232
DOC.
38 QUANTUM
THEORY
OF
RADIATION
hypotheses
in
§2
about the interaction between matter and radiation
by
means
of
processes
of
absorption
and emission
or,
respectively, spontaneous
and induced
radiation
processes.
I
was
led
to
these
hypotheses
from
a
desire
to
postulate
in the
simplest manner
the
quantum-theoretical
behavior
of
molecules in
a
manner
analogous
to
the classical
theory
of Planck's resonator. From the
general assumption
of
quanta
for
matter,
Bohr's second rule
(equation
9)
and Planck's radiation formula
followed
effortlessly.
The
result,
referring
to
the
momenta
transferred
to
molecules
by
spontaneous
and
induced radiation
processes, appears
to
me
to
be the
most
important
one.
If
we were
to
modify
one
of
our
postulates
about
momenta,
a
violation of
equation
(12)
would
[p.
61]
be the
consequence.
To
agree
with this
equation-which
is demanded
by
the
theory
of
heat-in
a
way
other than
by
our
assumptions, seems hardly possible.
Therefore,
we can
consider the
following
as
being
rather
certainly proven.
If
a
beam of radiation
effects the
targeted
molecule
to
either
accept
or reject
the
quantity
of
energy
hv in the form of radiation
by
an
elementary process (induced
radiation
process),
then there is
always
a
transfer of
momentum
hv/c
to the
molecule,
specifically
in the direction of
propagation
of the beam when
energy
is absorbed
by
the
molecule,
in the
opposite
direction
if
the molecule releases
the
energy.
If
the
molecule
is
exposed
to
the action of several directed beams of
radiation,
then
always
only one
of
them takes
part
in
an
induced
elementary
process;
only
this beam alone
determines
the direction of the
momentum
that is transferred
to
this molecule.
If
the molecule suffers
a
loss of
energy
in the amount of hv without external
stimulation, i.e., by emitting
the
energy
in
the form of radiation
(spontaneous
emission),
then this
process
too
is
a
directional
one.
There is
no
emission of radiation
in the form of
spherical waves.
The molecule suffers
a
recoil in the amount of
hv/c
during
this
elementary
process
of emission of
radiation;
the direction of the recoil
is,
at
the
present
state
of
theory,
determined
by
"chance."
The
properties
of the
elementary processes
that
are
demanded
by equation (12)
let the establishment
of
a quantumlike
theory
of
radiation
appear
as
almost
unavoidable. The
weakness of the
theory is, on
the
one hand,
that it does not
bring
us
closer
to
a
link-up
with the undulation
theory; on
the other
hand,
it also leaves
[21]
time of
occurrence
and direction of the
elementary processes
a
matter of "chance."
Nevertheless,
I
fully
trust
in the
reliability
of
the road taken.
Still
one
general
remark should find
its
place
here.
Nearly
all
theories of thermal
radiation
are
based
upon a
consideration
of
the interaction between radiation and
molecules. But in
general
one
is satisfied
with
a
consideration
of
the
energy
exchange
without consideration of the
exchange
of
momentum.
One feels
easily