xviii INTRODUCTION TO VOLUME
3
Zurich in the
summer
of
1912
when Einstein
accepted
a
call to
a
professorship
at
the
Swiss
Federal
Polytechnical
School
(ETH).[13]
III
When Einstein moved from the
Patent
Office to
the
university
in
October
1909,
the
problems
he had been
pondering
moved with him.
Foremost
among
these
was
surely
the
problem
of
radiation,
which
had
already
given
him
no
peace
for
several
years.
Only a
few weeks
before
moving
he
had
spoken on
this
subject
at
Salzburg
in
his first
major
address
at
a
scientific
meeting, arguing
that
a
new
theory
of radiation
was
needed.[14]
The
wave
theory
of light-and
Maxwell's
electromagnetic theory
which
provided
its
foundation-could
not
adequately
account
for
some phenomena
that
could, however, be
readily
understood
if
light
behaved
like
a
collection of
particles
of
energy.
A
particle
or
emission
theory
of
light,
on
the
other
hand,
failed to
account
for the
famil-
iar
phenomena
of
interference, diffraction,
and
so
forth
that
were explained
so
beautifully
by
the
wave
theory.
Einstein
expected
that
"the
next
phase
of
the
development
of theoretical
physics"
would
produce
"a
theory
of
light
that
can
be
interpreted as a
kind of fusion of the
wave
and emission
theories."[15]
He
based this
expectation
on
his
analysis
of the statistical fluctuations of the
properties
of
black-body
radiation
whose
average
behavior
is
described
by
the
experimentally
confirmed Planck distribution
law.
Einstein had carried
out two
independent
calculations of
such fluctuations,
one
using
Boltzmann's
principle
and the other
using
the
approach
of
his
own
theory
of Brownian motion. Both had
led to
fluctuations
demonstrating
the
presence
of
a
particlelike
structure
as
well
as a
wavelike
structure
in radiation.
These results had convinced Einstein that Maxwell's
electromagnetic theory,
which
predicted only
the wavelike
term
in the
fluctuations,
would have
to
be
modified
in
some
fundamental
way.
The fluctuation
arguments
had
per-
suaded him
that
the
quantum structure
in radiation
was a
necessary
conse-
quence
of Planck's distribution law and
not
just
an
assumption-"apparently
horrendous"[16] and
perhaps
avoidable-that
was
sufficient
for
deriving
this
distribution
law.
[13]See
Robert Gnehm
to Einstein, 23
January 1912,
and Einstein
to
Ludwig Forrer,
2
February
1912.
[14]Einstein
1909c
(Vol.
2,
Doc.
60).
[15]"...
daß die nächste
Phase der
Entwickelung
der
theoretischen
Physik
uns
eine Theorie
des Lichtes
bringen wird,
welche sich
als
eine
Art
Verschmelzung von
Undulations-
und Emis-
sionstheorie
des Lichtes auffassen läßt." Einstein
1909c
(Vol.
2,
Doc.
60),
pp.
482-483.
[16]"ungeheuerlich
erscheinenden Annahme."
Einstein
1909c
(Vol. 2,
Doc.
60), p.
495.
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