Diagonal And Vertical Time Travel

This is a bit of a thought dump where I try to work out the details of something I’m planning to use in a story, along with other bits I’ve adapted from elsewhere. It’s about time (travel).

I want to talk first of all about metaphors for the passage of time in language and, well, gestures I suppose, and it’s possible that this also comes into sign language. We tend to talk about going forwards and backwards in time, about “past” events and events “to come” and so forth. We also have links between the direction of writing, and what follows from that, and that of time. Our clocks move clockwise and progress bars on audio and video place the “early” direction on the left and “late” on the right. This has been going on for longer than the existence of easy video and audio playback on digital devices, because on tapes, for example, we have fast forward and play, with arrows pointing to the right, and rewind, with arrows pointing to the left. One thing I don’t know is whether Arabic and Hebrew use the same convention, even though their writing proceeds from right to left, but I would expect the dominance of Western culture to have dictated this. A less well-known aspect of this is somewhat more jarring for us as humans, as opposed to us as humans literate in Latin script. Languages are often described as having features to the right or the left, for example as having prefixes to the left and suffixes to the right, focussing on spoken language, but written language doesn’t always go that way, so it means that Hebrew, for example, might be described as having the definite article to the “left” of a noun even though as written that would be on the right. Incidentally, when people pause to think during a conversation, they will look to their right and up in cultures where writing is left-to-right and to their left and up where it’s right-to-left, and since this is learnt from other literate individuals and diffuses through culture that way, it’s even true of illiterate individuals such as small children. This illustrates how pervasive our time flow metaphors and conventions are. It should also be mentioned, for the sake of completeness as this probably won’t come up again here, that time can be thought of as linear or cyclical, and in theory cyclical time could be moving in any direction, although again this would depend on clockwise or anti-clockwise convention.

I could be wrong about this but I seem to remember that Chinese culture, inasmuch as it’s a single culture, agrees with my own view that the passage of time is vertical, falling from top to bottom. I presume the reason for this is the vertical direction of Chinese and related scripts, which brings me to wonder how it worked in Mongolia when it too had a vertical script. Another set of options for script direction exists in boustrophedon, where direction alternates and characters can be either inverted or mirrored, and I have no idea if that has any bearing on temporal metaphors. However, I want to put a case for a vertical time flow metaphor besides script direction, which seems fairly arbitrary, and then I can get on with this.

X, Y and Z axes are usually organised in order of left-to-right, front-to-back and low-to-high. On a graph, the Z axis usually points into the paper or screen and away from the viewer. The time axis is often referred to as W, because they ran out of letters and had to go backwards. As H.G. Wells pointed out in ‘The Time Machine’, gravity conventionally restricts our movement in the third dimension, i.e. height, in that if we consider the centre of, in our case, Earth as below us, we are liable to fall in that direction without support and have difficulty increasing our height above a reference level because it pulls us down. This difficulty is paralleled by our perception of the passage of time, because we are relentlessly propelled into later time from earlier, a process which is usually described as “forward” in time. Because of this similarity, it makes more sense to me to think of the passage of time like a waterfall, falling from top to bottom, and also as facing upwards since we have greater difficulty with accurately perceiving the future than the immediate past. This also has the advantage of working better in gestures and diagrams, because a vertical picture of the passage of time is the same for all observers, unlike time itself, whereas a time flow metaphor of forwards and backwards, or left and right, is reversed for someone opposite. That said, I’m pretty sure I don’t consistently gesture in this way. This vertical metaphor also works for the likes of family trees, and may also be used for timelines although this contradicts the order in which, for example, strata are laid down. A chronicle written in Latin script would, however, proceed vertically down the page, in a scroll, and in a codex (spined book) this would also be true when it was closed – the Book of Genesis is at the “top” of the Bible and Exodus is underneath it, in an English Bible, though in the Tanakh it’s the other way round. Family trees, incidentally, are actually the other way up compared to real trees.

Thinking of spacetime as a block, and simplifying space to two dimensions to make it possible to visualise it, it can be thought of as a kind of transparent cube with events embedded in it like flies in amber. The longer something goes on for, the longer its world-line is, from top to bottom, and at ordinary speeds, something moves around horizontally but is always earlier at the top than at the bottom. Then there’s the light-cone, which I regard as a crucial concept for time travel. The Sun is eight light minutes, Alpha Centauri four light years and Betelgeuse six hundred light years away. The upside-down, but still vertical, version of the light cone can be illustrated thus:

Flipping this over, events occurring above, such as the emission of light from a star, have only impinged upon an observer at the peaks of the cones if it happens within the top cone, and events occurring below, i.e. in the future, will only influence other things if they are within the light cone below the point where the peaks meet. The surface of the cone is defined by the speed of light because nothing can exceed it, and therefore there can be no causal connection between anything outside the light cone. Every location at a particular moment has these light cones.

This is where time travel comes in. The classical objection to travelling backwards in time is that it can create two types of paradoxes, or rather involves two types: the grandmother paradox and the ontological paradox. The grandmother paradox is illustrated most starkly by the possibility of becoming one’s own grandparent, leading to a loop in time with no cause and also a contradiction with recorded or otherwise “known” events. I know I’m not my own grandparent, or have a high degree of confidence that I’m not, because their identity and history aren’t mine. A slightly different paradox is the ontological one, exemplified by such events in fiction as the apparent eighteenth century CE glasses given to Captain Kirk by Bones in ‘Star Trek’ which were later pawned in 1986 in San Francisco, later possibly being acquired by McCoy and regifted. This means that certain complex objects have no origin at all, and raises questions such as what happens if someone sends a 21st century signed copy of the sheet music for Beethoven’s Fifth Symphony to Beethoven which he then copies instead of composing it for himself. It might be less disturbing if all that happens is a single electron or photon is transported in time.

My long-standing answer to this is to evoke what is called either the Novikov Self-Consistency Principle or the Blinovitch Limitation Effect, depending on whether you go with the real world or science fiction version of the effect. The Novikov Self-Consistency Principle holds that if an object travelling in time would cause a paradox of this nature, the probability of that occurring is zero. The Blinovitch Limitation Effect is similar and originated in ‘Doctor Who’, and means that either timelines cannot cross themselves or that if they do, some kind of catastrophe occurs. This happens for example in ‘Father’s Day’, where Rose encounters herself as a baby and it leads to damage to the time stream which can only be resolved by her father meeting his fate, that is, death under the wheels of a car.

My own solution to this has been to suppose that time travel “upwards” is only possible outside the light cone. That is, for example, you can move upwards in time but if you did so you would find yourself displaced at least the same distance in space from your point of origin as it would take light to travel to it by the moment of your departure. Hence if you travel three years upwards in time, say from 2023 to 2020 you might find yourself near Alpha Centauri because it then means that you can’t have any influence on where you came from. You might send a radio message warning people about Covid-19 but it wouldn’t get there until after the outbreak, for instance.

For this reason, I have tended to portray time travel as a “diagonal” process. You can’t stay in the same place and travel into the past, but will be displaced by the same distance in time and space: four light years for four years and so on. I also applied this to the idea of parallel universes, which I see as separated from us by extra dimensions, so your creation of a different history would simply mean that you had been transported to a parallel universe where that would always have been the case. The problem with this is that it kind of makes it look like any significant historical event was caused by time travel, or that there would be duplicate universes different only because in one a time traveller had turned up and made the same change as had happened without such intervention in another parallel universe. Thus in one universe, JFK isn’t shot because the guns jammed but in another someone came along and secretly did something to the guns in advance which caused them to jam. This seems suspicious.

There is a further problem with intersecting light cones, because events which can’t impinge on one light cone can do so on another, but I have to admit to being rather hazy about that, so I won’t go further. That, then, is diagonal time travel. There’s also vertical time travel.

Vertical time travel is what any object or person does when they’re sitting still, although of course real sitting still (or standing still) is not a trivial, or maybe not even a possible, thing because of things like orbits, rotation of planets and so forth, and the probable complete absence of an absolute frame of reference. Hence a vertical path through time is not a straight line, although it might look straight to observers who are stationary with respect to the object or event in question. Simplifying this, as in fact we always have to deal with this in other ways but generally think of ourselves as moving or stationary with respect to the scenery or neighbourhood as opposed to the reality that we are on a rotating, orbiting planet and in a solar system orbiting through the Galaxy, and so forth, time travel from the past to the future for most of us is almost perfectly vertical in nature. We are moving straight “down” in time, not diagonally. Sunlight is diagonal. In fact, all light in vacuo is diagonal because its velocity is the same for all observers.

Back to vertical time travel. Ignoring the complications mentioned just now, vertical time travel into the past should be impossible. Into the future it’s the norm. However, the conventional depiction of time travel in most popular culture is that of the protagonist moving through time, either into the future or the past. The latter clearly brings up the question of paradoxes, and conjures up the idea of a wise and intervening Universe for which there is no evidence stopping such things from happening. This is not, though, the only conceivable way an object or entity might move through time, and this is where my new ideas come in, and where this becomes a kind of world-building thought dump.

There is a second, less often used, version of time travel in fiction which is not used to explore paradoxes so far as I know, and it’s this which I’m going to call “vertical time travel”. This is illustrated only occasionally, as far as I know. The first example which comes to mind is found in Isaac Asimov’s ‘The Ugly Little Boy’. In this story (SPOILERS!) a Neanderthal child is brought into the twentieth century, although he eventually returns to his time with his H. sap. nurse. In this story, energy is required to hold objects in the modern world and budgetary constraints lead to them being returned to the past, including ultimately the Neanderthal boy, so this is not a one-way street and paradoxes are still possible. The TV series ‘Primeval’, which I haven’t seen, seems to have the premise of a similar effect where animals arrive in the present through temporal anomalies, but again this seems bidirectional. You also see the same kind of thing in «Les Visiteurs» and ‘Catweazle’, where a similar process brings characters from the Middle Ages into the twentieth century. Most of the time, perhaps to provide resolution in plot terms, the protagonists or objects are returned to the past and I can’t offhand think of any unidirectional time travel stories. They probably do exist as they allow for “fish out of water” stories. In fact most of the time this is satisfied by the idea of someone going into a state of suspended animation and being revived, as with ‘Rip van Winkel’, ‘Idiocracy’, the story that was based on, and Woody Allen’s ‘Sleeper’. This is a device allowing for time travel in a much more realistic manner than something like ‘Doctor Who’. However, actual vertical time travel allows for other possibilities which are quite intriguing and not available to such tales.

Time travel into the past can never be invented, only discovered. Look at it this way: the first time in history someone devises a time machine that goes upward in time, it will already have gone into the past. Hence it will have come into existence before it was invented. It might not be discovered in the sense that someone comes across it on its trip furthest back into the past, but the “inventor” has come across the machine at a later time than when it first sprang into existence. Alternatively, time travel might exist due to a peculiar combination of events and circumstances arising without the intentional use of technology, perhaps, for example, where a wormhole has orbited another at relativistic speeds due to one side being close to the event horizon of a black hole, resulting in an object able to capture masses and transport them to an earlier time in its history because time has passed more slowly at one end than the other. Incidentally, I’m not saying this is feasible, but such a situation could be discovered and wouldn’t’ve been invented. Hence there is a sense in which it’s impossible to invent a time machine completely aside from any technical reason why they would be impossible in themselves. Even possible time machines cannot be invented.

Given that, there is also a sense in which instances of time travel from the past might be discovered rather than deliberately or accidentally perpetrated, but I’ll come back to this. For now, imagine the following scenario. Objects move from the past to the present without ageing. By “ageing”, I mean any change due to the passage of time, so for example if a watch reading 4:45 pm moves an hour down the line, it still reads 4:45 pm on arrival, at 5:45 pm, and it still works. This amounts, I suppose, to the suspension of entropy, although it would seem inevitable that the object in question would not have a location, and in a sense would not exist, between the two occasions. It isn’t frozen in time: it has discontinuous existence. Otherwise any object on the surface of this planet, assuming it follows Earth, would remain in the location it succumbed to time travel and would seem to be a frozen item whose location could be discovered, and it would be observed. But maybe it wouldn’t, because this would involve interaction with the rest of the universe, and this would seem to imply change. Being subjected to millions of years of ionising radiation in an instant, and millions of years of thermal energy in the same time, would be likely to destroy such an object. Therefore suspension of entropy would mean the apparent absence of the object, perhaps existing in a merely latent state for geological epochs. After all, if time isn’t passing for an object, its electrical charges cannot attract or repel, its particles can’t change energy state, preventing their ability to emit or absorb photons, it has no weak interaction in its nuclei and so forth. It literally does nothing and does not interact with the Universe, although it has to be assumed that it retains momentum or it would end up buried inside the planet or in deep space.

This, then, is the fictional scenario I wish to present. There are objects which enter a kind of dormant state of existence for various intervals, or perhaps they simply enter a dormant state of existence until disturbed in some way. Then, after a period of time, a device comes along which can revive them back into existence. This does not occur causally, i.e. this is not a device which can actively retrieve objects from the past, although that is what it appears to do, because sub specie aeternitatis this is what would always have happened to such objects. Because of this, there can have been no interaction between such objects and the operator of the “discoverer” machine, or a paradox could be created. Even so, the operator of the discoverer subjectively experiences the decision to focus the machine on a selected target in the past which they would then bring into the present. That decision, though, was always casually determined, perhaps like all apparent instances of free will. Therefore, as far as the operator is concerned, they are making a decision to retrieve an object from the past, operating the machine and being presented with the intended object. So far, so good.

From this description, there seems to be no possibility of paradox, which makes it rather boring. Consider this though. If you have already interacted with an object in some way, you cannot bring it into the present. You can’t target last night’s dinner just after you’ve cooked it and eat it tonight, because if you did that you wouldn’t have had last night’s dinner. That said, what if you targeted someone else’s dinner from last night? You’ve had no causal interaction with that dinner. It could be in a kitchen you’ve never visited and be eaten by a complete stranger. What if that led to them developing a false memory that they had eaten the meal, because in one timeline they had but in yours they hadn’t?

To be highly specific, just suppose you went back to the set of ‘Moonraker’ and removed Branche Ravalec’s braces. You would then know what had happened, but everyone else would have the false memory of her having braces when they watched the film. In other words, and I would stress that this is fictional, this is an explanation for the Mandela Effect.

And it has legs! Now imagine a sealed box containing a cat. You have no knowledge whether this cat is dead or alive owing to a poison gas canister broken or remaining intact according to radioactive decay values. Also, you save the cat by removing her from her predicament before the canister is at all likely to be broken. A familiar example, but with a twist involving time travel. If you’d opened the box, you would never have succeeded in bringing the cat through time. Schroedingers Cat of course, but with a twist.

Now imagine a young woman shopping for groceries – yes, this is a heteronormative example. Her bag falls apart and her orange rolls along the pavement, to be picked up by a handsome young man. Their eyes meet and the rest is history. There are wedding bells and children. Then, one day a third party focusses their discoverer on the aforesaid orange and removes it to their present day. For that individual, this was always going to happen because they don’t know these people and it has no consequences for them. Meanwhile, spouses wake up in their own beds, perhaps at opposite ends of the country married to different people, with different children, with no memory of how this happened or anything, but with distinct memories of meeting each other, getting married, settling down and having children, who will now never exist. On the other hand, assuming they can do anything about this, and I suspect they can’t unless they can trace who did it and stop them without causally interacting with them, and how could that happen, they would end up back together but again, their own children conceived with their other partners would, again, never have existed.

I would maintain that all these possibilities are highly fruitful, and intend to write a story based on them. In the meantime, this post has served as a means of working out this detail of such a story for me. Thanks for your patience.

The Expanding Sphere Of Exploration

I am pessimistic about the future of the human race. The fact that we have come this far without dying out is a given of my existence, so it doesn’t follow that we will be resilient into the future. Maybe all we have been doing is running on luck. Perhaps out there in the Galaxy, life arises on hundreds of millions of planets, only to be wiped out by a gamma ray burst or series of impacts, and that hasn’t happened here, but why shouldn’t it? It so happens that there are reasons for this, but I’ll come to those later.

The other day, someone jokingly suggested that I believed ‘Star Trek’ was real. Whereas it would be nice to live in a post-scarcity society, I’m not actually specifically very keen on ‘Star Trek’ in the larger scheme of things and don’t find it at all believable, for reasons entirely other than the whole post-scarcity utopian space federation bit. My difficulty with suspending disbelief in ‘Star Trek’ is that it’s really what I’ve heard described as “mushy soft science fiction”, on the Mohs Scale Of Science Fiction Hardness, on which ‘Star Trek’ is apparently gypsum (and H2G2 is talc – not at all sure about that). I would actually swap those two. H2G2 is not very feasible at all of course, but for example, being initially a radio series it has plenty of non-humanoid aliens. ‘Star Trek’ TOS had more non-humanoid aliens in it than later series, and there’s an in-universe explanation for it which doesn’t really work, but that’s only one aspect. Others are faster than light travel (FTL) and gravity control. There are some advanced but feasible bits of tech in it too such as replicators, teleportation and androids.

Science fiction of a certain type basically has to have FTL, because if it hasn’t, events will be dominated by the difficulty everyone has in getting anywhere. This can be circumvented by making the story about the trip, setting events on a single planet or habitat or just not including space travel at all, but the sad thing is that as far as anyone knows, travelling faster than light is impossible in any meaningful way. There are jets of plasma shooting out of galaxies which seem to be travelling that fast but this is due to a foreshortening effect arising from the angle we see them at (I have to admit I’m not clear about what happens). Superluminal movement also happens due to the blurred nature of a wave function which means that when quantum tunnelling occurs a particle has a low but not zero probability of appearing on the far side of the barrier faster than it could have got there if it was moving at the speed of light, and if negative mass can exist, a warp drive is feasible. In fact, negative energy makes warp drives possible too, which is almost the same as nothing travelling faster than bad news. Douglas Adams again. Finally, the Universe expands faster than light, which is why space is black – light hasn’t had time to reach the observer over most of it, and may never do so.

It is of course feasible that we’ve just got hung up on our own version of physics and that there is another way of looking at these things which would make it possible to come up with a way of doing it, but there is no real reason to think this. This post, though, is not about what’s unfeasible, but about the prevalence of life, and possibly intelligent tool-using life or something that life has invented, in the Universe, or more specifically this galaxy.

The well-known Fermi Paradox is very simple to state: considering we live in such a vast Universe with unimaginably huge numbers of suitable stars for life and apparently also solar systems, where are all the aliens? Why don’t we constantly hear radio transmissions from them? Why can’t we see megastructures built around other stars? Why haven’t they visited us? A common conclusion to draw from this is that there just are no aliens, or at least no intelligent ones with spaceships and radios and stuff. It’s just us. There are a large number of other explanations offered, most of which have been heard many times, and new explanations are now emerging. One of these is the so-called “interdict scenario” offered by YouTuber and SF author John Michael Godier but based on an idea proposed by Martyn Fogg (I think) three dozen and a bit years ago where there was a chaotic belligerent phase early in Galactic history which has now settled down and civilisations in the Galaxy are now mature and welcome original thought, which arises best in species which haven’t been contacted yet. The Alpha Quadrant situation in ‘Star Trek’ is more like what was happening æons ago, perhaps even before Earth formed, and all of that’s now been sorted out. All of that diplomacy is extremely prehistoric and passée because it’s no longer necessary. Nowadays there are ancient protocols for first contact and the Prime Directive, and they’ve had time to get good at it.

The opposite to this is the Dark Forest scenario, 黑暗森林, suggested by the SF author Liu Cixin (刘慈欣) in his book of the same name, which I have yet to read. The details of the idea can be expressed through game theory, but rather than doing that it’s better to explain it via a metaphor. It can be summed up by the following quote:

The Universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds another life – another hunter, angel, or a demon, a delicate infant to tottering old man, a fairy or demigod – there’s only one thing he can do: open fire and eliminate them.

刘慈欣, ‘黑暗森林’

The situation does actually resemble that of terrestrial biomes quite closely, where many prey and predator species have evolved camouflage because those that didn’t either got picked off and killed or starved to death. If the laws of nature continue to operate with technologically advanced species in an interstellar environment, in a way it wouldn’t be surprising for them to be hiding away in this manner. In this scenario, humans are actually being quite reckless in that we make no attempt to disguise our presence and location, and consequently put ourselves at risk from alien attack.

I quote these two attempted solutions not so much to discuss them as to illustrate the contradictory ideas we have projected onto the Universe, which makes it even harder than the available data make it to work out what’s going on out there: the “real Galaxy” scenario. Another quite popular suggestion is that there’s just us, in various different ways. We could be the first of many, life could be really unlikely, it could be common but hardly ever becomes complex or our solar system might be unusual in some way which supports the development of our kind of life.

We will probably never even return to our natural satellite, let alone create space habitats or settle other planets. If we did, it would almost certainly make it very improbable that we were living before that time because of the vastness of the Universe and the number of places which could be settled or colonised, so any kind of space opera-style SF, no matter how carefully it respects scientific plausibility, is in the realm of fantasy if it includes human beings as a spacefaring species. But maybe, who knows?

We do have some limited information about how things are. There are only limited signs of this planet ever having hosted a technologically advanced civilisation, in the Eocene, whether that evolved here or elsewhere, so we can confidently assert that anyone who got here either trod amazingly careful or just didn’t come at all, perhaps because they never existed. Then again, maybe extraterrestrial civilisations aren’t interested in planets because they live in space arks or artificial colonies in space, possibly placed around stars we regard as hostile to life such as Rigel or the four-star Mizar system, which Earth astronomers never bother to look at in that way because of their apparent hostility for and implausibility as potential abodes for life’s evolution. It’s all unknowns. Nonetheless, I shall spin you a tale credible enough for me to entertain it briefly, at least for the purposes of this post.

In the early Universe, the Milky Way formed. Initially, its stars were mainly very large and short-lived and their planets were just balls of hydrogen and helium because heavier elements had yet to form in any quantity. During this period, metals (the astrophysical term for any heavier element) began to be forged in the cores of these stars and as they met their demise as supernovæ, they ejected these more massive atoms and the next generation of stars began to form with rocky planets and moons. Life arose or arrived on these bodies and over a period of æons, intelligent species evolved and began to develop technology. This took place in a kind of bell curve, with some appearing quite early because everything had gone “right” for them, and they found themselves apparently alone in the Universe because they were as of yet rare in the Cosmos. Nonetheless, some of them made their way into the Galaxy, explored it and settled on planets, perhaps making them more hospitable in the process or genetically modifying themselves to do so. Then the emergence of intelligent life reached its peak, around nine thousand million years ago, and many of them encountered each other. There was a long, indeterminate period of hostility and war, although it is difficult to conduct anything like a war between species living light years apart. This settled down after some time, perhaps millions of years, and galactic civilisation reached its mature phase. Systems were set up to share information and technology, and customs evolved to integrate newly-emerging species into the galactic social order. By the time Earth formed, over three thousand million years after the first interstellar civilisations arose, there were established and stable agreements not to interfere with primitive planets until they were ready. Yes, this is the interdict scenario. Very often in science fiction, the situation depicted is of a range of intelligent life forms who are of roughly the same stage of technological development as each other with a few who are so far advanced compared to the others that they’re like God. This seems unlikely because of the sheer age of the Galaxy. Civilisations might collapse and species go extinct of course, but even if this happens a lot, the period during which an interstellar culture could thrive might be very long indeed compared to current history. On the other hand, maybe technological progress reaches a plateau at some stage and the current phase of apparent rapid technological and scientific change is a brief “blip” between the more routine stages of the Palæolithic and an interstellar utopia. This confronts us with another paradox: that in fact we are the unusual ones and in a way the far future, if it happens, and the current state of most spacegoing civilisations in the Galaxy, actually have a lot in common with the Stone Age compared to what we have now. Having said all that, I haven’t fully thought through the implications.

Before I commit myself to this state of affairs, I want to describe another, very different one. It starts the same way as before, but instead of the early tail of the bell curve being in the distant past, it’s now. Humans are in fact unusually early as a technological species. When we go “out there”, we will find a sparsely-peopled Galaxy which does have other species capable of interstellar travel, but we’re all outliers and the glory days lie in the unimaginably distant future. Or alternatively, without anticipating the future, intelligent life is present but sparse in the Universe, so our nearest interstellar neighbours could be 576 light years away, and it has always been so and will continue to be like this in the future until all the stars go out. What then?

Humans have recently discovered that our very closest interstellar neighbour, Proxima Centauri, has a planet orbiting it with roughly the same mass as Earth’s, and in any case a maximum mass of less than three times ours, actually 2.77. Bear in mind that even that upper limit isn’t that much larger because if its density is the same, it gives it a diameter, with spurious accuracy, of 17 915 kilometres, only 40% larger, as would its gravity be – just slightly too high for comfortable habitation by humans. Its equilibrium temperature is only ten percent lower than Earth’s, which easily gives it a chance of having water oceans on its surface. We’ve kind of hit the jackpot first time, but incredibly, this isn’t big news for most people! However, this bizarre apathy isn’t the focus of my post. What is, is the fact that in a sphere around the Solar System with a radius of only 4.3 light years, there may be two habitable planets. There’s definitely one of course. If that “sample” were to be extrapolated by doubling the radius of the sphere to 8.6 light years, almost the distance to Sirius, it would octuple the volume and “statistics” would suggest eight habitable planets within that radius, which is clearly not so. There aren’t even eight star systems, including the Sun, within that sphere, and as could be expected three of the stars are red dwarfs, which are easier to detect so nearby, but more can be supposed elsewhere. It’s been said previously that red dwarfs appear to be surprisingly good candidates for habitable planets, so it’s not beyond all reason to assert that there might be some there, but they’re not Sun-like stars.

With a further increase of radius, the sphere could be expected to become a more accurate sample of the space in this region of the Galaxy. For argument’s sake, I’m going to assert the idea that there are five more habitable planets within six dozen light years, with the possibility that there might be more than one in some systems. For instance, if our own practically double planet system is in any way typical, and it probably isn’t, there might be another whose larger component is at the upper luimit of habitability and whose smaller one is at the lower, or perhaps a system with two habitable worlds, one very hot and one very cold. The TRAPPIST-1 system contains seven Earth-sized planets within a habitable zone, for example. Nonetheless, they could be expected to level out at less than two on average per system which contains any. Six such planets in a 72-light year radius sphere will have a mean distance of almost sixty-four light years from each other. However, at that rate the sphere of exploration would only need to be about three light years further for another to be found. Hence exploring space in this way, whether by going out there, sending probes or astronomical observation, would be exponentially more successful. And this ignores the idea of setting up further facilities on the outlying planets, or near the edges of explored space, increasing the chances of finding them more quickly and even further out.

John von Neumann, the mathematician, suggested that interstellar space could be explored relatively quickly and efficiently by releasing a space probe which, when it reached the target system, would build a copy or two of itself which would then be despatched on to more distant systems. Given that there are four hundred thousand million stars in the Galaxy, this would require around forty generations of probes, which is not hugely impractical-sounding, but the problem is that releasing such craft into the interstellar environment runs the risk of mutation and evolution into a swarm of “grey goo”-style craft attempting to convert all the suitable material in the Milky Way into copies of themselves. It would also still take a minimum of fifty thousand years for such a programme to cover the Galaxy, and a minimum of a further fifty thousand for all the data to be received. I will be returning to von Neumann’s devices in future, as they seem to have quite profound significance.

Combining the doubling of probes with the exponential growth of likely planets as the distance increases means that there is no impediment to a long period of such growth. Even if it takes five centuries to find the first five closest worlds, the sixth is likely to be found within five years of the fifth, assuming velocity is not a limit. This also means that the probability of finding a new one per year would have grown similarly in the intervening time. The longest wait could be expected at the start of this period. This assumes , though, a relatively homogenous Universe of infinite extent with regular distribution of potential homes for humanity. In the long term, this is unlikely to be so.

This is where the two constrasting scenarios of sparse relatively young and isolated civilisations and common mature confederated ones come into play. Deciding to assume that the average distance between spacefaring civilsation home worlds is 576 light years, which is exactly eight times six dozen, gives a sphere of influence (not really a sphere of course as there would be gaps) averaging out at 288 light years radius. This assumes also that the environmental requirements of different species are identical, raising the possibility of competition. This takes on a different significance when one considers that what may in fact have happened, for all we know, is that the biological entities who originally designed von Neumann machines may be long gone by now, and what we would in fact find is a galaxy dominated by machines of that kind, which would be a lot less fussy than animals about their requirements. Depending on the age and rate of exploration of our neighbours, we would of course be increasingly likely to meet. It’s been claimed that the nature of First Contact may be less important than the fact it happens at all, because of the impact of the realisation that we’re not alone in the Universe, but it would seem that if the Dark Forest explanation is true, that wouldn’t be the case as the chances are then that there would be a swift attempt at mutual annihilation, and that’s more significant than just discovering aliens. However, though a wonderful source of drama, the Dark Forest explanation is unlikely. This planet has had very obvious biosignatures for hundreds of millions of years or longer, for instance as the spectrographically detectable free oxygen in the atmosphere, and nothing has happened in that time which looks like an alien attack. Why not root out the possibility of intelligent life early rather than, say, wait for the industrial revolution? The increase in carbon dioxide since the Industrial Revolution is a technosignature, though possibly an ambiguous one. CFCs and the depletion of the ozone layer, however, are pretty much certain technosignatures because of the nature of fluorine and chlorine, which can only be handled and produced industrially in that way. We’ve been advertising our presence to a possibly empty Universe for a very long time. Incidentally, the transmission of routine radio signals is not an easily detectable sign because it isn’t as persistent as many people think it is. In fact, it disappears into the noise before it even gets to Proxima. I have to confess that I don’t know how far the Arecibo telescope message has reached or will reach.

I’m going to call the two scenarios I’ve already mentioned “Interdict” and “Early Tail”, and introduce a third, “Absent Aliens”, which is where there’s no other intelligent life in the Universe. In Interdict, the situation would probably arise where soon after First Contact, uplift would occur. I may be using this word unusually. By “uplift”, I mean the idea that advanced aliens will help us improve the level of technology and scientific information available to us, perhaps in Interdict in exchange for our original thinking and cultural artifacts. This has a kind of religious, cargo cult flavour to it, but in practical terms as far as discovery of new worlds is concerned, this could simply involve humanity being given a catalogue of every such world available to and suitable for us throughout the Galaxy. If this happens, a gradually increasing bubble of knowledge would just suddenly expand in a leap, and this would include other areas of knowledge.

In Early Tail, assuming friendly relations between us and aliens, and there are reasons to suppose that would be so, the bubble expands up until First Contact, after which information about suitable planets in other regions of space known to the aliens is shared. These aliens may or may not have similar requirements to our own, such as temperature, pressure, the presence of oxygen in the atmosphere, particular levels of gravity and so forth, and they may have incidentally discovered planets suitable for us but not for them. The situation could be intermediate or not. For instance, chlorine-breathing aliens who find oxygen poisonous are not in competition with us for habitats, although they may be for mineral resources or solar power, but extremophile-type aliens who thrive in conditions like those of Antarctica could have overlaps with us, and this could influence their attitude towards both sharing information and bothering to find out about it in the first place. I presume nobody with any influence is currently proposing the detection of planets with free chlorine in their atmospheres, so would we know about them in our own sphere of exploration by the time First Contact occurred or not? We might come across them accidentally though, and if we did, that would probably be a biosignature, though of very different life.

A simple Early Tail scenario is encountering aliens whose homeworld is 576 light years away with similar biology to our own, though of course probably not humanoid. If it takes five centuries to reach seventy-two light years from here during which five suitable worlds are visited and perhaps settled, and the aliens have explored at the same rate, we could expect to meet them when both spheres have a radius of 288 light years, which is four times that distance, and could therefore be expected to occur two millennia from now, by which time almost four hundred worlds would have been found by each. Assuming early friendly information-sharing, it involves an almost instant doubling of both species’ knowledge at that point. The rate of alien contact would also ramp up fast in the same way as the discovery of planets from this point because of the increase in explored volume. The idea of the average distance to intelligent aliens with interstellar civilisations being that far commits us to an estimate of the likelihood of a habitable planet having such life on it being around one in three thousand. Taking the window of habitability on our own planet as typical, i.e. around 1 500 million years, this would mean that the average time such a civilisation will exist, including the evolution of various species not directly connected to each other, would be around half a million years. However, this may not be how things really proceed, either here or elsewhere. Maybe there is instead a threshold in evolution after which intelligent life is a constant feature of the biosphere. There are a number of examples of species with somewhat similar intelligence to our own on this planet, including elephants, parrots, cetaceans, corvids, parrots and other simians. Maybe from now on, there will always be intelligent life here, and if this is so we can expect something like two-thirds of the suitable planets we encounter to have intelligent life on them. Using the density of habitability I’ve suggested, it could mean that the second world we come across will have intelligent life on it, or even the first.

Alternatively, intelligence and tool use could turn out to be an evolutionary dead end. Contrasting the polar opposites of releasing sheer clouds of young, like a coral, oyster, fish and to a lesser extent turtle, and not bothering with parental care, and having one or a very small number of young at a time which need extensive parental care and a learning period before being able to cope on their own, the second of these means that orphans tend not to thrive, it’s a long time before the next generation and we are unlikely to do much more than produce a few children. If an organism doesn’t need to mate, one individual in an environment empty of peers is likely to lead to a thriving population after very few generations, particularly if parental care is not required. Brains are also very resource-hungry. The human brain can account for up to 87% of resting basal metabolic rate. All this means that humans are vulnerable to major disasters and food shortages to a far greater extent than many other species with different reproductive and survival strategies, and it may be that other intelligent, technology using species have the same problem, meaning that they may not thrive for long. There’s also the evil twin of the Gaia Hypothesis, Peter Ward’s Medea Hypothesis. This is the idea that complex life tends to be fragile and vulnerable to extinction because of the activity of microörganisms, meaning that the more stable state of a biosphere is what was seen over most of Earth history, namely a load of microbes without much visible evidence of anything much going on and not even anything as advanced as an amœba. Medea in Greek mythology, found in ‘Jason And The Argonauts’, is the archetypal bad mother in the same way as Gaia is the archetypal good one, who murders her children. There have been a number of occasions in addition to the well-known mass extinctions of the Phanerozoic (the time since large hard-shelled animals evolved) which have almost destroyed all life on Earth: the oxygen catastrophe, snowball Earth, methane poisoning and a number of times hydrogen sulphide has been released in large quantities and nearly killed everything. Right now, anoxic zones are developing in the oceans which have in the geological record been associated with mass extinctions, and many organisms remove carbon from the atmosphere and deposit it in rocks such as chalk and fossil fuels, thereby making it unavailable as biomasse for a very long time. Therefore it could be that animals and complex plants are not a long-term prospect, and in fact they’re established not to be by the known course of our planet’s story. Hence intelligent life forms and even complex life forms of any kind might be rare even if microörganisms are not. Ward believes that the main immediate cause of mass extinctions is the release of hydrogen sulphide into the atmosphere by purple sulphur bacteria, which destroys the ozone layer and poisons ærobic life, caused by rapid global warming such as flood basalts releasing carbon dioxide, but also of course potentially by our own activities. If it’s common for this to happen when technological cultures become industrial, it could limit the length of civilisations dramatically, and it may be why we will never settle on other worlds or create large space habitats.

Assuming we survive, in this scenario the expanding sphere of exploration would simply proceed exponentially without any catalysts. However, we might not be finding anything interesting because although there could turn out to be plenty of planets like Earth, even with life on them, most of them would only ever have had simple, microbial life and many more would have permanently devastated environments, similarly with simple life but which once held complex life before the advent of intelligent life forms who managed to wipe almost everything out. Then there would be the occasional intelligent species which was still around but didn’t have long to go, probably too sparsely distributed for us ever to find any. And we are probably one of them.

Further limits emerge when expansion gets beyond about 1 500 light years. This is half the thickness of our spur of the local galactic arm. Beyond us are rifts in the Galaxy largely empty of stars or planets, and the neat sphere I’m imagining will no longer be spherical. A second factor may also come into play here. There may be a ring-shaped zone of habitable systems concentric with the galactic core. Stars near the centre of the Galaxy are very densely packed, but in any case, dense or not, not capable of supporting life on any planets they might have. They’re so close together that they’re likely to disrupt planetary orbits and the high level of radiation from the central black hole is likely to kill everything on a planet. Our galaxy is a barred spiral. This roughly means it has two arms projecting straight out of the centre before the spiral arms begin. The situation in the bars is about as hostile to life as the nucleus is. Further out, where the arms begin, heavier elements are quite common and consequently rocky planets likely to form, but also many gas giants, possibly so many of them that they will tend to fall towards their suns and become “Hot Jupiters”, which disrupt the orbits of the smaller, more hospitable, rocky planets. Also, there would be more cometary impacts because the stars are closer together and more likely to disrupt the orbits of the comets which orbit in a cloud around star systems far out. In addition, supernovæ are more likely closer to the planet than is safe, wiping out life in another way. Further out, stars are sparser and there are fewer heavy elements, meaning that planets would be simple gas giants consisting of hydrogen and helium but not much more than that. Hence the best distance in this Galaxy is likely to be between 25 000 and 33 000 light years from the centre. We are 27 000 light years out. It’s also fairly dangerous to be deep in a spiral arm – apparently we’re near the edge of one. This is because there are many blue supergiants there, which tend to go supernova after only a few million years.

Therefore what we’d probably find is that our “sphere” of exploration would cease to be one once it got beyond about 1 500 light years around the orbit of the Sun, two thousand light years hubward and five thousand towards the edge. However, if we’re on the edge of an arm, as we explored inward from that there would initially be a growth in the number of interesting worlds followed by a region where life had been obliterated given that it existed near the surface of the body concerned, but not necessarily if it was in a subsurface ocean, which judging by our own solar system can be expected to be very common, and rogue planets which have escaped from their suns which nonetheless have a source of heat in some way would also be immune from these risk factors. Hubward, that is, in the direction of Sagittarius from here, we could expect to find solar systems with more planets than ours but also more Hot Jupiters and with more frequent mass extinctions caused by comet bombardments until it became unfeasible for life to develop very far, assuming it does anyway. Towards the other edge of the Galaxy, which I presume is in the direction of Gemini, there would be more hospitable worlds more likely to harbour complex life forms but with less abundant heavy elements, so for example our own reliance on iodine to produce thyroid hormones would have no parallel. Biochemistry would become simpler with fewer trace elements, if possible.

However, of course a lot of this is guesswork. The scenarios I’ve considered here are that complex life is rare, the Dark Forest explanation of the Fermi Paradox, the idea that we are early in the history of intelligent life in the Galaxy, that complex life is unstable and short-lived, and that intelligent life is widespread and has reached a stable and peaceful maturity. It’s hard to extend this beyond mere speculation, and of course in the end we just don’t know what’s going on out there. We also have a tendency as a species to project out imagination onto the sky, and should be wary of that if we want to adopt a rational approach towards the prospect of intelligent life in the Galaxy. There are lots of fairly obvious things I haven’t considered, such as the idea that biological life is replaced by robots, that aliens and humans might prefer orbiting space stations to planets, or that there’s loads of intelligent life but it spends all its time on the local internet, but this is a fair sample of the ways things might go if we ever get our act together enough to go “out there”.