DOC.
60 391
it
can
be given
that is
not based
on an
assumption
as
horrendous-looking
as
Planck's
theory? Would
it
not be
possible
to
replace the
hypothesis
of light
quanta
by
another
assumption
that
would
also fit the
known phenomena?
If it
is
necessary
to
modify
the elements of the
theory,
would
it
not be possible to
retain
at
least the
equations
for the
propagation
of
radiation
and conceive
only
the
elementary processes
of emission
and
absorption
differently than
they
have been
until
now?
To
clarify these
matters,
we
will
try to
proceed
in
the
opposite
direc-
tion than that taken
by
Mr.
Planck in his radiation
theory.
We
consider
Planck's radiation formula
as
correct
and ask ourselves
whether
some
conclu-
sion
about the constitution
of
radiation
can
be
inferred
from it.
Of
two
considerations
I have
carried
out
in
this
sense,
I will here outline for
you
only
one
which,
because
of
its clarity,
seems
to
me
especially persuasive. [28]
Let
a
cavity contain
an
ideal
gas
as
well
as a
plate
made
of
a
solid
substance that
can
move
freely
only
perpendicular
to
its
plane.
Because of
the
irregularity
of the collisions
between the
gas
molecules
and
the plate,
the latter
will
be set
in
motion such
that its
average
kinetic
energy
equals
one-third of the
average
kinetic
energy
of
a
monoatomic
gas
molecule. This
is
a
conclusion
drawn
from
statistical mechanics.
We
now assume
that besides the
gas,
which
we
may
conceive
as
consisting
of
few
molecules, there is also
radiation
present in
the
cavity;
let this radiation
be the so-called
tempera-
ture
radiation
having
the
same
temperature
as
the
gas.
This will
be
the
case
if the walls
of
the
cavity have
the
definite
temperature
T,
are impermeable
to
radiation,
and
are
not
everywhere completely
reflecting
toward the
cavity.
Further,
we
shall
temporarily
assume
that
our
plate
is
completely
reflecting
on
both sides. In this
state of
affairs,
not
only
the
gas
but also the radia-
tion will
be acting
upon
the plate.
The
radiation will
exert pressure
on
both
sides of the plate.
The
forces
of
pressure
exerted
on
the
two
sides
are
equal
if
the plate is
at rest.
However,
if it is in
motion,
more
radiation will
be
reflected
on
the surface that is
ahead
during
the
motion
(front
surface)
than
on
the
back surface.
The backward-acting
force
of
pressure
exerted
on
the
front
surface is thus
larger
than the force
of
pressure
acting
on
the back.
Hence,
as
the resultant of the
two
forces, there
remains
a
force that
counteracts
the
motion
of the
plate
and
that increases with the
velocity
of
the plate.
We
will call
this resultant
"radiation
friction" in brief.