In 1993, J. Richard Gott III computed with scientific certainty that humanity would survive at least 5,100 more years. At the time, we took that as reason to relax, but Dr. Gott has now convinced us that we were 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 we 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.
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.