364
DOC. 20
LIGHT
QUANTUM
HYPOTHESIS
Doc.
20
Statement
on
the
Light
Quantum
Hypothesis
by
A.
Einstein
[IN: Naturforschende Gesellschaft
in
Zürich.
Sitzungsberichte (1911):
XVI.
Published
in
vol. 4
of
Vierteljahrsschrift
der
Naturforschenden Gesellschaft
in Zürich 56
(1911).
Minutes
of the
meeting
of
21 February
1911]
[1]
It
turns out
that
when Maxwell's
theory
of
electricity
and
the
molecular-kinetic
approach
are applied
to
some phenomena
of
light
production
and
conversion,
contradictions
with
observed
facts,
in
particular
those
concerning "black-body
radiation" and
generation
of
cathode
rays, come
to
light.
These contradictions
can
be removed
by
introducing
the
working hypothesis
that
in
the
propagation
of
light, energy
does not
fill
space
in
a
continuous
manner,
but
that, instead,
it consists
of
a
finite
number of
energy
quanta
localized
at
spatial
points,
which
move
without
dividing
and
can
be absorbed and
[2]
generated
only
as
a
whole.
If these
energy quanta
impinge on a
photoluminescent
substance,
then,
according
to
the
principle
of the conservation of
energy,
the radiant
energy
emitted
in
an
elementary process
must
be
equal
to
or
smaller
than the incident
radiant
energy,
and
from
the formula for the
energy
of
a light
quantum one
arrives in
a
simple manner
at
Stokes'
well-known
frequency
rule. When cathode
rays are
generated
by illuminating
solid
bodies,
the
energy
of the
light
quanta is
converted
into
the kinetic
energy
of
electrons,
and
only now
do
we
realize that the
quality
of
cathode
radiation,
i.e.,
the
velocity
of
electrons,
can
be
independent
of the
intensity
of the
exciting light,
while
the number of
ejected
electrons
is
proportional
to
the number of the
light
quanta.
But
from Planck's radiation formula
it
is
to
be inferred
that,
hand
in
hand
with
this,
there
has
to
come
about
a change regarding our conception
of the molecular-kinetic
mechanism
for the transfer of
energy
to ions
or
electrons
capable
of
oscillation
(resonators),
in
that
their
energy
can change only jumpwise by an
integral multiple
of
precisely one
quantum
of
light energy.
If
one
also
carries
this
mechanism
over
to
the
oscillations
of
the
material
molecules of
a
solid
body
that
are
due
to
the
body's
thermal
motion, one
arrives at
a
surprising
elucidation of the
change
of
specific
(molecular)
heats of
solids with
temperature,
which
until
now
had remained
a
total
mystery.
(End
of
the
session
10:30)