1. That's What They Thought 100 Years Ago.

Nine times out of 10, when I give my end of science spiel÷whether to a Nobel laureate in physics or to some poor soul that I'm trapped at a cocktail party÷the response is some variation of, "Oh, come on, that's what they thought 100 years ago." The reasoning behind this response goes like this: As the 19th century wound down, scientists thought they knew everything. But then Einstein and other physicists discovered relativity and quantum mechanics, opening up vast new vistas for modern physics and other branches of science.

The moral is that anyone who predicts science is ending will surely turn out to be as short-sighted as those 19th-century physicists were. Another popular anecdote involves the U.S. patent commissioner who, sometime in the 19th century, supposedly quit his job because he thought everything had been invented.

First of all, both of these tales are simply not true. No American patent official ever quit his job because he thought everything had been invented. And physicists at the end of the last century were engaged in debating all sorts of profound issues, such as whether atoms really exist.

What people are really implying when they say "that's what they thought 100 years ago" is that, because science has advanced so rapidly over the past century or so, it can and will continue to do so, possibly forever. This is an inductive argument, and as an inductive argument it is deeply flawed. Science in the modern sense has only existed for a few hundred years, and its most spectacular achievements have occurred within the last century. Because we were all born and raised in this era of exponential progress, we simply assume that it is an intrinsic, permanent feature of reality.

But viewed from an historical perspective, the modern era of rapid scientific and technological progress appears to be not a permanent feature of reality but an aberration, a fluke, a product of a singular convergence of social, intellectual and political factors. Ask yourself this: Is it really more reasonable to assume that this period of extremely rapid progress will continue forever rather than reaching its natural limits and coming to an end?

2. Answers Always Raise New Questions.

It is quite true that answers always raise new questions. But most of the answerable questions raised by our current theories tend to involve details. For example, when, exactly, did our ancestors begin walking upright? Was it three million years ago, or four million? On which chromosome does the gene for cystic fibrosis reside? The answers to such questions may be fascinating, or have enormous practical value, but they merely extend the prevailing paradigm rather than yielding profound new insights into nature

Other questions are profound but unanswerable. The big bang theory, for example, poses a very obvious and deep question: Why did the big bang happen in the first place, and what, if anything, preceded it? The answer is that we don't know, and we will never know, because the origin of the universe is too distant from us in space and time. That is an absolute limit of science, one forced on us by our physical limitations. There are lots of other unanswerable questions. Are there other dimensions in space and time in addition to our own? Are there other universes?

Then there is a whole class of what I call inevitability questions. Just how inevitable was the universe, or the laws of physics, or life, or life intelligent enough to wonder how inevitable it was? Underlying all these questions is the biggest question of all: Why is there something rather than nothing? None of these inevitability questions are answerable. You can't determine the probability of the universe or of life on earth when you have only one universe and one history of life to contemplate. Statistics require more than one data point. So, again, it is true that answers always raise new questions. But that does not mean that science will never end. It only means that science can never answer all possible questions, it can never quench our curiosity, it can never be complete.

Unanswerable questions, by the way, are what give rise to superstring theory, Gaia, psychoanalysis and other example of ironic science, as well as all of philosophy.

3. What About Life on Mars?

The day the life on Mars story broke last August, I walked into my office at Scientific American, and several colleagues immediately came up to me with big smirks and said, "So, what does Mr. No More Big Discoveries say now?"

As I said in my book, the discovery of extraterrestrial life would represent one of the most thrilling findings in the history of science. I hope to live long enough to witness such an event. But the so-called evidence presented last summer doesn't even come close. It consists of some organic chemicals and globule-shaped particles that vaguely resemble terrestrial microbes but which are subject to many alternative interpretations. Those scientists who are most knowledgeable about very old microfossils÷those who are the real experts in the origin of terrestrial life÷are also the most skeptical of the life-on-Mars interpretation. That's a very bad sign.

There is only one way we are going to know if there is life on Mars, and that is if we send a mission there to conduct a thorough search for it. Our best hope is to have a human crew drill deep below the surface, where there is thought to be enough liquid water and heat to sustain microbial life as we know it. It will be decades, at least, before we can muster the resources and money for such a project, even if society is willing to pay for it.

Let's say that we do eventually determine that microbial life existed or still exists on Mars. That would be fantastic, an enormous boost for origin-of-life studies and biology in general. But would it mean that science is suddenly liberated from all the limits that I have described? Hardly. If we find life on Mars, we will know that life arose in this solar system, and perhaps not even more than once. It may be that life originated on Mars and then spread to the earth, or vice versa.

More importantly, we will be just as ignorant about whether life exists elsewhere in the universe, and we will still be facing huge obstacles to answering that question. Let's say that engineers come up with a space transport method that boosts the velocity of spaceships by a factor of more than 10, to one million miles an hour. That spaceship would still require 3,000 years to reach the nearest star, Alpha Centauri.

It's possible that one of these days the radio receivers employed in our Search for Extraterrestrial Intelligence program, called SETI, will pick up electromagnetic signals÷the alien equivalent of Seinfeld÷coming from another star. But it's worth noting that most of the SETI proponents are physicists, who have an extremely deterministic view of reality. Physicists think that the existence of a highly technological civilization here on earth makes the existence of similar civilizations elsewhere highly probable.

The real experts on life, biologists, find this view ludicrous, because they know how much contingency÷just plain luck÷is involved in evolution. Stephen Jay Gould, the Harvard paleontologist, has said that if the great experiment of life were re-run a million times over, chances are it would never again give rise to mammals, let alone mammals intelligent enough to invent television.

For similar reasons Gould's colleague Ernst Mayr, who may be this century's most eminent evolutionary biologist, has called the search for extra-terrestrial life a waste of time and money. The U.S. Congress apparently agrees with Mayr, because they terminated the funding for the SETI program three years ago. It's now just getting by on private funds.