A fascinating calculation about how much longer humanity will survive is conducted by an astrophysicist at Princeton University.

January 31, 2007,  2:41 pm

Isn’t That Special? Copernicus Meets Doomsday

By John Tierney

Tags: bets, Copernican Principle, doomsday

With apologies to Dr. Johnson: When a reader knows the world is about to end, it concentrates his mind wonderfully. Or at least inspires him to post a comment. My bet that humanity will avert doomsday provoked plenty of intelligent disagreement. But there were a few optimists, too, and my favorite argument is the one by M. Skinner:

Imminent predictions of doomsday have been rife throughout human history. The unwritten subtext is that humanity deserves to be destroyed. This predilection with our self-destruction is just another manifestation of humanity’s hubris. It is the delusion we are so powerful we can cause the destruction of our entire species. In the past, we envisioned ourselves literally at the center of the universe. Now we see the unfamiliar and rapidly changing world we have created and are convinced that this will so offend the “natural order” that surely it will be our demise. We still have not quite grasped the fact that we are just not that powerful, but creating a better life, at least on a small scale, is something we have proven we can do.

A formal version of this “we’re not special” argument is called the Copernican Principle, which has been used to calculate how much longer humanity will survive — supposedly with 95-percent probability of being right, the scientific standard for precision. I know that sounds wacky, but it’s logical enough to have been published in the journal Nature.

The calculations were made by J. Richard Gott 3d, an astrophysicist at Princeton University. He reasoned that, just as we don’t occupy a special place in the universe, we also probably don’t occupy a special point in the history of our species. He assumed we’ve been around 200,000 years, and that there’s a 95-percent probability we’re neither in the first 2.5 percent of our species’ history nor in the final 2.5 percent. As Malcolm Browne explained it in The Times:

Based on this seemingly simple idea, Dr. Gott predicted that intelligent human beings would probably exist on Earth a maximum of 7.8 million years, a minimum of 5,128 years, or somewhere in between. This calculation, he said, requires no knowledge of how or why the human race might end, and is based solely on the fact that Homo sapiens first appeared on Earth about 200,000 years ago.

The prediction, he said, is in good agreement with the life spans of many comparable species, including Homo erectus, an ancestor of modern man, which lasted about 1.8 million years before becoming extinct. Predictions of this kind are a statistical way of saying that the longer something has been around, the longer it is likely to survive in the future.

Critics of this approach say it’s simplistic to pretend the only thing we know about our species is how long it’s been around. The Times’ James Glanz consulted with Carlton M. Caves, a physicist at the University of New Mexico, who did his own analysis and said of Gott: “Put succinctly, he rejects as irrelevant the process of rational, scientific inquiry, replacing it with a single, universal rule. That has to be wrong.”

But how do you make a “rational, scientific inquiry” into the odds of our species suriving? One possible approach would be to look at history and compute how long the average species has endured. But then how do you define your sample of species? If you’re looking only at dominant species, you could take heart from estimates that a dominant species goes extinct less than once every million years. (For more on the long debate on how to apply the Copernican Principle to humanity’s prospects, see Wikipedia’s article on the Doomsday argument.)

Doomsayers like to think they’ve made a rational, scientific inquiry and discovered that the past is an unreliable guide: Sure, we’ve survived all these millennia, but we’ve never faced global threats like the ones today. But I share M. Skinner’s belief that they’re being hubristic in assigning themselves such a special place in history: the first humans ever to accurately foresee the end. It’s always possible they’re right. It seems far more probable they’re like all the past prophets of doom who mistakenly thought they were special, too.

http://tierneylab.blogs.nytimes.com/2007/01/31/isnt-that-special-copernicus-meets-doomsday/