黑暗森林 – The Dark Forest

I’ve just heard an excellent podcast episode called ‘The 3 Body Problem Problem’, which you can listen to here. It’s very wide-ranging, and be warned, rather despair-inducing. I’m not going to go into too much depth about it, but I am going to talk about the Dark Forest Hypothesis in its social and political, and maybe psychological, setting, which is what that podcast already did.

The new Netflix series ‘The 3-Body Problem’ is an eight-part adaptation of 刘慈欣 (Liú Cíxīn)’s famous and award-winning novel, 三体, the first of a trilogy called ‘地球往事’, translated as ‘Remembrance of Earth’s Past’. In order to engage with this series in sufficient context, I feel like I’m going to have to zoom out so far that the actual trilogy itself is going to end up looking like an invisibly small dot on an invisibly small dot, and I don’t want that to happen so I’m going to have to break it down a bit. I am deliberately posting names and titles in 汉字 (Hanzi) because of the issues it raises. Two things about this: I am more used to Wade-Giles than pinyin romanisation and I prefer traditional Hanzi to simplified because the latter is trickier to associate with the ideas it represents. Looking at simplified Hanzi, which is what this is, is like having a migraine because there are bits missing from the characters which one really could do with being able to see. Yes this makes me a dinosaur, but non-avian dinosaurs would still be around today were it not for their “left hand down a bit” mishap 66 million years ago and there was basically nothing wrong with them.

I’ve read the first book of the trilogy. I didn’t so much not want to read the rest as find it an unnecessary financial outlay, so it ended there. Netflix too might end it there because they apparently haven’t had as much success out of this extremely expensive series as they’d hoped, so like several other series they may well cancel it way before time, while in the meantime adding lots of fluff to stories which were supposed to end like ’13 Reasons Why’, and while I’m at it, that book and series is interesting because it’s basically ‘An Inspector Calls’ for the twenty-first century and yet manages to be quite unfortunate in its implications regarding bereavement of people who have killed themselves (I don’t use the S-word because it’s not a crime). Anyway, before I get irredeemably off-topic I shall post a

Spoiler Warning!

and be done with it. So if you want to enjoy ‘The 3-Body Problem’, don’t read past here.

Before I get into the broader issues with the Netflix series and the book, I thought I’d explain what the Three Body Problem itself is. First of all, it’s fairly easy to work out where Cynthia (“the Moon”) and Earth are going to be at a given time, so for example we can easily work out when the phases happen, when it rises and sets, how far away they are from each other, when eclipses happen and how long lunar months are, and by extension the times of the tides. A lot of these things are also linked to Earth’s rotation, but the mathematics are fairly straightforward, although because both Earth and Cynthia move in ellipses relative to each other and the centre of mass (the “baycentre”) about which both orbit is not at Earth’s centre, and it would really help to know calculus, which I don’t, to make these calculations. Likewise with the Sun and Earth we know when the equinoctes and solstices are and how far away the barycentre of the two bodies is at any given moment to a high degree of accuracy. This is because Earth, Cynthia and the Sun and Earth are two bodies each when considered in that way. The fact that we can work out all this stuff in both cases also shows something else: that there are some straightforward pretty much accurate solutions for three bodies provided they’re in certain arrangements with each other. There are actually a lot of situations when the movement of three bodies fairly close to each other like the three mentioned here can be determined quite accurately. The case described here is simplified by the fact that Cynthia is both close to and much less massive than Earth and the Sun is much further away and more massive than either. Another very useful case is that of the Lagrange Points, where the balance between the gravity of two of the bodies is equal, leading to a stable point associated with them. Examples of this are sixty degrees behind or ahead of a planet or satellite in the same orbit, some cislunar point between a planet and its star or a planet and its satellite where the gravitational pulls are equal and cancel out, and some translunar point where the pull of Cynthia and Earth are again equal. As I’ve mentioned before on this blog, these points form a kind of “rapid transit system” around the Solar System which minimise the energy required to get between the various asteroids, moons and planets. There are other situations too. However, the Universe isn’t usually that neat and the majority of interactions between three bodies in fairly close proximity to each other are chaotic.

You really do need to look away now if you want to avoid spoilers.

The  三体 (Sān tǐ) are technologically competent aliens native to the Alpha Centauri system in the story. The Centauri system is in reality a ternary star system. Two Sun-like stars, one somewhat more massive and warmer than the other, orbit each other at a distance of between eleven and thirty-six times Earth’s distance from the Sun, whereas eleven thousand times the Earth-Sun distance, known as an AU (astronomical unit) from the barycentre orbits a much less massive red dwarf, Proxima, famously the closest star to the Sun except that since it takes half a million years to orbit the system so for some of the time it’s further from us than the other two, ignoring the fact that the entire system and the Solar System are both in their own orbits around the Galaxy. Right now, though, as its name suggests, its the closest. This situation, where two stars orbit each other much more closely than a third, is very common in the Universe and seems to be the most stable arrangement: the stars arrived in these positions after some chaotic behaviour and have now settled down. However, in 刘慈欣’s book, he imagines that a planet situated near these stars would have a chaotic orbit, some of the time getting too hot for complex life, sometimes getting too cold, sometimes being seriously perturbed by their gravity and sometimes almost being ripped apart by it and suffering severe volcanic eruptions. Life on such a planet could be imagined to be very difficult. It’s worth noting that this is not the real situation for most possible orbits of planets in the Centauri system, although it would be so for certain positions, such as for a planet halfway between the Sun-like pair or orbiting the Proxima far enough away to be strongly influenced by that pair’s gravity.

Due to the chaos of their home world, the 三体 decide to travel to Earth, and while doing so they also decide to harness the power of human intelligence by getting us to solve their world’s three-body problem through a VR video game where the player is put on the world in question, represented in a way humans can relate to, and has to find a solution to their predicament.

The first book, ‘三体’, begins in the 1960s during 毛泽东’s (Máo Zédōng’s) Cultural Revolution, where a scientist, 叶哲泰, is being denounced in a Struggle Session for his teaching of Einsteins Theories of Relativity. He is in fact killed in the process and his daughter, 叶文洁, is sentenced to hard labour followed by prison. This leads to her becoming very cynical about the human condition and our ability to improve things ourselves. Later on, she is employed as an indentured servant practicing science at a military base attempting to send and receive messages from any alien civilisations which might exist in other star systems, apparently focussing on the Centauri system. One day, she receives a message from an individual altruistic alien telling her that humans must at all costs cease to attempt broadcasting their existence and attempting to message aliens because it puts us all in danger. Because she now believes there is no way humans can sort out their own problems, 叶文洁 does the opposite, sending an enthusiastic message of welcome to the 三体, i.e. the aliens, and they proceed to plan to invade Earth, a process which will take four centuries because they can only travel at one percent of the speed of light.

There’s plenty more to both the series and the original trilogy, but this is enough to be going on with in terms of the details of the first book, and there is a particularly crucial point which is named after the middle novel of the trilogy: “黑暗森林”, or “The Dark Forest”. 刘慈欣 is not actually the first person to propose this idea.

Anyone who has read much of my blog will know that I think about the Fermi Paradox more than occasionally, but just in case you haven’t come across this, the Fermi Paradox, mentioned by the physicist Enrico Fermi in 1950 CE but not originally his idea, is this: the Universe is vast and there are innumerable Sun-like stars and planets orbiting them, and also æons old, so that life could have evolved from microbes to humans almost three times over or more given its age, and yet we hear nothing from intelligent aliens, are unaware even of the existence of life anywhere else in the Universe and have never been visited by them. In other words, “where is everybody?”. I’ve mentioned a few of the more interesting attempts at solving this problem in this blog. For instance, it might simply be that everyone else is really bad at maths and therefore there’s no rocket science on alien worlds, or it could be that the element phosphorus is always essential to life but is too scarce for it to happen very often, and when there are intelligent life forms, they can’t get out of their little oasis of phosphorus to reach other star systems, where in any case they’d have to take phosphorus with them to establish an outpost. One simple solution is that there’s no life anywhere else in the Universe at all. One I was keen on for a very long time was that other civilisations have something like the ‘Star Trek’ Prime Directive, that they can’t interfere with developing civilisations until they reach a certain stage of development. It could also be that there are plenty of civilisations which reach something like a twentieth century level of technological development but then end up wiping themselves out in a nuclear war, destroying themselves through climate change or developing artificial intelligence which then decides they’re a threat and kills them all. Note that I say “twentieth century level”: we could be living on borrowed time here.

Quite a lot of this is not at all reassuring. Perhaps even less reassuring is 黑暗森林, which is as I say not actually an original idea although it was 刘慈欣 who actually named it that. The exact metaphor was used by Greg Bear in the 1980s. The idea is this. There is silence out there because aliens elsewhere in the Universe are aware that broadcasting their presence would threaten their existence due to potentially hostile threats from other star systems, and humans are simply too naïve to realise what a bad idea it is to tell all and sundry we’re here. We don’t know any of them from Eve, and they could be really dangerous. They could just go, “ooh juicy, another race to enslave and another nice planet to conquer” and do something horrible to everyone. Another way of putting it: “it’s quiet. Too quiet.” It’s like the silence that falls over the clichéed hostile bar when someone from the Other Side enters.

Now I do not like this solution, to say the least. Obviously in saying that I could just be all weird about it and say, “well I don’t like this any more than you do, but facts is facts and it is what it is,” but that’s not what I’m saying. I might not like the course of a fatal disease or the policies of a particular political party, but it’s still possible to find that particular pathology interesting or the implementation of a particular set of policies fiendishly clever or elegant in a Machiavellian way. In this case, however, I see the solution itself as pathological, and apparently I’m not alone in that as you will find if you listen to that podcast. But I already had these misgivings before I heard it. The problem is that it’s very negative and cynical, which doesn’t necessarily make it unappealing, but more than that, it seems to be a reflection of the current state of the society, or perhaps world, in which it was written.

Because the thing is,  ‘地球往事’ is horribly, horribly grim and oppressive feeling. Suppose you look up at the skies and you see stars, an infinite horizon, endless hope and possibility and most of all for me the feeling that the atrocities and Hell we’ve made for ourselves on this small blue dot is as nothing compared to the hope the splendour of this unknown Universe around us shows. Even if it’s devoid of life entirely, it’s still magnificent and majestic, and moreover in spite of the actual Three Body Problem as opposed to the book, most of it works for pretty much of the time in one way or another. And if it isn’t devoid of life, there’s the optimism and awesomeness of a Cosmos replete with possibilities of friendship and fascinating variety. “Infinite variety in infinite combinations” as the Vulcans say.

There’s hardly any point in saying this, but just because something is appealing doesn’t make it plausible. I might be looking up at the sky with foolish, immature and groundless optimism. Absolutely, that could be so, and it’s very hard to decide whatbecause of the silence we all experience from the vast emptiness that surrounds us. So I don’t like it, but more importantly, what do the myths we make up say about us? What does it mean that 刘慈欣, in the 中华人民共和国 (People’s Republic of China) of the twenty-first century CE, is able to get this idea out to popular culture in the West via Netflix? Were there obstacles placed in front of him by the 中国共产党 (CCP) difficult to overcome, or were they not placed there in the first place because he perhaps has a knack of saying what they want him to say? Is he an establishment or an anti-establishment figure, and what does it mean that Netflix are apparently happy to stream what might be 中国共产党 propaganda? Or is it universal in some way, and if so is that universality a good thing or a bad thing?

‘ 三体’ has also been adapted by 腾讯 (Tencent) into a very different version. I know about 腾讯 on a personal level because someone close to me worked in 中国 (the Central State, i.e. China) for some time and the only way we could send messages to each other was through their app, QQ. Now I didn’t trust QQ very much at all and I was careful what I said on it, and I believe that was justified. One way of looking at this is that I’ve been duped by Western anti-Chinese propaganda, but it’s not that simple. QQ is their social media. Our social media are about as trustworthy, and this is not at all to say that 中国共产党 is better than the global megacorps. It’s more that they’re equally bad. It’s not about not trusting 中国. It’s about not trusting any big faceless organisation of any kind, because they simply will not have the interests of the ninety-nine percent at heart. We all know this.

Getting back to the actual Three Body Problem as understood in physics, it seems fairly clear that 刘慈欣 uses it as a metaphor for how unrestricted social systems are chaotic and unpredictable. A laissez-faire economic or social system, or a liberal or social democracy is just such a chaotic system, but it can be simplified by totalitarianism. If the likes of 中国共产党 and 腾讯, i.e. a few large organisations with a high degree of control over society, exist, we no longer have a chaotic Three-Body Problem but at least a special case of the problem like that of the Lagrangian Points or the Sun and Earth. Society can be made sense of and predicted. Likewise, in the West we have something like the social media firms, able to socially manipulate us all, and the US Republican Party, greatly simplifying the West through that extreme degree of control and gaslighting. So Netflix will be fine with streaming ‘The 3-Body Problem’ and by clamouring for a second season, which I must admit I personally want, we’re actually saying yes please, let’s have some more of that tasty propaganda.

There’s more than this though. ‘Star Trek’, and even more so Iain M Banks’s ‘Culture’ series and Ursula K Le Guin’s ‘The Dispossessed’, all provide a hopeful mythos for the nature of the wider Galaxy and optimism for the future. To quote from Banks’s ‘State Of The Art’:

Here we are with our fabulous GCU, our supreme machine; capable of outgenerating their entire civilization and taking in Proxima Centauri on a day trip…here we are with our ship and our modules and platforms, satellites and scooters and drones and bugs, sieving their planet for its most precious art, its most sensitive secrets, its finest thoughts and greatest achievements…and for all that, for all our power and our superiority in scale, science, technology, thought and behaviour, here was this poor sucker, besotted with them when they didn’t even know he existed, spellbound with them, adoring them; and powerless. An immoral victory for the barbarians.

Not that I was in a much better position myself. I may have wanted the exact opposite of Dervley Linter, but I very much doubted I was going to get my way, either. I didn’t want to leave, I didn’t want to keep them safe from us and let them devour themselves; I wanted maximum interference…I wanted to see the junta generals fill their pants when they realized that the future is––in Earth terms––bright, bright red.

Instead of such a myth, we are now asked to adopt 黑暗森林 as the explanation for the silence of the heavens, and maybe beyond that to accept that that silence justifies fear of the Other, and through that fear, as occurs later in the trilogy, that totalitarianism is the only answer. Does that sound at all familiar? Does it perhaps sound like certain members of the Republican Party rejecting democracy and freedom of the press in favour of Project 2025? And yes, it most definitely sounds like something coming out of 中国, but it’s equally at home in the West, and I happen to be mentioning the US Republican Party here but it applies just as much to many other Western countries, including Britain.

You may have struggled with my incessant use of 汉字 in this post but all that really is, most of the time, is a way of transcribing ideas into ideograms like our &’s and @’s. Just as we might look over at that country and think that the Central State has essentially foreign ideas based on the thoughts of “Chairman” Mao, we might also imagine that the capitalist West is free from such things. But it isn’t. It suits the West just fine actually. Nor is the Central State in any wise Communist, because by definition any economy with a stock market isn’t Communist. It’s just as capitalist as we are and it’s actually better at it, to the extent that certain people could learn from them how to be even more capitalist than they are already. Oceania has always been at war with Eastasia.

We are aware that encounters between White people from Western Europe and racialised people elsewhere, such as in Afrika, the Americas and Oceania, have not generally ended well for the latter, and this has often been associated with a mismatch in technology. We might attempt to deduce that this is also what would happen if another species from elsewhere in the Universe with superior technology encountered humanity. However, that makes the rather major and unwarranted assumption that aliens are like us. This is unlikely, partly because they’re alien but also because in this scenario they’ve reached another star system. It also assumes that the greed and materialism dictated by the European-derived economic system is a law of nature and that there’s no other way things can proceed.

This, though, is how I see things going. Here we are on Earth with increasing threats to our civilisation, mostly self-inflicted, such as the use of weapons of mass destruction, anthropogenic climate change and artificial intelligence, among other more prosaic problems. In the meantime, we haven’t been back to Cynthia for over fifty years and there’s no sign of us building large space colonies or going to Mars. Hence we’re missing out on the Overview Effect, or Arthur C Clarke’s ‘Rocket To The Renaissance’, both of which could stand a good chance of changing global consciousness, we have no orbital solar power stations which could satisfy all of our energy needs and enrich Third World nations around the Equator, and various calamities could, and probably will, befall us which space exploration and settlement would’ve prevented. On the other hand, suppose a civilisation out there somewhere has thriven and got past this, or hasn’t got itself into such a pickle in the first place. Those are the kinds of civilisation which we’re likely to end up contacting, because the others simply aren’t viable. Which kind of civilisation we are remains to be seen to some extent, although I know which one I think we are. Or maybe every species of this kind just ends up annihilating itself.

The attempt to contact aliens depicted early on in this series and book is an act of hope, of optimism, which is depicted as bringing down utter catastrophe upon the world. Well no, I’m not going to adopt that view, particularly when it seems to suit certain social forces exceedingly well. I prefer the other. Hence if technological cultures exist elsewhere, they would be of the following kinds: unable or unwilling to leave their planet and perhaps quite healthily uninterested in doing so, in which case they’re not a threat; capable of space travel but also wiping themselves out before leaving their solar system, and yes those would be hostile but are not a threat; able to leave their systems but unwilling to contact us for various reasons; able to leave their systems, peaceful, coöperative and friendly. Or, there could just not be any intelligent life anywhere else. Any of these options has nothing to do with the Dark Forest, is more inspiring than that and is less likely to be useful for political oppression. So there!

Vulcan And Vulcan

If you say “Vulcan” to most people nowadays in an Outer Space context, the chances are they’ll think of Spock, and that’s an entirely valid thing to do. However, if you were to say it to anyone with much knowledge of astronomy in the nineteenth century, it would’ve called something completely different to mind: a planet which orbits the Sun even more closely than Mercury. I’m going to cover both in this post.

Firstly, the ‘Star Trek’ Vulcan, whose Vulcan name is Ni’Var. This is reputed to orbit the star 40 Eridani A, a member of a trinary star system also known as ο² Eridani (Omicron-2 Eridani – that isn’t an “O”) sixteen and one quarter light years from here, and therefore also quite close to 82 Eridani, which is said to be one of the most suitable nearby stars for life, around which a possibly habitable planet orbits in real life. Of the stars, A is an orange dwarf, B a white dwarf and C a red dwarf which is also a flare star. Because B would previously have been a red giant and exploded, the chances are that any habitable planets orbiting A would have been sterilised by B’s outburst, and since C is a flare star, this is also unsuitable, although there would be nothing to stop an interstellar civilisation settling a planet in A’s habitable zone, which would of course be Vulcan.

As I’ve mentioned, I don’t pay much attention to either ‘Discovery’ or the new ‘Star Trek’ films, but I’m aware that Vulcan has been destroyed in revenge for the destruction of Romulus. I find this a bit annoying and I’m not sure what the point of it was plot-wise, but it doesn’t alter the in-universe fact that Vulcan was the homeworld of the first species to make open contact with humans when Zefram Cochrane first activated the warp drive. I’m also aware that that is inconsistent with the depiction of Cochrane in TOS. It is interesting, though, that any real planet in the habitable zone of 40 Eridani A would have been severely damaged by the 40 Eridani B supernova.

I understand Vulcan to have no moons, higher gravity than Earth and no surface oceans. I’m also aware that Romulans and Vulcans are the same species. It irritates me that they’re humanoid but also interests me that some of their anatomy and physiology is known, such as their copper-based respiratory pigment. Then again, although the in-universe explanation of widespread humanoid aliens is that we are all descended from humanoid ancestors who existed around the time our own Solar System formed, it’s also conceivable that convergent evolution would lead to similar body forms among sentient tool-using species. Back to Vulcan itself though. It has a thinner atmosphere than Earth’s, which I think justifies the copper-based blood pigment, and the sky and much of the surface is red. There are seas, i.e. large landlocked lakes, rather than oceans continuous with each other. Depending on the total surface coverage of bodies of water, I think this would probably make the planet uninhabitable for humans although clearly not for native life. 40 Eridani A is a K-type star, with a longer lifetime than the Sun’s in terms of being able to support a habitable planet, which, if orbiting at the distance necessary to receive the same quantity of light and head from its primary as we do from our Sun as a planet, would have a mean orbital radius of about 0.68 AU, i.e. sixty-eight percent of Earth’s distance from the Sun, and 223-day year. However, Vulcan is supposed to be hotter than Earth and might therefore be closer to its sun or have more greenhouse gases in its atmosphere, or it could just reflect less heat back into space, and in fact it probably would due to less ice on its surface. The difficult thing to account for with Vulcan is the combined higher gravity and thinner atmosphere, but there is another reason than gravity why a body might lose some of the gas surrounding it, which is consistent with what we “know” about Vulcan. Earth’s strong magnetic field is generated by our own large moon, Cynthia, which raises tides in our iron-nickel core and magnetises it like stroking a bar of iron with a magnet does, and that generates our magnetosphere, which traps ionising radiation from the solar wind which might otherwise reach Earth’s surface and strip away our atmosphere. Hence Vulcan, with no pre-existing satellites, would not have this benefit but would on the other hand still be able to hold on to some atmosphere because of its higher gravity, so maybe that is in fact realistic. Venus has no magnetic field but an extremely dense atmosphere, although not one hospitable to life at the solid surface, due to photolysis – the action of light on rocks releasing carbon dioxide gas. However, we’re basically aware that Vulcan’s atmosphere has enough oxygen to support human life without their own oxygen supply, and not enough carbon dioxide to poison us, which is 0.5% at our own atmospheric pressure. 170 millibars partial pressure of oxygen is required for this and CO₂ cannot be making a significant contribution to the pressure, so we can surmise that the rest of Vulcan’s atmosphere substantially consists of other gases. It isn’t pure oxygen. In fact, it’s quite likely to be nitrogen if Vulcan physiology is anything like ours and their bodies consist partly of protein, as the nitrogen has to come from somewhere, so I’m going to say the mean surface air pressure is about 0.25 bars. I’ve plucked this figure out of the air, so to speak. There probably is no such thing as sea level there because of the various lakes with different presumed depths and heights, so this would be defined as some kind of mean distance from the centre of the planet or a level at which gravitational pull is close to a particular standard. The boiling point of water on Vulcan is therefore about 60°C, but we know from McCoy’s mouth that Vulcan is very hot compared to Earth, so this puts an upper limit on its surface temperature unless it’s so hot at the equator that it causes water to evaporate.

40 Eridani A is orange. The sky is likely to be close to a complementary colour, such as teal, given that, but because of the dusty surface it’s entirely feasible that it would in fact be pinkish due to small particles high in the atmosphere. Also, the general ruddiness of the planet as shown on screen gives the impression of heat and dryness, so artistically that does seem to be a good decision. The same features make some people think of Mars as a hot planet when in fact it’s often colder than Antarctica. Regarding sparse water cover, a thin atmosphere might make sense here too, particularly if water is regularly evaporating from the surface at the equator, since some might then be lost into space.

Vulcan would also lack plate tectonics if it’s like this, since that’s fuelled by water. The planet has no continents as such, but it does have active volcanoes and lava fields, which is to some extent to be expected as it corresponds to the “hot spot” situation in the centre of the Pacific plate on Earth, where magma seems to need to vent. Here, this results in Hawaiʻi, but on Vulcan a mountain range could be expected because there are no oceans. There would be nothing like the Pacific Ring Of Fire, and also no fold mountains because those are caused by the collision of continental plates.

Vulcan’s colour is depicted differently in different manifestations of the series. In TOS and Enterprise, it’s red. In TAS it’s yellower, and in TNG brownish. However, on Mars there is variation in colour from space due to a dust storm season, and this can be imagined on Vulcan too. Maybe one way to think of Vulcan is as a larger, hotter version of Mars.

The real 40 Eridani A does have a planet. This is, as usual, called “b”, and orbits much closer to the star than the inner edge of the habitable zone. It has a roughly circular orbit 0.22 AU from the star and a mass estimated at 8.5 times Earth’s (both those figures are rounded off). At Earth’s density, this would give it a diameter of around 25 000 kilometres, which is a type of planet unknown in our own solar system at any distance from us, and it’s classed as a “Super-Earth”, but it has a period of 43 days and would be like Mercury on its surface during the day, if it rotates at all. It’s also the closest known Super-Earth. Its orbit differs considerably from Mercury’s, which will become relevant later in this post, in being much less elliptical, which to me, in my probable naïveté, suggests there are no planets larger than it in at least the inner solar system.

This brings me to the other Vulcan. In the nineteenth Christian century, the French astronomer Urbain Le Verrier came up with a particularly accurate model of planetary motion within the Solar System. It had been noted that the most recently discovered planet, Uranus, tended to drift slightly behind and ahead of its predicted position given its distance from the Sun and shape of its orbit. From this, Le Verrier calculated mathematically that there was likely to be another planet further out pulling at it, and predicted its position, which turned out to be correct. In fact he almost had it named after him, but they eventually decided to call it Neptune. This established his reputation and consequently, when he turned his attention to the orbit of Mercury, people paid attention and took his views seriously.

Mercury’s orbit is quite unusual compared to the other planets, particularly if you ignore the period of time when Pluto was regarded as one. It’s the most eccentric orbit by a long way compared to the others, with a variation in distance from the Sun of around twenty percent. Le Verrier also noted that the movement of the “points” of the orbit precessed around the Sun much faster even when compared to its year of eighty-eight days than those of other planets. Just as he had with Neptune, Le Verrier proposed that there was either an as-yet undiscovered planet even closer to the Sun or a number of smaller bodies like asteroids within the orbit of Mercury, and since it would’ve been so close and so hot, he called it Vulcan after the Roman god of fire, Vulcanus. The planet’s existence could be confirmed in two ways. Either it could be detected in transit, as most planets are detected at the moment, or it could possibly be glimpsed during a total solar eclipse. A number of astronomers then reported that they had indeed seen this planet transiting the Sun. For instance, Edmond Lescarbault, a doctor, described a tiny black spot moving across the Sun faster than a sunspot, moving with the rotation of the Sun, would, and also lacking a sunspot’s penumbra. The observations even seemed to confirm Le Verrier’s prediction of Vulcan’s size and orbit. However, it was difficult to predict when these transits would occur because that depended on the tilt of Vulcan’s orbit compared to ours. Mercury, for example, can only be seen to transit the Sun in May or November because only then is the tilt of both its and our orbits aligned such that it can get between us and the Sun. The observations did seem to occur fairly randomly, but at first glance Mercury’s do as well, if you didn’t know anything about its movements already.

There was a total eclipse of the Sun in 1883, shortly after Le Verrier’s death in 1877, during which Vulcan was not observed. It was still possible that the planet was either behind or transiting the Sun at the time, but six further such observations, the last in 1908, also failed to turn it up, making it increasingly improbable that the planet existed. However since that time astronomers have claimed that close ups of the Sun’s surface do sometimes show small black dots which are not sunspots, although these may be imperfections of photographic plates, and there are asteroids which approach the Sun more closely than Mercury does, such as Icarus. It strikes me that it’s not only possible but probable that there are asteroids which orbit entirely within the orbit of Mercury, although they would have to be very small and would be difficult to observe or confirm. These are known as Vulcanoids, and would have to be between six kilometres and a couple of hundred metres in diameter. Every region of the Solar System which is not severely perturbed by the gravity of known objects has been found to contain objects like asteroids or comets, so if the innermost region of the system doesn’t have any this must be due to a non-gravitational effect. It is in fact possible that the light from the Sun is so strong at that distance that it would push smaller bodies away from it over a long period of time, so this may be the explanation. This might sound far-fetched, but it’s been proposed that this effect could be used to divert asteroids which would otherwise crash into Earth by painting them white in order that the pressure of light from the Sun would change their orbits, and this is also the principle used in a solar sail. The MESSENGER probe took photographs of the region but this was limited because damage from sunlight needed to be avoided. Much closer in than Mercury, asteroids are likely to vaporise of course.

Vulcan was considered to orbit 26 million kilometres from the Sun, giving it a sidereal period (“year”) of twenty-six days. At another point, observations appeared to show it had a year of 38.5 days. I think it was also supposed to be very small but I can’t track this down: possibly about a thirtieth the mass of Mercury, which with the same density would’ve given it a diameter of around 1 600 kilometres, probably meaning that if it had been found to exist it would’ve been demoted from planethood by now in the same way as Pluto was. In fact, if it did exist, it would indeed have perturbed the orbit of Mercury but the other factors which turned out to be the explanation for this phenomenon would still be in play, meaning that there would’ve been an even greater anomaly unless the planet happened to be exactly the right mass and in exactly the right place, and possibly retrograde. Some kind of pointless immense astroengineering project could probably achieve that to some extent, but why? Possibly to prevent us from being aware of relativity?

The fact is that the planets don’t simply orbit the Sun alone without influencing one another and the Sun. This is the famous three-body problem, that it’s impossible to work out in almost all cases how three bodies would orbit each other, and even more so the much larger number of massive bodies in the Solar System. It’s possible to work out how much gravitational influence the planets would have on each other if they were the only two bodies in the Universe though, and if initial conditions are known. For instance, Venus and Earth approach each other to within fifty million kilometres and have roughly the same mass, so left to themselves they would orbit each other at roughly twenty-five million kilometres from their centre of gravity once in forty-five millennia if I’ve calculated that correctly, and at the closest approach, which would be during a transit of Venus, that’s the gravitational pull we’re exerting on each other – about forty-five thousand times less than the Sun’s. Mercury is the least massive planet, being just over half the mass of Mars, the next smallest. Pluto is of course far lower in mass, and if Cynthia is considered a planet in its own right, that would be considerably less massive. Anyway, this means that Mercury is pulled around a lot by the other planets. Venus approaches it to within about 38 million kilometres but without doing the maths it isn’t clear if that’s the biggest gravitational influence because of Jupiter being so much more massive than the other planets, even though it’s far further away. Jupiter is over three hundred times the mass of Earth but would get within 4.8 AU of Mercury, which actually gives it roughly the same influence as Venus. But this is not the only reason Mercury’s orbit precesses as much as it does.

Albert Einstein listed a number of ways to test his theory of general relativity, one of which was the orbit of Mercury. The pull of the other planets is insufficient to explain precession in Newtonian terms. There’s still a bit left over if you try to do this. It’s at least seven percent larger than it “should” be. The explanation for this was instrumental in getting general relativity accepted. Einstein made three suggestions about how general relativity could be corroborated. One was that light would be red shifted if it passed through a gravity well. Remarkably, although it took something like four decades, the observation of 40 Eridani B eventually showed that this was so, I’m guessing because of the other stars in its system. Gravity stretches light because it distorts space. The second proposition was that stars observed near the Sun during a total solar eclipse (Again! They’re useful things) would appear to be in a different position because their light would be bent by the solar gravity, and this was indeed found to be so a few years later. However, the world had to wait for these two findings. The other one was that Mercury’s orbit would precess at the rate it did having taken into account the perturbations of all the other planets, and again this was found to be so, but in this case it was already known that this happened because Le Verrier had observed it in the previous century and the existence of Vulcan had been refuted. The reason this happens, I have to admit, I don’t really understand, but I can provide a kind of visual model of it which could show this.

The Rubber Sheet Theory is a model of space as if it’s two dimensional left to itself with weights representing stars and planets which, if placed on such a sheet would create dents in it. Obviously this is not an adequate explanation as such of general relativity for several reasons, one of which is that it uses gravity to explain gravity – that’s what’s pulling down the weights. It also makes space appear to be a substance, something which physicists had worked heavily against when they disproved the existence of the luminiferous æther, which since it was supposed to be extremely rigid wouldn’t work in this situation anyway. It shouldn’t be mistaken for Einstein’s theory itself, but it is a useful way of looking at it. In any case, if you imagine the kind of dent which shows up in the title sequence of Disney’s ‘The Black Hole’:

. . . which is like one of those charity coin collection things, space around the Sun is distorted to a limited extent like that, and attempting to do a “wall of death”-style orbit around it, which would in any case be elliptical rather than perfectly circular because the Universe is imperfect like that, would lead to your bike describing a series of ellipses which were not perfectly congruent with each other but were more like a spirograph pattern. Having written that paragraph with its references to a number of very ’70s things makes me wonder if it’s going to make any sense to someone born after Generation X.

Now I can see that this does happen, but I am also puzzled by it. Whereas I’m sure that I couldn’t aim a coin at one of those charity collection things in such a way that it would just circle around at the same level until friction interfered, and that at best if I could make it describe an elliptical path for a few revolutions, the bits of the ellipse furthest from and closest to the hole would precess, I would put that down to the fact that I, and anyone else to a lesser extent, can’t aim perfectly rather than simply due to the geometry of the hole. Nevertheless, this appears to be what I’m being asked to believe with this: that it isn’t only one’s inability to aim perfectly, or for that matter the friction the coin (or ball bearing – let’s take the instability of the coin out of the picture), that leads to this precession. But apparently not. Apparently, if you were to have too much time on your hands and designed some kind of precision ball bearing throwing machine for charity coin collectors, and it wouldn’t be popular because they want coins, not ball bearings, it would do the wobble thing even if it stayed circular enough not to fall down the hole immediately, and it would wobble more the closer it was to the whole. So they say, and this is what got general relativity accepted.

There have been other Vulcans. For instance, one of the many hypothetical planets in Western astrology is the intramercurial Vulcan, seen as the soul ruler of Taurus and orbiting once every twenty days. This Vulcan would go retrograde more often than Mercury. It’s fiery and urges the individual to look for non-physical knowledge, which makes sense given its history in astronomy. It was also suggested in a poll as the name of one of the moons of Pluto, and actually won the most votes but that was then named Kerberos after the Hadean dog, which was the runner up. Vulcan actually doesn’t seem like a very good name for a moon of an icy planet way out in the outer reaches of the Solar System, but I don’t know the reasons it wasn’t used. Maybe the IAU just didn’t want to be reminded of what they might regard as an embarrassing phase in the recent history of their science. In the Second Doctor story ‘The Power Of The Daleks’, there’s a planet called Vulcan which is settled by humans and highly volcanic with pools of fuming mercury on its surface. Doesn’t sound very nice at all really. There does not, however, seem to be an asteroid named Vulcan, which is quite surprising.

I’ve sometimes wondered if there’s a story behind the naming of the ‘Star Trek’ Vulcan and if it’s in any way connected to the hypothetical planet, but I don’t know. How about you?