Living on a small planet in a big universe exposes us to all manner of existential problems, but what are the worst, and what are the weirdest?

Sitting here watching the sluicing downpour from Tropical Storm Etau as it passes southwest of Tokyo makes me realize that there's something about being in Japan that prompts questions on human frailty. Perhaps it's just one too many late-night monster movies. Or it could be the contrast of this country's potent geophysical activity with the exquisite beauty of so much of its landscape and culture.

Whatever the reason, thinking about the most disturbing events or discoveries that might come our way - the existential nightmares - has a morbid fascination. I'd also argue that there is some scientific value to mulling these over. We devote a lot of time to seeking answers to origins - of universes, planets, life - but not so much time to the other side of the equation. But bad stuff is important. You can't dodge a ten-ton-truck if you're not even aware that trucks exist.

In that spirit, here's a proposal for a list of a few major existential crises, roughly ordered by increasing weirdness.

Filthy cage syndrome: Irreversible environmental change caused by us

This one is, as they say, a no-brainer. Really, it is. Ever had a pet in a cage? It eats, it poops, it scuffles stuff up, it gnaws, it sheds. Were it not for your constant efforts to restore the environment to a clean and pristine state, your fluffy companion would keel over in short order and you'd have some awkward explaining to do to the younger family members.

Now scale this up to, say, a planet. All that scuffling and gnawing is going to shift the environment towards some new state, a state that is different from the one that gave rise to your species in the first place.

And the rub is that a planetary environment - its climate and chemical state - may not readily return to what it was, no matter what you do. Bio-engineering and gene-editing, for whatever purposes, may just contribute to the speed and size of the perturbation. Cue filthy cage syndrome: hot, flooded, food-chain challenged, diseased. Suddenly you're a new bump of soil in the rose garden about-which-we-shall-not-speak-of-again.

An unfortunate confluence of natural phenomena

When we're a global species, wringing just about all we can from a planet in terms of food and resources, it doesn't take much tinkering to bring things tumbling down. If a whole bunch of tinkering happened at the same time it would be catastrophic for the likes of us.

Existential crisis #2 is when volcanoes, hurricanes and typhoons, drought, flooding, solar flares, algal blooms, earthquakes, wildfires, and viral epidemics all hit on a Monday morning. By Friday we're experiencing an inescapable slide back to the Pleistocene, and it's time to dig out the manuals for rebuilding civilization.

An academically interesting aspect of this scenario is that it's caused by a chain of individually low probability events. We tend to ignore stuff like this, because after all, how likely is it for all of these events to happen together? Mathematically very unlikely. Yet, we live in a cosmos where the improbable happens all the time.

Asteroids, supernova, dark matter caustics

Asteroids have hit the Earth for the past 4 billion years, they're the afterthought of planetary formation - the very, very late arrivals at the party. There's no sign at present of a civilization-killing asteroid (at least 10 km in size) crossing Earth's orbit at the wrong time in our near future - but there will be, eventually.

Stars go bang. And when they do it's the worst of times and the best of times. Some supernova can push about 1046 Joules of energy out into the cosmos, most of it in the form of neutrinos, but at least 1% as the kinetic energy of expelled matter, and perhaps 0.01% as electromagnetic radiation. Not good for any planetary systems that are close by (within say 200 light years or so).

For us the closest potential supernova progenitor may be a star called IK Pegasi, 150 light years away. If it goes pop, we might be toast, and there'd be almost no warning. Someone would look up and say 'Oh, look at that bright light', and those would be our last words.

The positive is that supernova help generate and disperse heavy elements, and may have played a critical role in triggering the formation of the solar system. Although I think one would have to be remarkably philosophical to draw comfort from that as our world ends.

Dark matter caustics may occur if dark matter is what we speculate it to be - non-interacting, 'collisionless', heavy sub-atomic particles. Ever watch the way sunlight makes patterns of high and low intensity at the bottom of a pool of rippling water? This is related to the mathematical construct of a 'caustic', a formal singularity or concentration in phase space. Turns out that dark matter sloshing about in a galaxy may form caustics, and these concentrations of mass might sweep through stellar systems perturbing objects like asteroids, comets, even planets. Nasty.

The construction of an AI (maybe)

I've written about this before on these pages. The parenthesized 'maybe' is because it's not clear to me that an artificial intelligence will be a threat long enough to continue to do serious damage to our species.

But, as in some of the previous items, a short sharp shock could be enough to reduce our existence to dust. And a software AI might produce just that - going from stone age to space age in microseconds and deciding that its survival requires taking on all the resources that humans like to think are theirs.

Finding life on Mars (according to some)

You look puzzled. How can finding life on Mars be an existential problem? While it's true that this might not cause the immediate downfall of our species, it could be a portent of trouble ahead. The philosopher Nick Bostrom has laid this out in detail, and I'll summarize the argument here.

Let's suppose that we find evidence of a separate origin of life on Mars. That instantly boosts the probability for life arising anywhere across the cosmos. Except, we haven't found any signs of intelligent or technological life elsewhere. That is puzzling, because the age and scale of our galaxy is such that even relatively modest interstellar travel capabilities should have enabled life to spread pretty much everywhere in just a few tens of millions of years. This is the so-called Fermi Paradox.

Bostrom's gloomy assessment is that there would be something that consistently happens to stem the tide of life, to cut it short. There would be a 'Great Filter' preventing intelligent life (whatever that really means) from surviving. And finding life on Mars (extinct or extant) adds serious weight to the notion that we've yet to be eliminated by that filter, but that it's coming.

Getting a message from aliens

We would like to know if we're alone in the universe. We'd especially like to know if we're the only thinking beings. Such knowledge would go a long way to quell concerns over the previous item as well.

But beware the power of ideas.

I delved into this issue a couple of times recently in another forum, but the bottom line is that alien messages could - perhaps - be hugely problematic. What if, for example, the message says 'We are coming to eat you!' I know we've all seen that famous Twilight Zone episode, but really, what would our species do if it got precisely that warning, beamed across the cosmos with undoubtedly sophisticated and powerful means?

It would, I think, class as a doozy of an existential crisis. It wouldn't even require the aliens' harvesting ship to show up. We'd have self-destructed in panic long before.

Discovering that there really are multiverse versions of all of us

Now we're getting into some richly weird, yet perhaps not crazy, terrain. Some multiverse theories suggest that there are such a vast number of pocket universes that they are most meaningfully counted in terms of how many would be genuinely distinguishable by us. 

One estimate is that there could be 10-to-the-10-to-the-16 of these universes. That's enough that there might be versions of all of us wondering around some of those other realities, different only in the most subtle ways.

How does this constitute an existential crisis for humans? Well, strictly speaking it is more of an existential crisis for individual humans. What, for example, happens to morals when there are a gazillion versions of you? Want to tear up that parking ticket? Go ahead, it's ok because in a ton of other realities you actually do the right thing and pay the fine.

I think this is a bit silly, and others have dissected such daftness with much more sophistication. But it doesn't matter what I think, what matters is if we discovered all of this to be true, and some humans decide it's the ultimate sign of a manifest destiny. They might be suddenly unfettered by the usual rules that have (more or less) kept our species from self-destruction.

Discovering that the universe is a simulation

Arguments have been made that many outstanding existential puzzles would be resolved if, in fact, we live inside a simulation. Yes, it's a version of the Matrix answer. It's also not entirely new (related speculations go back to the ancient Greeks) but it has a little more gloss now because we have a hazy impression of where our own technology is headed, and because it would fix items like the Fermi Paradox and perhaps our continued puzzlement over quantum phenomena like entanglement and causality.

Of course this may all be wrong. But for a moment just suppose that we had proof that everything we knew - the cosmos around us, its physics, its mathematics, and our own existence - was simply an emergence within a simulation. And that instead of this happening inside our heads, our heads are actually inside the simulation.

That, I think, would bring on a pretty brutal type of existential crisis. Not only are we not in a real universe, we are not real either - whatever 'real' actually means.

Would there be anything to do about this? Not really, because where would we go?