|MadSci Network: Astronomy|
Hi, Martin. You have certainly asked a very speculative question. As with most speculative questions, the answer is "it depends."
Let's figure out a few numbers first of all. A pretty good estimate of the galaxy-wide average distance between sun-like stars is 10 lightyears. Assuming that sun-like stars close to the center of the galaxy have an average separation distance of 1000 AU, I figure that sun-like stars at the very rim of the galaxy (100,000 lightyears from the center) are separated, on average, by about 300 lightyears. By a similar calculation, sun-like stars in our neck of the woods (about 1/3 of the way from the center to the rim) are separated on average by 10 or so lightyears.
So here we run into the first problem answering your question. This is the issue of where our hypothetical alien race's home star is located. Obviously, aliens who start out close to the center of the galaxy have a much easier time colonizing neighboring star systems. Let's assume for the sake of argument that these aliens live in a part of the galaxy that is about average.
The second issue is: given that a nearby star our little green men want to colonize is "sun-like," what's the likelihood that it has one or more planets that are "earth-like," in the sense that the aliens would be able to live on them? The problem is, we don't know what fraction of sun-like stars have earth-like planets. Our current technology can't yet detect planets the size of earth around other stars. So we have to make another assumption here. Let's be optimistic and say that every sun-like star has one habitable planet. The actual number is probably somewhat lower, but you can adjust the figure up or down to suit your tastes.
The third question to answer is: how fast can our aliens travel? Of course, everything in the universe is limited to the speed of light or less. However, as a practical matter the maximum speed at which one can travel through space is probably much lower. To get some feel for this, the Apollo spacecraft traveled about 11,000 meters per second. (The speed of light in a vacuum is about 300 million meters per second, for comparison.) Let's assume that our aliens can achieve velocities two orders of magnitude (100 times) higher than that achieved by the spacecraft in the Apollo program.
Okay. So now we have a hypothetical race of little green men living somewhere in our galaxy. They can travel at speed of approximately 1,100,000 meters per second, or 2.5 million miles per hour. This is approximately 0.37% of the speed of light. Let's assume that our aliens live about halfway out from the center of the galaxy towards the rim. We can safely assume that stars here have separation distances about equal to the galaxy-wide average, which I stated earlier was about 10 lightyears. Therefore, the travel time from one star to another is, on average, about 2725 years. (Of course, it's possible that the aliens might travel 10 or even 100 times faster than this, but the amount of energy they would have to expend in order to do this would be phenomenal. Here is a description of the Saturn V rockets used to launch the Apollo spacecraft to the Moon. As you can see, accelerating the payload to 11,000 meters per second required a mass of fuel more than 50 times higher than the mass of the payload. Even getting to our velocity of 1 million meters per second would require a tremendous amount of fuel, and at velocities not much higher than this, relativistic effects such as increasing mass with increasing velocity start to become important.)
So, could these aliens colonize anything? Well, yes. The amount of fuel required to send people to another star using a conventional chemical rocket is extremely large, but it is not infinite. One could posit some circumstances (such as extreme overpopulation or environmental catastrophes) which could cause the civilization to want to pick up and put down roots on another planet. So, as you stated in your question, settlement of other worlds is possible. It certainly takes a very long time and is very energy-expensive. However, advanced propulsion methods might be able to solve one or both of these problems. The question is whether any type of civilization could be maintained between the two worlds. My initial inclination is to say no - after all, since the stars we are talking about are ten lightyears apart, any roundtrip communication would take twenty years. To me, this seems too long for any meaningful connection between the two worlds, since any important decisions requiring input from the other planet would have to made long before the reply could be received. On the other hand, who knows? Maybe these aliens have life spans 100 times longer than ours and live at a proportionally slower speed. Suddenly, 20 years doesn't seem like such a long time.
All of the above is a very long-winded way of saying that I don't know the answer to your question. No one does. But some back-of-the-envelope calculations show that, while it is unlikely that any civilization made up of aliens who were much like human beings could be maintained across multiple star systems, it is at least possible.
I hope that answers your question, or at least goes a long ways toward helping you form an educated opinion.
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