392
DOC.
395
OCTOBER
1917
it
is
forced
upon one
when
something
of
urgency
is
waiting
to be
done,
and
in
this
case,
one
is
not
in the
mood
to
comprehend
it
well. I
shall also
have
this
letter transcribed
again so
that
perusal proceeds
as
painlessly
as
possible.
Even
though
I
have
made
an
effort to become somewhat versed in
physics,
my knowledge
is nevertheless
quite limited,
even more so
my ability
to work
independently
with the
subject
matter. For
a long
time it
has
been
a
great
wish
of mine to become not
only
a
philosopher
but
also
a
trained theoretical
physicist;
but
I
see
increasingly
that
although
I
am
able to
understand the
mathematics,
I
cannot master it
to
the
point
that
I
can
work
mathematically.
Thus
I
have
become
acquainted
with
the
general
theory
of
relativity
not
only
through popular
descriptions
but
also
through studying
your original papers.
I
may
be able
to
understand
covariant
theory, etc., yet
I
do not have
an
overview of it.
That
is
how
it
came
about
that
in
my
letters to
you
I overlooked
some
things I
had
learned
and
which
are
self-evident, of
course,
such
as,
that
according
to
the
general
principle
of
relativity,
the
law
of
the
conservation of
momentum must be
valid for
every
coordinate
system. Besides,
from
your
letter
I
saw
clearly
what
is
involved in
the
selection of
a
coordinate
system
and in
what
way
the
laws of
momentum conservation do not work. From
it
I
also
learned,
in
particular,
what
I
had heard
many
times
already
but
what
a philosopher
repeatedly
forgets, namely,
that
an
idea
lacking
mathematical
formulation has
no
value in
theoretical
physics
as
yet,
and that
expressions
like
“most
closely Euclidean,”[2] etc., merely
state
the
problem.
As
concerns
electron
theory
and
Ehrenhaft,
certain critical considerations
by
Ehrenhaft,
quite
aside
from the
experiments,
do
seem
to
me
very cogent.
Ehren-
haft thinks
virtually nothing
of all theories
and,
even
though
he does
go
too far
in
this,
he
certainly
shares
this
fault
with
the
great
Mach.[3]
Ehrenhaft
points
out
legitimately
that
a
and
ß
rays merely
measure
the
rela-
tion
e/m.
He considers it
unproven
that
a
particles
were
helium
atoms
with
two
elementary
charges.[4]
The fact
that
helium
emerges
from it
is not adequate proof
for him. Even
so,
this
certainly
does
seem
to be
a very plausible hypothesis.
On
the other
hand,
his criticism
concerning
ß
rays really is very convincing. Indeed,
it
is
completely unproven,
but nonetheless
obviously
not
necessarily false,
that
ß
particles carry
the
elementary charge
and
that their
mass
is
1/2000
of
the
hydrogen
atom.
But
since
it
is
acknowledged,
at
least,
that
matter sometimes holds several
units
of
elementary charge,
the
already so firmly
rooted
belief
that
we
knew of
masses
that
were
1/2000th
of
the
hydrogen
atom is
quite
unfounded.
I
believe
this
very simple
critical
objection
is
certainly justified
if
it
can
be assumed
that the
mass
of
ß
particles
is
smaller
than that
of
the
hydrogen
atom.
It
certainly
is
a