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What analogies and metaphors might be useful in describing the processes of planets and stars in our ...
2021-04-01 09:00:00

Baby Earth, Infant Sun
The Life of Stars and Planets

Baby Earth, Infant Sun
Baby Earth, Infant Sun

The young sun cavorts wildly about, and the just-formed planet spits out magma like a spoiled little boy. In their old age they’ll fall into torpor, cool down, and their frozen remnants will wander through the galaxy for aeons. A strained analogy, or is there more to it?

Read in 11 minutes

Metaphor is a great tool for ordering reality. That’s because we juxtapose one domain with another, which immediately makes us sensitive to the similarities between them – as well as the differences. When somebody tells me I’m just like my father, there’s a good chance that in my vehement denial of this inappropriate accusation, I’ll learn a lot about myself. When in the first half of the 17th century Descartes suggested quite seriously that the world is like a mechanism, and in the second half of that century Newton expressed this precisely, scientists and philosophers threw themselves into alternately confirming and refuting this metaphor. After several centuries of such struggles, we can now say quite a bit about the degree to which the world resembles a wound-up watch, and the degree to which it doesn’t.

In this analogy, what works is that many natural systems can be understood after taking them apart. In this way, we managed to figure out, for example, the solar system, and even to predict the existence of Neptune and Pluto. Since the movements of the heavenly bodies result from simple physical reactions, which in addition can be added up – entirely like the movements of cogs and springs – missing force always means some kind of missing object. Another great success was the programme of listing the ‘cogs’ of our universe, and today physicists know the catalogue of atoms just as well as a watchmaker knows the catalogue of spare parts.

But from the very start, this metaphor has been missing something. Until today, the main bone of contention has been life – not to mention its very specific manifestation: humanity. The cat doesn’t seem to behave like a mechanical watch either, even though Descartes firmly maintained that animals don’t truly feel pain, and a dog’s writhing and yelping in reaction to when it’s poked in the side is no different from the whine of the springs of a cuckoo clock when it’s hit with a hammer.

In fact, even completely dead objects, such as planets and stars, don’t seem to behave in a ‘mechanical’ way. For example, though the sun demonstrates a regular 11-year activity cycle, perhaps recalling the beating of a heart, a closer look reveals that each of these cycles has a slightly different duration, and what’s more, there are ‘stronger’ and ‘weaker’ cycles. If a star is a mechanism, it’s certainly a capricious one.

Finally, in the 20th century something like a counter-attack occurred: biological language began to flow into all areas of the natural sciences. Today, astronomers shamelessly speak of the life and death of stars, and geologists of the evolution of planets, while the more brave express this even in the form of metaphor: the world is a living organism. But is it really?

A black dwarf from the future

Life, as they say, is just a transitory stage between birth and death. Today we’re no longer surprised that everything which surrounds us once was born, but 150 years ago this was still not obvious. In fact, for the majority of the 20th century, there was even a discussion under way about whether the universe itself had a beginning, or whether it has always existed. The generality that everything once didn’t exist is much bolder than it might seem at first glance. In fact, even today we’re so used to the enduring nature of the world that it’s hard for us to imagine Saturn without rings – and we now know that they too had their beginning and will someday pass. The birth of icebergs, rivers or volcanoes can sometimes be observed, but the birth of new continents, mountain ranges, planets or stars can be hard to imagine. Yet it seems that everything, even atoms, must have come to exist, and for leaven, the universe was equipped only with a gigantic dose of chaotic energy, though relatively little – or in fact zero! – formed structures.

So can the emergence of galaxies, stars or planets actually be compared to the birth of living organisms? Here, the concept of reproduction enters: a human comes from other humans, and an amoeba takes its start from another being. It doesn’t seem that – with rare exceptions – anything like this happens in the non-living world. Planets form from galactic dust clustering around young stars, not from other planets. It’s similar with the stars, which are concentrations of galactic gas. There’s nothing like ‘reproduction’ of stars. Some people sometimes point out that the explosion which sometimes accompanies a star’s last moments shakes the surrounding gas out of balance and initiates its collapse into the form of a new star; it can happen that the material of the ‘old’ star is partly mixed with that of the ‘new’ one. That’s true. But it’s still not reproduction in the biological sense, particularly as there’s no way to speak of any kind of inheritance. Rather, the situation is analogous to how an avalanche begins: the shocks caused by one avalanche can ‘awaken’ the snow that lies on a neighbouring slope, and so on. The ‘births’ of the enlivened structures are thus not much more than their emergence from some earlier state.

The question of death is even more subtle. We begin by recalling that scientists still haven’t decided whether our universe faces a clearly defined end in the future. What’s accepted as the most likely scenario is that its current expansion, diffusion and cooling won’t end, but will in fact speed up, as a result of which it will finally become a boundless, cold, black expanse. If this is really how it happens, the things that surround us have any amount of time to ‘die’. But in what sense will the sun or Earth, for example, ‘die’?

Individual stars end their period of shining in various ways. Fundamentally, after a more or less spectacular end, what’s left of them is only a cooling ember. Our sun will thus transform, after a few tens of billions of years, into a so-called black dwarf: a cold ball of carbon-oxygen material more or less the size of Earth, on which at that time practically no physical processes will be taking place. Earth and the other planets, if they’re not eaten by ‘their’ stars, can in principle circle around such ‘corpses’ for any length of time, also cooling and dying – though with each successive billion years, the likelihood increases that they’ll leave the embrace of gravity and join the chaotic galactic swarm of objects thrown off by planetary systems.

These stories make us aware that biological death is also not in fact a ‘disappearance’, but rather the end of active maintenance of a certain form. ‘My’ matter doesn’t disappear after death; it’s distributed across the surface of the planet – though the part of it that’s least appetizing to other organisms, the skeleton, may last even millions of years if we’re lucky. So the death of a living organism means rather that the fate of a certain portion of matter ceases to be actively controlled, and the part of it that isn’t taken up by another living organism, or that isn’t shattered in an avalanche, can last for a practically unlimited time.

Let’s look at the fate of our planet from this perspective. After a few billion years, most likely the reserves of heat necessary to maintain plate tectonics will run out, and thus the system of continents and seas will stabilize. Simultaneously, the formation of new mountains will end, and volcanism will die out; thus the surface will gradually start to flatten and smooth out. On a similar timescale, the liquid core will cool, and thus the surface will cease to be protected against the solar wind by the cocoon of the magnetosphere. On other planets this has led, over time, to the atmosphere blowing away and the ‘baking’ of the hydrosphere, element by element, which is also how it will certainly happen on Earth. I assume life will die out. So our planet will freeze in motionlessness: dry, weathered and smoothed by the last rains and winds. Because there is no equivalent of scavengers in cosmic space that will throw themselves on this carcass, we can think of it as a skeleton cast into the desert.

Those beloved tectonics

Finding planetary and cosmic analogies for biological birth and death isn’t excessively difficult – particularly when we allow ourselves the flexibility and flowery language papering over all kinds of conflicts that is customary in such situations. But why stop? After all, life between our two end points isn’t a gruel-like, homogenous sequence of identical days, weeks, months and years. Looking just a little bit closer, we see something like youth, maturity and old age. Do the planets and stars also have their puppy years?

With the stars, the situation presents itself as follows. Our sun, for example, emerged over more or less a few million years, a time when matter from the surrounding cloud of galactic gas was collecting. At a certain moment, its thermonuclear heart ‘ignited’, and continues beating until today. Interestingly, initially it wasn’t such an orderly furnace as it is now: the first few million years of stars of the sun’s type are the so-called T-Tauri phase, during which energy is still produced quite chaotically. Such a star emits a very strong wind, spitting out a relatively large amount of matter in the form of hot, aggressive ‘coughs’ – their victims, by the way, are the planets forming in parallel in the disk that surrounds the star. Only with time does the star’s internal structure stabilize, and a certain model emerge, characteristic for it (and different for different kinds of stars), of spatial transport of energy to the surface.

It’s similar with planets. In light of the presence of radioactive isotopes diffused in all rocks (which heat more strongly at the beginning, and after 4.5 billion years are to a large degree ‘burnt out’), and because of the brutal circumstances in which these globes are formed, they are initially hot, chaotic objects. Earth, we suspect, was once so heated that its coat of rock melted into the form of a global ocean of magma. The gigantic reserve of energy collected in the hot rocks and metal was freed up in the form of chaotic streams of boiling matter – the Earth’s atmosphere, by the way, was made up at the time of evaporated rock. Only with time did the magma ocean cool, the crust of Earth emerge and the still-mysterious process that is plate tectonics begin. Let us note in passing what a ‘civilized’ form of energy release these tectonics ensure for us: when we lay out on a map of our globe the places where volcanoes erupt, almost all the red spots trace the thin lines dividing the plates. It’s really quite nice of the planet that it chose such a mature, orderly method, and we, in Poland, don’t have to worry that one day a volcano will begin to sprout up under our favourite fishing tackle store. The residents of Sicily and Hawaii, on the other hand… well, at least they know what they’re signing up for.

The miserable fate of Mars

Here a certain model is formed: first a portion of matter collects in one place, and after a certain moment it begins to ‘live its own life’, i.e. it cuts its umbilical cord to the rest of the world and uses a source of energy to act on its surroundings. Initially, this energy is freed up intensively and chaotically – sometimes, in fact, this poses a threat to the body’s surroundings – but in the end the object organizes its structure in such a way that it manages to emit energy in a uniform, rhythmic way. During this time the basic form remains unchanged, though there’s a cost: to maintain this form (in the purely philosophical sense, of course), it’s necessary to distribute the work (in the purely physical sense, of course). But when the source of energy starts to run out, it increasingly begins to neglect the form, until in the end it completely ceases to be supported and the matter is no longer actively controlled. If in the surroundings there are forces able to capture it – such as the solar wind removing the atmosphere from the surface of the planet, when the energy to generate the magnetic poles and top up the gases runs out – it happens without a fight. Let’s note that something similar happened to our ‘external’ neighbour, Mars. This planet once had quite a large ocean and most likely a decent atmosphere, but lacked a magnetic pole that could protect these precious resources against the gusts of the solar wind; they were removed from the surface, and the Red Planet today is almost completely dry.

Unruly metaphors

So can we compare planets and stars to living organisms? Of course we can. We undoubtedly lose some of the information – planets and stars, for example, don’t have parents, and in fact they don’t reproduce at all. The world of living creatures, in the sense of the Earth’s biosphere, also has one characteristic with fundamental significance. It’s equipped with a mechanism of genetic encoding of information about itself, and an imperfect transmission of this information to its descendants, which sets off evolution – and not in the loose, metaphorical sense in which one speaks of ‘the evolution of galaxies’, but in the narrow, precise biological sense.

In the history of natural thought, we have many times found ourselves in a similar situation: something ‘seems to be associated with something’, like the solar system reminded Kepler and Newton of a mechanical watch. In principle, there are two possibilities: either it’s a random association, which for fun we can stretch out as far as we want, drawing from it at best aesthetic pleasure, a philosophical feeling of depth, or actually something in the way of an ethical imperative (‘we must care for the planet as for a living organism’) – or it conceals a deep truth about reality. Perhaps there exists some reason – deep, capable of precise expression – for which some (and perhaps all) natural objects seem to develop in a similar way?

There are several suggestive keywords that appear in this fantasy: particularly ‘energy’, ‘order’ and ‘self-organization’. These are the important, almost foundational concepts of the thermodynamics of irreversible processes – relatively young, as they were born in the second half of the 20th century, the heritage of research seeking to discover order precisely in processes as complex as the cooling of the planets. On the one hand, this science achieved a range of precise results, used today by physicists and chemists to predict the behaviour of the systems they research. On the other hand, it exerts such a strong influence on philosophers and other dreamers, seeking in it inspiration for a new metaphor for the universe. Perhaps ‘life’ really isn’t the best choice. But we’ve already had our fill of ‘mechanism’. What will come next?

Translated from the Polish by Nathaniel Espino

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Łukasz Lamża

is a philosopher of nature who writes popular scientific articles, particularly in “Tygodnik Powszechny”. He’s specifically trained in astrophysics and cosmology, but fortunately he sometimes comes down to Earth.