The flip side is that we are crunchy and taste good with Sriracha sauce?

Not everyone agrees that finding life elsewhere would be good news.

That would be Nick Bostrom, a philosopher at the University of Oxford and director of the Future of Humanity Institute there, one of the great pessimists of this or any other age.

In an article published in Technology Review in 2008, Professor Bostrom declared that it would be a really bad sign for the future of humanity if we found even a microbe clinging to a rock on Mars. “Dead rocks and lifeless sands would lift my spirit,” he wrote.

Why?

It goes back to a lunch in 1950 in Los Alamos, N.M., the birthplace of the atomic bomb. The subject was flying saucers and interstellar travel. The physicist Enrico Fermi blurted out a question that has become famous among astronomers: “Where is everybody?”

The fact that there was no evidence outside supermarket tabloids that aliens had ever visited Earth convinced Fermi that interstellar travel was impossible. It would simply take too long to get anywhere.

The argument was expanded by scientists like Michael Hart and Frank Tipler, who concluded that extraterrestrial technological civilizations simply didn’t exist.

The logic is simple. Imagine that one million years from now Earthlings launch a robot to Alpha Centauri, the closest star system to our own. It gets there in a few years, and a million years later sends off probes to two other star systems. A million years after that, each of those sends off two more probes. Even allowing for generous travel times, in 100 million years roughly a nonillion stars (1030) could be visited. The galaxy contains maybe 200 billion stars, so each could be visited more than a trillion times in this robot crisscrossing.

The interstellar probe part of this is not so crazy, by the way. Serious people are already contemplating sending a probe to another star, using technology that could be achievable in the near future. See, for example, the Defense Advanced Research Projects Agency and its 100-Year Starship Study.

There are billions of potentially habitable planets in the galaxy, moreover. If only a small fraction of these develop life and technology, that would be enough to turn the whole galaxy into Times Square.

The Milky Way is 10 billion years old. So where are those aliens or their artifacts? We’ve found zilch. If life is so easy, someone from somewhere must have come calling by now. This is known as the Fermi paradox.

There are many loopholes in this argument, including the possibility that we wouldn’t sufficiently recognize alien life if it camped in our front yards. The simplest explanation, Dr. Bostrom and others say, is that there are no other spacefaring civilizations.

There must be something, he concludes, that either stops life from starting at all, or shuts it down before it can conquer the stars. He calls it the Great Filter.

You can imagine all kinds of bottlenecks in the evolution of life and civilization — from the need for atoms to first combine into strands of RNA, the genetic molecule that plays Robin to DNA’s Batman, to nuclear war, climate change or a mishap of genetic engineering — that could constitute a Great Filter.

The big question for Professor Bostrom is whether the Great Filter is in our past or our future, and for the answer he looks to the stars. If there is nothing else out there, then maybe we have survived whatever it is. As bizarre as it sounds, we are the first ones in the neighborhood to have run the cosmic obstacle course.

If there is company out there, it means the Great Filter is ahead of us. We are doomed.

This is a staggeringly existential piece of knowledge to have obtained at what seems to be a tender age as a species, based on a cursory examination of a sliver of our cosmic neighborhood. It is also a truly brave exercise of the power of human reason.

Why IS the sky dark at night?

There is a precedent of sorts in an old riddle known as Olbers’ Paradox, after Heinrich Wilhelm Olbers, a 19th-century amateur astronomer who enunciated a problem that had bothered some astronomers since the 16th century: Why is the sky dark at night? In an infinite eternal universe, every line of sight would end on a star, the thinking went, and even dust clouds would glow as bright as day.

Luminaries as disparate as the Scottish physicist Lord Kelvin and the writer Edgar Allan Poe suggested that the dark night sky was a clue to the fact that the universe is finite, at least in time, and had a beginning, a notion now cemented by the Big Bang.

If Olbers saw the dawn of time, perhaps Fermi and Bostrom have seen the sunset. We should hardly be surprised. Nothing lasts forever. The fathers of SETI, Carl Sagan and Frank Drake, stressed that a key unknown element in their equations was the average lifetime of technological civilizations. Too short a lifetime would eliminate the possibility of overlapping civilizations. Forget about the mythical brotherhood of the galaxy. The Klingons left the building long ago.

The best we could have hoped for was to be another evolutionary phase in the zigzag development of earthly life on the way to who knows what. But in a few billion years, the sun will die, and so will the earth, and our descendants — if they are still on it. The universe will not remember us or Shakespeare or Homer.

We can’t really blame Professor Bostrom for that. But he has a history of disturbing thoughts. In 2003, he argued that we were probably all living in a computer simulation, something he said would be easy for “technologically mature” civilizations to do.

What his and other sci-fi-style calculations have in common is that they are extrapolations, of the doubling of chip capacity decreed by Moore’s Law in the case of computer simulations, or the doubling of space probes over the eons. Believe them at your peril. Chips can’t get smaller forever. Untended machines far, far from home break or forget why they are there. Apple can’t keep doubling iPhone sales eternally.

As the great science writer and biologist Lewis Thomas liked to say, we are an ignorant species. This is why we do the experiment.