A Survival Imperative for Space Colonization

Xman

In 1993, J. Richard Gott III computed with scientific certainty that humanity would survive at least 5,100 more years. At the time, I took that as reason to relax, but Dr. Gott has now convinced me I was wrong. He has issued a wake-up call: To ensure our long-term survival, we need to get a colony up and running on Mars within 46 years.

If you’re not awakened yet, I understand. It’s only prudent to be skeptical of people who make scientific forecasts about the end of humanity. Dr. Gott, a professor of astrophysics at Princeton, got plenty of grief after he made his original prediction in 1993. But in the ensuing 14 years, his prophetic credentials have strengthened, and not merely because humanity is still around.

Dr. Gott has used his technique to successfully forecast the longevity of Broadway plays, newspapers, dogs and, most recently, the tenure in office of hundreds of political leaders around the world. He bases predictions on just one bit of data, how long something has lasted already; and on one assumption, that there is nothing special about the particular moment that you’re observing this phenomenon. This assumption is called the Copernican Principle, after the astronomer who assumed he wasn’t seeing the universe from a special spot in the center.

Suppose you want to forecast the political longevity of the leader of a foreign country, and you know nothing about her country except that she has just finished her 39th week in power. What are the odds that she’ll leave office in her 40th week? According to the Copernican Principle, there’s nothing special about this week, so there’s only a 1-in-40 chance, or 2.5 percent, that she’s now in the final week of her tenure.

It’s equally unlikely that she’s still at the very beginning of her tenure. If she were just completing the first 2.5 percent of her time in power, that would mean her remaining time would be 39 times as long as the period she’s already served — 1,521 more weeks (a little more than 29 years).

So you can now confidently forecast that she will stay in power at least one more week but not as long as 1,521 weeks. The odds of your being wrong are 2.5 percent on the short end and 2.5 percent on the long end — a total of just 5 percent, which means that your forecast has an expected accuracy of 95 percent, the scientific standard for statistical significance.

And you can apply this Copernican formula to lots of other phenomena by assuming they’re neither in the first 2.5 percent nor the final 2.5 percent of their life spans.

Now, that range is so broad it may not seem terribly useful to you, and Dr. Gott readily concedes that his Copernican formula often is not the ideal method. The best the formula could do regarding Bill Clinton, who had been president for 127 days when the 1993 paper in Nature was published, was predict he would serve at least three more days but not more than 13.6 more years. You could have gotten a narrower range by using other information, like actuarial data from previous presidencies, or factoring in the unlikelihood that the Constitution would be changed so he could serve more than two terms.

But the beauty of the Copernican formula is that it allows you to make predictions when you don’t have any other information, which is how Dr. Gott managed to predict the tenure of virtually every other nation’s leader that day in 1993 — a total of 313 leaders. If none of those still in power stays in office beyond age 100, Dr. Gott’s accuracy rate will turn out to be almost exactly 95 percent.

Some philosophers and experts in probability theory have argued that Dr. Gott is making unwarranted deductions from past life spans, and that it is wrong to assume there is nothing special about the moment we’ve chosen to make a forecast. (See www.tierneylab.com for details of the debate.) But last year two philosophers, Bradley Monton and Brian Kierland, analyzed the criticisms and concluded in an article in the Philosophical Quarterly that Dr. Gott had indeed come up with a useful tool for difficult situations — like trying to forecast doomsday without data from other planets.

The Copernican formula predicts, based solely on our 200,000-year track record, that the human race is likely to survive at least 5,100 more years but not longer than 7.8 million — roughly the same prediction you’d make based on the longevity of past mammals on Earth, Dr. Gott says.

That upper limit is a disappointment to those of us who imagine humans multiplying across the universe for billions of years. Dr. Gott doesn’t rule out that possibility, but the Copernican Principle makes him conclude it is unlikely.

After all, if colonization is common and there’s nothing special about our civilization, why haven’t we already colonized other worlds? Why aren’t we colonists ourselves from a civilization somewhere else?

If you think of yourself as a randomly chosen individual among all the intelligent beings who ever lived in the universe, then the odds are you’re living in one of the larger and older civilizations — simply because a lot more people have lived in those than in small, short-lived civilizations.

“The sobering facts,” Dr. Gott says, “are that in a 13.7 billion-year-old universe, we’ve only been around 200,000 years, and we’re only on one tiny planet. The Copernican answer to Enrico Fermi’s famous question — Where are the extraterrestrials? — is that a significant fraction must be sitting on their home planets.”

It might seem hard to imagine that humans would invent rockets and then never use them to settle other worlds, but Dr. Gott notes that past civilizations, notably China, abandoned exploration. He also notes that humans have been going into space for only 46 years — a worrisomely low number when using Copernican logic to forecast the human spaceflight program’s longevity.

Since there’s a 50 percent chance that we’re already in the second half of the space program’s total lifespan, Dr. Gott figures there is a 50 percent chance it will not last more than another 46 years. Maybe the reason civilizations don’t get around to colonizing other planets is that there’s a narrow window when they have the tools, population and will to do so, and the window usually closes on them. “In 1970 everyone figured we’d have humans on Mars by now, but we haven’t taken the opportunity,” Dr. Gott says. “We should it do soon, because colonizing other worlds is our best chance to hedge our bets and improve the survival prospects of our species. Sooner or later something will get us if we stay on one planet. By the time we’re in trouble and wish we had that colony on Mars, it may be too late.”

You could argue that he’s being too pessimistic about space exploration. The space program may be only 46 years old, but humans have been exploring new territory for tens of thousands of years, so by Copernican logic perhaps they’ll keep it doing it far into the future. But given recent trends — after going to the Moon, we now barely send humans into orbit — he’s right to be worried.

If it’s true that civilizations normally go extinct because they get stuck on their home planets, then the odds are against us, but there’s nothing inevitable about the Copernican Principle. Earthlings could make themselves the statistical anomaly. When extinction is the norm, you may as well try to be special.

http://www.nytimes.com/2007/07/1 ... gewanted=2&_r=1

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