Symposium on unidentified flying objects: hearings before the Committee on Science and Astronautics, U.S. House of Representatives, Ninetieth Congress, second session, July 29, 1968.

90
I
should
first
emphasize
that the
distances
between
the
stars
are
absolutely
huge.
Light,
faster
than which
nothing
can
travel,
takes
4
1
/
2
years to get
from
here to
the
nearest
star.
Mr.
Rousx.
Excuse
me,
isn't
that
a
rather
arbitrary
statement?
Dr.
SAGAN.
I
don't
think
so.
Perhaps
you
can
tell
me
why
you
think
it
might
be,
then
I
can
tell
you
why
I
think
it isn't.
Mr.
ROI7SH.
In
my
opening
statement
I
referred
to the
new
audacity
of
imagination
John
Dewey
had
spoken
of.
I'm
thinking
of imagina-
tive
terms,
not
factual
terms.
Dr.
SAGAN. Let
me
say
in
a
sentence,
why
most
physicists believe
no
material
object
can
travel
faster
than
light.
That
takes
us
into
ques-
tions
of
the theory
of
relativity,
which has
had
previous encounters
with
congressional
committees,
and
perhaps
we
don't
want
to
go
into
that
in
very great
detail.
But
the
essential
point
is
that
in
making
a few,
very
few
assump-
tions,
one of which was,
the
one
we
are talking
about,
nothing goes
faster,
Einstein
was
able
to
then
derive
a
whole
body
of
predictions
which
are
confirmed
in vast
detail.
Therefore,
if
someone
says that
is
not
a good
idea,
that things
can
travel faster
than
light,
then they
have
to
come
up
with
a
physical
theory
which
explains
everything
we
know
in
a
way
that
is
consistent
with
the
idea
that
you
can
travel faster
than
light.
No
one has
succeeded in
doing
that.
Many
physicists,
have
tried.
Therefore,the
present
belief
is
that
you
can't.
But
that,
of
course,
is
a
time-dependent
statement.
It
may
be
that
this
isn't
the
ultimatetruth.
In
physics,
as in
much
of
all
science,
there are
no
permanent
truths,
There
is
a
set
of
approximations,
getting
closer
and
closer,
and
people
must
always be ready
to
revise
what
has
been
in
the past
thought
to
be
the
absolute
gospel
truth.
If
I
might
say, to
revise
opinions,
is
one
which
is
frequent
in
science,
and
less
frequent
in
politics.
[Laughter.]
Dr. SAGAN.
So
'
in the context
of contemporary
science,
I'm
obviously
speaking
in that context,
one cannot
travel
faster
than
light.
So
the
distances
between
the
stars
are extremely
large.
Of
course,
any
contemporary
space
vehicle
would
take
a
ridiculous
amount
oftime
to
get
from
here to
anywhere
else,.
but
we
are
not
talking
about
contemporary
space
vehicles.
The
queston,"Is
there
any
conceivable
method of
traveling
from
one
place to
another very
close
to the speed
of
light,
and
therefore
get
reasonable
transit
times?"
involves extrap-
olations
of
technology
of
a
very
difficult
sort.
However,
let
me
merely
say
at
least
some
people
who
have
looked
into
the
subject
have
con-
cluded that
it
is
not out of
the
question,
even with contemporary
principles
of
science,
to
imagine
vehicles
capable
of
traveling
close
to
the speed of
light,
between the
stars.
This
doesn't
mean
that
it
happens.
There
may
in fact
be
insuperable
engineering
difficulties
we
don't
know
about, but
there
is
nothing
in
the physics that
prohibits
interstellar
space
flight.
So
any
estimate
of
how
likely it is
that
we
would be
visited
by an
extraterrestrial intelligent civilization,
depends not only
on how many
of them
are there,
but on what kind of transport they have,
and how
often they launch
their
space
vehicles,
even very
optimistic estimates
for
all
these
numbers,
gives
a
conclusion that
an advance
civilization
comes here very rarely.
But I
again emphasize the great uncertainty in
any of these numerical estimates, as they involve parts of science
we
don't
know very
much
about.