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New Paper Offers Additional Reasoning for Fermi's Paradox
KentuckyFC writes "If the universe is teeming with advanced civilizations capable of communicating over interstellar distances, then surely we ought to have seen them by now. That's the gist of a paradoxical line of reasoning put forward by the physicist Enrico Fermi in 1950. The so-called Fermi Paradox has haunted SETI researchers ever since. Not least because if the number of intelligent civilizations capable of communication in our galaxy is greater than 1, then we should eventually hear from them. Now one astrophysicist says this thinking fails to take into account the limit to how far a signal from ET can travel before it becomes too faint to hear. Factor that in and everything changes. Assuming the average communicating civilization has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 communicating civilizations in the Milky Way to ensure that you'll see one of them. Any less than that and the chances are that they'll live out their days entirely ignorant of each other's existence. Paradox solved, right?"
Did you read the summary? The point is that outside of our galaxy no intelligible signal is going to reach us. Therefore, the rest of the universe doesn't even enter into it.
You are using English. Please learn the difference between loose and lose; they're, there, and their; your and you're.
(See this wiki article as an example of a slightly technical description of why)
Think about the transit time, think about the number that would be lost. You can't really assume a straight geometric progression for something so incredibly fraught.
Well, almost, at least for the purposes of ballpark calculations.
Now, we have to make a couple of assumptions -- such as that they have the technology to send out self-replicators that will last long enough to get to the next star, which is a function of speed and durability. For the sake of argument, let's assume that the Voyager spacecraft (which just left the Solar System) are capable of self-replication, have a very long-lived power supply (long half-life radioisotope, for example) and their electronics will survive long exposure to galactic cosmic rays. (All big assumptions, but imaginably within range of our technology.)
Also assume an average spacing of about five light years apart for stars.
At the current speed (about 16 km/sec), it would take a Voyager about 90,000 years to reach the next start. Allow 10,000 years for the laborious process of self-replicating from raw materials and launching another of itself on its way, for a total of 100,000 years per generation. Assume each vehicle replicates itself only twice, and stays put (perhaps assembling large black monoliths on the local planets for the mystification of any eventual inhabitants). So we have a doubling rate of once per 0.1 million years.
Assume about 100 billion stars in our galaxy (this is the number I found most frequently mentioned), it would take between 36 and 37 doublings to send a probe to every star in the galaxy (less because stars are closer nearer the core). Call it 40 to allow for probe loss.
So in a mere four million years, self-replicating probes travelling no faster than Voyager could visit every star in the galaxy -- except for the speed problem. That growth rate can be maintained initially, but like any spreading colony (such as bacteria in a petri dish) the edge of the colony can only advance at a certain speed, and the doubling rate has to fall off (it's ludicrous to think that the number of visited stars could go from half the galaxy to the whole galaxy in a mere 100,000 years, the probes would have to be approaching lightspeed for that).
Take the galaxy diameter as 100,000 light years, it'd take nearly 2 billion years for a Voyager-speed probe to cross it, or near 3 billion to go around half the circumference (to avoid the black hole at the core). The galaxy is old enough that there probably sun-like stars (our Sun being a second-generation star, necessary if you want enough heavy elements for terrestrial planet formation) a couple of billion years older than ours. (And if we assume faster travel speed, say 0.01 c instead of 0.000055 c, the numbers get a lot better.)
So Fermi's question was simply "where are they?". If they're really not around (vs simply ignoring us or being undetectable to us), then the above assumptions are too optimistic.
-- Alastair
The Catholic Church also accepts evolution as "how God did it" so good luck trying to get through to the Creationists who obviously don't want anything to do with St Peter's successor.