April 17, 2013  
Is Anybody Out There?

Maybe not. When Enrico Fermi and associates casually mused about the probability of extra-terrestrial intelligence, Fermi asked "Where are they?". He quickly calculated that they should have been here by now and yet they are not. This is known as the Fermi paradox.

Enrico Fermi, extraordinary physicist

First of all, let's dispense with the science fiction paradigm of space travel. Nothing can travel at the speed of light and there isn't any reason whatsoever to think that we could create worm-holes in space or travel at warp speed. If we voyage among the stars, it is definitely going to take a while. The nearest star system is 25 trillion, 666 million miles away. Voyager 1 holds the speed record for flight out of the solar system at 38,400 mph. At that speed, Alpha Centauri is only 76,300 years away (Voyager is not headed that way). Let's assume an advanced civilization manages to travel about a thousand times faster or five to ten percent of the speed of light.

I believe Fermi guessed that an advanced civilization might reach and colonize a nearby star system in 500 years, before moving on. I'm going to be more conservative and use 10,000 years. It seems to me that interstellar planet hopping would take hundreds of years with robotic probes before investing in a human expedition. This assumes advances that cut travel time down to five or six decades. It would be amazing to arrive at a nearby star system and find a planet perfectly suitable to our needs, so we will likely need to make modifications. Perhaps we'll need to redirect a few comets onto collision course in order to add water, for example. And we'd deploy tailored made microbes to adjust atmospheric conditions. It would take a while before even an advanced civilization could push outward to yet another star.

If the galactic empire were centered on Earth, and assuming an average advance of 10 light years, it would take 500,000 years to reach the top and bottom of the galactic disk. The closet edge of the galactic disk is about 25,000 light years away and would be colonized within 25 million years. And the far edge of the galaxy would be part of the dominion by 75 million years. There would be time to infiltrate every corner of the galaxy in less than 100 million years. The metal rich (those elements besides hydrogen and helium) galactic disk is about 8 billion years old. The earth formed 4.6 billion years ago and had more than enough heavy elements to form life. If we were to make the extremely conservative assumption that there weren't enough metals for life to form until 5 billion years ago and that intelligent life takes 4.6 billion years to evolve, then there would still have been time for the galaxy to have been explored four times over. A less conservative view would mean that the galaxy should have fully inhabited scores of times. So where are they?

The answer could be that intelligent life is either rare or transient. It could be that other intelligent life has no innate desire to explore. Or perhaps we are quarantined until we tame our violent nature. Maybe interstellar travel is impractical no matter how advanced the civilization. Or maybe we are the most advanced. One of the more troubling possibilities is that the galaxy is a dangerous place and the continuity of life is unlikely -- against the odds. We've had a number of extinction events including chunks of rocks from space. There is speculation that some could have been caused by a gamma ray burst from a nearby supernova which toasted our ozone layer. A close encounter with another star could disrupt the Oort cloud sending tens of thousands of comets to visit the inner solar system. A close encounter with an OB association of blue hot stars (like the Orion Nebula stars) during our 250 million orbit about the galaxy would be very uncomfortable for life on earth.

If our galaxy is a dangerous place and if intelligent life is quite rare, then there are two clear imperatives: 1. Understand all those things that could threaten us with extinction, 2. Get some of us off this rock and settled in another stellar system as soon as possible. Life's prime directive is that life must continue. In order to survive we must eventually reach the stars. Certainly none of those living today will ever set foot on an exoplanet. And perhaps we have a billion years under a gradually warming sun to extend life beyond our solar system. But we must consider the possibility that our window of opportunity could be much smaller than we currently imagine. We individually will never know all the wonders that lie beyond, but our species has a chance at salvation and (nearly) eternal life as we claim for ourselves the heavens.