Halloween explanation of Fermi question


John Harris proposed a radical solution to the KIC 8462852 problem: it is a Halloween pumpkin.

A full Dyson sphere does not have to be 100% opaque. It consists of independently orbiting energy collectors, presumably big flat surfaces. But such collectors can turn their thin side towards the star, letting past starlight. So with the right program, your Dyson sphere could project any pattern of light like a lantern.

Of course, the real implication of this is that we should watch out for trick-or-treating alien super-civilizations. By using self-replicating Bracewell probes they could spread across the Milky way within a few million years: they ought to be here by now. And in this scenario they are… they are just hiding until KIC 8462852 suddenly turns into a skull, and suddenly the skies will swarming with their saucers demanding we give them treats – or suffer their tricks…

There is just one problem: when is galactic Halloween? A galactic year is 250 million years. We have a 1/365 chance of being in the galactic “day” corresponding to Halloween (itself 680,000 years long). We might be in for a long night…


Likely not even a microDyson

XIX: The Dyson SunRight now KIC 8462852 is really hot, and not just because it is a F3 V/IV type star: the light curve, as measured by Kepler, has irregular dips that looks like something (or rather, several somethings) are obscuring the star. The shapes of the dips are odd. The system is too old and IR-clean to have a remaining protoplanetary disk, dust clumps would coalesce, the aftermath of a giant planet impact is very unlikely (and hard to fit with the aperiodicity); maybe there is a storm of comets due to a recent stellar encounter, but comets are not very good at obscuring stars. So a lot of people on the net are quietly or not so quietly thinking that just maybe this is a Dyson sphere under construction.

I doubt it.

My basic argument is this: if a civilization builds a Dyson sphere it is unlikely to remain small for a long period of time. Just as planetary collisions are so rare that we should not expect to see any in the Kepler field, the time it takes to make a Dyson sphere is also very short: seeing it during construction is very unlikely.

Fast enshrouding

In my and Stuart Armstrong’s paper “Eternity in Six Hours” we calculated that disassembling Mercury to make a partial Dyson shell could be done in 31 years. We did not try to push things here: our aim was to show that using a small fraction of the resources in the solar system it is possible to harness enough energy to launch a massive space colonization effort (literally reaching every reachable galaxy, eventually each solar system). Using energy from already built solar captors more material is mined and launched, producing an exponential feedback loop. This was originally discussed by Robert Bradbury. The time to disassemble terrestrial planets is not much longer than for Mercury, while the gas giants would take a few centuries.

If we imagine the history of a F5 star 1,000 years is not much. Given the estimated mass of KIC 8462852 as 1.46 solar masses, it will have a main sequence lifespan of 4.1 billion years. The chance of seeing it while being enshrouded is one in 4.3 million. This is the same problem as the giant impact theory.

A ruin?

An abandoned Dyson shell would likely start clumping together; this might at first sound like a promising – if depressing – explanation of the observation. But the timescale is likely faster than planetary formation timescales of 10^510^6 years – the pieces are in nearly identical orbits – so the probability problem remains.

But it is indeed more likely to see the decay of the shell than the construction by several orders of magnitude. Just like normal ruins hang around far longer than the time it took to build the original building.

Laid-back aliens?

Maybe the aliens are not pushing things? Obviously one can build a Dyson shell very slowly – in a sense we are doing it (and disassembling Earth to a tiny extent!) by launching satellites one by one. So if an alien civilization wanted to grow at a leisurely rate or just needed a bit of Dyson shell they could of course do it.

However, if you need something like 2.87\cdot 10^{19} Watt (a 100,000 km collector at 1 AU around the star) your demands are not modest. Freeman Dyson originally proposed the concept based on the observation that human energy needs were growing exponentially, and this was the logical endpoint. Even at 1% growth rate a civilization quickly – in a few millennia – need most of the star’s energy.

In order to get a reasonably high probability of seeing an incomplete shell we need to assume growth rates that are exceedingly small (on the order of less than a millionth per year). While it is not impossible, given how the trend seems to be towards more intense energy use in many systems and that entities with higher growth rates will tend to dominate a population, it seems rather unlikely. Of course, one can argue that we currently can more easily detect the rare laid-back civilizations than the ones that aggressively enshrouded their stars, but Dyson spheres do look pretty rare.

Other uses?

Dyson shells are not the only megastructures that could cause intriguing transits.

C. R. McInnes has a suite of fun papers looking at various kinds of light-related megastructures. One can sort asteroid material using light pressure, engineer climate, adjust planetary orbits, and of course travel using solar sails. Most of these are smallish compared to stars (and in many cases dust clouds), but they show some of the utility of obscuring objects.

Duncan Forgan has a paper on detecting stellar engines (Shkadov thrusters) using light curves; unfortunately the calculated curves do not fit KIC8462852 as far as I can tell.

Luc Arnold analysed the light curves produced by various shapes of artificial objectsHe suggested that one could make a weirdly shaped mask for signalling one’s presence using transits. In principle one could make nearly any shape, but for signalling something unusual yet simple enough to be artificial would make most sense: I doubt the KIC transits fit this.

More research is needed (duh)

In the end, we need more data. I suspect we will find that it is yet another odd natural phenomenon or coincidence. But it makes sense to watch, just in case.

Were we to learn that there is (or was) a technological civilization acting on a grand scale it would be immensely reassuring: we would know intelligent life could survive for at least some sizeable time. This is the opposite side of the Great Filter argument for why we should hope not to see any extraterrestrial life: life without intelligence is evidence for intelligence either being rare or transient, but somewhat non-transient intelligence in our backyard (just 1,500 light-years away!) is evidence that it is neither rare nor transient. Which is good news, unless we fancy ourselves as unique and burdened by being stewards of the entire reachable universe.

But I think we will instead learn that the ordinary processes of astrophysics can produce weird transit curves, perhaps due to weird objects (remember when we thought hot jupiters were exotic?) The universe is full of strange things, which makes me happy I live in it.

[An edited version of this post can be found at The Conversation: What are the odds of an alien megastructure blocking light from a distant star? ]

Time travel system

By Stuart Armstrong


I’ve been looking to develop a system of time travel in which it’s possible to actually have a proper time war. To make it consistent and interesting, I’ve listed some requirements here. I think I have a fun system that obeys them all.

Time travel/time war requirement:

  • It’s possible to change the past (and the future). These changes don’t just vanish.
  • It’s useful to travel both forwards and backwards in time.
  • You can’t win by just rushing back to the Big Bang, or to the end of the universe.
  • There’s no “orthogonal time” that time travellers follow; I can’t be leaving 2015 to go to 1502 “while” you’re leaving 3015 to arrive at the same place.
  • You can learn about the rules of time travel while doing it; however, time travel must be dangerous to the ignorant (and not just because machines could blow up, or locals could kill you).
  • No restrictions that make no physical sense, or that could be got round by a human or a robot with half a brain. Eg: “you can’t make a second time jump from the arrival point of a first.” However, a robot could build a second copy of a time machine and of itself, and that could then jump back; therefore that initial restriction doesn’t make any particular sense.
  • Similarly, no restrictions that are unphysical or purely narrative.
  • It must be useful to, for instance, leave arrays of computers calculating things for you then jumping to the end to get the answer.
  • Ideally, there would be only one timeline. If there are parallel universes, they must be simply describable, and allow time-travellers to interact with each other in ways they would care about.
  • A variety of different strategies must be possible for fighting the war.

Consistent time travel

Earlier, I listed some requirements for a system of time travel – mainly that it be both scientifically consistent and open to interesting conflicts that aren’t trivially one-sided. Here is my proposal for such a thing, within the general relativity format.

So, suppose you build a time machine, and want to go back in time to kill Hitler, as one does. Your time machine is a 10m diameter sphere, which exchanges place with a similarly-size sphere in 1930. What happens then? The graph here shows the time jump, and the “light-cones” for the departure (blue) and arrival (red) points; under the normal rules of causality, the blue point can only affect things in the grey cone, the red point can only affect things in the black cone.

time_travel01The basic idea is that when you do a time jump like this, then you “fix” your points of departure and arrival. Hence the blue and red points cannot be changed, and the universe rearranges itself to ensure this. The big bang itself is also a fixed point.

All this “fixed point” idea is connected to entropy. Basically, we feel that time advances in one direction rather than the other. Many have argued that this is because entropy (roughly, disorder) increases in one direction, and that this direction points from the past to the future. Since most laws of physics are symmetric in the past and the future, I prefer to think of this as “every law of physics is time-symmetric, but the big bang is a fixed point of low entropy, hence the entropy increase as we go away from it.”

But here I’m introducing two other fixed points. What will that do?

Well, initially, not much. You go back into time, and kill Hitler, and the second world war doesn’t happen (or maybe there’s a big war of most of Europe against the USSR, see configuration 2 in “A Landscape Theory of Aggregation”). Yay! That’s because, close to the red point, causality works pretty much as you’d expect.

However, close to the blue point, things are different.


Here, the universe starts to rearrange things so that the blue point is unchanged. Causality isn’t exactly going backwards, but it is being funnelled in a particular direction. People who descended from others who “should have died” in WW2 start suddenly dying off. Memories shift; records change. By the time you’re very close to the blue point, the world is essentially identical to what it would have been had there been no time travelling.

Does this mean that you time jump made no difference? Not at all. The blue fixed point only constrains what happens in the light cone behind it (hence the red-to-blue rectangle in the picture). Things outside the rectangle are unconstrained – in particular, the future of that rectangle. Now, close to the blue point, the events are “blue” (ie similar to the standard history), so the future of those events are also pretty blue (similar to what would have been without the time jump) – see the blue arrows. At the edge of the rectangle, however, the events are pretty red (the alternative timeline), so the future is also pretty red (ie changed) – see the red arrows. If the influence of the red areas converges back in to the centre, the future will be radically different.

(some people might wonder why there aren’t “changing arrows” extending form the rectangle into the past as well as the future. There might be, but remember we have a fixed point at the big bang, which reduces the impact of these backward changes – and the red point is also fixed, exerting a strong stabilising influence for events in its own backwards light-cone).

So by time travelling, you can change the past, and you can change part of the future – but you can’t change the present.

But what would happen if you stayed alive from 1930, waiting and witnessing history up to the blue point again? This would be very dangerous; to illustrate, let’s change the scale, and assume we’ve only jumped a few minutes into the past.

time_travel03Maybe there you meet your past self, have a conversation about how wise you are, try and have sex with yourself, or whatever time travellers do with past copies of themselves. But this is highly dangerous! Within a few minutes, all trace of future you’s presesence will be gone; you past self will have no memory of it, there will be no physical or mental evidence remaining.

Obviously this is very dangerous for you! The easiest way for there to remain no evidence of you, is for there to be no you. You might say “but what if I do this, or try and do that, or…” But all your plans will fail. You are fighting against causality itself. As you get closer to the blue dot, it’s as if time itself was running backwards, erasing your new timeline, to restore the old one. Cleverness can’t protect you against an inversion of causality.

Your only real chance of survival (unless you do a second time jump to get out of there) is to rush away from the red point at near light-speed, getting yourself to the edge of the rectangle and ejecting yourself from the past of the blue point.

Right, that’s the basic idea!

Multiple time travellers

Ok, the previous section looked at a single time traveller. What happens when there are several? Say two time travellers (blue and green) are both trying to get to the red point (or places close to it). Who gets there “first”?

time_travel04Here is where I define the second important concept for time-travel, that of “priority”. Quite simply, a point with higher priority is fixed relative to the other. For instance, imagine that the blue and green time travellers appear in close proximity to each other:


This is a picture where the green time traveller has a higher priority than the blue one. The green arrival changes the timeline (the green cone) and the blue time traveller fits themselves into this new timeline.

If instead the blue traveller had higher priority, we get the following scenario:


Here the blue traveller arrives in the original (white) timeline, fixing their arrival point. The green time traveller arrives, and generates their own future – but this has to be put back into the first white timeline for the arrival of the blue time traveller.

Being close to a time traveller with a high priority is thus very dangerous! The green time traveller may get erased if they don’t flee-at-almost-light-speed.

Even arriving after a higher-priority time traveller is very dangerous – suppose that the green one has higher priority, and the blue one arrives after. Then suppose the green one realises they’re not exactly at the right place, and jump forwards a bit; then you get:

time_travel07(there’s another reason arriving after a higher priority time traveller is dangerous, as we’ll see).

So how do we determine priority? The simplest seems time-space distance. You start with a priority of zero, and this priority goes down proportional to how far your jump goes.

What about doing a lot of short jumps? You don’t want to allow green to get higher priority by doing a series of jumps:time_travel08This picture suggests how to proceed. Your first jump brings you a priority of -70. Then the second adds a second penalty of -70, bringing the priority penalty to -140 (the yellow point is another time traveller, who will be relevant soon)

How can we formalise this? Well, a second jump is a time jump that would not happen if the first jump hadn’t. So for each arrival in a time jump, you can trace it back to the original jump-point. Then your priority score is the (negative) of the volume of the time-space cone determined by the arrival and original jump-point. Since this volume is the point where your influence is strongest, this makes sense (note for those who study special relativity: using this volume means that you can’t jump “left along a light-beam”, then “right along a light-beam” and arrive with a priority of 0, which you could do if we used distance travelled rather than volume).

Let’s look at that yellow time traveller again. If there was no other time traveller, they would jump from that place. But because of the arrival of the green traveller (at -70), the ripples cause them to leave from a different point in space time, the purple one (the red arrow shows that the arrival there prevents the green time jump, and cause the purple time jump):time_travel09So what happens? Well, the yellow time jump will still happen. It has a priority of 0 (it happened without any influence of any time traveller), so the green arrival at -70 priority can’t change this fixed point. The purple time jump will also happen, but it will happen with a lower priority of -30, since it was caused by time jumps that can ultimately be traced back to the green 0 point. (note: I’m unsure whether there’s any problem with allowing priority to rise as you get back closer to your point of origin; you might prefer to use the smallest cone that includes all jump points that affected you, so the purple point would have priority -70, just like the green point that brought it into existence).

What other differences could there be between the yellow and the purple version? Well, for one, the yellow has no time jumps in their subjective pasts, while the purple has one – the green -70. So as time travellers wiz around, they create (potential) duplicate copies of themselves and other time travellers – but those with the highest priority, and hence the highest power, are those who have no evidence that time jumps work, and do short jumps. As your knowledge of time travel goes up, and as you explore more, your priority sinks, and you become more vulnerable.

So it’s very dangerous even having a conversation with someone of higher priority than yourself! Suppose Mr X talks with Mrs Y, who has higher priority than him. Any decision that Y does subsequently has been affected by that conversation, so her priority sinks to X’s level (call her Y’). But now imagine that, if she wouldn’t have had that conversation, she would have done another time jump anyway. The Y who didn’t have the conversation is not affected by X, so retains her higher priority.

So, imagine that Y would have done another time jump a few minutes after arrival. X arrives and convinces her not to do so (maybe there’s a good reason for that). But the “time jump in an hour” will still happen, because the unaffected Y has higher priority, and X can’t change that. So if the X and Y’ talk or linger too long, they run the risk of getting erased as they get close to the “point where Y would have jumped if X hadn’t been there”. In graphical form, the blue-to-green square is the area in which X and Y’ can operate in, unless they can escape into the white bands:time_travel10So the greatest challenge for a low priority time-traveller is to use their knowledge to evade erasure by higher priority ones. They have a much better understanding of what’s going on, they may know where other time jumps likely end up at or start, they might have experience at “rushing at light speed to get out of cone of danger while preserving most of their personality and memories” (or technology that helps them do so), but they are ever vulnerable. They can kill or influence higher priority time-travellers, but this will only work “until” the point where they would have done a time jump otherwise (and the cone before that point).

So, have I succeeded in creating an interesting time-travel design? Is it broken in any obvious way? Can you imagine interesting stories and conflicts being fought there?


The Biosphere Code

Let's build a smarter planetYesterday I contributed to a piece of manifesto writing, producing the Biosphere Code Manifesto. The Guardian has a version on its blog. Not quite as dramatic as Marinetti’s Futurist Manifesto but perhaps more constructive:

Principle 1. With great algorithmic powers come great responsibilities

Those implementing and using algorithms should consider the impacts of their algorithms.

Principle 2. Algorithms should serve humanity and the biosphere at large.

Algorithms should be considerate of human needs and the biosphere, and facilitate transformations towards sustainability by supporting ecologically responsible innovation.

Principle 3. The benefits and risks of algorithms should be distributed fairly

Algorithm developers should consider issues relating to the distribution of risks and opportunities more seriously. Developing algorithms that provide benefits to the few and present risks to the many are both unjust and unfair.

Principle 4. Algorithms should be flexible, adaptive and context-aware

Algorithms should be open, malleable and easy to reprogram if serious repercussions or unexpected results emerge. Algorithms should be aware of their external effects and be able to adapt to unforeseen changes.

Principle 5. Algorithms should help us expect the unexpected

Algorithms should be used in such a way that they enhance our shared capacity to deal with shocks and surprises – including problems caused by errors or misbehaviors in other algorithms.

Principle 6. Algorithmic data collection should be open and meaningful

Data collection should be transparent and respectful of public privacy. In order to avoid hidden biases, the datasets which feed into algorithms should be validated.

Principle 7. Algorithms should be inspiring, playful and beautiful

Algorithms should be used to enhance human creativity and playfulness, and to create new kinds of art. We should encourage algorithms that facilitate human collaboration, interaction and engagement – with each other, with society, and with nature.

The algorithmic world

World gross economic productThe basic insight is that the geosphere, ecosphere, anthroposphere and technosphere are getting deeply entwined, and algorithms are becoming a key force in regulating this global system.

Some algorithms enable new activities (multimedia is impossible without FFT and CRC), change how activities are done (data centres happen because virtualization and MapReduce make them scale well), or enable faster algorithmic development (compilers and libraries). Algorithms used for decision support are particularly important. Logistics algorithms (routing, linear programming, scheduling, and optimization) affect the scope and efficiency of the material economy. Financial algorithms the scope and efficiency of the economy itself. Intelligence algorithms (data collection, warehousing, mining, network analysis but also human expert judgement combination methods), statistics gathering and risk models affect government policy. Recommender systems (“You May Also Enjoy…”) and advertising influence consumer demand.

Since these algorithms are shared, their properties will affect a multitude of decisions and individuals in the same way even if they think they are acting independently. There are spillover effects from the groups that use algorithms to other stakeholders from the algorithm-caused  actions. And algorithms have a multitude of non-trivial failure modes: machine learning can create opaque bias or sudden emergent misbehaviour, human over-reliance on algorithms can cause accidents or large-scale misallocation of resources, some algorithms produce systemic risks, and others embody malicious behaviours. In short, code – whether in computers or as a formal praxis in an organisation – matters morally.

What is the point?

Photo codeCould a code like the Biosphere Code actually do anything useful? Isn’t this yet another splashy “wouldn’t it be nice if everybody were moral and rational in engineering/politics/international relations?”

I think it is a first step towards something useful.

There are engineering ethics codes, even for software engineers. But algorithms are created in many domains, including by non-engineers. We can not and should not prevent people from thinking, proposing, and trying new algorithms: that would be like attempts to regulate science, art, and thought. But we can as societies create incentives to do constructive things and avoid known destructive things. In order to do so, we should recognize that we need to work on the incentives and start gathering information.

Algorithms and their large-scale results must be studied and measured: we cannot rely on theory, despite its seductive power since there are profound theoretical limitations about our predictive abilities in the world of algorithms, as well as obvious practical limitations. Algorithms also do not exist in a vacuum: the human or biosphere context is an active part of what is going on. An algorithm can be totally correct and yet be misused in a harmful way because of its framing.

But even in the small, if we can make one programmer think a bit more about what they are doing and choosing a better algorithm than they otherwise would have done, the world is better off. In fact, a single programmer can have surprisingly large impact.

I am more optimistic than that. Recognizing algorithms as the key building blocks that they are for our civilization, what peculiarities they have, and learning better ways of designing and using them has transformative power. There are disciplines dealing with parts of this, but the whole requires considering interdisciplinary interactions that are currently rarely explored.

Let’s get started!

Universal principles?

Essence of ethicsI got challenged on the extropian list, which is a fun reason to make a mini-lecture.

On 2015-10-02 17:12, William Flynn Wallace wrote:
> ​Anders says above that we have discovered universal timeless principles.​ I’d like to know what they are and who proposed them, because that’s chutzpah of the highest order. Oh boy – let’s discuss that one.

Here is one: a thing is identical to itself. (1)

Here is another one: “All human beings are born free and equal in dignity and rights.” (2)

Here is a third one: “Act only according to that maxim whereby you can, at the same time, will that it should become a universal law.” (3)

(1) was first explicitly mentioned by Plato (in Theaetetus). I think you also agree with it – things that are not identical to themselves are unlikely to even be called “things”, and without the principle very little thinking makes sense.

I am not sure whether it is chutzpah of the highest order or a very humble observation.

(2) is from the UN declaration of universal human rights. This sentence needs enormous amounts of unpacking – “free”, “equal”, “dignity”, “rights”… these words can (and are) used in very different ways. Yet I think it makes sense to say that according to a big chunk of Western philosophy this sentence is a true sentence (in the sense that ethical propositions are true), that it is universal (the truth is not contingent on when and where you are, although the applications may change), and we know historically that we have not known this principle forever. Now *why* it is true quickly branches out into different answers depending on what metaethical positions you hold, not to mention the big topic of what kind of truth moral truth actually is (if anything). The funny thing is that the universal part is way less contentious, because of the widely accepted (and rarely stated) formal ethical principle that if it is moral to P in situation X, then the location in time and space where X happens does not matter.

Chutzpah of the highest order? Totally. So is the UN.

(3) is Immanuel Kant, and he argued that any rational moral agent could through pure reason reach this principle. It is in many ways like (1) almost a consistency requirement of moral will (not action, since he doesn’t actually care about the consequences – we cannot fully control those, but we can control what we decide to do). There is a fair bit of unpacking of the wording, but unlike the UN case he defines his terms fairly carefully in the preceding text. His principle is, if he is right, the supreme principle of morality.

Chuzpah auf höchstem Niveau? Total!

Note that (1) is more or less an axiom: there is no argument for why it is true, because there is little point in even trying. (3) is intended to be like a theorem in geometry: from some axioms and the laws of logic, we end up with the categorical imperative. It is just as audacious or normal as the Pythagorean theorem. (2) is a kind of compromise between different ethical systems: the Kantians would defend it based on their system, while consequentialists could make a rule utilitarian argument for why it is true, and contractualists would say it is true because the UN agrees on it. They agree on the mid-level meaning, but not on the other’s derivations. It is thick, messy and political, yet also represents fairly well what most educated people would conclude (of course, they would then show off by disagreeing loudly with each other about details, obscuring the actual agreement).

Philosopher’s views

Do people who think about these things actually believe in universal principles? One fun source is David Bourget and David J. Chalmers’ survey of professional philosophers (data). 56.4% of the respondents were moral realists (there are moral facts and moral values, and that these are objective and independent of our views), 65.7% were moral cognitivists (ethical sentences can be true or false); these were correlated to 0.562. 25.9% were deontologists, which means that they would hold somewhat Kant-like views that some actions are always or never right (some of the rest of course also believe in principles, but the survey cannot tell us anything more). 71.1% thought there was a priori knowledge (things we know by virtue of being thinking beings rather than experience).

My views

Do I believe in timeless principles? Kind of. There are statements in physics that are invariant of translations, rotations, Lorenz boosts and other transformations, and of course math remains math. Whether physics and math are “out there” or just in minds is hard to tell (I lean towards that at least physics is out there in some form), but clearly any minds that know some subset of correct, invariant physics and math can derive other correct conclusions from it. And other minds with the same information can make the same derivations and reach the same conclusions – no matter when or where. So there are knowable principles in these domains every sufficiently informed and smart mind would know. Things get iffy with values, since they might be far more linked to the entities experiencing them, but clearly we can do analyse game theory and make statements like “If agent A is trying to optimize X, agent B optimizes Y, and X and Y do not interact, then they can get more of X and Y by cooperating”. So I think we can get pretty close to universal principles in this framework, even if it turns out that they merely reside inside minds knowing about the outside world.