The Milky Way galaxy is large, the visible disk of stars is around 100,000
light years across (around 6x10<sup>17</sup> miles). The nearest star to
the Earth, excluding the Sun, is <font color=magenta>Proxima Centauri
</font> which is around 4.3 light years from the Earth. So, if a spacecraft
could travel at the spped of light, <i>c</i>, it would take 4.3 light years
for the ship to travel from Proxima Centauri to the Earth and it would take
~100,000 years for it to travel across the Galaxy. This simple argument would
suggest that perhaps a very technically advanced civilization could easily 
explore the Galaxy over the lifetime of the Galaxy, > 10 billion years.
<p><hr><p>
If the civilization is roughly like ours and uses space probes similar to ours,
this changes the argument. For example, we live in a planetary system dominated 
by the Sun, our star. When we send probes into space, we try to do it in an
economical way; we launch the probes just fast enough to escape the Earth and
to reach their desired destinations. If we want a probe to escape the Solar
System, we would give it the escape velocity from the Solar System. From the
orbit of the Earth, this corresponds to a speed of ~xx,xxx kilometers per hour.
If the extraterrestrial civilization lives in a planetary system to 
ours and is also interested in economy, then we would expect that it would
also send out probes with speeds on the order of xx,xxx kilometers per hour or
so.  In this case, the trip from <font color=magenta>Proxima Centauri</font>
would take
<center>
Trip = (length of trip)/(speed) = xx,xxx years
</center>
and a trip across the Galaxy would require xxx millions of years. Such
times are both shorter than the age of the Galaxy and thus still plausible
that extraterrestrials could explore the Galaxy over the lifetime of the 
Galaxy.
<p><hr><p>
From a psychological and sociological viewpoint, this is likely a
multi-generational exercise and motivation and drive play roles in whether 
any civilization is willing to devote such time, resources, and effort. <p>
<font color=magenta>Are there  
ways to go about exploration which remove psychological and sociological 
questions?</font>
<p><hr><p>
<font color=magenta>Von Neumann Machines</font>:
<p><hr><p>