Wired has an article about the CSER Existential Risk Conference in December 2016, rather flatteringly comparing us to superheroes. Plus a list of more or less likely risks we discussed. Calling them the “10 biggest threats” is perhaps exaggerating a fair bit: nobody is seriously worried about simulation shutdowns. But some of the others are worth working a lot more on.

## High-energy demons

I am cited as talking about existential risk from demon summoning. Since this is bound to be misunderstood, here is the full story:

As noted in the Wired list, we wrote a paper looking at the risk from the LHC, finding that there is a problem with analysing very unlikely (but high impact) risks: the probability of a mistake in the analysis overshadows the risk itself, making the analysis bad at bounding the risk. This can be handled by doing multiple independent risk bounds, which is a hassle, but it is the only (?) way to reliably conclude that things are safe.

I blogged a bit about the LHC issue before we wrote the paper, bringing up the problem of estimating probabilities for unprecedented experiments through the case of Taleb’s demon (which properly should be Taylor’s demon, but Stigler’s law of eponymy strikes again). That probably got me to have a demon association to the wider physics risk issues.

The issue of how to think about unprecedented risks without succumbing to precautionary paralysis is important: we cannot avoid doing new things, yet we should not be stupid about it. This is extra tricky when considering experiments that create things or conditions that are not found in nature.

## Not so serious?

A closely related issue is when it is reasonable to regard a proposed risk as non-serious. Predictions of risk from strangelets, black holes, vacuum decay and other “theoretical noise” caused by theoretical physics theories at least is triggered by some serious physics thinking, even if it is far out. Physicists have generally tended to ignore such risks, but when forced by anxious acceleratorphobes the arguments had to be nontrivial: the initial dismissal was not really well founded. Yet it seems totally reasonable to dismiss some risks. If somebody worries that the alien spacegods will take exception to the accelerator we generally look for a psychiatrist rather than take them seriously. Some theories have so low prior probability that it seems rational to ignore them.

But what is the proper de minimis boundary here? One crude way of estimating it is to say that risks of destroying the world with lower probability than one in 10 billion can safely be ignored – they correspond to a risk of less than one person in expectation. But we would not accept that for an individual chemistry experiment: if the chance of being blown up if someone did it was “less than 100%” but still far above some tiny number, they would presumably want to avoid risking their neck. And in the physics risk case the same risk is borne by every living human. Worse, by Bostrom’s astronomical waste argument, existential risks risks more than 1046 possible future lives. So maybe we should put the boundary at less than 10-46: any risk more likely must be investigated in detail. That will be a lot of work. Still, there are risks far below this level: the probability that all humans were to die from natural causes within a year is around 10-7.2e11, which is OK.

One can argue that the boundary does not really exist: Martin Peterson argues that setting it at some fixed low probability, that realisations of the risk cannot be ascertained, or that it is below natural risks do not truly work: the boundary will be vague.

## Demons lurking in the priors

Be as it may with the boundary, the real problem is that estimating prior probabilities is not always easy. They can vault over the vague boundary.

Hence my demon summoning example (from a blog post near Halloween I cannot find right now): what about the risk of somebody summoning a demon army? It might cause the end of the world. The theory “Demons are real and threatening” is not a hugely likely theory: atheists and modern Christians may assign it zero probability. But that breaks Cromwell’s rule: once you assign 0% to a probability no amount of evidence – including a demon army parading in front of you – will make you change your mind (or you are not applying probability theory correctly). The proper response is to assume some tiny probability $\epsilon$, conveniently below the boundary.

…except that there are a lot of old-fashioned believers who do think the theory “Demons are real and threatening” is a totally fine theory. Sure, most academic readers of this blog will not belong to this group and instead to the $\epsilon$ probability group. But knowing that there are people out there that think something different from your view should make you want to update your view in their direction a bit – after all, you could be wrong and they might know something you don’t. (Yes, they ought to move a bit in your direction too.) But now suppose you move 1% in the direction of the believers from your $\epsilon$ belief. You will now believe in the theory to $\epsilon + 1\% \approx 1\%$. That is, now you have a fairly good reason not to disregard the demon theory automatically. At least you should spend effort on checking it out. And once you are done with that you better start with the next crazy New Age theory, and the next conspiracy theory…

## Reverend Bayes doesn’t help the unbeliever (or believer)

One way out is to argue that the probability of believers being right is so low that it can be disregarded. If they have probability $\epsilon$ of being right, then the actual demon risk is of size $\epsilon$ and we can ignore it – updates due to the others do not move us. But that is a pretty bold statement about human beliefs about anything: humans can surely be wrong about things, but being that certain that a common belief is wrong seems to require better evidence.

The believer will doubtlessly claim seeing a lot of evidence for the divine, giving some big update $\Pr[belief|evidence]=\Pr[evidence|belief]\Pr[belief]/\Pr[evidence]$, but the non-believer will notice that the evidence is also pretty compatible with non-belief: $\frac{\Pr[evidence|belief]}{\Pr[evidence|nonbelief]}\approx 1$ – most believers seem to have strong priors for their belief that they then strengthen by selective evidence or interpretation without taking into account the more relevant ratio $\Pr[belief|evidence] / \Pr[nonbelief|evidence]$. And the believers counter that the same is true for the non-believers…

Insofar we are just messing around with our own evidence-free priors we should just assume that others might know something we don’t know (maybe even in a way that we do not even recognise epistemically) and update in their direction. Which again forces us to spend time investigating demon risk.

## OK, let’s give in…

Another way of reasoning is to say that maybe we should investigate all risks somebody can make us guess a non-negligible prior for. It is just that we should allocate our efforts proportional to our current probability guesstimates. Start with the big risks, and work our way down towards the crazier ones. This is a bit like the post about the best problems to work on: setting priorities is important, and we want to go for the ones where we chew off most uninvestigated risk.

If we work our way down the list this way it seems that demon risk will be analysed relatively early, but also dismissed quickly: within the religious framework it is not a likely existential risk in most religions. In reality few if any religious people hold the view that demon summoning is an existential risk, since they tend to think that the end of the world is a religious drama and hence not intended to be triggered by humans – only divine powers or fate gets to start it, not curious demonologists.

## That wasn’t too painful?

Have we defeated the demon summoning problem? Not quite. There is no reason for all those priors to sum to 1 – they are suggested by people with very different and even crazy views – and even if we normalise them we get a very long and heavy tail of weird small risks. We can easily use up any amount of effort on this, effort we might want to spend on doing other useful things like actually reducing real risks or doing fun particle physics.

There might be solutions to this issue by reasoning backwards: instead of looking at how X could cause Y that could cause Z that destroys the world we ask “If the world would be destroyed by Z, what would need to have happened to cause it?” Working backwards to Y, Y’, Y” and other possibilities covers a larger space than our initial chain from X. If we are successful we can now state what conditions are needed to get to dangerous Y-like states and how likely they are. This is a way of removing entire chunks of the risk landscape in efficient ways.

This is how I think we can actually handle these small, awkward and likely non-existent risks. We develop mental tools to efficiently get rid of lots of them in one fell sweep, leaving the stuff that needs to be investigated further. But doing this right… well, the devil lurks in the details. Especially the thicket of totally idiosyncratic risks that cannot be handled in a general way. Which is no reason not to push forward, armed with epsilons and Bayes’ rule.

That the unbeliever may have to update a bit in the believer direction may look like a win for the believers. But they, if they are rational, should do a small update into the unbeliever direction. The most important consequence is that now they need to consider existential risks due to non-supernatural causes like nuclear war, AI or particle physics. They would assign them a lower credence than the unbeliever, but as per the usual arguments for the super-importance of existential risk this still means they may have to spend effort on thinking about and mitigating these risks that they otherwise would have dismissed as something God would have prevented. This may be far more annoying to them than unbelievers having to think a bit about demonology.

Emlyn O’Regan makes some great points over at Google+, which I think are worth analyzing:

1. “Should you somehow incorporate the fact that the world has avoided destruction until now into your probabilities?”
2. “Ideas without a tech angle might be shelved by saying there is no reason to expect them to happen soon.” (since they depend on world properties that have remained unchanged.)
3. ” Ideas like demon summoning might be limited also by being shown to be likely to be the product of cognitive biases, rather than being coherent free-standing ideas about the universe.”

In the case of (1), observer selection effects can come into play. If there are no observers on a post demon-world (demons maybe don’t count) then we cannot expect to see instances of demon apocalypses in the past. This is why the cosmic ray argument for the safety of the LHC need to point to the survival of the Moon or other remote objects rather than the Earth to argue that being hit by cosmic rays over long periods prove that it is safe. Also, as noted by Emlyn, the Doomsday argument might imply that we should expect a relatively near-term end, given the length of our past: whether this matters or not depends a lot on how one handles observer selection theory.

In the case of (2), there might be development in summoning methods. Maybe medieval methods could not work, but modern computer-aided chaos magick is up to it. Or there could be rare “the stars are right” situations that made past disasters impossible. Still, if you understand the risk domain you may be able to show that the risk is constant and hence must have been low (or that we are otherwise living in a very unlikely world). Traditions that do not believe in a growth of esoteric knowledge would presumably accept that past failures are evidence of future inability.

(3) is an error theory: believers in the risk are believers not because of proper evidence but from faulty reasoning of some kind, so they are not our epistemic peers and we do not need to update in their direction. If somebody is trying to blow up a building with a bomb we call the police, but if they try to do it by cursing we may just watch with amusement: past evidence of the efficacy of magic at causing big effects is nonexistent. So we have one set of evidence-supported theories (physics) and another set lacking evidence (magic), and we make the judgement that people believing in magic are just deluded and can be ignored.

(Real practitioners may argue that there sure is evidence for magic, it is just that magic is subtle and might act through convenient coincidences that look like they could have happened naturally but occur too often or too meaningfully to be just chance. However, the skeptic will want to actually see some statistics for this, and in any case demon apocalypses look like they are way out of the league for this kind of coincidental magic).

Emlyn suggests that maybe we could scoop all the non-physics like human ideas due to brain architecture into one bundle, and assign them one epsilon of probability as a group. But now we have the problem of assigning an idea to this group or not: if we are a bit uncertain about whether it should have $\epsilon$ probability or a big one, then it will get at least some fraction of the big probability and be outside the group. We can only do this if we are really certain that we can assign ideas accurately, and looking at how many people psychoanalyse, sociologise or historicise topics in engineering and physics to “debunk” them without looking at actual empirical content, we should be wary of our own ability to do it.

So, in short, (1) and (2) do not reduce our credence in the risk enough to make it irrelevant unless we get a lot of extra information. (3) is decent at making us sceptical, but our own fallibility at judging cognitive bias and mistakes (which follows from claiming others are making mistakes!) makes error theories weaker than they look. Still, the really consistent lack of evidence of anything resembling the risk being real and that claims internal to the systems of ideas that accept the possibility imply that there should be smaller, non-existential, instances that should be observable (e.g. individual Fausts getting caught on camera visibly succeeding in summoning demons), and hence we can discount these systems strongly in favor of more boring but safe physics or hard-to-disprove but safe coincidental magic.

# Best problems to work on?

80,000 hours has a lovely overview of “What are the biggest problems in the world?” The best part is that each problem gets its own profile with a description, arguments in favor and against, and what already exists. I couldn’t resist plotting the table in 3D:

There are of course plenty of problems not listed; even if these are truly the most important there will be a cloud of smaller scale problems to the right. They list a few potential ones like cheap green energy, peace, human rights, reducing migration restrictions, etc.

I recently got the same question, and here are my rough answers:

• Fixing our collective epistemic systems. Societies work as cognitive systems: acquiring information, storing, filtering and transmitting it, synthesising it, making decisions, and implementing actions. This is done through individual minds, media and institutions. Recently we have massively improved some aspects through technology, but it looks like our ability to filter, organise and jointly coordinate has not improved – in fact, many feel it has become worse. Networked media means that information can bounce around multiple times acquiring heavy bias, while filtering mechanisms relying on authority has lost credibility (rightly or wrongly). We are seeing all sorts of problems of coordinating diverse, polarised, globalised or confused societies. Decision-making that is not reality-tracking due to (rational or irrational) ignorance, bias or misaligned incentives is at best useless, at worst deadly. Figuring out how to improve these systems seem to be something with tremendous scale (good coordination and governance helps solve most problems above), it is fairly neglected (people tend to work on small parts rather than figuring out better systems), and looks decently solvable (again, many small pieces may be useful together rather than requiring a total perfect solution).
• Ageing. Ageing kills 100,000 people per day. It is a massive cause of suffering, from chronic diseases to loss of life quality. It causes loss of human capital at nearly the same rate as all education and individual development together. A reduction in the health toll from ageing would not just save life-years, it would have massive economic benefits. While this would necessitate changes in society most plausible shifts (changing pensions, the concepts of work and life-course, how families are constituted, some fertility reduction and institutional reform) the cost and trouble with such changes is pretty microscopic compared to the ongoing death toll and losses. The solvability is improving: 20 years ago it was possible to claim that there were no anti-ageing interventions, while today there exist enough lab examples to make this untenable. Transferring these results into human clinical practice will however be a lot of hard work. It is also fairly neglected: far more work is being spent on symptoms and age-related illness and infirmity than root causes, partially for cultural reasons.
• Existential risk reduction: I lumped together all the work to secure humanity’s future into one category. Right now I think reducing nuclear war risk is pretty urgent (not because of the current incumbent of the White House, but simply because the state risk probability seems to dominate the other current risks), followed by biotechnological risks (where we still have some time to invent solutions before the Collingridge dilemma really bites; I think it is also somewhat neglected) and AI risk (I put it as #3 for humanity, but it may be #1 for research groups like FHI that can do something about the neglectedness while we figure out better how much priority it truly deserves). But a lot of the effort might be on the mitigation side: alternative food to make the world food system more resilient and sun-independent, distributed and more robust infrastructure (whether better software security, geomagnetic storm/EMP-safe power grids, local energy production, distributed internet solutions etc.), refuges and backup solutions. The scale is big, most are neglected and many are solvable.

Another interesting set of problems is Robin Hanson’s post about neglected big problems. They are in a sense even more fundamental than mine: they are problems with the human condition.

As a transhumanist I do think the human condition entails some rather severe problems – ageing and stupidity is just two of them – and that we should work to fix them. Robin’s list may not be the easiest to solve, though (although there might be piecemeal solutions worth doing). Many enhancements, like moral capacity and well-being, have great scope and are very neglected but lose out to ageing because of the currently low solvability level and the higher urgency of coordination and risk reduction. As I see it, if we can ensure that we survive (individually and collectively) and are better at solving problems, then we will have better chances at fixing the tougher problems of the human condition.

# Survivorship curves and existential risk

In a discussion Dennis Pamlin suggested that one could make a mortality table/survival curve for our species subject to existential risk, just as one can do for individuals. This also allows demonstrations of how changes in risk affect the expected future lifespan. This post is a small internal FHI paper I did just playing around with survivorship curves and other tools of survival analysis to see what they add to considerations of existential risk. The outcome was more qualitative than quantitative: I do not think we know enough to make a sensible mortality table. But it does tell us a few useful things:

• We should try to reduce ongoing “state risks” as early as possible
• Discrete “transition risks” that do not affect state risks matters less; we may want to put them off indefinitely.
• Indefinite survival is possible if we make hazard decrease fast enough.

# Simple model

A first, very simple model: assume a fixed population and power-law sized disasters that randomly kill a number of people proportional to their size every unit of time (if there are survivors, then they repopulate until next timestep). Then the expected survival curve is an exponential decay.

This is in fact independent of the distribution, and just depends on the chance of exceedance. If disasters happen at a rate $\lambda$ and the probability of extinction $\Pr(X>\mathrm{population}) = p$, then the curve is $S(t) = \exp(-p \lambda t).$

This can be viewed as a simple model of state risks, the ongoing background of risk to our species from e.g. asteroids and supernovas.

## Correlations

What if the population rebound is slower than the typical inter-disaster interval? During the rebound the population is more vulnerable to smaller disasters. However, if we average over longer time than the rebound time constant we end up with the same situation as before: an adjusted, slightly higher hazard, but still an exponential.

In ecology there has been a fair number of papers analyzing how correlated environmental noise affects extinction probability, generally concluding that correlated (“red”) noise is bad (e.g. (Ripa and Lundberg 1996), (Ovaskainen and Meerson 2010)) since the adverse conditions can be longer than the rebound time.

If events behave in a sufficiently correlated manner, then the basic survival curve may be misleading since it only shows the mean ensemble effect rather than the tail risks. Human societies are also highly path dependent over long timescales: our responses can create long memory effects, both positive and negative, and this can affect the risk autocorrelation.

## Population growth

If population increases exponentially at a rate $G$ and is reduced by disasters, then initially some instances will be wiped out, but many realizations achieve takeoff where they grow essentially forever. As the population becomes larger, risk declines as $\exp(- \alpha G t).$

This is somewhat similar to Stuart’s and my paper on indefinite survival using backups: when we grow fast enough there is a finite chance of surviving indefinitely. The growth may be in terms of individuals (making humanity more resilient to larger and larger disasters), or in terms of independent groups (making humanity more resilient to disasters affecting a location). If risks change in size in proportion to population or occur in different locations in a correlated manner this basic analysis may not apply.

# General cases

Overall, if there is a constant rate of risk, then we should expect exponential survival curves. If the rate grows or declines as a power $t^k$ of time, we get a Weibull distribution of time to extinction, which has a “stretched exponential” survival curve: $\exp(-t/ \lambda)^k.$

If we think of risk increasing from some original level to a new higher level, then the survival curve will essentially be piece-wise exponential with a more or less softly interpolating “knee”.

## Transition risks

A transition risk is essentially an impulse of hazard. We can treat it as a Dirac delta function with some weight $w$ at a certain time $t$, in which case it just reduces the survival curve so $\frac{S(\mathrm{after }t)}{S(\mathrm{before }t)}=w$. If $t$ is randomly distributed it produces a softer decline, but with the same magnitude.

## Rectangular survival curves

Human individual survival curves are rectangularish because of exponentially increasing hazard plus some constant hazard (the Gompertz-Makeham law of mortality). The increasing hazard is due to ageing: old people are more vulnerable than young people.

Do we have any reason to believe a similar increasing hazard for humanity? Considering the invention of new dangerous technologies as adding more state risk we should expect at least enough of an increase to get a more convex shape of the survival curve in the present era, possibly with transition risk steps added in the future. This was counteracted by the exponential growth of human population until recently.

## How do species survival curves look in nature?

There is “van Valen’s law of extinction” claiming the normal extinction rate remains constant at least within families, finding exponential survivorship curves (van Valen 1973). It is worth noting that the extinction rate is different for different ecological niches and types of organisms.

However, fits with Weibull distributions seem to work better for Cenozoic foraminifera than exponentials (Arnold, Parker and Hansard 1995), suggesting the probability of extinction increases with species age. The difference in shape is however relatively small (k≈1.2), making the probability increase from 0.08/Myr at 1 Myr to 0.17/Myr at 40 Myr. Other data hint at slightly slowing extinction rates for marine plankton (Cermeno 2011).

In practice there are problems associated with speciation and time-varying extinction rates, not to mention biased data (Pease 1988). In the end, the best we can say at present appears to be that natural species survival is roughly exponentially distributed.

# Conclusions for xrisk research

Survival curves contain a lot of useful information. The median lifespan is easy to read off by checking the intersection with the 50% survival line. The life expectancy is the area under the curve.

In a semilog-diagram an exponentially declining survival probability is a line with negative slope. The slope is set by the hazard rate. Changes in hazard rate makes the line a series of segments.
An early reduction in hazard (i.e. the line slope becomes flatter) clearly improves the outlook at a later time more than a later equal improvement: to have a better effect the late improvement needs to reduce hazard significantly more.

A transition risk causes a vertical displacement of the line (or curve) downwards: the weight determines the distance. From a given future time, it does not matter when the transition risk occurs as long as the subsequent hazard rate is not dependent on it. If the weight changes depending on when it occurs (hardware overhang, technology ordering, population) then the position does matter. If there is a risky transition that reduces state risk we should want it earlier if it does not become worse.

### Acknowledgments

Thanks to Toby Ord for pointing out a mistake in an earlier version.

# Appendix: survival analysis

The main object of interest is the survival function $S(t)=\Pr(T>t)$ where $T$ is a random variable denoting the time of death. In engineering it is commonly called reliability function. It is declining over time, and will approach zero unless indefinite survival is possible with a finite probability.

The event density $f(t)=\frac{d}{dt}(1-S(t))$ denotes the rate of death per unit time.

The hazard function $\lambda(t)$ is the event rate at time $t$ conditional on survival until time $t$ or later. It is $\lambda(t) = - S'(t)/S(t)$. Note that unlike the event density function this does not have to decline as the number of survivors gets low: this is the overall force of mortality at a given time.

The expected future lifetime given survival to time $t_0$ is $\frac{1}{S(t_0)}\int_{t_0}^\infty S(t)dt.$ Note that for exponential survival curves (i.e. constant hazard) it remains constant.

# The case for Mars

On practical Ethics I post about the goodness of being multi-planetary: is it rational to try to settle Mars as a hedge against existential risk?

The problem is not that it is absurd to care about existential risks or the far future (which was the Economist‘s unfortunate claim), nor that it is morally wrong to have a separate colony, but that there might be better risk reduction strategies with more bang for the buck.

One interesting aspect is that making space more accessible makes space refuges a better option. At some point in the future, even if space refuges are currently not the best choice, they may well become that. There are of course other reasons to do this too (science, business, even technological art).

So while existential risk mitigation right now might rationally aim at putting out the current brushfires and trying to set the long-term strategy right, doing the groundwork for eventual space colonisation seems to be rational.

# Dampening theoretical noise by arguing backwards

Science has the adorable headline Tiny black holes could trigger collapse of universe—except that they don’t, dealing with the paper Gravity and the stability of the Higgs vacuum by Burda, Gregory & Moss. The paper argues that quantum black holes would act as seeds for vacuum decay, making metastable Higgs vacua unstable. The point of the paper is that some new and interesting mechanism prevents this from happening. The more obvious explanation that we are already in the stable true vacuum seems to be problematic since apparently we should expect a far stronger Higgs field there. Plenty of theoretical issues are of course going on about the correctness and consistency of the assumptions in the paper.

# Don’t mention the war

What I found interesting is the treatment of existential risk in the Science story and how the involved physicists respond to it:

Moss acknowledges that the paper could be taken the wrong way: “I’m sort of afraid that I’m going to have [prominent theorist] John Ellis calling me up and accusing me of scaremongering.

Ellis is indeed grumbling a bit:

As for the presentation of the argument in the new paper, Ellis says he has some misgivings that it will whip up unfounded fears about the safety of the LHC once again. For example, the preprint of the paper doesn’t mention that cosmic-ray data essentially prove that the LHC cannot trigger the collapse of the vacuum—”because we [physicists] all knew that,” Moss says. The final version mentions it on the fourth of five pages. Still, Ellis, who served on a panel to examine the LHC’s safety, says he doesn’t think it’s possible to stop theorists from presenting such argument in tendentious ways. “I’m not going to lose sleep over it,” Ellis says. “If someone asks me, I’m going to say it’s so much theoretical noise.” Which may not be the most reassuring answer, either.

There is a problem here in that physicists are so fed up with popular worries about accelerator-caused disasters – worries that are often second-hand scaremongering that takes time and effort to counter (with marginal effects) – that they downplay or want to avoid talking about things that could feed the worries. Yet avoiding topics is rarely the best idea for finding the truth or looking trustworthy. And given the huge importance of existential risk even when it is unlikely, it is probably better to try to tackle it head-on than skirt around it.

# Theoretical noise

“Theoretical noise” is an interesting concept. Theoretical physics is full of papers considering all sorts of bizarre possibilities, some of which imply existential risks from accelerators. In our paper Probing the Improbable we argue that attempts to bound accelerator risks have problems due to the non-zero probability of errors overshadowing the probability they are trying to bound: an argument that there is zero risk is actually just achieving the claim that there is about 99% chance of zero risk, and 1% chance of some risk. But these risk arguments were assumed to be based on fairly solid physics. Their errors would be slips in logic, modelling or calculation rather than being based on an entirely wrong theory. Theoretical papers are often making up new theories, and their empirical support can be very weak.

An argument that there is some existential risk with probability P actually means that, if the probability of the argument is right is Q, there is risk with probability PQ plus whatever risk there is if the argument is wrong (which we can usually assume to be close to what we would have thought if there was no argument in the first place) times 1-Q. Since the vast majority of theoretical physics papers never go anywhere, we can safely assume Q to be rather small, perhaps around 1%. So a paper arguing for P=100% isn’t evidence the sky is falling, merely that we ought to look more closely to a potentially nasty possibility that is likely to turn into a dud. Most alarms are false alarms.

However, it is easier to generate theoretical noise than resolve it. I have spent some time working on a new accelerator risk scenario, “dark fire”, trying to bound the likelihood that it is real and threatening. Doing that well turned out to be surprisingly hard: the scenario was far more slippery than expected, so ruling it out completely turned out to be very hard (don’t worry, I think we amassed enough arguments to show the risk to be pretty small). This is of course the main reason for the annoyance of physicists: it is easy for anyone to claim there is risk, but then it is up to the physics community to do the laborious work of showing that the risk is small.

The vacuum decay issue has likely been dealt with by the Tegmark and Bostrom paper: were the decay probability high we should expect to be early observers, but we are fairly late ones. Hence the risk per year in our light-cone is small (less than one in a billion). Whatever is going on with the Higgs vacuum, we can likely trust it… if we trust that paper. Again we have to deal with the problem of an argument based on applying anthropic probability (a contentious subject where intelligent experts disagree on fundamentals) to models of planet formation (based on elaborate astrophysical models and observations): it is reassuring, but it does not reassure as strongly as we might like. It would be good to have a few backup papers giving different arguments bounding the risk.

# Backward theoretical noise dampening?

The lovely property of the Tegmark and Bostrom paper is that it covers a lot of different risks with the same method. In a way it handles a sizeable subset of the theoretical noise at the same time. We need more arguments like this. The cosmic ray argument is another good example: it is agnostic on what kind of planet-destroying risk is perhaps unleashed from energetic particle interactions, but given the past number of interactions we can be fairly secure (assuming we patch its holes).

One shared property of these broad arguments is that they tend to start with the risky outcome and argue backwards: if something were to destroy the world, what properties does it have to have? Are those properties possible or likely given our observations? Forward arguments (if X happens, then Y will happen, leading to disaster Z) tend to be narrow, and depend on our model of the detailed physics involved.

While the probability that a forward argument is correct might be higher than the more general backward arguments, it only reduces our concern for one risk rather than an entire group. An argument about why quantum black holes cannot be formed in an accelerator is limited to that possibility, and will not tell us anything about risks from Q-balls. So a backwards argument covering 10 possible risks but just being half as likely to be true as a forward argument covering one risk is going to be more effective in reducing our posterior risk estimate and dampening theoretical noise.

In a world where we had endless intellectual resources we would of course find the best possible arguments to estimate risks (and then for completeness and robustness the second best argument, the third, … and so on). We would likely use very sharp forward arguments. But in a world where expert time is at a premium and theoretical noise high we can do better by looking at weaker backwards arguments covering many risks at once. Their individual epistemic weakness can be handled by making independent but overlapping arguments, still saving effort if they cover many risk cases.

Backwards arguments also have another nice property: they help dealing with the “ultraviolet cut-off problem“. There is an infinite number of possible risks, most of which are exceedingly bizarre and a priori unlikely. But since there are so many of them, it seems we ought to spend an inordinate effort on the crazy ones, unless we find a principled way of drawing the line. Starting from a form of disaster and working backwards on probability bounds neatly circumvents this: production of planet-eating dragons is among the things covered by the cosmic ray argument.

Risk engineers will of course recognize this approach: it is basically a form of fault tree analysis, where we reason about bounds on the probability of a fault. The forward approach is more akin to failure mode and effects analysis, where we try to see what can go wrong and how likely it is. While fault trees cannot cover every possible initiating problem (all those bizarre risks) they are good for understanding the overall reliability of the system, or at least the part being modelled.

Deductive backwards arguments may be the best theoretical noise reduction method.

# The end of the worlds

George Dvorsky has a piece on Io9 about ways we could wreck the solar system, where he cites me in a few places. This is mostly for fun, but I think it links to an important existential risk issue: what conceivable threats have big enough spatial reach to threaten a interplanetary or even star-faring civilization?

This matters, since most existential risks we worry about today (like nuclear war, bioweapons, global ecological/societal crashes) only affect one planet. But if existential risk is the answer to the Fermi question, then the peril has to strike reliably. If it is one of the local ones it has to strike early: a multi-planet civilization is largely immune to the local risks. It will not just be distributed, but it will almost by necessity have fairly self-sufficient habitats that could act as seeds for a new civilization if they survive. Since it is entirely conceivable that we could have invented rockets and spaceflight long before discovering anything odd about uranium or how genetics work it seems unlikely that any of these local risks are “it”. That means that the risks have to be spatially bigger (or, of course, that xrisk is not the answer to the Fermi question).

Of the risks mentioned by George physics disasters are intriguing, since they might irradiate solar systems efficiently. But the reliability of them being triggered before interstellar spread seems problematic. Stellar engineering, stellification and orbit manipulation may be issues, but they hardly happen early – lots of time to escape. Warp drives and wormholes are also likely late activities, and do not seem to be reliable as extinctors. These are all still relatively localized: while able to irradiate a largish volume, they are not fine-tuned to cause damage and does not follow fleeing people. Dangers from self-replicating or self-improving machines seems to be a plausible, spatially unbound risk that could pursue (but also problematic for the Fermi question since now the machines are the aliens). Attracting malevolent aliens may actually be a relevant risk: assuming von Neumann probes one can set up global warning systems or “police probes” that maintain whatever rules the original programmers desire, and it is not too hard to imagine ruthless or uncaring systems that could enforce the great silence. Since early civilizations have the chance to spread to enormous volumes given a certain level of technology, this might matter more than one might a priori believe.

So, in the end, it seems that anything releasing a dangerous energy effect will only affect a fixed volume. If it has energy $E$ and one can survive it below a deposited energy $e$, if it just radiates in all directions the safe range is $r = \sqrt{E/4 \pi e} \propto \sqrt{E}$ – one needs to get into supernova ranges to sterilize interstellar volumes. If it is directional the range goes up, but smaller volumes are affected: if a fraction $f$ of the sky is affected, the range increases as $\propto \sqrt{1/f}$ but the total volume affected scales as $\propto f\sqrt{1/f}=\sqrt{f}$.

Self-sustaining effects are worse, but they need to cross space: if their space range is smaller than interplanetary distances they may destroy a planet but not anything more. For example, a black hole merely absorbs a planet or star (releasing a nasty energy blast) but does not continue sucking up stuff. Vacuum decay on the other hand has indefinite range in space and moves at lightspeed. Accidental self-replication is unlikely to be spaceworthy unless is starts among space-moving machinery; here deliberate design is a more serious problem.

The speed of threat spread also matters. If it is fast enough no escape is possible. However, many of the replicating threats will have sublight speed and could hence be escaped by sufficiently paranoid aliens. The issue here is if lightweight and hence faster replicators can always outrun larger aliens; given the accelerating expansion of the universe it might be possible to outrun them by being early enough, but our calculations do suggest that the margins look very slim.

The more information you have about a target, the better you can in general harm it. If you have no information, merely randomizing it with enough energy/entropy is the only option (and if you have no information of where it is, you need to radiate in all directions). As you learn more, you can focus resources to make more harm per unit expended, up to the extreme limits of solving the optimization problem of finding the informational/environmental inputs that cause desired harm (=hacking). This suggests that mindless threats will nearly always have shorter range and smaller harms than threats designed by (or constituted by) intelligent minds.

In the end, the most likely type of actual civilization-ending threat for an interplanetary civilization looks like it needs to be self-replicating/self-sustaining, able to spread through space, and have at least a tropism towards escaping entities. The smarter, the more effective it can be. This includes both nasty AI and replicators, but also predecessor civilizations that have infrastructure in place. Civilizations cannot be expected to reliably do foolish things with planetary orbits or risky physics.

# The 12 threats of xrisk

The Global Challenges Foundation has (together with FHI) produced a report on the 12 risks that threaten civilization.

And, yes, the use of “infinite impact” grates on me – it must be interepreted as “so bad that it is never acceptable”, a ruin probability, or something similar, not that the disvalue diverges. But the overall report is a great start on comparing and analysing the big risks. It is worth comparing it with the WEF global risk report, which focuses on people’s perceptions of risk. This one aims at looking at what risks are most likely/impactful. Both try to give reasons and ideas for how to reduce the risks. Hopefully they will also motivate others to make even sharper analysis – this is a first sketch of the domain, rather than a perfect roadmap. Given the importance of the issues, it is a bit worrying that it has taken us this long.

# Existential risk and hope

Toby and Owen started 2015 by defining existential hope, the opposite of existential risk.

In their report “Existential Risk and Existential Hope: Definitions” they look at definitions of existential risk. The initial definition was just the extinction of humanity, but that leaves out horrible scenarios where humanity suffers indefinitely, or situations where there is a tiny chance of humanity escaping. Chisholming their way through successive definitions they end up with:

An existential catastrophe is an event which causes the loss of most expected value.

They also get the opposite:

An existential eucatastrophe is an event which causes there to be much more expected value after the event than before.

So besides existential risk, where the value of our future can be lost, there is existential hope: the chance that our future is much greater than we expect. Just as we should work hard to avoid existential threats, we should explore to find potential eucatastrophes that vastly enlarge our future.

## Infinite hope or fear

One problem with the definitions I can see is that expectations can be undefined or infinite, making “loss of most expected value” undefined. That would require potentially unbounded value, and that the probability of reaching a certain level has a sufficiently heavy tail. I guess most people would suspect the unbounded potential to be problematic, but at least some do think there could be infinite value somewhere in existence (I think this is what David Deutsch believes). The definition ought to work regardless of what kind of value structure exists in the universe.

There are a few approaches in Nick’s “Infinite ethics” paper. However, there might be simpler approaches based on stochastic dominance. Cutting off the upper half of a Chauchy distribution does change the situation despite the expectation remaining undefined (and in this case, changes the balance between catastrophe and eucatastrophe completely). It is clear that there is now more probability on the negative side: one can do a (first order) stochastic ordering of the distributions, even though the expectations diverge.

There are many kinds of stochastic orderings; which ones make sense likely depends on the kind of value one uses to evaluate the world. Toby and Owen point out that this what actually does the work in the definitions: without a somewhat precise value theory existential risk and hope will not be well defined. Just as there may be unknown threats and opportunities, there might be surprise twists in what is valuable – we might in the fullness of time discover that some things that looked innocuous or worthless were actually far more weighty than we thought, perhaps so much that they were worth the world.

# Cool risks outside the envelope of nature

How do we apply the precautionary principle to exotic, low-probability risks?

The CUORE collaboration at the INFN Gran Sasso National Laboratory recently set a world record by cooling a cubic meter 400 kg copper vessel down to 6 milliKelvins: it was the coldest cubic meter in the universe for over 15 days. Yay! Applause! (And the rest of this post should in no way be construed as a criticism of the experiment)

## Cold and weird risks

I have not been able to dig up the project documentation, but I would be astonished if there was any discussion of risk due to the experiment. After all, cooling things is rarely dangerous. We do not have any physical theories saying there could be anything risky here. No doubt there are risk assessment of liquid nitrogen or helium practical risks somewhere, but no analysis of any basic physics risks.

Compare this to the debates around the LHC, where critics at least could point to papers suggesting that strangelets, small black holes and vacuum decay were theoretically possible. Yet the LHC could argue back that particle processes like those occurring in the accelerator were already naturally occurring almost everywhere: if the LHC was risky, we ought to see plenty of explosions in the sky. Leaving aside the complications of correcting for anthropic bias, this kind of argument seems reasonably solid: if you do something that is within the envelope of what happens in the universe normally and there are no observed super-dangerous processes linked to it, then this activity is likely fine. We might wish for careful risk assessment, but given that the activity is already happening it can be viewed as just as benign as the normal activity of the universe.

However, the CUORE experiment is actually going outside of the envelope of what we think is going on in the universe. In the past, the universe has been hotter, so there would not have been any large masses at 6 milliKelvins. And with a 3 Kelvin background temperature, there would not be any natural objects this cold. (Since 1995 there have been small Bose-Einstein condensates in the hundred nanoKelvin range on Earth, but the argument is the same.)

How risky is it to generate such an outside of the envelope phenomenon? There is no evidence from the past. There is no cause for alarm given the known laws of physics. Yet this lack of evidence does not argue against risk either. Maybe there is an ice-9 like phase transition of matter below a certain temperature. Maybe it implodes into a black hole because of some macroscale quantum(gravity) effect. Maybe the alien spacegods get angry. There is an endless number of possible hypotheses that cannot be ruled out.

We might think that such “small theories” can safely be ignored. But we have some potential evidence that the universe may be riskier than it looks: the Fermi paradox, the apparent absence of alien intelligence. If we are alone, it is either because there are one or more steps in the evolution of life and intelligence that are very unlikely (the “great filter” is behind us), or there is a high likelihood that intelligence disappears without a trace (a future great filter). Now, we might freely assign our probabilities to (1) that there are aliens around, (2) that the filter is behind us, and (3) that it is ahead. However, given our ignorance we cannot rationally give zero probability to any of these possibilities, and probably not even give any of them less than 1% (since that is about the natural lowest error rate of humans on anything). Anybody saying one of them is less likely than one in a million is likely very overconfident. Yet a 1% risk of a future great filter implies a huge threat. It is a threat that not only reliably wipes out intelligent life, but also does it to civilizations aware of its potential existence!

We then have a slightly odd reason to be slightly concerned with experiments like CUORE. We know there is some probability that intelligence gets reliably wiped out. We know intelligence is likely to explore conditions not found in the natural universe. So a potential explanation could be that there is some threat in this exploration. The probability is not enormous – we might think the filter is behind us or the universe is teeming with aliens, and even if there is a future filter there are many possibilities for what it could be besides low-temperature physics – but nearly any non-infinitesimal probability multiplied by the value of our species (at least 7 billion lives) tends to lead to a too large risk.

## Precaution?

At this point the precautionary principle rears its stupid head (the ugly head is asleep). The stupid head argues that we should hence never do anything that is outside the natural envelope.

The ugly head would argue we should investigate before doing anything risky, but since in this case the empirical studying is causing the risk the head would hence advice just trying out theoretical risk scenarios – not very useful given that we are dealing with something where all potential risk comes from scenarios unconstrained by evidence!

We cannot obey the stupid head much, since most human activity is about pushing the envelope. We are trying to have more and happier people than has ever existed in the universe before. Maybe that is risky (compare to Stapledon’s Last and First Men where it turned out to be dangerous to have too much intelligence in one spot), but it is both practically hard to prevent and this kind of open-ended “let’s not do anything that has not happened in the past” seems unreasonable given that most events are new ones and generally do not lead to disasters. But the pushing of the envelope into radically new directions does carry undefinable risk. We cannot avoid that. What we can do is to discuss whether we are willing to take on such hard to pin down risk.

However, this example also shows a way precaution can break down. Nobody has, to my knowledge, worried about cooling down matter besides me. There is no concerned group urging precaution since there is no empirical nor normative reason to think there is anything wrong specifically with CUORE: we only have a general Fermi paradox-induced inchoate worry. Yet proper precaution requires considering weak possibilities. I suspect that most future big new disasters will turn out to have avoided precautionary considerations just because there was no obvious reason to invoke the principle.

## Conclusion?

Many people are scared more by uncertainty than actual risk. But we cannot escape it. Especially if we want to reduce existential risk, which tends to be more uncertain than most. This little essay is about some of the really tricky limits to what we can know about new risks. We should expect them to be unexpected. And we should expect that the standard decision methods will not behave sensibly.

As for the CUORE team, I wish them the best of luck to find neutrinoless double beta decay. But they should keep an eye open for weird anomalies too – they have a chance to peek outside the envelope of the natural in a well controlled setting, and that is valuable.