It's easy to despair at the gulf between stars and the millennia of time it would take to get a ship there. The fastest spacecraft humans have yet produced was Helios 2, which after a slingshot maneuver in 1989 was moving at about 103 kilometers per second. There are two things to note about that statement. The first is that it was a slingshot maneuver, not an acceleration achieved under its own power (which is always the case in space exploration.) The second is that breakneck speed would deliver Helios 2 to our nearest neighbor Alpha Centauri right around 13,000 years from now. (This also means there are stars in our neighborhood that even if we aimed our fastest-yet probe at them, we could never reach, because they're moving away from us faster than our fastest spacecraft.)
Those frustratingly quarantine-like time spans suddenly seem much shorter when we consider the geologically brief time spans between close passes of the sun and nearby stars, resulting from proper motion. These changes in interstellar geography actually occur much faster than plate tectonics. In the space of a few tens of thousands of years we might go from having our nearest neighbor five light years away, to close enough to impinge on the Oort cloud and send comets falling toward the inner system. It's amazing to think but since our ancestors were first using fire, multiple close-passes between other stars have occurred. Merely 70,000 years ago we had a star 0.82 light years away (Scholz's Star), and in another 1.3 million, we'll have another (Gliese 710).
There are a number of clear inferences to be drawn from the frequency of such close passes.
1) It bears repeating, in geologic time, 70,000 years is really not long. Humans may have already started leaving Africa when this occurred. We should assume the sun is not unique in this, and our close-pass rate is about once per million years. That means, since the solar system formed, this has occurred 4,500 times. The Wild-2 comet, from which we retrieved material that we've analayzed on Earth, has a nitrogen isotope ratio that strongly suggests it's a comet formed around another star. It also has the amino acid glycine.
2) Impact ejecta from large bodies like Earth can make it into orbit. We have Martian rocks here on Earth from such events having happened on Mars. Space is not hospitable, but even metazoans have survived fairly harsh exposures; for example, C. elegans worms from space shuttle Columbia experiments survived uncontrolled re-entry and were found alive on the ground weeks after the crash. They weren't even protected inside large space rocks. Some exobiologists expect that for this reason, if we do find life elsewhere in the solar system, it will be related to life on Earth (ejected and diffused during the Archaean?) essentially a long-lost branch of archaebacteria. While such a process would be less likely, i.e. take longer in the much greater volume of the outer solar system, if it is not less than 1 in 4,500, it has probably already occurred.
3) It is very likely that the amazingly short 66 years from first manned powered flight to first human on the Moon occurred in part because of the Moon's relative proximity. More than twice that duration has now elapsed and we are still only in the talking stages about a landing on Mars. A species that has the good fortune to "come of age" in terms of space faring technology, when the next closest star is a mere 0.82 light years from their own, has an easier task of proving the possibility of interstellar space flight than we do, coming of age when we're about equidistant from everything. We might therefore narrow our search for intelligent life to super-Earths around sun-like stars with close neighbors. (Of course, there is also a not-unreasonable argument to be made that close passes, or any distant large bodies disturbing the local Oort cloud, increase the chance of major impact events and decrease the chance of the kind of complexity developing that would allow long-distance space travel.) It's worth noting that in view of the high frequently of close passes, a problem for the Oort shower hypothesis of mass extinctions is that it does not happen more frequently, i.e., every million years.)
4) Recalling that our fastest spacecraft have all used gravitational slingshot maneuvers - while we might speculate wildly about the amazing propulsion technology visiting aliens would have, we can be 100% certain that they will have gravity maneuvers at their disposal, because we use it. It's easy and cheap (free, really.) Therefore, there may be interstellar "backwaters" that will not necessarily be empty spots in the galaxy, but places that are difficult to approach from nearby stars and then slingshot away from to another nearby star. If you're a species in such a backwater, you're not going to get visited very much, and you'll ask "Where is everybody?" As the stars shift, your interstellar geography status may change quickly, within a few thousand years. One of the problems with detecting aliens, particularly well-advanced ones, is we don't know what we're looking for. We may be looking right at evidence of their existence and miss it because they don't use our provincial communication methods, or because we're used to it and we explain it in terms of the background operation of "dumb matter". Or, the periodic mass extinctions that are sometimes claimed to be associated with close passes could in fact be associated with close passes - but because of an ecosystem-collapsing alien visitation as Stephen Hawking envisions, rather than because of Oort cloud impactors.
Previous post on alien evolution, There's (at least) a 1-in-3 Chance of Life on Europa
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