DOC.
18
DISCUSSION OF DOC.
17 353
conception,
and
it
is
what disturbs
many physicists, namely,
to have
to
assume
that
rigid
bodies
do not exist. But I
think that
this
should
not be
interpreted as meaning
that
a
body
must
deviate from
rigidity
in all
possible
directions because motions take
place
in
all
possible
directions,
but,
rather,
that the
invariance holds
only
when
considering
a
certain direction
of
motion. It will be the
business
of the
mathematicians to formulate
more exactly
the conditions of
rigidity
in
these
systems.
As
for the
rest,
it
is
difficult to
decide whether
all
the
consequences
of the
relativity
principle
agree
with
experience,
because the
deviation from what mechanics
yields
for
v2
systems
at rest
is
always
of
the relative
magnitude
_.
This
is
a quantity
that
always
remains
small.
It
shows
up
in
the
discussion
of the
electromagnetic masses
of electrons
of
longitudinal
and
transverse
mass.
These
have
been calculated
according
to
the
ideas
of
the
relativity principle,
but it
must be said
that
this matter
has
not
yet really
been
decided.
But
it is to
be
expected,
as
my
colleague
Mr.
Einstein
indicated, that,
in
the
course
of
time,
there
will
be
experimental
findings
that
will
decide the
matter.
Perhaps
only
the
following
remains
to
be
said in this
area,
something
which
proves
to
be
a
difficult matter
for the
physicist
in discussions
of
this
relativity
principle.
You
have
heard that
we
must
give
up
the notion that
an
ether
exists.
Perhaps
we
should
say
that
it
is
not too
bad about the ether. We
came
to know it
as an
ad hoc
hypothesis by
means
of
which to
explain
all sorts
of
phenomena.
One
has
saddled
it with
more
and
more
incomprehensible properties.
But
the
fact still
remains that
we are
supposed
to
discuss
propagations
without
having any
idea
in what
these
propagations
consist.
The
velocity
of
propagation
of
light, wave
motions
that
are
propagated,
the
whole
theory
of
interference,
all
these
were
hitherto based
on
certain
conceptions
that
are now gone.
We
are
supposed
to
speak
about
propagation
in
a
medium
which is
not
a medium,
and about
which
we
do
not know
a
thing.
I
think that
this
is
a
gap
that
must
be
filled,
because
scientific,
and
especially physical,
discussions
that
operate
with
formulas that
cannot
be
linked
to
any
mental
image
cannot
be maintained forever.
So,
as
far
as
the
principle designated
as
the
principle
of
relativity
is
concerned,
I
think that
this is
something
that has
been
needed, that,
for
once,
simply
stipulates some
things, some
unclarities
to which
we
have not
given
any
thought
at
all,
and
puts
them
in
certain order. Time
will
probably
show
what
sort
of
difficulties
are
involved,
but
whatever
they
are,
they will probably
find
their solutions.
Prof.
Einstein:
First of
all,
I wish to
thank
Prof.
Kleiner
for his kind words.
For the
rest, I wish to
say a
few
things
in
response
to
the
things
he
brought
up.
According
to
the
theory
of
relativity, a rigid body
cannot exist at all.
Let
us imagine a
rod of
a
certain
length.
If
we
pull
on one side,
the other end
will start to
move
at
once.
This would be
a signal
that
moves
with infinite
speed
and that
could
be used
to
define
time,
which leads
to
highly
improbable consequences
for
reasons
that
cannot be
explained
here
in
greater
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