3

WHAT WE’LL
NEVER KNOW

While struggling to prepare my lectures for BBC radio, I had a fantasy. Suppose I had a time machine. I could fast forward into the future, turn on the radio, listen to the broadcast version, take notes—and then reverse back to the present and start writing. Well, there was plainly no such quick fix—but could there ever be?

Arthur C. Clarke noted that any sufficiently advanced technology is indistinguishable from magic. We can’t now envision what artifacts might exist centuries hence, any more than a Roman could imagine today’s SatNav and mobile phones. Nevertheless, a physicist would confidently assert that time machines will remain forever fiction. That’s because changing the past would lead to paradoxes—­infanticide would violate logic as well as ethics if the victim was your grandmother. So, what is the demarcation between concepts that seem crazy now but might be realized eventually and things that are forever impossible? Are there limits to how much we can ever predict? Are there scientific problems that will forever baffle us—phenomena that simply transcend human understanding?

Einstein averred that “the most incomprehensible thing about the universe is that it is comprehensible.” He was right to be astonished. Our minds evolved to cope with life on the African savannah, but they can also comprehend the microworld of atoms and the vastness of the cosmos. We marvel at the fact that the universe isn’t anarchic—that atoms obey the same laws in distant galaxies as in the lab. Our cosmic horizons have vastly enlarged. Our Sun is one of a hundred billion stars in our galaxy, which is itself one of many billion galaxies in range of our telescopes. And this entire panorama emerged from a hot, dense beginning nearly 14 billion years ago. Some inferences about the early universe are as evidence-based as anything a geologist might tell you about the history of our Earth; we can make confident and precise statements about how hot and dense things were in the first few seconds of our universe’s expansion, even just a microsecond after the big bang. But, as always in science, each advance brings into focus some new questions that couldn’t previously have even been posed. We now confront the mystery of the very beginning (if indeed there was one). It’s a mystery because, right back in the first tiny fraction of a second, conditions would have been far hotter and denser than we can simulate in the lab. We don’t know the physical laws that then prevailed, so we lose any foothold in experiment.

Einstein himself made one of the biggest advances in our comprehension. More than 200 years before him, Isaac Newton had shown that the gravity that makes apples fall is the same force that holds planets in their orbits. Einstein went much further. He didn’t prove Newton wrong—Newton’s mathematics is good enough to program space flights to distant planets—but he transcended Newton by offering insights into gravity that made it seem more natural and linked it to the nature of space and time, and the universe itself.

The other great twentieth-century intellectual revolution, quantum mechanics, tells us, completely counter to all intuition, that on the atomic scale nature is intrinsically fuzzy; nonetheless, atoms behave in precise mathematical ways when they emit and absorb light, or link together to make molecules. Quantum theory is the basis for much of modern technology: it is vindicated every time you take a digital photograph, surf the Internet, or use any gadget—a DVD player, or a supermarket bar code—that involves a laser. Quantum mechanics fully matched the importance of Einstein’s work, but was a collective rather than individual effort.

General relativity and quantum theory are the twin pillars of twentieth-century physics, but at the deepest level they contradict each other—they haven’t yet been meshed into a single unified theory. In most contexts, this does not impede us because their domains of relevance do not overlap. Astronomers can ignore quantum fuzziness when calculating the motions of planets and stars. Conversely, chemists can safely ignore gravitational forces between individual atoms in a molecule because these are nearly forty powers of ten feebler than electrical forces. But during the very earliest instants after the big bang, when everything was squeezed smaller than a single atom, quantum fluctuations could shake the entire universe.

Mysteries of the cosmos and microworld

To confront the overwhelming mystery of what banged and why it banged, Einstein’s theory isn’t enough because it treats space and time as smooth and continuous. Success will require new insights into what might seem the simplest entity of all: “mere” empty space. We know that no material can be chopped into arbitrarily small pieces: eventually you get down to discrete atoms. Likewise, even space and time can’t be divided up indefinitely. There are powerful reasons to suspect that space has a grainy and atomic structure—but on a scale a trillion trillion times smaller than atoms. This is key unfinished business for twenty-first-century science. According to the most favored theory the fundamental entities are not points but tiny loops, or strings, and the various subnuclear particles are different modes of vibration—different harmonics—of these strings. The particles that physicists study are “woven” from space itself. Moreover, these strings are vibrating not in our ordinary space (with three spatial dimensions, plus time) but in a space of ten or eleven dimensions.

We are three-dimensional beings: we can go left or right, forward or backward, up or down, and that is all. So how are the extra dimensions, if they exist, concealed from us? This may be because they are wrapped up tightly. A long hose-pipe may look like a line (with just one dimension) when viewed from a distance, but from closer up we realize that it is a long cylinder (a two-dimensional surface) rolled up tightly; from still closer, we realize that this cylinder is made from material that isn’t infinitely thin, but extends in a third dimension. By analogy, every apparent point in our three-dimensional space, if hugely magnified, may actually have some complex structure: a tightly wound origami in six extra dimensions. Some of the extra dimensions may be more loosely wrapped, so that their effects show up on a microscopic scale in laboratory experiments: indeed there are optimists who believe that the new Large Hadron Collider in Geneva could reveal clues to these extra dimensions.

The microstructure of space manifests itself on scales far smaller than any we can directly probe. Likewise, at the other extreme, our cosmological theories offer intimations that the universe is vastly—perhaps even infinitely—more extensive than the patch we can observe with our telescopes. The domain that astronomers call “the universe”­—the space, extending more than 10 billion light-years around us and containing billions of galaxies, each with billions of stars, billions of planets (and maybe billions of biospheres)—could be an infinitesimal part of the totality. There is a definite horizon to direct observations: a spherical shell around us, such that no light from beyond it has had time to reach us since the big bang. But there is nothing physical about this horizon, any more than there is anything special about the horizon here on the Earth. If you were in the middle of an ocean, it’s not likely that, just beyond your horizon, the water actually ends. And there are reasons to suspect that our universe—the aftermath of our big bang—extends hugely further than we can see. And that is not all.

Our big bang may not be the only one. Some have speculated that other universes could exist alongside ours. Imagine ants crawling around on a large sheet of paper (their two-dimensional universe). They would be unaware of a similar sheet that’s parallel to it. Likewise, there could be another entire universe (with three-dimensional space, like ours) less than a millimeter away from us, but we’d be oblivious to it if that millimeter were measured in a fourth spatial dimension, while we are imprisoned in just three.

It was perhaps self-indulgent to start this chapter with the most remote and speculative topics. But the bedrock nature of space and time, and the structure of our entire universe, are surely among science’s great open frontiers. They exemplify intellectual domains where we’re still groping for the truth—where, in the fashion of ancient cartographers, we must still inscribe “here be dragons.” A unified theory, if achieved, would complete a unification program that started with Newton, who identified the universal force of gravity, and continued through Faraday and Maxwell, who showed that electric and magnetic forces were intimately linked, and their successors. It might even realize the Pythagorean vision of reducing all nature’s complexities to geometry. Until we have such a theory we won’t understand one of the deepest mysteries that astronomy has revealed—that there is dark energy latent even in empty space, which pushes galaxies apart at an accelerating rate. And our successors will need to address questions that we can’t yet formulate: Donald Rumsfeld’s famous “unknown unknowns” (what a pity, incidentally, that Rumsfeld didn’t stick to philosophy!).

Einstein himself worked on an abortive unified theory till his dying day; in retrospect it is clear that his efforts were premature—too little was then known about the forces and particles that govern the subatomic world. Cynics have said that he might as well have gone fishing from 1920 onward, but there’s something rather noble about the way he persevered, reaching beyond his grasp. (Likewise, Francis Crick, the driving intellect behind molecular biology, shifted, when he reached sixty, to the “Everest” problems of consciousness and the brain even though he knew he’d never get near the summit.)

The cumulative advance of science requires new technology and new instruments—in symbiosis, of course, with theory and insight. The Large Hadron Collider at CERN in Geneva is the world’s biggest and most elaborate scientific instrument. Its completion in 2009 generated enthusiastic razzmatazz and wide public interest; but at the same time questions were understandably raised about why such a large investment was being made in seemingly recondite science. But what is special about this branch of science is that its practitioners in many different countries have chosen to commit much of their resources over a time span of nearly twenty years to construct and operate a single vast instrument in a Europe-led collaboration. Britain’s annual contribution amounts to about 2 percent of its overall budget for academic science, which doesn’t seem a disproportionate allocation to a field so challenging and fundamental (and in which Britain and the United States have a specially strong record, and can aspire to more than their pro rata share of the discoveries). This global collaboration on a single project to probe some of nature’s most fundamental mysteries—and push technology to its limits—is surely something in which our civilization can take pride.

Twenty-first-century challenges

We are witnessing stronger links between two frontiers of science: the very large (the cosmos) and the very small (the quantum). But only a tiny proportion of researchers are cosmologists or particle physicists. There’s a third frontier, the very complex—and that’s where 99 percent of scientists deploy their efforts. Our everyday world presents intellectual challenges just as daunting as those of the cosmos and the quantum. It may seem incongruous that scientists can make confident statements about galaxies billions of light-years away, while being baffled about issues close at hand that we all care about—diet and common diseases, for instance. But this is because our environment is so immensely complicated. Even the smallest insect, with its layer upon layer of intricate structure, is far more complex than either an atom or a star.

The different sciences are sometimes likened to successive levels of a tall building: physics on the ground floor, then chemistry, then cell biology, and all the way up to psychology­—with the economists in the penthouse. There is a corresponding hierarchy of complexity: atoms, molecules, cells, organisms, and so forth. But the analogy fails in a crucial respect. In a building, insecure foundations imperil everything above; but the higher-level sciences dealing with complex systems aren’t imperiled by an insecure base. The uncertainties of subatomic physics are irrelevant to biologists and environmentalists. To those who study how water flows—why it goes turbulent, or why waves break—it’s irrelevant that water is molecules of hydrogen and oxygen. An albatross returns to its nest after wandering 10,000 miles in the southern oceans—and it does this predictably. But it would be impossible, even in principle, to calculate this behavior “bottom up” by regarding the albatross as an assemblage of atoms.

Everything, however complicated—breaking waves, migrating birds, and tropical forests—is made of atoms and obeys the equations of quantum physics. But even if those equations could be solved, they wouldn’t offer the enlightenment that scientists seek. Each science has its own autonomous concepts and laws. Reductionism is true in a sense. But it’s seldom true in a useful sense. Problems in biology, and in environmental and human sciences, remain unsolved because it’s hard to elucidate their complexities, not because we don’t understand subatomic physics well enough.

Let us focus now on some specifics. If I were to conjecture where the scientific cutting edge will advance fastest, I’d plump for the interface between biology and engineering. Practitioners of the new science of synthetic biology can construct a genome from small stretches of DNA. And another burgeoning discipline, nanotechnology, aims to build up inorganic structures atom by atom, leading to even more compact devices that will enhance computer processing and memory and could enable nanorobots. Computers are already transformational, especially in fields where we can’t do real experiments. In the “virtual world” inside a computer, astronomers can mimic galaxy formation or crash another planet into the Earth to see if that’s how our Moon might have formed; meteorologists can simulate the atmosphere, for weather forecasts and to predict long-term climatic trends; brain scientists can simulate how neurons interact. Just as video games get more elaborate as their consoles get more powerful, so, as computer power grows, these virtual experiments become more realistic and useful.

Some things, like the orbits of the planets, can be calculated far into the future. But such cases are actually atypical. In most contexts, there’s a fundamental limit to how far ahead we can predict. That’s because tiny contingencies­—like whether a butterfly flaps its wings—have consequences that grow exponentially. For reasons like this, even the most fine-grained computation cannot normally forecast British weather even a few days ahead. (But—and this is important—this doesn’t stymie predictions of long-term climate change, nor weaken our confidence that it will be colder next January than it is in July.) So there are limits to what can ever be learned about the future, however powerful computers become. But what can we conjecture, more broadly, about how science will develop in the rest of this century?

Understanding the brain—the most complicated thing we know about in the universe—is of course a supreme challenge. Scanning techniques are revealing how our brains develop, and how our thoughts and emotions are processed. But already new debates are opening up about personal responsibility and freedom. The US National Academy of Sciences recently gave a special award for a project entitled “Neural Correlates of Admiration and Compassion.” This is scary. If scanners can reveal our emotions and obsessions, when we are sincere and when we are bluffing, that’s the ultimate invasion of our privacy.

One thing that’s changed little for millennia is human nature and human character. Before long, however, new cognition-enhancing drugs, genetics, and cyborg techniques may alter human beings themselves. That’s something qualitatively new in recorded history—and disquieting because it could portend more fundamental forms of inequality if these options were open only to a privileged few. And we are living longer. Ongoing research into the genetics of aging may explain why—indeed, a real wild card in population projections is that future generations could achieve a really substantial enhancement in lifespan. This is still speculation—mainstream researchers are cautious about the prospect of improvements that are more than incremental. (And of course whether a longer lifespan is indeed an “improvement” depends on whether it is the years of full activity or those of senile decrepitude that are prolonged.) But such caution hasn’t stopped cryonic enthusiasts, worried that they’ll die before this nirvana is reached, from bequeathing their bodies to be frozen, hoping that some future generations will resurrect them, or download their brains into a computer. For my part, I’d rather end my days in an English churchyard than a California refrigerator.

Will computers take over? Back in the 1990s, IBM’s Deep Blue beat Kasparov, the world chess champion. A mobile phone, suitably programmed, can now beat a grand master; and a more advanced IBM computer (dubbed “Watson”) competed successfully with humans on a TV game show. But robots can’t yet recognize and move the pieces on a real chessboard as adeptly as a child can. Later this century, however, their more advanced successors may relate to their surroundings (and to people) as adroitly as we do through our sense organs. Moral questions then arise. We accept an obligation to ensure that other human beings can fulfill their natural potential; and we even feel the same about some animal species. But what is our obligation toward sophisticated robots, our own creations? Should we feel guilty about exploiting them? Should we fret if they are underemployed, frustrated, or bored?

Be that as it may, robots surely have immense potential in arenas that humans can’t readily reach—in mines, oil rigs, and suchlike. Health care may be aided by nanorobots voyaging inside our bodies. And where they might really come into their own is way beyond the Earth—in aiding the long-term human aspiration to explore outer space.

Probing beyond the Earth

Newton realized that a projectile would escape Earth’s gravity, and go into orbit, if it reached a speed of 18,000 miles per hour. But it wasn’t, of course, until the 1950s that rockets achieved such speeds; the first artificial satellite, the Soviet Sputnik, was launched in 1957, followed by a succession of further launches from the USSR and the United States. Humans soon followed. In the 1960s manned space flight went from cornflakes packet to reality. Neil Armstrong’s “small step” on the Moon came only twelve years after Sputnik—and only sixty-six years after the Wright brothers’ first flight.

Had the pace been sustained there would by now have been a lunar base, even an expedition to Mars. But the Moon race was an end in itself, driven by the urge to beat the Russians; there was no motive to sustain the huge investment and maintain the pace of the 1960s. Only the middle-aged can remember when men walked on the moon. Films like Apollo 13 and In the Shadow of the Moon were for me (and I suspect for many others of similar vintage) an evocative reminder of historic episodes that we followed anxiously at the time. But to the young, the outdated gadgetry and right-stuff values portrayed in these films seem as antiquated as those of a traditional Western.

Post-Apollo, hundreds of astronauts have circled the earth in low orbits, but none has gone further. Instead, unmanned space technology has flourished, giving us GPS, global communications, environmental monitoring, and other everyday benefits. And scientific exploration has burgeoned too. Probes to Mars and to the moons of Jupiter and Saturn have beamed back pictures of varied and distinctive worlds. I hope that during this century the entire solar system will be explored by flotillas of miniaturized unmanned craft. One can imagine robotic fabricators, building large structures, or perhaps mining rare materials from asteroids.

But will people venture there too? The need weakens with each advance in robots and miniaturization—that’s my view, as a practical scientist. But as a human being, I’m nonetheless an enthusiast for manned missions, as a long-range adventure for (at least a few) humans. The next humans to walk on the Moon may be Chinese: China has the resources, the dirigiste government, and, perhaps, the willingness to undertake an Apollo-style program. Americans have downgraded the priority of manned space flight: their firm plans don’t even include a return to the Moon. The main impediment for NASA is that it’s constrained by public and political opinion into being too risk-averse. The space shuttle failed twice in 135 launches. Although this represents a level of risk that astronauts or test pilots would willingly accept, the shuttle had been promoted as a safe vehicle for civilians. Each failure caused a national trauma and was followed by a hiatus in the program while costly efforts were made (with very limited effect) to reduce the risk still further.

I don’t think future expeditions to the Moon and beyond will be politically and financially viable unless they are cut-price ventures, spearheaded by individuals prepared to accept high risks—perhaps even “one-way tickets.” And these may have to be privately funded; no Western governmental agency would expose civilians to such a hazardous venture. It is now US policy to encourage private companies to undertake launches—rendering NASA more like an airport authority and less like an airline. The Falcon 9 rocket, developed by the SpaceX company led by the entrepreneur Elon Musk, has successfully launched a payload into orbit. The involvement in space projects of Elon Musk, Jeff Bezos (founder of Amazon), and others in the high-tech community with credibility and resources is surely a positive step. And Google has offered a prize for whoever can build and launch a robotic lunar lander that can carry out specific tasks on the Moon. This is another stimulus—leveraging far more money than the prize itself offers.

There is a step change in cost and technical challenge between orbital flights that circle the Earth and expeditions to the Moon and beyond. But it’s surely not unrealistic to envisage private sponsorship at the multi-billion-dollar level: this is within reach even of some individuals. A comparison might be Formula One car racing, where leading teams have each had budgets of around $400 million a year.

There may be a parallel here with terrestrial exploration, which was driven by a variety of motives. The explorers who set out from Europe in the fifteenth and sixteenth centuries were bankrolled mainly by monarchs, in the hope of recouping the expenditure in exotic merchandise or by colonizing new territory. Some, for instance Captain Cook’s three eighteenth-century expeditions, were publicly funded, at least in part as a scientific enterprise. And for some, generally the most foolhardy of all, the enterprise was primarily a challenge and adventure: the same motive that drives test pilots, mountaineers, and round-the-world sailors. And it will be dangerous. Remember that nowhere in our solar system offers an environment as clement, even, as the Antarctic or the top of Everest. It is foolish to claim, as some do, that emigration into space offers a long-term escape from Earth’s problems.

A century or two from now, however, small groups of intrepid adventurers may be living independently from the Earth. Whatever ethical constraints we impose here on the ground, we should surely wish such pioneers good luck in genetically modifying their progeny to adapt to alien environments. This might be the first step toward divergence into a new species: the beginning of the posthuman era. And machines of human intelligence could spread still further. Whether the long-range future lies with organic posthumans or with intelligent machines is a matter for debate. Would it be appropriate to exploit Mars, in a manner akin to that of the pioneers who advanced westward across the United States? Should we send seeds for plants genetically engineered to grow and reproduce there? Or should the Red Planet be preserved as a natural wilderness, like the Antarctic? The answer should depend on what the pristine state of Mars actually is. If there were any life there already—especially if it had different DNA, testifying to quite separate origin from any life on Earth—then there would be widely voiced insistence that Mars should be preserved unpolluted.

Is there life out there already?

And this leads to one of the other great unknowns. Do we really expect to find any living creatures out there already? Firm evidence for even the most primitive bugs or bacteria would be immensely significant. But what really fuels popular imagination is the prospect of advanced life—the aliens familiar from science fiction. (I’m discounting, of course, that aliens in UFOs have already visited us. The claimed manifestations—crop circles and the like—are as banal and unconvincing as the messages from the “other side” routinely reported in the heyday of spiritualism a hundred years ago.) Mars is a frigid desert with a very thin atmosphere. There may be simple life there, or remnants of creatures that lived early in the planet’s history; and there could conceivably be life, too, in the ice-covered oceans of Jupiter’s moon Europa, but nobody expects a complex biosphere in those locations. Suppose, however, we widen our gaze beyond our solar system.

The Italian monk and scholar Giordano Bruno, burned at the stake in 1600, conjectured that the stars were other suns, each with their retinue of planets. Four hundred years later, science confirms this: our Sun is just one star among billions in the vastness of space far beyond our own solar system. Astronomers have learned (but only since the 1990s) that other stars indeed have planets circling around them, just as the Earth, Mars, and Jupiter circle around our own star, the Sun. These planets have not yet actually been seen. The first few hundred to be discovered were inferred indirectly by measuring the wobble induced in the motion of their parent star by their gravitational pull. The extrasolar planets discovered by this technique are very big, rather like Jupiter and Saturn, the giants of our own solar system. But we’ll be especially interested in possible twins of our Earth—planets the same size as ours, orbiting other Sun-like stars, on orbits with temperatures such that water neither boils nor stays frozen. Detecting Earth-like planets, hundreds of times less massive than Jupiter, is a real challenge. They induce motions of merely centimeters per second in their parent star—too small for current techniques to measure.

There’s another way to search for such planets: we can look for their shadow. A star would dim slightly when a planet was in transit in front of it; an orbiting planet would cause regularly repeating dimming, occurring once per orbit. NASA’s Kepler spacecraft, launched in March 2009, has been designed to detect this phenomenon. It carries a modest-sized telescope that points steadily at the Cygnus and Lyrae constellations. It monitors the brightness of about 150,000 stars in its field of view, and repeats this every half hour. It is sensitive enough to detect a dimming by just 1 part in 10,000 (which is what would be expected from the transit of a planet whose diameter was 100 times smaller than that of the star). Kepler can thereby reveal planets no bigger than the Earth and tell us how commonly they occur. In February 2011, tentative detections of planets around a thousand different stars were released (and one star was discovered to have no fewer than six planets). The planets so far discovered are closer to their parent star than the Earth is to the Sun; it will take another two years to identify planets that are in slower Earth-like orbit, but there is every expectation that these, too, will be numerous.

But we’d really like to see these planets directly—not just their shadow—and that’s hard. To realize just how hard, suppose an alien astronomer with a powerful telescope was viewing the Earth from, say, 30 light-years away—the distance of a nearby star. Our planet would seem, in Carl Sagan’s phrase, a “pale blue dot,” very close to a star (our Sun) that outshines it by many billions: a firefly next to a searchlight. But if the hypothetical aliens could detect the Earth, they could learn quite a bit about it. The shade of blue would be slightly different depending on whether the Pacific Ocean or the Eurasian land mass were facing them. They could infer the length of the day, the seasons, that there are oceans, the gross topography, and the climate. By analyzing the faint light, they could infer that Earth has a biosphere. Within twenty years, huge telescopes, in space or on the ground, will allow us to draw such inferences about planets the same size as our Earth, orbiting other Sun-like stars.

In his 1584 book On the Infinite Universe and Worlds, Bruno’s speculations went further: on some of those planets, he conjectured, there might be creatures “as magnificent as those upon our human earth.” On this issue, we’ve little more evidence than Bruno had. Could some of these extrasolar planets harbor life-forms far more interesting and exotic than anything we might find on Mars? Could they even be inhabited by beings that we could recognize as intelligent? Our cosmos would then seem far more interesting: we would look at a distant star with renewed interest if we knew it was another sun, shining on a world as intricate and complex as our own.

We still know too little to say whether alien life is likely or unlikely; indeed the origin of life on Earth is a key unsolved problem. As often in science, lack of evidence leads to polarized opinions in the community, but I think utter open-mindedness is the only rational stance while we know so little about how life might start and what evolutionary paths it might take.

Even the most firmly Earth-bound scientist would accept that one of the great challenges is to understand how life gets started. There’s an enormous variety of life on Earth—from slime mold to monkeys (and, of course, humans as well). Life seems to have been present from the very earliest times and survives in the most inhospitable corners of our planet—inside arid desert rocks, deep underground, and in the highest reaches of the atmosphere. We know that all these diverse species, many millions of them, share a genetic code based on DNA. But the basic transition from nonliving to living—the origin of the very first life—is almost as mysterious today as it was in Darwin’s time. What led from amino acids to the first replicating systems, and to the intricate protein chemistry of monocellular life? Laboratory experiments that try to simulate the soup of chemicals on the young Earth may offer clues; so might computer simulations. Darwin envisaged a “warm little pond.” We are now more aware of the immense variety of niches that life can occupy. The ecosystems near hot sulphurous outwellings in the deep oceans tell us that not even sunlight is essential. So life’s beginnings may have occurred in a torrid volcano, deep underground, or even in the rich chemical mix of a dusty interstellar cloud.

Within our solar system, Earth is the Goldilocks planet—not too hot and not too cold. Were it too hot, even the most tenacious life would fry; if too cold then the processes that created life would have happened far too slowly. But the correct temperature is not the only important thing. Everywhere you find life on Earth you find water: not necessarily oxygen, nor always sunlight—but always water. A source of energy and water seem to be the bare necessities for life. Analysis of interstellar space has shown that water is abundant throughout the universe and that starlight is also in great supply. It seems that the basic ingredients are out there, but is there life? The origin of life on Earth might have involved a fluke so rare that it happened only once in the entire galaxy—like shuffling a whole pack of cards into a perfect order. On the other hand, it might turn out that the process was almost inevitable given the right environment. So perhaps the cosmos teems with life.

Incidentally, if any signs of life were found elsewhere in our solar system—and (an important proviso) if we could be sure that it was based on a different kind of DNA, implying that it had a separate origin from terrestrial life—then we could immediately conclude that life was widespread in the universe. Something that had happened twice around a single star must have happened on millions of planets elsewhere in the galaxy.

Or even intelligent life?

Even if simple life is common, it is of course a separate question whether it’s likely to evolve into anything that we might recognize as intelligent or complex. Indeed, evolutionists don’t agree on how differently our own biosphere could have developed if contingencies like ice ages and meteorite impacts had happened differently. If, for instance, the dinosaurs hadn’t been wiped out, the chain of mammalian evolution that led to humans may have been foreclosed and it’s not clear whether another species would have taken our role.

Complex biospheres like the Earth’s could be rare because of some bottleneck, some key stage in evolution, that is hard to transit. Perhaps it is the transition to multicellular life. (The fact that simple life on Earth seems to have emerged quite quickly, whereas even the most basic multicellular organisms took nearly 3 billion years, suggests that there may be severe barriers to the emergence of any complex life.) Or the bottleneck could come later. Perhaps, more ominously, there could be a bottleneck at our own present evolutionary stage—the stage when intelligent life starts to develop technology. If so, the future development of life on (and perhaps beyond) the Earth depends on whether humans survive this critical phase. This requires avoidance of a cataclysm that wipes us out—unless, before this happens, some humans or advanced artifacts have spread beyond our home planet.

Maybe the search for life shouldn’t restrict attention to planets with biospheres like that of the Earth. Science-­fiction writers have other ideas: balloon-like creatures floating in the dense atmospheres of Jupiter-like planets, swarms of intelligent insects, nanoscale robots, and more. (And it’s often better to read first-rate science fiction than second-rate science—it’s far more stimulating, and perhaps no more likely to be wrong.) Indeed it is surprising that depictions of aliens show limited variety—they are predominantly envisaged as mammalian bipeds. The aliens may not be organic at all. The most durable form of life may be machines whose creators have long ago been usurped or become extinct. An intelligent race of aliens could have manufactured self-­reproducing machines that spread through the cosmos while their creators stayed at home. The machines could have intelligence­—even superhuman intelligence—but they would not necessarily have conscious feelings. And they could be much smaller than us. They may be nanorobots­—immensely complex, but almost too small to be seen. Indeed, perhaps they are here already!

The detection of extraterrestrial intelligence would be an immense culture shock for humanity—it would mean that we were part of a Galactic club and that it would be worth searching for further examples of alien life by all astronomical techniques. On the other hand, it would be a blow to humanity’s cosmic self-esteem. However, were our biosphere unique it would disappoint the searchers, but, in compensation, we could be less cosmically modest: our Earth, tiny though it is, would be uniquely important in the galaxy.

We may learn this century whether biological evolution is unique to the pale blue dot in the cosmos that is our home, or whether Darwin’s writ runs through a wider universe that teems with life—even with intelligence. But even in the latter case, such intelligence could be unimaginably different from our own. Some “brains” may package reality in a fashion that we can’t conceive and may have a quite different perception of reality. Others could be uncommunicative: living contemplative lives, perhaps deep under some planetary ocean, doing nothing to reveal their presence and having no motive for interstellar travel. Still other brains could actually be assemblages of superintelligent social insects. There may be a lot more out there than we could ever detect. Absence of evidence wouldn’t be evidence of absence.

In his Philosophical Investigations, Ludwig Wittgenstein famously wrote, “If a lion could speak, we couldn’t understand him.” So would the culture gap with aliens be unbridgeable? Not necessarily. They may come from planet Zog and have seven tentacles but they’d be made of the same kind of atoms as us. If they had developed advanced technology, they would share with us an understanding of physics, math, and astronomy. They’d gaze out, if they had eyes, at the same cosmos—they’d trace their origins back to the same big bang. But they might find string theory a doddle—and understand things that are beyond our grasp.

Could humans eventually understand everything?

This thought takes us back to the question raised at the beginning of this chapter: Are there intrinsic limits to our understanding, or to our technical capability? Could some branches of science come to a halt because we bump up against the inherent limits of our brainpower, rather than because the subject is exhausted? Humans are more than just another primate species. We are special: our self-­awareness and language were a qualitative leap, allowing cultural evolution, and the cumulative diversified expertise that led to science and technology. But some aspects of reality—a unified theory of physics, or a full understanding of consciousness—might elude us simply because they’re beyond human brains, just as surely as Einstein’s ideas would baffle a chimpanzee. Perhaps complex aggregates of atoms, whether brains or machines, can never understand everything about themselves.

Simulations, using ever more powerful computers, will extend scientists’ capacity to understand processes that can be neither studied in our laboratories nor observed directly. Future discoveries may be made by brute force rather than by insight. Computers with human-level capabilities will accelerate science, even though they won’t think like we do. Deep Blue beat Kasparov by exploiting its higher processing speed to explore millions of alternative series of moves and responses before deciding an optimum move; likewise, machines will make scientific discoveries that have eluded unaided human brains. For example, some substances are perfect conductors of electricity when cooled to very low temperatures (superconductors). There is a continuing quest to find the recipe for a superconductor that works at ordinary room temperatures (that is nearly 300 degrees above absolute zero; the highest superconducting temperature achieved so far is 120 degrees). This quest involves a lot of trial and error, because nobody understands exactly what makes the electrical resistance disappear more readily in some materials than in others.

Suppose that a machine came up with such a recipe. It might have succeeded in the same way that Deep Blue trounced Kasparov, by testing millions of possibilities rather than by having a theory or strategy. But it would have achieved something that would get a scientist a Nobel Prize. Moreover, its discovery would herald a technical breakthrough that could, among other things, lead to still more powerful computers—an example of the runaway acceleration in technology, worrying to some futurists, that could be unstoppable when computers can augment or even supplant human brains.

Toward the far future

One final question: Are there special perspectives that astronomers can offer to science and philosophy? I think there are. Astronomers are disclosing insights that New Agers would welcome and be attuned to. Not only do we share a common origin, and many genes, with the entire web of life on Earth, but we are linked to the cosmos. All living things depend on the stars: they are energized by the heat and light from the Sun; they are made of atoms that were forged from pristine hydrogen, billions of years ago, in faraway stars.

More significantly, astronomers can offer an awareness not only of the immensity of space but also of the immense time spans that lie ahead. The stupendous time spans of the evolutionary past are now part of common culture (apart from in creationist circles). Our present biosphere is the outcome of about 4 billion years of evolution. But most people still somehow think we humans are necessarily the culmination of the evolutionary tree. That hardly seems credible to an astronomer, aware of huge time horizons extending into the future as well as into the past. Our Sun formed 4.5 billion years ago, but it’s got 6 billion more before the fuel runs out. And the expanding universe will continue—perhaps forever—becoming (according to the best current long-range forecast) ever colder, ever emptier. As Woody Allen said, “Eternity is very long, especially towards the end.” So, even if life were now unique to Earth, there would be scope for posthuman evolution—whether organic or silicon-based—on the Earth or far beyond.

It won’t be humans who witness the Sun’s demise; it will be entities as different from us as we are from a bug. We can’t conceive what powers they might have. But there are some things they couldn’t do, like travel back in time. So they can never tell us what (if anything) still perplexes them.