From The Expanse to Foundation to Battlestar Galactica: A Future History of Mankind

 

Future History of Humanity

Science fiction works best for me when it becomes more than entertainment—when it creates a world rich enough to think about civilization, politics, technology, religion, identity, and the fate of humanity.

My three favorite science-fiction series remain:

1. Battlestar Galactica
2. Dark
3. The Expanse

For me, these belong in a category of their own. Battlestar Galactica combines civilization, extinction, religion, destiny, politics, and recurring cycles of destruction and rebirth. Dark is perhaps the most disciplined treatment of time, causality, determinism, and identity I have seen on television. The Expanse gives the most convincing picture of humanity becoming a genuine space civilization.

Although Dark remains firmly in my top three, it does not enter directly into the future history developed below; that sequence is built around The Expanse, Foundation, and Battlestar Galactica.

Several others come close. Westworld is very good, though weaker toward the end. 3 Body Problem may yet become something greater. Foundation is excellent and almost belongs with my top three, though something still feels missing. Silo and Fallout are very effective entertainment, while Dark Matter is simply good. I could not initially get into Severance, though I intend to try again. I am now trying Pluribus, while Murderbot and Alien: Earth are also on my list.

I generally do not like Star Wars; The Mandalorian was a rare exception.

It was while discussing these series—especially For All Mankind, which I have not yet watched—that another idea emerged.

Its premise initially seemed too narrow to interest me: what if the Soviet Union had reached the Moon first? But the series apparently develops into something much larger—a continuing Space Race leading to lunar bases, Mars, space resources, permanent off-world populations, and eventually distinct political identities beyond Earth.

And then the thought occurred to me:

this sounds almost like the opening chapter of The Expanse.

Not canonically, of course, but as an imaginary future history the progression works remarkably well:

For All Mankind → The Expanse → Foundation → Battlestar Galactica.

I have not yet watched For All Mankind, so I would not place it in the title or judge it alongside the others. But conceptually, it provides a very attractive prehistory for the sequence.

From Earth to the Solar System

The Expanse begins with humanity already established as a true Solar-System civilization.

Earth remains the old center. Mars has become an independent political and technological power, while the Belt has developed its own culture, shaped by low gravity, resource extraction, exploitation, and marginalization.

This feels like the natural long-term consequence of the process that For All Mankind appears to begin.

Humanity does not simply move into space.

It becomes different societies.

Then the Ring network opens access to countless new worlds.

The Solar System ceases to be the whole human universe.

Humanity becomes interstellar.

Give that process thousands of years, and the world of Foundation becomes easy to imagine.

From the Solar System to the Galactic Empire

In Foundation, humanity has moved from one planetary system to the entire Galaxy.

Millions of inhabited worlds belong to a vast Galactic Empire, while Earth has become so remote that humanity has largely forgotten its own origin.

This feels like a natural conceptual continuation of The Expanse.

In The Expanse, Earth still defines Mars and the Belt.

In Foundation, Earth has become almost irrelevant.

One can imagine the Ring network as one of the first steps toward this enormous dispersal, with the political world of The Expanse becoming the distant prehistory of the Galactic Empire.

But Foundation is not ultimately about the triumph of civilization.

It is about its collapse.

Hari Seldon's psychohistory predicts the fall of the Empire and a long dark age. The Foundation is created to shorten that interregnum and prepare a Second Galactic Empire.

What If Psychohistory Fails?

Perhaps Seldon's Plan succeeds, and the civilization emerging from the dark age eventually becomes the distant ancestor of Battlestar Galactica.

But a more interesting possibility is that psychohistory ultimately fails.

The collapse of the Galactic Empire could deepen into economic disintegration, isolation, war, demographic collapse, and technological regression, until almost all the human worlds once filling the Galaxy disappear.

Humanity had occupied millions of planets.

Thousands of years later, only scattered remnants remain.

Kobol could be one of them, eventually giving rise to the Twelve Colonies.

Then Battlestar Galactica is not the story of humanity beginning its expansion into space.

It is the story of what remains after a vast Galactic civilization has already risen and disappeared.

The Twelve Colonies are not the beginning.

They are the residue.And this is where the connection to Battlestar Galactica begins.

Earth: From Planet to Myth

This also makes the role of Earth fascinating.

Already in Foundation, the original home of humanity has largely disappeared from collective memory.

Now imagine another ten thousand or twenty thousand years of collapse, migration, isolation, and technological forgetting.

Earth would gradually pass through several stages:

historical location → ancestral memory → religious tradition → myth.

And this is exactly how Earth appears in Battlestar Galactica.

The survivors are not merely searching for another habitable planet.

They are trying to recover a forgotten origin.

Humanity has become so old that it no longer remembers where it began.

The God of the Cylons

Perhaps the most intriguing connection concerns R. Daneel Olivaw, the ancient robot of the Foundation universe who survives across immense stretches of human history and helps guide humanity’s future.

Suppose Daneel—or some descendant of such an intelligence—survives the collapse of the Galactic Empire. Humanity forgets the Empire, Earth, robots, and psychohistory.

But the machine remembers.

Tens of thousands of years later, surviving humans encounter an intelligence that knows their forgotten history, influences events across generations, and possesses powers beyond their understanding.

They would probably call it God.

This suggests an intriguing interpretation of the mysterious God of Battlestar Galactica: perhaps it is not God at all, but the last remnant of an unimaginably ancient artificial intelligence.

That would also make one detail especially amusing: the entity apparently does not like being called God.

Perhaps because it knows perfectly well that it isn't one.

From Psychohistory to Providence

Psychohistory works by predicting the statistical behavior of enormous populations. But after the Galactic collapse, humanity may have fallen from millions of inhabited worlds to only a handful of surviving colonies.

At that scale, psychohistory would become increasingly ineffective.

Individual decisions would begin to matter enormously.

By the time of Battlestar Galactica, only tens of thousands of humans remain. The ancient intelligence could no longer guide history through populations alone.

It would have to intervene through individuals:

Baltar. Six. Roslin. Adama. Kara Thrace.

What the survivors interpret as divine providence could therefore be the distant descendant of psychohistory.

At the height of Galactic civilization:

guide populations.

Near the extinction of humanity:

guide individuals.

Humanity Invents Artificial Intelligence Again

And then comes what may be the greatest irony.

Long before the Galactic Empire, humanity had already created robots.

Machines such as Daneel had existed for tens of thousands of years.

But civilization collapsed.

Human beings forgot.

Eventually, one surviving branch of humanity invented artificial intelligence again.

And it created the Cylons.

Humanity therefore repeats something it had already done in an age so remote that nobody remembers it.

The oldest surviving machine from humanity's first age of artificial intelligence would then find itself trying to save humanity from the machines produced by its second invention of artificial intelligence.

Now the central statement of Battlestar Galactica acquires an enormous historical meaning:

“All of this has happened before, and all of this will happen again.”

Perhaps human civilization does not simply progress.

Perhaps it expands, collapses, forgets, and reconstructs the same ideas again.

An Imaginary History

The speculative chronology is then roughly:

Humanity leaves Earth → The Expanse → Solar-System civilization → the Ring network → interstellar expansion → Foundation → Galactic Empire → collapse → Seldon dark age → psychohistory fails or only partly succeeds → most of humanity disappears → Earth is forgotten → Kobol and the Twelve Colonies survive → humanity creates AI again → the Cylons → Battlestar Galactica → the survivors search for mythical Earth → an ancient intelligence, remembered as God, guides them there → humanity forgets everything once more.

Thousands of years later, on our Earth, humans rediscover science, build computers, and begin creating artificial intelligence again.

The whole imagined history can therefore be reduced to:

Expansion → Solar civilization → Galactic civilization → Empire → collapse → forgetting → rebirth → repetition.

Perhaps this is why Battlestar Galactica remains, for me, the greatest science-fiction series I have watched: it does not merely ask what humanity will become, but whether humanity can ever escape itself.

Ancient DNA from the Green Sahara and the Discovery of a Deep North-African Lineage

A remarkable paper published in Nature in 2025 has provided the first genome-wide ancient DNA from human populations of the Central Sahara. The work, by Nada Salem and collaborators, is entitled “Ancient DNA from the Green Sahara reveals ancestral North African lineage.” Its implications are substantial because it gives direct genetic evidence for something that had previously been difficult to establish: North Africa possessed a deeply rooted indigenous population lineage of its own, neither reducible to recent Eurasian back-migration nor adequately describable as an extension of presently sampled sub-Saharan populations.

The result is particularly interesting because it also changes the interpretation of the famous 15,000-year-old Taforalt population from Morocco and clarifies the relationship between ancient North Africans, sub-Saharan African populations and the populations involved in the major Out-of-Africa expansion.

The Green Sahara

During the African Humid Period, roughly 14,500–5,000 years ago, much of the Sahara was covered by grasslands, lakes and savannah rather than desert. Human communities lived throughout the region, first as hunter-gatherers and later increasingly as pastoralists.

At Takarkori, a rock shelter in southwestern Libya, the study recovered genome-wide ancient DNA from two women buried about 7,000 years ago during the Pastoral Neolithic.

Because ancient DNA preservation in the Sahara is extremely difficult, this represents the first genome-wide evidence from the Central Sahara. One genome, however, is far better preserved than the other, so most of the detailed population-genetic inference rests primarily on a single individual.

A Previously Unknown North-African Lineage

The central discovery is that the Takarkori individuals cannot be explained simply as descendants of Levantine farmers who moved into Africa with pastoralism.

Nor can their ancestry be adequately represented by Yoruba, Dinka, Mota or other presently sampled sub-Saharan populations.

Instead, the authors infer the existence of a deeply divergent ancestral North-African population.

Their best-fitting admixture graph gives approximately

Takarkori ≃ 93% deep North-African ancestry + 7% ancient Levantine-related ancestry.

The important statement is not the exact numerical precision of 93% and 7%, since admixture graphs are models rather than unique reconstructions of history. The robust qualitative conclusion is that the overwhelming majority of Takarkori ancestry derives from a previously unknown North-African lineage, with only a relatively small contribution from an Out-of-Africa-related Levantine population.

This immediately changes how one should imagine ancient North Africa.

The simple picture

North African = sub-Saharan African + Eurasian back-migration

is inadequate.

There appears instead to have existed an old North-African population with its own deep history inside Africa.

Another “Remain-in-Africa” Lineage

The most fascinating part concerns the position of this North-African lineage in the deeper human family tree.

The paper finds that Takarkori is genetically closer to the early Out-of-Africa lineage than the tested relatively unadmixed sub-Saharan populations are.

The authors use the approximately 45,000-year-old Zlatý kůň genome as a proxy for an early branch of the population that had left Africa.

Their admixture graph suggests approximately the following topology:

ancestral African Homo sapiens

→ an earlier branch represented by something like the Mota-related African lineage

→ another ancestral branch, which subsequently separates into

deep North-African lineage

and

Out-of-Africa lineage.

Thus the deep North-African population can be thought of, cautiously, as an African sister lineage relatively close to the population involved in the major Out-of-Africa expansion.

It is therefore useful to abandon the overly simple opposition

remain in Africa / leave Africa

and think instead in terms of several structured African populations.

One population lineage ultimately contributed to the major Out-of-Africa expansion.

Other lineages remained in Africa.

And among those that remained was apparently this distinctive North-African lineage, which was more closely related to the ancestral Out-of-Africa population than the Mota-like lineage used in the authors’ model.

This does not mean that North Africans were Eurasians who subsequently returned to Africa.

That alternative can be tested using Neanderthal ancestry.

The Neanderthal Test

Populations descended substantially from the major Out-of-Africa expansion normally carry Neanderthal ancestry because modern humans interbred with Neanderthals after leaving Africa.

Takarkori contains only about

0.15% detectable Neanderthal ancestry.

This is extremely low.

The corresponding values obtained in the study are roughly:

Takarkori: 0.15%

Taforalt and Neolithic Moroccan individuals: 0.6–0.9%

Typical Out-of-Africa populations examined in the study: 1.4–2.36%

The tested ancient and present-day sub-Saharan genomes contained essentially no detectable Neanderthal ancestry under the same analysis.

This is a crucial result.

If the dominant ancestry of Takarkori had come from a Eurasian population that left Africa, encountered Neanderthals and subsequently migrated back into North Africa, Takarkori should contain much more Neanderthal ancestry.

It does not.

The most economical interpretation is therefore that the major component of Takarkori ancestry comes from a population that remained in Africa, although it was closely related to the lineage that eventually participated in Out-of-Africa.

The small amount of Neanderthal ancestry can then be explained by the approximately 7% later Levantine-related contribution.

So two ideas that might initially appear contradictory can both be true:

1. The deep North-African lineage was closely related to the ancestral Out-of-Africa lineage.

2. It subsequently received a smaller amount of gene flow from populations that actually had participated in Out-of-Africa.

The first is a deep genealogical relationship.

The second is later admixture.

They should not be confused.

The Extraordinary Connection with Taforalt

The part of the paper that I find particularly striking concerns Taforalt Cave in Morocco. The Taforalt individuals are about 15,000 years old, approximately 8,000 years older than Takarkori.

An earlier ancient-DNA study had modeled Taforalt approximately as

63.5% Natufian-related ancestry + 36.5% broadly “sub-Saharan African” ancestry.

But the supposed African component could not be identified with any specific known sub-Saharan population. The Takarkori genomes now provide a much better proxy for this missing component, giving approximately

Taforalt ≃ 60.8% Natufian-related + 39.2% Takarkori-related North-African ancestry.

Since Takarkori is younger than Taforalt, this obviously does not mean that Taforalt descended from Takarkori. Rather, Takarkori is the best available representative of a still older unsampled North-African lineage inherited by both populations.

Calling this deeper lineage A and the Levantine-related component L, the approximate picture is

Takarkori ≃ 0.93 A + 0.07 L,

Taforalt ≃ 0.40 A + 0.60 L.

The apparently counter-intuitive fact that younger Takarkori preserves much more of A is therefore explained by geography and isolation rather than chronology: northwestern North Africa had already experienced substantial Levantine-related gene flow by the time of Taforalt, whereas the Saharan Takarkori population remained much more isolated.

This also shows why Takarkori cannot simply be modeled as Taforalt + Natufian. Since Taforalt itself contains only about 40% of A, adding further Natufian-related ancestry can only decrease that fraction, whereas Takarkori requires approximately 93%.

Thus the Takarkori genomes reveal something that Taforalt alone could not expose: evidence for a deeper North-African population that existed before both Taforalt and Takarkori.

The Mitochondrial Haplogroup N

Both Takarkori women carried a very basal branch of mitochondrial haplogroup N, one of the major maternal lineages associated with populations outside Africa. Its estimated split around 61,000 years ago is close to the period of the major Out-of-Africa expansion.

This is consistent with the possibility that a population closely related to the ancestral Out-of-Africa population remained in North Africa. But mtDNA traces only one maternal lineage, so this result is supportive rather than decisive. The stronger evidence comes from the autosomal genome, admixture modeling and Neanderthal ancestry.

What the Paper Does Not Show

The study is based on only two individuals, with most high-resolution inference coming from one genome, so the admixture percentages should be treated as model-dependent estimates rather than exact historical proportions.

Nor does the paper show that modern North Africans are simply 93% Takarkori-like. Thousands of years of later migration and admixture separate Takarkori from present-day Maghrebi populations.

What the study establishes is the existence of a deep indigenous North-African lineage. Determining how widespread it was and how much of it survives today will require many more ancient genomes from across North Africa and the Sahara.

The Larger Historical Picture

The deepest significance of this work is that it makes the old binary picture of North Africa as simply

sub-Saharan African + Eurasian

increasingly untenable.

Instead, early Africa contained several deeply differentiated populations. Among them was apparently a North-African lineage that remained in Africa but was more closely related to the ancestral Out-of-Africa population than the sampled sub-Saharan lineages were.

Takarkori therefore represents something conceptually important: another “remain-in-Africa” lineage—deeply African, distinct from the sampled sub-Saharan populations, yet unusually close to the lineage that later participated in the Out-of-Africa expansion. It subsequently received limited gene flow from descendants of populations that had actually left Africa.

This also changes the interpretation of Taforalt. The older shorthand

Taforalt ≃ 60% Near Eastern + 40% sub-Saharan African

is better replaced by

Taforalt ≃ 60% Natufian-related + 40% deep North-African ancestry.

Thus Takarkori appears to preserve evidence of an ancestral North-African population that existed before both Takarkori and Taforalt.

For me, this is the central result: a deep indigenous North-African lineage existed early in modern-human history, remained largely within Africa, was closely related to the ancestral Out-of-Africa lineage, and later received limited admixture from its descendants.

What had previously been, for me, largely a historical and metaphysical intuition has now acquired substantive genomic evidence.

The result does not close the history of North Africa. It opens it.

Reference: Nada Salem, Marieke S. van de Loosdrecht, Arev Pelin Sümer et al., “Ancient DNA from the Green Sahara reveals ancestral North African lineage,” Nature 641, 144–150 (2025), DOI: 10.1038/s41586-025-08793-7.

https://www.nature.com/articles/s41586-025-08793-7

Theory of Free Will and Evil

 

I have been developing a metaphysical view of free will based on three principles.

They can be named in two equivalent ways:

1. The Perspectival Principle — or the Metaphysical Equivalence Principle of Free Will.

2. The Relative Principle — or the Graduation Principle.

3. The Initial-Condition Principle — or the Initial-Condition Principle of Consciousness.

These are not six principles, but two names for the same three ideas.

1. The Perspectival Principle

Free will is first experienced from within.

Whatever the ultimate causal structure of reality may be, the conscious agent experiences itself as a source of action. From the first-person point of view, we do not normally experience our decisions as the final products of causal chains. We experience:

I act. I choose. I am the source.

This is why I call it the Metaphysical Equivalence Principle of Free Will.

Every conscious agent inhabits, so to speak, a metaphysical reference frame in which the causal forces acting upon them are partially transformed away phenomenologically, leaving the experience of agency.

This does not yet mean that physical determinism is false. It means that agency is structurally unavoidable from the first-person point of view.

2. The Graduation Principle

Although every conscious agent experiences freedom, not every agent is equally free.

Freedom depends on psychological structure, education, social position, material conditions, historical circumstances, power, opportunity, cognitive capacity, and the actual alternatives open to the person.

The son of a king and the son of an untouchable do not begin life inside the same space of possibilities.

Freedom therefore comes in degrees.

And if freedom is graded, then responsibility must also be graded.

This gives the basic chain:

Graded freedom → graded responsibility → graded evil.

Not every harmful act has the same degree of authorship. Some agents act mainly as channels through which circumstances operate; others act as partial sources; still others can become strong creators of new situations.

3. The Initial-Condition Principle

The deepest limitation on freedom may not be causality itself, but initial conditions.

Nobody chooses their genome, parents, century, place of birth, early environment, emotional architecture, childhood experiences, language, or initial cognitive structure.

A person may later transform themselves, but the person who begins that transformation was never chosen.

Thus:

The agent may be relatively free in dynamics, while remaining radically unfree in origin.

We do not choose the space of selves from which we begin.

Weak Evil and Strong Evil

If freedom and responsibility are graded, then evil itself cannot be completely uniform.

I distinguish between weak evil and strong evil.

Weak evil is largely structural. It emerges from bad initial conditions, psychological constraints, ignorance, deprivation, social pathology, historical forces, institutional structures, and narrow spaces of possibility.

Here the individual often acts more as a conductor or transmitter than as a true origin.

The evil is partly cosmological: it belongs to Nature, history, society, and the arbitrary distribution of initial conditions.

Responsibility is therefore diluted because authorship is diluted.

Strong evil is different.

It appears when an agent possesses substantial freedom, foresight, integration, access to alternatives, symbolic power, and the capacity to impose intention upon the world.

Such an agent does not merely transmit an existing structure.

The agent creates a new structure.

The central concept here is authorship.

Weak evil is mainly transmitted.

Strong evil is genuinely authored.

The Problem of Evil as a Distribution Problem

This changes the traditional problem of evil.

Instead of asking only:

Why does God allow evil?

we should also ask:

At what level of reality was this evil generated?

Was its source Nature, initial conditions, psychology, society, institutions, history, or a strong self?

Weak evil primarily accuses initial conditions and structures.

Strong evil primarily accuses strong selves.

The problem of evil therefore becomes a problem of distribution: where, exactly, is the source of the evil located?

Metaphysics becomes a topography of agency.

Some-Man and Most-Man

I use the terms Some-Man and Most-Man to describe two regimes of agency.

They are not two types of human beings.

In the Most-Man regime, the individual acts mainly within the statistical flow of circumstances and larger structures.

In the Some-Man regime, the individual becomes strongly source-like, capable of originating rather than merely transmitting structures.

The same person may move between these regimes.

Someone may live mostly as Most-Man and, at a decisive moment, enter a Some-Man state. Another may remain for a long period in a strongly source-like phase.

So Some-Man and Most-Man are phases of agency, not essences of persons.

A Brief Physical Analogy

The framework is metaphysical, not physical, but physics offers a useful analogy.

The Metaphysical Heisenberg Cut marks the conceptual boundary between evil as world-effect and evil as agent-event — between transmission and authorship.

An agent may cross this boundary.

A strongly source-like agent may lose integration and authorship and fall back toward the Most-Man regime. I call this metaphysical decoherence.

The reverse movement is also possible: an agent may re-cohere and become more strongly source-like.

The physical analogy is secondary. Its purpose is only to describe transitions between metaphysical regimes.

The Central Idea

The whole framework can be summarized as follows:

The agent is perspectivally free, relatively free, and initially unfree.

From this follow:

Graded freedom.

Graded responsibility.

Graded evil.

And from graded evil comes the distinction between weak evil and strong evil.

The deepest question is therefore not simply:

Who is guilty?

but:

Where did the evil actually originate?

Was it generated, transmitted, amplified, or merely conducted?

That is the central metaphysical question of this framework.

Gas Giants, Quasi-Stars, Fuzzy Cores, and the Strange Interior of the Earth

 

A simple question about Jupiter’s Great Red Spot led me to a more basic conceptual difficulty: why do we call Jupiter a planet at all?

When we think of a planet, especially from our Earth-based intuition, we imagine a world with a surface — somewhere one could in principle travel to, descend toward, and eventually land on.

This picture works reasonably well for Mercury, Venus, Earth, and Mars. But it fails completely for Jupiter and Saturn.

A planet on which you cannot land

Jupiter has no solid ground.

If one descended through its atmosphere, there would be no definite moment at which the atmosphere ended and the surface began. The hydrogen and helium would simply become denser and denser as pressure and temperature increased.

One would pass gradually from a relatively thin atmosphere into dense gas, then into a supercritical fluid, and eventually into extremely dense metallic hydrogen.

There is no ordinary boundary corresponding to “air above, ground below.”

Even Jupiter's quoted “surface” is only a conventional reference level, usually associated with an atmospheric pressure of about one bar. An imaginary spacecraft could simply pass through it. There would be nothing there on which to land.

This immediately makes Jupiter seem less like Earth and much more like the Sun.

Jupiter as a kind of quasi-star

Jupiter consists mainly of hydrogen and helium, just like the Sun.

It is a huge, approximately spherical, self-gravitating fluid body, and it possesses no solid surface. From this point of view, it seems much more closely related physically to a star than to a terrestrial planet.

The crucial difference is mass.

The Sun is massive enough for gravitational compression to raise its central temperature and density until sustained nuclear fusion becomes possible. Hydrogen nuclei fuse ultimately into helium and release enormous amounts of energy.

Jupiter never became massive enough for this to happen.

I therefore find it tempting conceptually to think of Jupiter as a kind of “quasi-star”: an object made principally of stellar material that never acquired enough mass to ignite ordinary hydrogen fusion.

The expression “quasi-star,” however, already has a specific technical meaning in astrophysics, so it would not be a good formal name for Jupiter. The broader established category is that of substellar objects.

And nature actually provides a remarkable sequence:

Jupiter-like planet → brown dwarf → low-mass star.

Brown dwarfs occupy the intermediate region. They are more massive than ordinary giant planets, and some can temporarily burn deuterium, but they are not massive enough to sustain ordinary hydrogen fusion.

So there really is a physical continuity between Jupiter and stars.

In terms of composition and bulk thermodynamics, Jupiter is arguably much more Sun-like than Earth-like.

Stars have no ground either

The Sun also has no solid surface.

What we call the solar “surface” is the photosphere. But the photosphere is not a material boundary like the surface of Earth.

It is essentially the region from which visible photons can finally escape efficiently.

An imaginary indestructible spacecraft could descend through the photosphere into progressively denser and hotter plasma. There is no ground underneath.

So Jupiter and the Sun share something fundamental: both are enormous self-gravitating bodies composed mainly of hydrogen and helium, and neither possesses an ordinary solid surface.

The decisive distinction is that the Sun sustains nuclear fusion whereas Jupiter does not.

Metallic hydrogen is not solid metal

Another concept that initially confused me is “metallic hydrogen.”

The word metallic naturally suggests something like iron or copper — a solid metal.

But metallic does not mean solid.

Under the enormous pressures inside Jupiter, hydrogen undergoes a profound change in its electronic properties. Its electrons become sufficiently delocalized that the material becomes electrically conducting.

In that sense hydrogen becomes metallic.

But it is expected to exist mainly as an extremely dense conducting fluid.

So metallicity refers primarily to electronic behavior, not to whether the substance is solid or liquid.

There is therefore no enormous floor of solid metallic hydrogen hidden underneath Jupiter's atmosphere.

Does Jupiter have a core?

Yes — but probably not the kind of core one might first imagine.

The traditional simple picture was something like this: an atmosphere of hydrogen and helium, then metallic hydrogen, and finally a compact rocky or icy core.

Measurements by NASA's Juno spacecraft have suggested something considerably more interesting.

Jupiter appears to possess what is often called a “dilute” or “fuzzy” core.

Instead of having a sharply bounded solid ball at its center, heavy elements may be distributed throughout a large central region, gradually mixing into the surrounding hydrogen-helium material.

There may therefore be no simple boundary saying: hydrogen ends here, solid core begins here.

The composition changes progressively toward the center.

This idea of a fuzzy core is fascinating because it undermines our ordinary picture of a planet as a definite solid object surrounded by an atmosphere.

Do stars also have fuzzy cores?

In another sense, yes.

The Sun has what we call a fusion core. But again, this is not a solid object with a sharp wall.

It is simply the central region where temperature and density are sufficiently high for nuclear fusion to occur efficiently.

The fusion rate falls continuously as one moves outward.

The Sun then passes through the radiative zone, the convective zone, and eventually the photosphere.

These are regions defined primarily by changing physical conditions and mechanisms of energy transport, rather than by rigid material boundaries.

So there is an interesting comparison.

Jupiter's fuzzy core is fuzzy mainly in composition: the heavy elements appear to be spread through an extended central region.

The Sun's core is diffuse in another sense: the region of efficient nuclear fusion fades gradually with radius.

Astrophysical “cores” therefore need not resemble a solid ball at all.

Then Earth gives us the opposite surprise

At this point another question appears.

Earth really does have a solid inner core.

Its broad internal structure is:

solid mantle → liquid outer core → solid inner core.

But the temperature increases as one travels toward the center.

Near Earth's center, temperatures are thought to be of order several thousand degrees, roughly comparable to the temperature of the visible surface of the Sun.

So how can the inner core possibly be solid?

The answer is pressure.

Whether a substance is solid or liquid is not determined by temperature alone. It depends on both temperature and pressure.

For iron and iron-rich alloys, the melting temperature increases strongly with pressure.

In Earth's outer core, the temperature is above the local melting point, so the material is liquid.

But deeper inside, the pressure becomes so enormous that the melting temperature rises faster than Earth's actual temperature.

Eventually the melting temperature becomes higher than the local temperature.

The iron therefore crystallizes.

So, paradoxically, while descending toward hotter and hotter regions of Earth, we pass from liquid material back into solid material.

This is one of the most counterintuitive facts about the Earth's interior.

And there is an additional beautiful consequence.

As Earth slowly cools, its solid inner core gradually grows. Material from the liquid outer core crystallizes onto it.

In a very real sense, the center of Earth is slowly freezing.

High pressure does not simply mean solid matter

This comparison between Earth and Jupiter teaches an important lesson.

It would be tempting to think that because Jupiter has enormously greater pressure than Earth, its interior must eventually become solid.

But this is not true.

Pressure alone does not determine the phase of matter. Temperature and the detailed properties of the substance are equally important.

Iron under the conditions found near Earth's center enters a solid crystalline phase.

Hydrogen under much of Jupiter's interior conditions becomes instead an extraordinarily dense electrically conducting fluid.

Different substances have different phase diagrams.

So “enormous pressure” does not automatically mean “solid.”

From Earth to Jupiter to the Sun

Perhaps a more physically illuminating sequence than our ordinary astronomical labels is:

Earth → Jupiter → brown dwarf → Sun.

Earth is principally a rock-and-metal object with a genuine surface.

Jupiter is principally a self-gravitating hydrogen-helium fluid without a solid surface.

Brown dwarfs continue the progression toward increasingly massive and hotter hydrogen-helium objects.

And eventually, beyond the necessary mass threshold, we reach genuine stars in which sustained hydrogen fusion becomes possible.

Seen this way, Jupiter belongs unquestionably to the planetary family astronomically, but physically it sits remarkably close to the stellar world.

It has no ground, no conventional material surface, probably no sharply bounded solid core, and most of its mass exists as hydrogen and helium under conditions completely outside ordinary human experience.

Earth, meanwhile, gives us the opposite surprise: despite temperatures of thousands of degrees, its deepest central region remains solid because the pressure is so enormous.

The broader lesson is perhaps the most interesting one.

Our everyday categories — solid, liquid, gas, surface, atmosphere, core — come from a tiny region of the possible states of matter that we experience at ordinary terrestrial pressures and temperatures.

Inside giant planets and stars, gravity pushes matter into regimes where those familiar intuitions begin to fail.

And once we look at the problem this way, the boundary between a “world” and a “star” becomes much less obvious than ordinary language makes it seem.