If your first language is English and you’re over about thirty-six, I’m guessing, you may well remember this song. If you’re a bit older you’ll also remember their song about Arthur Daley, and you’ll also know who that was. I only realised recently that there was a claymation video for it though, because in 1987 CE I wasn’t watching television, with the exception of the repeat of ‘The Hitch-Hikers’ Guide To The Galaxy’.
One of the oft-repeated lines, alleged to be a quote from the original series, was Spock saying “Well, it’s life Jim, but not as we know it. . . Captain”, with a distinctive pause at the end of the verse before the Spock impersonator says the final word. It won’t surprise you to know that just as Kirk or anyone else never said “beam me up, Scotty”, this is a misquotation. The closest Spock comes to saying it is “no life as we know it” in ‘Devil In The Dark‘, when in fact he says it twice, and that episode in particular refers to a very common suggestion regarding “life, Jim, but not as we know it” – silicon-based life.
I have already discussed this here:
Without re-watching the video, my conclusion was that there are two ways in which life which could be said to be silicon-based are possible. Because silicon compounds are often bioactive, and silicon-based structures do exist in organisms on this planet, a situation could arise where much of an organism’s biochemistry involves silicon compounds, even including hormones and much of the hard parts of their body, but at core still carbon-based. The alternative is that a narrower range of silicon compounds which are however particularly versatile could be used in a manner similar to the difference between binary and higher-based ways of representing numbers. It can still be done, but the binary representation of the number eleven takes four bits but only one duodecimal digit. Hence silicon-based life could still exist but be more “long-winded” than carbon-based, and consist of relatively larger molecules than the already very large macromolecules found in terrestrial life such as muscle protein and cellulose. However, although I think it’s likely to be possible, I don’t think it would emerge of its own accord or be able to survive outside a specially designed environment, mainly because silicates are very stable compounds and once silicon has entered such a state it would be difficult to remove it. If you watch an exposed microchip under a microscope, you can see it visibly degrading and not-so-gradually oxidising. An environment containing silicon-based life of this kind would have to be free of oxygen and water. Silicon in water liberates hydrogen and combines with oxygen to become silica, and this may not happen with the silicon compounds used for life, but there’s a risk of producing bonds which are so strong that ordinary biochemistry can’t sever them all the way through silicon-based biochemical pathways.
Only the lightest atoms are able to produce more than single bonds. This includes boron, carbon, nitrogen and oxygen. As far as I know, no heavier element is able to form more than single covalent bonds. This means that the structures of molecules made up of chains or rings of silicon atoms may be rather limited, although silenes do contain double silicon bonds. Moreover, the stability of longer chains of silicon atoms is lower than that of carbon chains with the same number of atoms in them. Nonetheless, I do believe silicon-based biochemistry is possible and I will now cover some of the possibilities.
DNA is a well-known helical polymer used to store genetic information. Silicates have various forms, consisting of sheets of hexagonally-arranged silica tetrahedra sharing vertices, alternating such tetrahedra with opposite orientations, simple chains of the same, or helically-arranged such chains. This is interestingly close to DNA, although it’s only a single helix and the unit is silica groups. There are also double chains known as amphiboles, linked via shared oxygen atoms. Some asbestos minerals are amphiboles. If these are able to form extra bonds with bivalent atoms of various kinds, the result would be a very similar molecule to DNA, with a potentially readable code, although how it would come uncoupled, be transcribed, what it would be transcribed into, and how it could replicate and recouple are different questions, which may not have answers. Nevertheless, this is a potential storage medium, perhaps one which would need to have more “steps” than DNA per codon. This illustrates what I mean by molecules needing to be relatively larger, although on the other hand the actual rungs are smaller because they only consist of two atoms.
Closer analogues with organic compounds are the silanes and silanols. The former are silicon-based versions of the alkanes such as methane, butane and propane. Like them, silanes are flammable, and become more flammable the longer their chains are because longer chains are less stable. In the case of alkanes, similar substances can be derived from them in the form of long-chain fatty acids, with a -COOH group on one end. These are just ordinary organic acids which happen to have very long chains, and in organisms they’re often joined together by a glycerol residue at one end into kind of E-shaped molecules, used to store energy and form cell membranes. Eicosapentanoic acid is quite a well-known essential fatty acid. It has mixed double and single bonds, like all polyunsaturated fatty acids, and twenty carbons per molecule. By contrast, the highest silane has only six carbons, and is entirely saturated because they only have single bonds.
Silanols are the silicon-based versions of alcohols, although many of them are in fact organosilicon compounds rather than containing no carbon. They’re more acidic than their organic equivalents and can be used in shampoos to improve the pH balance of the scalp and hair. Another class of silicone compounds in common use is the cyclosiloxanes, which are hormonally active, found in cosmetics and toiletries and are persistent in the environment. As far as this particular biosphere is concerned, these substances are concerning, but their bioactivity suggests that in other circumstances they could be functional as biochemicals. Many of these contain oxygen bonds, which may be why they aren’t broken down. It may not be that they are merely difficult to process in organic biochemistry, but just difficult to do so in any circumstances conducive to chemically-based life, and if that’s so, the chances are that compounds with silicon-oxygen bonds may break down in other ways but not in such a way as to become usable again by living systems. This would ultimately result in a silicon-based biosphere having all its silicon locked up with oxygen, which is how things are here.
It would be interesting to attempt to replicate the Miller-Urey experiment with silane instead of methane. This was an attempt to replicate the chemical conditions of Earth soon after formation to discover whether biochemicals found in life today would form, and it succeeded. It used water, ammonia, methane and hydrogen, and resulted in the production of such compounds as the amino acid glycine and the sugar ribose. However, although this is a fruitful exercise, it doesn’t reflect the kind of conditions likely to exist in any real situation. The interstellar medium does contain silane and several other silicon compounds, so this is not entirely unrealistic, but the concentration of silane and other silicon-based substances is much lower than their carbon-based equivalents.
A strong argument against silicon-based life could be made on the basis of the existence of organic life on this planet. Silicon is almost a thousand times more abundant that carbon, and yet life developed based on this far less widespread element. This might, though, result from other conditions being unsuitable such as the presence of water, which is however the most abundant compound in the Universe. For silicon to combine with ease in other ways, oxygen would have to be relatively scarce. On Earth, oxygen is the most abundant element in the crust, much of it combined with silicon, and I may be wrong but I find it hard to imagine a rocky planet whose situation is different enough to enable silicon to form other compounds routinely. Oxygen is the third most abundant element in the Universe as a whole, and is more than a dozen times as common as silicon. It isn’t necessarily that silicon-based life is impossible so much that other factors make it unlikely ever to happen on its own, without intervention.
That said, there might be a reason for manufacturing silicon-based microbes, for example, to reclaim plastic waste or as part of a manufacturing process. In a sense, microbes are highly complex nanotech devices, and organic life forms can only do so much because the range of conditions in which they can function is limited. The same applies to silicon-based life, but this is a potential advantage as it prevents the “grey goo scenario” of self-replicating devices eating up the planet. This brings up the issue of exotic solvents.
Although silicon biochemistry is bound to be very different to its carbon-based equivalent, what I’ve described so far is substantially similar, and as such it would require a solvent. It can’t use water unless silicon enzymes are able to catalyse in those conditions without being damaged. Silicon-based life attempting to use water is like carbon-based life attempting to use mercury as a substitute for water, which would combine with the sulphur in proteins and destroy their structure, except more severely because any silicon atom with free electrons would bind to the oxygen, liberating hydrogen incidentally, but also knackering the structure of its macromolecules. This is the big problem with silicon-based life in fact, and although I’ve suggested that entirely artificial silicon-based life forms could be used to clean up the environment, they would have to be exposed to water to do their job, which would be very harmful to them. Although glass, silica and many other silicon compounds are excellent at keeping water out without reacting, some kind of solvent would need to get in there with the molecular machinery that’s actually doing the living, i.e. the metabolism, if the biochemistry has any similarity to that of life as we know it.
One possibility here is methanol. This is the simplest alcohol, with this structure:
Somewhat ironically, the reason why this might work is probably that it has a carbon atom bound to the oxygen, which is stronger than a silicon-oxygen bond, so silicon will not pull it away from the molecule and annihilate its own structures. Methanol is found in the interstellar medium, and there’s no reason why it wouldn’t occur on planets. Its melting point is -97°C, so in an atmosphere it’s entirely possible that liquid methanol could exist on the surface of a planet, though above the melting point of water ice it would do so as a mixture with water, which would be unsuitable, so if there are worlds with methanol oceans that might be a start. However, methanol is not as abundant as water, unlike another candidate solvent, sulphuric acid. Sulphuric acid is widely distributed in our own solar system, on Venus, Mars and Io for example, and this suggests that there are whole planets out there with sulphuric acid oceans, or at least lakes. Sulphuric acid seems to be much better at supporting diverse silicon chemistry than water is.
It isn’t entirely true that silicon never forms double bonds. This occurs in silene, for example. I have to be honest here and confess that I don’t know if the existence of a double bond in silene means it can occur in any other situations. While bonds are under consideration, it’s worth looking a bit more closely at the differences between carbon and silicon chemistry. The covalent radius of a silicon atom is 117 picometres compared to carbon’s 77, but the length of the silicon-oxygen bond is 163 pm compared to carbon’s 143, from which the radii must be subtracted, leading to a shorter bond length outside the covalent radius for silicon, which makes the molecules more potentially crowded for the latter and so less diverse. There just isn’t the space around silicon atoms for there to be as much variety per atom.
That said, one thing I haven’t considered yet is the question of siloxanes. Is it possible that rather than envisaging an oxygen-free environment for silicon-based life, we should be thinking in terms of just letting it happen and seeing where that would lead us? Siloxanes include both cyclical compounds and silicon oils and rubbers, all of which have their parallels in carbon-based biochemistry. Silanols are siloxanes in a sense because they have the silicon-oxygen bond, and it’s possible to build other compounds from them. Oxidation leads to the formation of silica, so this may ultimately be an unstable situation for life to exist in, but perhaps for a short period it could.
Life as we don’t quite know it, that is, still relying on complex chemistry and solvents, has certain requirements. By definition, it needs a solvent, which in our case is water but could hypothetically be various other compounds including the aforementioned sulphuric acid and methanol, but also potentially formaldehyde, ammonia or hydrogen sulphide, which I’ll talk about in a bit. It needs enough different kinds of chemicals to perform various functions, and it needs the right balance between chemical stability and reactivity. It can’t afford to have a “doomsday pathway” where an essential and irreplaceable function causes a reaction to practically inert compounds, because this would end up locking all of the central elements up in those substances, and this appears to be what silica is in the case of silicon-based life. On the other hand, this could be useful in biotechnology to protect the environment, as it amounts to artificial organisms cleaning themselves up after their work is done.
Hence I would say that silicon-based life of this kind is possible if it was carefully designed and functioned in a highly specialised and protected environment, but it could never kick-start itself. It’s feasible that a sealed vessel in a lab could be provided with an appropriate solvent, be free of water and oxygen and be seeded with a variety of silicon-based chemicals by an intelligent life form or machine intelligence and then either spontaneously assemble into simple life forms or be purposefully manufactured as such, and there are also reasons for doing this, but there is very probably no world anywhere in the Universe where this happened on its own. In fact, if we did happen to find a planet or moon with silicon-based life on it, it would be good evidence for the existence of intelligent life somewhere.
Having said all that, there are other ways in which life could be silicon-based than simply imagining the mimicry of organic biochemistry, and perhaps all these stringent requirements just mean that biochemistry as such only really applies to our own very specific kind of life. Nowadays, classical computers are silicon-based, with doping from other elements, e.g. traces of arsenic to alter the atomic structure and allow them to function as arrays of transistors. However, this requires conditions where elemental silicon can form or be deposited, which are hard to imagine. It’s also interesting from the perspective of whether intelligence has to be alive. It’s possible to imagine some kind of crystalline process occurring on a planet where transistors or other switches grow out of inorganic materials which never fit the criteria of life but nevertheless evolve and increase their information processing capacity until they count as intelligent. This, however, is evolution and that would arguably make something alive. For instance, it would reproduce. A much more straightforward way in which intelligent machines could appear would be through what we’re doing now with our development of artificial intelligence, and many have claimed that a solution to the Fermi Paradox, for instance, is that interstellar intelligence is entirely machine-based. Considering the current trends in AI, space travel, nanotech and genetic engineering, a combination of applications of these in the long term could lead to self-replicating intelligent spacecraft who would be very much at home in interplanetary space if not interstellar, the problem there being energy sources, which could be addressed by going into a sleep mode and coasting, perhaps for centuries, to reach resources, which is far more practical for a machine designed to do that than a human, although for all we know other intelligent life forms might be absolutely fine with dormancy over long periods due to the conditions they evolved to cope with. Such entities probably wouldn’t consist of biological materials as we understand them and the usual restrictions on biochemistry assumed above wouldn’t apply. No solvent would be needed, a few inorganic compounds and elements would be sufficient and so forth.
Returning to “life as we know it” to some extent, there are several possible biochemical options which have not been mentioned yet. Prominent among these is boron. Like carbon and silicon, boron avidly forms covalent compounds though with three bonds rather than four. It forms three covalent bonds and is a metalloid, with some properties typical of metals and some of non-metals. The compounds it forms are often based on polyhedral forms, either closed or like a basket, in which atoms of other elements can be incorporated. It has at least eight different pure forms (allotropes), similar to carbon’s diamond, graphite, fullerenes and bucktubes. On a molecular level it tends to form icosahedra and there is a fullerene-like form consisting of forty atoms. It also has an extensive chemistry with an affinity for ammonia.
Hence boron chemistry can go in two directions towards complex structures with behaviour. On the one hand, it can be like conventional biochemistry, but with ammonia rather than water as a solvent, so it can have complex carbon-like chemistry. This might involve boranes, which are explosive in oxidising environments such as our own lower atmosphere but would be more stable in a reducing atmosphere, such as one mainly consisting of hydrogen. Boranes are boron hydrides, but differ in shape from hydrocarbons because they tend to form the basket shapes mentioned already, or icosahedra. It can form double bonds like carbon. Hence it’s possible to imagine boron-based life living in a hydrogen-rich atmosphere in conditions where ammonia is liquid and acting as a solvent, on a relatively cold world whose temperature is around -77°C or above, how high exactly depending on the atmospheric pressure. Such conditions are possible on the super-Earth/sub-Neptune planets which are in fact the most common of all types of planet but are not found in this Solar System, which is a puzzle, provided they are just outside the “Goldilocks Zone” for our kind of life.
On the other, there’s the possibility, which also exists for carbon, of nano-“machines” being built from the structures which pure boron forms. Hence there is perhaps another route into life which is not really biochemical. The interesting thing about boron, and it can be interesting in spite of its name, is that it kind of straddles biochemistry and nanotech in a way carbon doesn’t. With carbon, the structures of biochemistry are of course exceedingly useful and versatile, but they generally consist of polymers. Boron is able to form molecules with unusual structures which are kind of quasi-crystalline and can work both mechanically and chemically.
There is, however, a big issue with boron I haven’t mentioned yet. It’s rare. It’s less common than the next fifteen elements, up to scandium, and also the second rarest of the four lightest elements, beryllium being much more scarce due to the difficulty of its formation. Hence, whereas boron has exciting possibilities as a basis for life, like silicon it’s unlikely ever to happen without intervention. If anything, it’s more likely even than silicon to act as the basis for biochemistry, but silicon is the seventh most abundant element and boron the thirty-fourth. It’s possible that shortcomings in silicon chemistry prevent silicon-based life, but in the case of boron it could merely be its scarcity compared to carbon.
This probably exhausts the possibilities of elements as the basis for life using similar biochemistry to our own, but it doesn’t have much relevance to other possibilities for life. I’ve already mentioned plasma-based life, to which chemistry isn’t very relevant and it is possible that we’ve got a bit too hung up on chemistry as the only possibility. Another couple are associated with neutron stars. Firstly, there is the option explored by Robert L. Forward in his ‘Dragon’s Egg’ novels. Forward imagines that neutrons on the surface of such a star could combine in various ways like atoms do into molecules, and have their own equivalent to chemistry. They would however have lives millions of times faster than ours, and he also supposed that their life expectancy would be around half an hour, which is quite reasonable. There is a second suggestion concerning the interior of a neutron star which was explored by Stephen Baxter in his Xeelee series, which I haven’t read and don’t understand, and there is also the Orion’s Arm version, which may be related, of Hildemar’s Knots. These are quite difficult to understand and explain, but seem to depend on the probable fact that the interior of a neutron star is likely to be superfluid and have quantised microvortices of rotation. In order to explain this, helium II is a little closer to everyday experience.
Helium is the only baryonic matter with no solid state under pressures encountered routinely on the surface of this planet. This is a little abstract as it also has the lowest boiling point of any substance, and therefore can’t be stably surrounded by a gas under pressure since everything else is solid at that temperature. There are two common isotopes of helium, helium-4 and helium-3, and because of the way spin works, one consists of bosons and the other of fermions. Above a certain temperature, all helium behaves rather like an ordinary liquid except for being almost invisible, but below it, the helium-4 isotope becomes what’s referred to a little confusingly as helium II. At a much colder temperature, helium-3 also enters this state, which is referred to as superfluidity, and is a macroscopic quantum state. It behaves as a mixture of ordinary fluid and a fluid with no viscosity at all. It can climb vertical surfaces and it flows more easily through small holes than large ones. It conducts heat at the speed of sound, a feature also found in superconductors which means that effectively a small amount of helium II is always at the same temperature throughout. With respect to Hildermar’s Knots, the important property of superfluids is that when they’re stirred, they continue to rotate forever because they have no viscosity, and the vortex formed is quantised, and due to the peculiar nature of half-integral spin things then become really confusing. Neutron stars spin very fast and this, I think, stirs the interior superfluid neutronium into quantised vortices, each exhibiting a single quantum of angular momentum and also of magnetic flux. This results in a dense tangle of filaments. Something called the Urca Process involved in the cooling of neutron stars leads to an excess of left-handed electrons which become spin currents. The topology of these filaments changes if they touch, leading to a wave being emitted through the medium. Braids of these filaments amount to life because they can consume left-handed electrons, the braids can store information and the waves propagate signals like nerve impulses. In Orion’s Arm, Hildemar’s Knots can’t relate to the Universe outside their neutron star and regard it as an abstract mathematical problem rather than the Universe. Likewise, attempting to get one’s head round what quantised vortex filament braids in neutronium within neutron stars actually are is very like trying to solve an abstract mathematical problem. It isn’t clear to me how much of this is handwaving, but if it isn’t, it’s an interesting observation because it means that both modes of life regard the other as arcane and abstract. Also, neither can approach or exist in the other’s realm. This goes beyond “environment” because the interior of a neutron star and the kind of space compatible with atomic matter are so different that they hardly make sense to each other. Although a neutron star is only the size of a medium-sized city considered from the outside, they will distort space to some extent on the inside, and the amount of matter within them is enough to make half a million Earth-sized planets. Moreover, all of this would be happening on an absolutely minute scale.
There’s a second kind of theoretical filamentous life which may exist within less extreme stars, made of cosmic strings. Before I launch into this, I want to point out that I have my doubts about the very basis of this life form. There is an issue with magnetic monopoles, and it goes like this. We’re used to the idea of positive and negative electrical charges and the idea that one can exist without the other. There are positrons, protons, electrons and muons, all of which have isolated charges which are not paired by their opposites, although they attract each other. It might be thought that south and north magnetic poles could also exist alone, but this has never been found, and if a bar magnet is cut in half sideways it just becomes two smaller bar magnets with a north and south pole each. This seems to go on no matter how many times it’s done. Remarkably, this was discovered in the twelfth Christian century and the reason for it has never been discovered. Physics as it stands today often insists that magnetic monopoles must exist somewhere, but if they do, none have ever been detected and I personally don’t believe they exist. If they do, they would form a kind of exotic matter whose orbitals would be much smaller than those of electrons in atoms, but they would be somewhat like them nevertheless and crowd together like atoms, and consequently they would be extremely dense and yet not at all of the kind of matter which makes up superdense objects we know about such as white dwarfs and neutron stars. Because they’re so dense, it’s possible that they’re only found inside massive objects such as planets and stars. That’s the first bit.
The second bit can be explained by combing hair. I have a double crown, meaning that I can’t really have a parting. There are two whorls on my head. Most people have only one because their bodies are not covered in the kind of hair which grows, or grew, on their head, but if it was everyone would have a double crown. This is presumably the case for other species of ape, and of course for cats and dogs, to take two examples. These are known as topological defects and are thought to exist in the Universe because of the way it formed. Space only appears to be Euclidean. Two parallel lines do not in fact stay the same distance apart but will meet at a finite distance, but it appears to be Euclidean to most observers outside an immensely strong gravity well. However, space is markèdly non-Euclidean near a cosmic string because it’s a defect in space, such that a circle around such an object would have less than 360°. This peculiarity makes them very dense but their width is similar to that of a proton. They behave as one-dimensional objects, and a kilometre long stretch of a cosmic string would be more massive than Earth. This gives it an unimaginably huge density, and it may be that they are responsible for the clumping of matter seen in the Universe, where galaxies form into clusters and filaments because they have all fallen towards the strings. That said, there could also be cosmic strings whose “non-Euclideanness” is opposite, such that circles around them would have more than 360°, and these would have immensely negative mass. If these exist, it would be possible to move away from them faster than light, so they probably don’t, but that doesn’t mean the ones with psitive mass don’t either.
Both magnetic monopoles and cosmic strings are topological defects like partings and hair whorls in geometry, so they have an affinity to each other. Monopoles might appear as if threaded onto a cosmic string, and when this happens, the resultant beaded “necklace” could have something like chemistry. Neither may it have escaped your attention that this sounds once again rather like DNA, the difference being that magnetic monopoles can only be of two types, south and north. At this point it’s necessary to define the nature of life, which for the purposes of this suggestion has three characteristics. It can encode information. This is the thing which it’s difficult to understand how plasma life cells would be able to do along with the microspheres mentioned in that same post, and which for now constitutes a problem in imagining how plasma-based life is possible. Our own life solved it with DNA and RNA, initially at least. Another characteristic is that such information carriers must be able to replicate before they’re destroyed. This has an interesting dynamism with it which appeals to my Marxist brain, as it means life can only be considered as having a history from cradle to grave. The final characteristic is a surce of free energy, which considering we’re talking about life inside living stars is not exactly in short supply.
Were it as simple as magnetic monopoles lined up on a cosmic string, equilibrium would prefer north and south poles to alternate and no real information could be stored on the necklace, but each monopole could be further split into two semi-poles, making four possible semi-poles which do not necessarily annihilate each other as hey would if they were simply monopoles and their corresponding antiparticles. This makes four base-pairs which can exist on their own strings, which is remarkably similar to the structure of DNA, with four possible encoding items each of which can couple with precisely one other partner situated on a different string or strand.
This whole possibility is not intended so much to represent a real situation, although it may, as to show how different life might be to how we generally understand it. If a situation that different to carbon-based proteinaceous organic life using water as a solvent can exist, the sheer variety of possible life forms is enormous, which multiplies the possible locations suitable for life many times. The way things are here, including all these possibilities, the habitable zones for life around stars has been expanded, the types of planets or moons involved are also more varied, there are at least two possibilities on and in neutron stars and there’s a further possibility inside ordinary stars. Then there are possibly multitudinous other types of life that haven’t even been thought of. So once again, even in a Universe where there were very few Earth-like planets, many other possibilities exist and the Cosmos could once again be seen to be teeming with life. The only problem is how to recognise it.
A Box Of Chocolates 