The gravitational density wave theory of Lin and his co-workers involves the formation of stars along a rather concentrated shock front which co-rotates with the spiral pattern as a rigid body around the galactic center. The locus of star formation is expected to move according to the pattern velocity which - in view of the differential rotation of the Galaxy - generally departs from the circular velocity of stars and interstellar clouds (except at a critical distance from the center: at the co-rotation radius) and thus one could expect to find systematic effects in the shifts of spiral arm tracers of different ages. If we assume that the case of mankind is about average and accept the idea that the longevity of a civilization might be limited with high probability by catastrophic events, threatening during the crossing of the galactic arms, intelligent life is presumably concentrated on a belt in the Galaxy which is a narrow annulus including the co-rotation circle and the galactic orbit of our sun. If the "Galactic Belt of Intelligent Life" is a reality at least the first and last factors in the 'Drake Equation' must be reassessed. From heliocentric point of view the distribution of our potential extraterrestrial partners is highly anisotropic: in a small solid angle around the line of sight there are about thousand times as many of them in the tangential directions than towards the galactic center or anticenter.
What is the distribution of intelligent life in the Galaxy? A simple question, an exciting question and clearly a question whose answer is most difficult to achieve. But it is a question that surely should be asked and answered in order to elaborate a suitable search strategy, and a question which is impossible to answer professionally without taking into account the geometric and kinematic properties of our stellar system. Estimates of the likelihood, galactic distribution and accessibility of extra-terrestrial civilizations generally contain three shortcomings: They treat our Galaxy as a homogeneous, isotropic and steady-state system and not as an object of well-known kinematics and reasonably well understood morphology and path of evolution.
The gravitational density wave theory of Lin [1] is one of the most suitable of theories which can provide an acceptable quantitative viewpoint from which it is possible to explain the large-scale geometry and dynamics of the galactic spiral structure in a coherent way. This theory involves the formation of stars along a rather concentrated shock front that co-rotates with the spiral pattern as a rigid body around the galactic center. Even if one finds that clumping is the most typical feature of galactic disk distribution, there is an overall large-scale pattern of spiral arms, which can roughly be approximated by logarithmic curves of the form:
) between us and another planetary
system with heliocentric galactic coordinates r, l, b
can be written in the form:
is the angular
pattern speed
(
), 
is the galactic rotation
curve, 
is the surface density
distribution in the galactic plane and 
is the radial
velocity dispersion of the stars].
The angular pattern speed is one of the most important model parameters and
according to Lin's group equals 
(in satisfactory agreement with the empirical data).
In the early seventies L. S. Marochnik and his co-workers [2] pointed
out that since the bulk of the galactic mass is concentrated in subsystems
with a large velocity dispersion, the properties of the spiral density-waves
are actually governed not by the total mass of the Galaxy, but by a small
portion of it associated with the extremely flat sub-system of stars having
a small velocity dispersion. Equ. (1) contains therefore only the density
of this flat subsystem which differs strongly from the total surface density
in both magnitude and space distribution. Marochnik et al. found that only
with close to 40
and
close to 23 are we able to
obtain a spiral structure which is in good agreement with the large scale
distribution of neutral hydrogen in the Galaxy.
It appears, therefore, that not the outermost HII regions but the objects
in the solar neighborhood lie in the zone of co-rotation. The rather small
value of 
is
empirically supported by an early
work of A. Blaauw [3] who found that the associations in the
solar neighborhood (d<1kpc)
consist of a number of subsystems of
different ages. The distances between the subsystems (10-40pc) are
of the same order of magnitude as the sizes of the subsystems themselves,
the age difference between the neighboring subsystems is about
years.
In view of this the physical mechanism which initiated the birth of this
subsystems must have had a linear velocity between 5 and 10
which is in good agreement with a small 
,
but entirely incompatible with the
large relative velocity proposed by Lin.
The smallness of is also supported
e.g. by a former work of A. H. Nelson and T. Matsuda [4],
investigating one-dimensional galactic spiral shocks, by B. A. Balázs [5],
investigating the spatial distribution of open clusters of various ages and
by recent papers of P. Grosbol and P. A. Patsis [6] (BVIK surface photometry
of five ordinary spiral galaxies) and Yu. N. Mishurov at al. [7-8]
(investigation of the line-of-sight velocity field of Cepheids).
It is easy to see that in this case the age of the solar system is
comparable with the period during which the Sun remains between two
spiral arms. Its radial phase (
)
between the Sagittarius
and Perseus arms corresponds to an value of
24.7
(if 
= 25). As it is known that in the vicinity of the Sun the galactic
rotation curve is linear with a slope of 
, where A is Oort's
galactic rotation constant, the co-rotation radius comes to
= 10kpc and A = 15 
comes
to 10.1
.
(Taking say 8kpc for the galactocentric distance of our Sun,
the whole picture shrinks, but the striking proximity remains.)
Now, if we following Shklovskii, Clark, Clube, Marochnik [9-12] and others accept the idea, that the longevity of a civilization might be limited with a high probability by close supernova explosions and impacts of large comets, the life expectancy of advanced civilizations is the time which their system spends between two neighboring spiral arms where the occurrence of fatal cosmic events is rendered unlikely and the belt of extraterrestrial civilizations presents itself as a surprisingly narrow one!
With regard to the age of the solar system, objects with
which left the Sagittarius arm or the Perseus one,
in the direction of the galactic rotation 
years
ago have by now just reached the other arm and conversely, objects with the same
age and parent arm, but with 
have similarly traveled
the whole way in the opposite direction between the two spirals.
Figure 1: The ``Galactic Belt of Life'' in the Galaxy.
G is the galactic center; C, 
, D, 
, E, 
are the intersections of the spiral arms with the circle of
corotation and the belt edges.
Denoting the galactocentric radius of these objects by 
and
making use of eqn. (2) we get
If we take into account that there is a sort of incubation spread in the
process of star formation and therefore stars which were born owing to the
same large-scale compression of the interstellar medium (by the density wave)
are not completely coeval, the half width of the belt becomes only
insignificantly different from the previous one. Changing the unit of
into nanoradian/yr eqn (2) leads to
and inserting
we get
yr, eqn (7) leads to a minor change
of only 0.005kpc in .
It is known that basically as a reaction against exaggerated subservience to the copernican principle Brandon Carter [13] in the early seventies introduced the so called anthropic principle, which in its weak form declares that we must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.
Now, if we assume that the case of mankind is about average and accept the idea that highly developed planet-dwelling life is not likely to survive the catastrophic events threatening during the crossings of spiral arms, our very presence and the possibility to discuss extra-terrestrial contact problems shows that we and our potential non space-faring partners live close to the galactic circle of co-rotation ('kinematic ecosphere').
There are thus morphological arguments favoring the concept of a belt of civilizations based on a kinematic ecosphere in the Galaxy in the form of an annulus with a breadth of roughly 0.5kpc including the galactic orbit of our Sun. We should concentrate our efforts to contact extraterrestrial beings on this narrow belt (Fig. 1).
If the galactic belt of intelligent life is a reality the first and last
factors in the famous Drake Equation (see f.i. Kreifeldt [14]) must be
reconsidered. As far as 
the number of potentially
suitable parent stars
is concerned, only stars in the belt of life can be regarded.
As the volume of the ring comes about
6
, the stellar density 
0.13*/
and roughly 10% of the stars have long lasting habitable zones
(see f. i. Tucker [15]), is only about 
!
One can see at the first glance, that from heliocentric point of
view the distribution of the suitable stars is highly anizotropic.
Choosing f. i. a typical solid angle of 
sr
and taking again D = 0.13*/, one deals with
in the
first case and 
objects in the second one.
As only around 10% of the stars have long lasting habitable zones
is in our case 
and 4, respectively.
As regards 
the lifetime of a planet during which highly developed
species can exist on it, can also be judged by astronomical methods.
It is basically limited by the evolution of the parent star and by
stellar kinematic factors.
Limitations of the first type are governed by the mass M of a star. Stars with nearly the mass of the sun have main-sequence lifetimes (cp. Zuckerman [16]):
is limited by the crossing time
which a planetary system spends between two spiral arms. Changing the unit
of into 
yrs eqn. (4) leads to
,
is inversely proportional to the distance
from the co-rotation circle:
