DOCS.
272,
273
NOVEMBER
1916 261
a
Lorentz
transformation but
degenerates
into another transformation
at
infinity.
Naturally,
all of
this
is
not in conflict with the
principle
of
relativity,
I
ac-
knowledge.
But
if
I
am
to believe all
of
this,
your theory
will
have lost
much
of
its
classical
beauty
for
me.
With
it
an “explanation”
of
the
origin
of
inertia
is
gained,
which
is
actually
not
an
explanation,
for
it
is
not
an
explanation
from
known
or
verifiable facts
but
from
masses
invented ad
hoc.[5]
I
am
convinced–
but this
is
only
a
belief
that
cannot be
proven,
of
course-that
these
masses
will
go
the
way
of
the
“ether wind.”
New
efforts
will
continually
be made
actually
to observe
them,
but
it
will
never
work until
we
finally
come
to
the
conviction
that
they
do not exist.
[The
stars and nebulae
obviously
are
not
part
of
the
“envelope,”
because
there the
gij's are
still
approximately Galilean; otherwise,
we
could not
possibly identify
spectral
lines.][6]
Is
it
not
possible
that,
in the
end,
the
explanation
for
inertia
must be
sought
in
the
infinitely
small
rather than
in
the
infinitely large?
I
am
not
a
physicist,
and this
is
probably
just
an
entirely
meaningless
hallucination.
But it
is
very
hard
for
me
to believe in
the distant
masses.
I
would
prefer having
no
explanation
for
inertia
to
this
one.
I would not
dare
to
write
you
all
this
if
I
did not
know,
from
the
very
pleasant
hours
we
spent together,
that
you
will not
take it amiss. You know
that
it
is
only
my deep
admiration
for
your
theory
which
impels
me
to
do
so.
With
all
good wishes, yours very truly,
W.dS.
273. To
Willem
de
Sitter
[Berlin,]
4
November
1916
Dear
Colleague,
Your
letter,[1]
which
puts
me
back in time
to
the
pleasurable Leyden
days,[2]
I
read
with
great
interest and
am
looking
forward
to
your
popular
paper
in
English,
which
will
probably
also be
arriving
soon.[3] I
am sorry
for
having
placed
too
much
emphasis
on
the
boundary
conditions
problem
in
our
discussions. This
is
purely
a
matter of taste which
will
never
gain
scientific
significance.
I
must add
right
away,
though,
that
I
never
did
think
of
a
temporally
finite extension
of
the
world;
also
spatially, finite
extension
is
not
the
issue.[4]
Rather,
my
need to
generalize
just
led
me
to
the
following
interpretation:
Let it be
possible
to indicate
a
spatial
envelope
(a
massless
geometric
surface)
(in
four
dimensions,
a
tube),
outside of which
a
gram
atomic
weight
can
have
as
little
inertia
as
I
please.[5]
Then
I
can say
that,
within the
envelope,
the inertia
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