I. The Unseen Particle

1934. Osaka Imperial University. A twenty-seven-year-old lecturer is turning a problem over in his mind.

An atomic nucleus contains protons and neutrons. Protons carry positive charge, and like charges repel. How can so many protons stay bound together in such a small space?

Something else must be at work. At nuclear distances it can overcome electrical repulsion; farther away it rapidly weakens. Giving this strong, short-range interaction a name does not yet explain it.

Hideki Yukawa asks whether the interaction could be described through the exchange of an undiscovered particle. Later explanations often picture two people tossing a ball. That makes exchange imaginable, but it does not explain attraction. Ordinary ball throwing will not pull the throwers together. The real work belongs to quantum theory.

What, then, is the “ball”?

Relating the force’s range to the particle’s mass, Yukawa estimates something heavier than an electron but lighter than a proton. This is not a name casually inserted into a blank. The new entity has to answer to the nuclear scale already known.

No experiment has seen it. But physics does not know only three particles: photons and positrons are already part of the picture. Yukawa is not adding a magical fourth member. He is proposing a new one with a particular job to do.

His paper, On the Interaction of Elementary Particles, appears in 1935. The later name meson carries the sense of something intermediate. Today’s account of nuclear forces is more complex, but the path he opened proved genuinely productive.

The paper does not immediately attract wide attention. Discussion, experiments and waiting separate the proposal from recognition. We need not stage a collective Western sneer to appreciate a young researcher’s isolation.

In the later 1930s, a particle found in cosmic rays seems promising. It turns out to be the muon, without the expected strong-interaction role. In 1947, work by Lattes, Occhialini, Powell and others identifies the pion. Similar mass is not enough. How the particle behaves also matters.

In 1949 Yukawa receives the Nobel Prize in Physics, Japan’s first Nobel laureate. A society rebuilding after war finds more than professional distinction in the award: the possibility of creating something again.

But the prize is not our only concern. Before a discovery, how does someone make room for what has not yet been seen?

II. Zhuangzi

As a child, Yukawa reads the Four Books with his grandfather: the Analects, Mencius, the Great Learning and the Doctrine of the Mean. First he repeats the words, not necessarily understanding them. Old sentences enter life as sounds. Meaning can arrive later.

At secondary school he turns toward Laozi and Zhuangzi. The Zhuangzi becomes a lasting companion in his reading.

His essays will later appear in the English collection Creativity and Intuition: A Physicist Looks at East and West. A man who writes equations also asks a question less easily put into one: how does imagination grow?

What did Zhuangzi give him?

The butterfly dream unsettles the waking self. The happy fish make “How do you know?” newly interesting. A supposedly useless tree interrupts our usual measure of usefulness. These stories do not abolish every distinction. They let you notice that you have been looking from somewhere all along.

Yukawa wrote about the fish’s happiness and brushed the characters for “knowing the joy of fish” in calligraphy. There is affection for nature here, a willingness to approach it sympathetically. Yet imagining the fish’s happiness does not establish that it is happy. Listening to the world and already knowing the answer are different things.

For a future proposer of a new particle, such reading can be soil. Soil is not an instruction manual for the fruit. It cannot derive a mass for physics. It might, however, help someone keep a question open before an answer arrives.

III. A Crack That Can Open

Since Comte, this group of essays has kept asking what counts as knowing. Requiring knowledge to face tests has real force. Shutting out every question that does not yet fit our measures carries a danger of its own.

But Descartes, Kant, Comte and the Vienna Circle cannot be marched into a single file, all repeating: if the eye cannot see it, thought must not touch it.

Nor can we say that the East never drew boundaries.

Traditions contain disputes, adventurous speculation and rigid exclusions. The reading that forms one person is more complicated than a map divided down the middle.

The Doctrine of the Mean’s sincerity, Buddhist emptiness and Zhuangzi’s transformations do not all name the same thing. “Form is emptiness” does not mean visible and invisible are indistinguishable. It cannot guarantee an undiscovered particle’s existence.

Another possibility remains. An old book can loosen a habit and make another way of asking available. Its gift need not be an answer. Sometimes it is permission to turn your head.

Yukawa’s physics, meanwhile, does not bypass earlier work. Nuclear forces, quantum fields, Heisenberg and Fermi all belong to his problem. European physicists also proposed unobserved entities. Dirac, whom this series has just considered, is an obvious example.

Yukawa’s courage consists not in dispensing with evidence, but in proposing a question that later evidence can answer. The difference between muon and pion is a reminder: reality has no obligation to accommodate a beautiful guess.

The crack, then, belongs to no single civilization. Someone opens it and remains willing to be corrected by the world. Imagination and testing need not face each other as enemies.

IV. Yukawa and Dirac

Dirac predicted antimatter; Yukawa predicted the meson. Both allowed theory to point toward a new particle before experimental confirmation. That resemblance is worth keeping.

Their emphases differ, though.

With Dirac, difficulties inside an equation come sharply into view. Negative-energy solutions did not automatically announce the positron. Between the formal result and its physical interpretation lay hesitation, revision and imagination.

With Yukawa, the puzzle of a force both powerful and short-ranged stands forward. What field and what particle mass could account for that reach? Calculation and images push each other along. A picture of two ball throwers does not give birth to the theory by itself.

Where, in this, is Zhuangzi?

Perhaps somewhere earlier and harder to trace. Someone accustomed to stories of transformation and shifting viewpoints may not look only in old drawers for a new answer. We can understand reading this way without claiming to reconstruct every thought at the moment of discovery.

The Zhuangzi contains no meson formula. Saying it nourished Yukawa is very different from deriving his theory from the butterfly dream. Literary inspiration need not dress as scientific proof to deserve respect.

Dirac needed imagination too. Yukawa too was constrained by equations. Assigning mathematics to one man and Eastern intuition to the other makes both smaller.

Put them at neighboring desks instead. At one, mathematical form presses a question upon physics. At the other, a physical problem searches for form. Both must look up to see whether the world agrees.

V. Nishida Kitarō

In Kyoto we also encounter Nishida Kitarō, already present in this series.

During Yukawa’s student years, Nishida’s philosophy was a major intellectual presence there. Kyoto University’s account also records Yukawa attending his lectures. The contact matters; it does not furnish a philosophical origin for every later discovery.

Nishida’s absolute nothingness invites an especially tempting bridge. An unseen particle becomes a discovered particle: does that not sound like being emerging from nothing? Yet unobserved describes a state of knowledge. Absolute nothingness is a philosophical question, not a warehouse awaiting a shipment of particles.

Nāgārjuna’s emptiness is not that warehouse either. It is not a substance filled with ideas, waiting to become matter. Bringing different kinds of “nothing” together can start a question. Treating them as one thing erases the differences.

More interesting here is a young person able to listen to philosophy and study physics without first surrendering half of himself. Hard questions do not always respect departmental boundaries. Nor does a reader grow in only one language.

A question carried out of one classroom may accompany him into another. Not to answer the work being done there, but to keep him asking.

VI. The Traveler

Yukawa calls his autobiography Tabibito: The Traveler.

The name fits. He works in Kyoto and Osaka, later goes to Princeton and Columbia, and returns to Kyoto. Alongside these journeys run others, between books and between disciplines.

Not every crossing needs to end with East and West becoming one. A person can carry the sound of old books, training in modern mathematics, solitary doubts and colleagues’ objections. They do not always harmonize. They can nevertheless take part in the same act of creation.

Our question about knowing grows a little wider. Qu Yuan has already brought another voice; Nāgārjuna is not merely an extreme negation. Yukawa now brings us back to a particular piece of work, where reading from different sources meets calculation and experiment.

A traveler does not announce that two banks are identical because he has crossed a bridge. He simply need not leave every book he has read forever at his point of departure.

VII. After the War

August 1945. Hiroshima. Nagasaki.

Beside those names, understanding the nucleus can no longer remain a sentence on a desk. What institutions will knowledge enter? Whose power will it become, and whose lives will it reach? A correct equation cannot answer those questions for us.

Still, no simple line runs from Yukawa’s meson paper to the atomic bomb. Fission is not the tearing apart of the strong force. A heavy nucleus splits into products that are, in total, more tightly bound, releasing energy. Meson theory was not the direct technical principle behind building the bomb.

The 1949 prize remains an important encouragement in postwar Japan. It does not automatically erase a country’s suffering or responsibilities. It shows many people that creative work is still possible after devastation.

In 1955 Yukawa signs the Russell–Einstein Manifesto and later participates in Pugwash peace activities. He is not asking humanity to stop understanding nuclei. He is asking it to face what nuclear weapons might destroy.

Nobel’s name brings another tension among knowledge, technology and peace to mind. But neither life needs the same tidy story of atonement. Discovery is not enough. Choices afterward, and the distribution of their consequences, require scientists, publics and institutions to answer together.

VIII. The Bridgehead

On this imagined bridge, Yukawa arrives quietly.

He likes books and needs solitude. That need not erase colleagues, discussion and collaboration from his creativity. Quiet people too live among other people’s questions and replies.

He steps onto the bridge and looks around.

Comte, Popper, Fichte, Thatcher, Woolf. Qu Yuan and Nāgārjuna too. These people cannot simply be assigned to European and Asian teams. Each has brought a path of their own.

Yukawa pauses in the middle. Not an umpire between civilizations. Someone accustomed to walking back and forth between different pages.

He sees Dirac. We can imagine neither hurrying to compare who had an image first and who had an equation. Both know that a page full of symbols remains at a distance from the world. Only its response can tell them where they have found a way through.

He sees Nāgārjuna. No emptiness full of consciousness waits here for physicists to extract particles. Perhaps there is only a reminder: do not mistake your explanation for something that no longer depends on conditions.

He takes an old book from his pocket. The Zhuangzi. We have put this worn volume into the scene; no biography is being invoked to prove he carried this particular copy all his life.

He opens the butterfly dream, then closes the book. Water moves below the bridge. Perhaps there are fish.

A butterfly is not a meson, a dream not an experiment. The story did not anticipate nuclear physics. It can first make a person pause, and notice that the familiar world still has room for another way of asking.

Yukawa puts the book away. The bridge continues ahead. An old book has accompanied him this far. What remains is for him to keep asking, and for the world to answer.^[1][2][3]

Han Qin (秦汉) · @hqinjarsy · Great Lives