and chemical phenomena
I'm not sure item (e) was very widely recognized as being
problematic in 1900, but in retrospect we can see it as such.
We all know that sodium light is yellow, and neon light is
red. In modern physics we explain the electromagnetic
behavior of atoms using pretty much the same laws as for
the electromagnetic behavior of macroscopic antennas. We
know that if you had an atom with electrons going around
a nucleus in accordance with classical laws, like planets
around the sun, the electrons would radiate, losing energy
rather rapidly until they spiral down into the nucleus.
As far as I know, spectroscopy didn't make it onto the
list of "known" paradoxes circa 1900, because people didn't
believe in atoms, and/or didn't feel the need to apply
the laws of physics to atoms. So it seems that just
recognizing the existence of atoms
_as objects subject to the laws of physics_ was really
quite a big deal, much bigger than most people realize
nowadays.
QM and relativity are still considered "weird", whereas
atoms are taken for granted in popular culture nowadays,
even if encrusted with tremendous misconceptions.
We should remember there were _three_, not two, revolutions
in physics in the late 1800s and early 1900s:
-- atoms,
-- relativity, and
-- quantum mechanics.
Tangential note: In honor of the year '05, I was
re-reading some of Einstein's papers. In particular
I wondered whether it was worth making a big fuss
over the Brownian motion papers. Now and/or then,
who cares about Brownian motion? The answer is
that nobody much cared about Brownian motion *until*
Einstein connected it to atoms. The idea that
atomic-scale processes were subject to the known
laws of physics was a very big deal.
It was also not obvious to Einstein! In 1905 he had
a theory of atomic fluctuations, and he suspected
that the theory could be applied to Brownian motion,
but he wasn't sure. The 1905 paper stimulated
experimental work, so by the next year (1906) he
was able to write a paper that had Brownian Motion
in the title.
BTW the exceedingly-useful equation connecting the
mobility to the diffusion constant is in the 1906
Brownian motion paper, not anywhere in 1905.
Returning to the main point of this note: If/when you
accept atoms as objects whose internal structure is
subject to the laws of physics, then the very existence
of the periodic table is evidence for quantum behavior.
Without the exclusion principle, fluorine, neon, and
sodium would all have rather similar properties.
(The unmodern approach, I suppose, is to just say
"atoms exist" and to make 92 exceptions to the laws of
physics -- one exception for each type of atom.)
One thing I'd sorta forgotten until yesterday is that once
you accept atoms _as objects subject to the laws of physics_,
the existence of ordinary permanent magnets is powerful
evidence for non-classical behavior. Suppose you have
a disk of steel, magnetized N on one face and S on the
other face. How can that possibly be? If you try to
explain it in terms of Amperian currents, it would be
energetically favorable for the currents to stop. What
keeps them from stopping?
The modern-physics answer is that ferromagnetism is due
to the _spin_ of the electrons ... and spin is a completely
non-classical concept. The electron can't stop spinning.
Secondly we note that in terms of potential energy only,
it would be energetically favorable for the electrons to
arrange themselves alternating up,down,up,down with zero
macroscopic magnetization. The only thing that makes
them arrange themselves up,up,up,up in a ferromagnet is
kinetic energy, in conjunction with the exclusion principle
... which again is completely non-classical.
=============================
References:
The University of Augsburg has put up nice facsimiles
i.e. PDF images of the papers Einstein published in
Annalen der Physik. There are 49 papers, from 1901
through 1922.
http://www.physik.uni-augsburg.de/annalen/history/Einstein-in-AdP.htm
These are wonderful if you can read German, even a
little bit, and make good conversation pieces otherwise.
I have not been able to find a similar jackpot of
English translations online. The only ones I know
of are the four 1905 papers, and his dissertation
(submitted 1905, published 1906):
http://www.fourmilab.ch/etexts/einstein/specrel/www/
http://www.fourmilab.ch/etexts/einstein/E_mc2/www/
http://lorentz.phl.jhu.edu/AnnusMirabilis/AeReserveArticles/eins_lq.pdf
http://lorentz.phl.jhu.edu/AnnusMirabilis/AeReserveArticles/eins_brownian.pdf
http://lorentz.phl.jhu.edu/AnnusMirabilis/AeReserveArticles/eins_diss.pdf
Conspicuously missing are translations of the 1906
Brownian motion paper, the 1910 critical point paper,
and the 1916 GR paper.
Suggestion: If some students are looking for an
interesting project that would be a big service to
the community, they could track down copyright-expired
English translations of classic papers, key them in,
and make them available as HTML.
I suggest you bookmark the Augsburg URL, because the
originals are harder to search for than you might think.
Presumably that's because the papers are scanned images,
which means the search engines can't recognize the text
as text, which means that even knowing what the papers
say doesn't help you find them.
A useful collection of links to Einstein source material
can be found at:
http://lorentz.phl.jhu.edu/AnnusMirabilis/
============
I remind people (again :-) that the history of science is
a poor guide to the teaching of science. Good introductory
pedagogy is simple and straightforward; history is not.
What we need are good _contemporary_ hands-on experiments
students can do (or at least demos we can do) to illustrate
the point that atoms are reeeally small, but not infinitely
small.
The most readily-available experimental evidence I can
think of is to measure the viscosity of air, and connect
that to the size of molecules via kinetic theory. Simply
exhaling through a soda straw suffices to make the point
that the viscosity of air is neither zero nor infinite.
Using chem-lab equipment you can measure pressure using
a water manometer, and measure d(volume)/dt using a
collecting jar and a stopwatch. Given an estimate of the
viscosity, you can estimate the size (i.e. cross section)
of the molecules.
I don't think more than a rough estimate is really needed,
but you can make this quantitative if you want:
http://thunder1.cudenver.edu/chemistry/classes/chem4518/viscosity.htm
I did ye olde oleic-acid-on-water experiment in HS chemistry,
but I didn't find it very convincing. For one thing, it
seemed (at best) valid as an upper bound on the size of
atoms, but not a lower bound. Secondly, it required taking
a lot on faith about the structure and behavior of oleic
acid, so much so that it seemed more logical to just take
atoms on faith without bothering with the experiment.
So ... does anybody have any better ideas for hands-on
experiments involving atoms?