4 DOC.
1
PRINCIPLE OF RELATIVITY
interpretation
that
light
in
empty space always propagates
with the
same velocity,
irrespective
of the
state
of motion of the
light source.
Yet this result
seems
to
be in
contradiction with the aforementioned
principle
of
relativity.
After
all,
when
a
beam
of
light
travels
with
a
stated
velocity
relative to
one
observer,
then-so
it
seems-a
second
observer who is
himself
traveling
in the direction
of
the
propagation
of the
light
beam should find the
light
beam
propagating
at
a
lesser
velocity
than the first
observer does.
If
this
were
really
true,
then the law
of
light propagation
in
vacuum
would not be the
same
for two observers who
are
in
relative,
uniform motion
to
each
other-in
contradiction
to
the
principle
of
relativity
stated above.
This
is
where the
theory
of
relativity comes
in. This
theory
shows that the law
of
constancy
of
light propagation
in
vacuum can
be satisfied
simultaneously
for two
observers,
in relative motion
to
each
other,
such that the
same
beam of
light
shows
the
same velocity
to
both of them.
The
possibility
for such
an
at
first
glance paradoxical interpretation
can
be
understood from
a more
detailed
analysis
of the
physical meaning
of
spatial
and
temporal
statements.
Of
special importance
to
this
question
is
the
recognition
of
the
relativity
of the
concept
of
simultaneity.
Before the
theory
of
relativity,
it
was
believed that the
statement
"two
events
happening
at two
different
places
are
simultaneous" had
a
clear
meaning-clear
without
a special
need
to
define the
concept
of
simultaneity.
A
more
detailed
investigation,
which
does
not
skirt
the
issue
of
defining simultaneity, showed, however,
that the
simultaneity
of
two events is
not
absolute,
but instead
can
only
be defined relative
to
one
observer
of
a
given
state of
motion. It turns out that two
events
which
are
simultaneous with
respect
to
one
observer
are,
in
general,
not
simultaneous
with
respect
to
a
second observer who is
moving
relative
to
the first
one.
This
signifies
a
fundamental
change
in
our
concept
of
time.
(This
is
the
most
important,
and also the
most
controversial theorem
of
the
new
theory
of
relativity.
It is
impossible
to enter
here into
an
in-depth
discussion of
the
epistemological
and
"naturphilosophischen" assumptions
and
consequences
which
evolve from this basic
principle.)*
Those
who want
to
familiarize themselves with
a
more
detailed substantiation and
justification
can
find sufficient
instruction-without
[1]
difficult mathematical
derivations-in
E.
Cohn's
pamphlet, "Physikalisches
über
Raum und
Zeit,"
and in
an
essay
by
Jos.
Petzoldt,
"Die Relativitätstheorie der
[2]
Physik,"
in the most
recent
issue of the
Zeitschrift
für
positivistische Philosophie.
By combining
the
principle
of
relativity
with the results of
the
constancy
of
light-
velocity
in
vacuum,
one
arrives
by
a
purely
deductive
manner
at
what is
today
called
*Translator's note. The German
"Naturphilosophie"
embraces far
more "metaphysics"
than the
(Baconian) English
"natural
philosophy";
thus,
a
literal translation would be
very
misleading.
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