I’ve never been a big fan of the idea that scientists, because they are so rational, should run the world. First, scientists are often as nutty as the rest of us, or nuttier. Second, leadership requires more than reason. Think Captain Kirk versus Mr. Spock. But recent events make the idea of a Scientist King more appealing. Also, I think of certain scientists as wise, even statesmanlike. They possess a quiet authority that makes you trust their judgment, and not just on scientific matters. Physicists Hans Bethe and Steven Weinberg come to mind, and so does cosmologist Martin Rees. Rees, who is based at the University of Cambridge and is Britain’s Astronomer Royal, has had an illustrious career, which you can read about here. In addition to his decades of contributions to astrophysics and cosmology, he has also spoken out on threats facing humanity. I first met Rees in 1990 at a cosmology conference in Sweden. We have crossed tracks a couple of times since then, most recently at a meeting in Switzerland that Rees attended via cyberspace. After I wrote a column on Rees’s remarks, we had the following email exchange. –John Horgan

Deep down in your gut, do you think we're doomed?

No, I don’t. But we’ll surely have a bumpy ride through this century. The world’s growing and more demanding population puts our natural environment under strain, and could trigger dangerous climate change and mass extinctions if “tipping points” are crossed. But to reduce these risks, we don’t need to put the brakes on technology; on the contrary, we need to enhance our understanding of nature, and deploy appropriate technology (zero-carbon energy, for instance) more urgently.

Novel technologies – bio, cyber and AI – could be transformative. They may offer new solutions; on the other hand, they create new vulnerabilities. It’s hard to be optimistic about a peaceful world if a chasm persists, as deep as it is in today’s geopolitics, between the welfare levels and life-chances in different regions.

Which apocalypse worries you most these days?

I wouldn’t use the word “apocalypse” but I think we have new grounds to be anxious. In the next decade or two what worries me most are issues I highlighted in my book Our Final Century 15 years ago: the empowerment of individuals or small groups by powerful technologies, whose actions, whether errors or intentionally malicious, can cascade globally. Cyberattacks are already high on our radar. And I’d guess that biothreats soon will be too. Back in the early days of recombinant DNA research, leading biologists met in Asilomar, California, and agreed guidelines on what experiments should and shouldn’t be done. But now, 40 years after Asilomar, things are very different: the research community is far more broadly international, and more influenced by commercial pressures. I’d worry that whatever regulations are imposed, on prudential or ethical grounds, can’t be enforced worldwide – any more than the drug laws can --- or the tax laws. Whatever can be done will be done by someone, somewhere.

And that’s a nightmare. An atomic bomb can’t be built without large scale special-purpose facilities. But biotech involves small-scale, dual-use equipment. Indeed, biohacking is burgeoning even as a hobby and competitive game. We know all too well that being tech-savvy doesn’t guarantee balanced rationality. The global village will have its village idiots and they’ll have global range. Such threats will pose an intractable challenge to governments and aggravate the tension between freedom, privacy and security.

These threats are of course additional to the risks of nuclear war or of state-level cyber-attacks on national infrastructure.

Should we start a colony on Mars in case things get really bad here?

I disagree strongly with Elon Musk, and with my Cambridge colleague Stephen Hawking, who advocate rapid build-up of large-scale Martian communities. It’s a dangerous delusion to think that space offers an escape from Earth's problems. We’ve got to solve these here. Coping with climate change may seem daunting, but it’s a doddle compared to terraforming Mars. There’s no ‘Planet B’ for ordinary risk-averse people.

However, I think (and hope) that there will be bases on Mars by 2100. But the practical case for sending people gets weaker as robots improve. So the only manned ventures will be high-risk, cut price, and privately sponsored –undertaken by thrill-seekers prepared to accept one-way tickets. They’re the people who will venture to Mars.

But we (and our progeny here on Earth) should cheer on these brave adventurers. The space environment is inherently hostile for humans, So, precisely because they will be ill-adapted to their new habitat, the pioneer explorers will have a more compelling incentive than those of us on Earth to re-design themselves. They’ll harness the super-powerful genetic and cyborg technologies that will be developed in coming decades. These techniques will, one hopes, be heavily regulated on Earth; but “settlers” on Mars will be far beyond the clutches of the regulators. This might be the first step towards divergence into a new species. So it’s these spacefaring adventurers, not those of us comfortably adapted to life on Earth, who will spearhead the post-human era. So we (or our Earthbound progeny) should cheer them on.

You've been talking about science bumping into limits lately. Is this a new concern?

It’s absolutely not a new concern for me. In fact I said in my very first BBC radio talk, given when I was still a graduate student, that some discoveries might have to await the emergence of “a species more intelligent than ourselves.”

In every subject there will, at every stage, be “unknown unknowns.” (Donald Rumsfeld was famously mocked for saying this in a different context – but of course he was right, and it might have been better for the world had he become a philosopher). But there is a deeper question. Are there things that we’ll never know, because they are beyond the power of human minds to grasp? Are our brains matched to an understanding of all key features of reality? I’m confident that answers to many current mysteries will come into focus in the coming decades. But maybe not all: there may be phenomena, crucial to a full understanding of physical reality, that we are not aware of, any more than a monkey comprehends the nature of stars and galaxies.

So I think we should be open-minded about the possibility that some fundamental truths about nature could be too complex for unaided human brains ever to fully grasp. Indeed, perhaps we'll never understand the mystery of these brains themselves -- how atoms can assemble into “grey matter” that can become aware of itself and ponder its origins. Or perhaps any universe complicated enough to have allowed our emergence is for just that reason too complicated for our brains to understand.

The quest for a unified theory of physics seems to have stalled. Could it turn out to be a dead end?

The history of physics of course features successive “leaps” and unifications: Newton, Maxwell, Einstein, quantum mechanics, quantum electrodynamics, and the “standard model.” There are surely deeper regularities that still elude us. But these future unifications are likely to be harder to find than the earlier ones. This is partly because there may not be such direct experimental tests, and partly because they involve an especially big jump in scales. For instance, any unification of gravity and quantum theory is likely to involve the Planck length – twenty powers of ten smaller than an atomic nucleus. That’s the scale on which empty space may have some intricate structure. This may be of the kind envisaged by string theory or loop quantum gravity -- or it could be something quite different. The optimists hope that some such theory may one day gain credibility by accounting for the unexplained numbers in the standard model, or perhaps by new cosmological observations that probe closer to the Planck scale. We don’t know which, if any, of current ideas may be on the right lines.

I think it’s important that some people should continue to tackle this “Everest problem” – to seek a testable theory from many perspectives.. It’s presumptuous (as some people like Peter Woit have done) to deride the way some manifestly brilliant people choose to dedicate their scientific lives. [See Woit's response to this comment.] But we must be open-minded about the possibility that the true theory may simply be too difficult – but we need to explore far longer and harder before concluding that. In that sense – and that sense alone – the quest may be deemed a “dead end” by far-future historians.

Is inflation, the theory of cosmic creation, dead?

It’s probably best to think of inflation as a “scenario” rather than a specific theory. In that generic sense it’s very much alive – indeed it remains the best idea we have to account for the scale of the cosmos and the character of the fluctuations. But the trouble is that we have no firm ideas on the physics that prevailed at the huge energies where inflation would have occurred (a trillion times higher than can be reached in a particle accelerator). Measurements of the tilt, the gaussianity and the tensor/scalar ratio of the fluctuations set constrains on that physics and may lead to some new tests or refutations but “inflation” is a great concept and remains a good bet.

Are multiverse theories testable? If not, why does anyone take them seriously?

There are some variants of “inflation” – for instance Andrei Linde’s “eternal inflation”--that lead to many “big bangs” and not just one. But as I’ve just noted, we don’t know whether the actual physics at the relevant ultra-high energies has the specific features that his model requires. Nor do we know whether the separate big bangs (if they occurred) would have cooled down to be governed by the same laws that we observe. There are some theories that allow more than a googol of different vacuum states, in each of which the microphysics would be different.

Could this be true? A challenge for 21st century physics is to answer two questions. First, are there many “big bangs” rather than just one? Second – and this is even more interesting -- if there are many, are they all governed by the same physics?

Until about 50 years ago we weren’t sure whether there was a big bang at all. Now we have enough evidence to delineate cosmic history back to the ultra-dense first nanosecond – with as much confidence as a geologist inferring the early history of our Earth and with precision better than a few percent. So in 50 more years, it’s not overoptimistic to hope that we may have a “unified” physical theory, corroborated by experiment and observation in the everyday world, that is broad enough to describe what happened in the first trillionth of a trillionth of a trillionth of a second – where the densities and energies were far higher than the range where current theories apply. If that theory predicts multiple big bangs we should take that prediction seriously even though it can’t be directly verified (just as we give credence to what Einstein’s theory tells us about the unobservable insides of black holes, because the theory has survived many tests in domains we can observe).

It’s sometimes claimed that domains that are in principle unobservable aren’t part of science. But not even the most conservative astronomer would take this line. We’re in an accelerating universe where distant galaxies will disappear over a horizon, and their far future would never be in principle observable. So it’s natural to suppose that there are galaxies that are already beyond the horizon and so forever unobservable. If you’re in the middle of the ocean, you’d be surprised if its boundary lay just beyond your horizon. Likewise, astronomers are confident that the volume of space-time within range of our telescopes -- what astronomers have traditionally called 'the universe' – is only a tiny fraction of the aftermath of our big bang. We'd expect far more galaxies located unobservably beyond the horizon.

If we’re in a multiverse, it would imply a fourth and grandest Copernican revolution; we’ve had the Copernican revolution itself, then the realization that there are billions of planetary systems in our galaxy; then that there are billions of galaxies in our observable universe. But now that’s not all. The entire panorama that astronomers can observe could be a tiny part of the aftermath of ‘our’ big bang, which is itself just one bang among a perhaps-infinite ensemble.