I have been amazed to explore the archives on Hideki Yukawa, which have been systematically categorized and meticulously maintained by Osaka University in Japan. My sincere thanks and acknowledgment to the Yukawa Memorial.
Below are a few gems from their public archives :
Draft of the paperwritten in 1934 – The making of the groundbreaking paper of Yukawa, which eventually led to his Nobel Prize in 1949.
The archive draft is accompanied by a note which reads:
Yukawa had not published any paper before then. In 1933, Yukawa began working at Osaka Imperial University and tackled the challenge of elucidating the mystery of nuclear forces while Seishi Kikuchi and other prominent researchers were producing achievements in nuclear physics and quantum physics. The idea of γ’ (gamma prime) that Yukawa came up with in early October led to the discovery of a new particle (meson) that mediates nuclear forces. The idea of introducing a new particle for the purpose of explaining the forces that act between particles was revolutionary at that time. Yukawa estimated the mass of the new particle and the degree of its force. No other physicists in the world had thought of this idea before.
2. Letters between Tomonaga and Yukawa
Sin-Itiro Tomonaga was a legendary theoretical physicist from Japan, who independently formulated the theory of quantum electrodynamics (apart from Feynman and Schwinger) and went on to win the Nobel Prize in physics in 1965.
Both these theoreticians were intensely working on interrelated problems and constantly exchanged ideas. The archival note related to the letter has to say the following:
During this period, Yukawa and Tomonaga concentrated on elucidating nuclear forces day in and day out, and communicated their thoughts to each other. In this letter, before starting the explanation, Tomonaga wrote “I am presently working on calculations and I believe that the ongoing process is not very interesting, so I omit details.” While analyzing the Heisenberg theory of interactions between neutrons and protons, Tomonaga attempted to explain the mass defects of deuterium by using the hypothesis that is now known as Yukawa potential. The determination of potential was arbitrary and the latest Pegrum’s experiment at that time was taken into consideration. Tomonaga also compared his results with Wigner’s theorem and Majorana’s theory.
3. Rejection letter from Physical Review
Which physicist can escape a rejection from the journal Physical Review?
Even Yukawa was not spared :-) Below is a snapshot of a rejection letter from 1936, and John Tate does the honours.
The influence of Yukawa and Tomonaga can be seen and felt at many of the physics departments across Japan. Specifically, their influence on nuclear and particle physics is deep and wide, and has inspired many in Japan to do physics. As the archive note says:
Yukawa and Tomonaga fostered the theory of elementary particles in Japan from each other’s standpoint. Younger researchers who were brought up by them, so to speak, must not forget that the establishment of Japan’s rich foundation for the research of the theory of elementary particles owes largely to Yukawa and Tomonaga.
4. Lastly, below is a picture of the legends from the archive: Enrico Fermi, Emilio Segrè, Hideki Yukawa, and James Chadwick.
From the archive note on the picture from September 1948:
Yukawa met Prof. Fermi and other physicists of the University of Chicago who were staying in Berkeley for the summer lectures. From the left: Enrico Fermi, Emilio Segrè, Hideki Yukawa, and James Chadwick.
Tomorrow, I will conclude my third trip to Japan. I always take a lot of inspiration from this wonderful country. As usual, I have not only met and learnt a lot from contemporary Japanese researchers, but also have metaphorically visited the past masters who continue to inspire physicists like me across the world.
In the previous essay, we discussed engineering civilizations. Specifically, we discussed how philosophy and technology have played a critical role in the betterment of civilization. In addition, the fact that thoughts and tools have direct implications on how human beings live cannot be overstated.
Over the centuries, there have been many people who have played a critical role in advancing philosophical thoughts and engineering tools that have directly or indirectly influenced the development of physics. Before we really get into the specifics of the science of mechanics and its historical developments, we need to investigate some of the ideas that were part of the discussion in ancient civilizations. Given that mechanical tools date back to the prehistoric era, it is not surprising that human beings took an interest in understanding how the world works within their proximity.
In such a development, a human being is always curious to understand and harness nature. To do this effectively, one will have to systematically think about how nature works. According to the existing literature from a history of science viewpoint, the Greek philosophers occupy a very special position.
Part of the reason is that their thoughts were recorded in some form or another, and these thoughts date back to a period as early as 300 to 400 BCE or even before. This also coincides with the time at which writing became an important tool to propagate ideas, and therefore, one can see the emergence of historical evidence from this era. The Greek philosophers have a great reputation in Western philosophy and have influenced the development of scientific thinking for a very long period.
This does not mean that people from other civilizations did not think about tools and philosophical ideas. Just that the availability of written records, either direct or indirect, has been one of the hallmarks of Greek civilization. Thanks to the accumulation of these texts, either in the primary or in the secondary source form, it has played a critical role in identifying a specific point in Western civilization(1).
Physics, as we know it in the 21st century, has a very different avatar compared to its origin. Among the many Greek philosophers who seriously thought about natural philosophy was Aristotle(2). Aristotle has had a long legacy of being the student of some great thinkers from the Hellenistic era.
Aristotle was a student of Plato, and Plato was a student of Socrates. So, these three gentlemen have contributed to Western philosophy in such a great way that one cannot discuss anything about philosophical discourse without bringing them into the picture. The origins of physics, too, have some connection to these gentlemen, specifically to Aristotle(3), who was guided by some processes of thinking developed by Socrates and his direct guru, Plato.
The Plato school of thinking deeply influenced Aristotle, and yet he paved his own way in Western philosophy. He was also one of the first writers among these philosophers to seriously think about natural philosophy in the form and shape that resembles the current day science. His questions were related to natural phenomena, and it is by studying them that we realize Aristotle’s contribution. As I always keep saying, it is not just the answers that make great thoughts; it is also the questions that elevate the answers and make them profound. In that way, Aristotle’s questions were profound.
Therefore, I need to emphasize the quality of the questions that Aristotle asked instead of just emphasizing the answers he gave in the process of this questioning. It also indicates that current-day physics, as we now know it is not in the form Aristotle thought about, but we can see a glimpse of some interesting ideas in Aristotle, which turned out to be extremely important. It became so critical that for the next thousand years from the time of Aristotle, the Western philosophy and the science that emerged out of it kept Aristotle as an important benchmark and developed its thought either in agreement or in rejection of Aristotle’s idea.
So much so that this thinking process in reference to Aristotle’s idea not only influenced Western civilization but also had a very deep implication for Islamic civilizations. Although Aristotle’s ideas became critical in ancient Europe and the ancient Islamic world, its ideas and categorizations of knowledge found relevance across various civilizations.
So, let’s try to get a glimpse of the man himself – Aristotle. Let us look at the biographical details of this remarkable individual and learn about his work.
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The Life of Aristotle
Aristotle was born in Stagira, Macedonia, which is in the northern part of Greece, in 384 BCE. His father was a physician to one of the kings in Macedon, and Aristotle was probably influenced by his father’s way of thinking about nature and natural phenomena. Of course, this is all conjecture because nobody knows the exact nature of the interaction between Aristotle and his father. However, this reconstruction is generally accepted based on historical texts(4).
When Aristotle was around 17 years old, in 367 BCE, he was sent to Athens to join the remarkable institution known as Plato’s Academy. During that period, Plato’s Academy was one of the leading intellectual centers in Europe, playing a critical role in shaping the thinking of various philosophers, including Aristotle.
It was at this place that Aristotle honed his skills in philosophical argumentation and the observation of nature and natural phenomena. This foundation played a crucial role in shaping his later works. For approximately 20 years, Aristotle remained at Plato’s Academy.
After Plato died in 348 BCE, Aristotle moved elsewhere. Historical texts suggest two possible reasons for his departure. One possibility is that he had intellectual differences with Plato’s nephew, who took over the Academy. The second possible reason is that the political atmosphere in Athens had become increasingly hostile toward people from Macedonia. These reasons, of course, remain speculative, although they are mentioned in historical literature.
In 343 BCE, King Philip II of Macedon invited Aristotle to tutor his son, Alexander. This is the same Alexander whom history remembers as Alexander the Great, though we will refer to him simply as Alexander. Aristotle played a crucial role in educating Alexander for more than five to seven years, deeply influencing the future conqueror’s thought process.
After about seven to eight years, Aristotle returned to Athens and established the Lyceum, a school of philosophy that had a profound impact on Western thought. This school had a unique feature—philosophers often lectured while taking long walks. As a result, the school came to be known as the Peripatetic School, recognized for its tradition of philosophical discourse while on the move.
During this period, around 330 BCE, Aristotle produced some of his most significant works, particularly in the context of physics. Even before leaving Athens, he had made substantial contributions to topics related to biology. However, in this second phase of his career, his focus broadened to natural philosophy, logic, ethics, aesthetics, rhetoric, and even music. This vast intellectual repertoire is one of Aristotle’s most remarkable features(4).
Around 323 BCE, Aristotle left Athens again following the untimely death of Alexander. During that period, anti-Macedonian sentiment was widespread in Athens, which likely motivated his departure. Within a year of leaving, around 322 BCE, Aristotle passed away at the age of 62. The probable cause of his death appears to have been natural.
All this information, of course, is based on historical sources that have been passed down through the centuries(4). One should always be cautious when interpreting ancient texts, as most of these sources survive only in translation rather than their original form. There is always a possibility of inaccuracies, and one must approach these accounts with care.
Aristotle’s intellectual journey covered a vast spectrum of human inquiry, from the heavens, as in astronomy, to logic within the framework of mathematics and, of course, physics. His interests also overlapped with observational phenomena and the life around him, which is evident from the questions he explored. Among his many areas of interest, we will focus primarily on Aristotle and his contributions to physics.
Aristotle’s writings on natural philosophy cover a vast range of topics, from celestial bodies to the nature of matter and motion. His works, though sometimes speculative by modern standards, laid the foundation for centuries of philosophical and scientific thought(5). Much of what we know about Aristotle comes from the preserved writings of later scholars, as very few primary sources from Aristotle himself remain.
Before I discuss Aristotle’s book, a few words on the origin of the word Physics. The word physics is derived from a Greek word called phusikḗ, which means natural science. This word has its origin in another word, and it is called phúsis, which means nature in Greek. The modern definition of physics is essentially derived from the Latin word physikā, which essentially means the study of nature.
This word was adapted by the old French, which was further adapted by English to be transformed into the word physics, as in natural philosophy.
In the context of Aristotle, it is the Greek word phúsis, meaning nature, is central to this discussion.
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The Eight Books of Physics
Aristotle’s Physics is divided into eight books(6), each exploring different aspects of nature and motion. Specifically, he was interested in the principles and causes of change and motion of objects. Let me give you an overview of his books.
Book One introduces Aristotle’s fundamental concepts of matter and form. A key discussion in this book is the doctrine of the four causes: the material cause, which explains what something is made of; the formal cause, which defines its shape or essence; the efficient cause, which refers to the force or agent that brings about change; and the final cause, which represents the purpose or end goal of an object or process. These causes provide a framework for understanding physical interactions and transformations.
In Book Two, Aristotle argues for teleology—the idea that nature has inherent purposes. He defends the study of nature as a legitimate form of scientific inquiry and lays out a methodological foundation for observing and interpreting natural phenomena.
Book Three explores the nature of change. Aristotle distinguishes between actuality, the realized state of an object brought about by an external force, and potentiality, the inherent capability of an object to become something else. This distinction is significant in understanding motion and transformation.
In Book Four, Aristotle explores the nature of voids, rejecting the concept of a vacuum. He also presents an interesting perspective on time, defining it as a measure of change.
Book Five classifies different kinds of motion, an important step in distinguishing natural movement from forced motion.
In Book Six, Aristotle discusses the nature of continuity and the concept of infinity. He argues against the existence of actual infinity, emphasizing the finiteness of physical reality.
Book Seven introduces the concept of the Prime Mover—an external force that initiates movement. This idea becomes central to Aristotle’s broader cosmology.
Book Eight expands on the idea of the Prime Mover as the ultimate cause of cosmic motion. This notion of an external force influencing the universe had a lasting impact on Western thought, intertwining scientific and theological perspectives for centuries.
Related to these books was Aristotle’s book titled On the Heavens. In there, Aristotle discussed his celestial theories. This four-part work, On the Heavens, explores the nature of celestial bodies and their motion.
He proposes that heavenly bodies are made of ether, a perfect and unchanging substance.
Aristotle also argues that the Earth is spherical and that celestial objects move in circular orbits—a belief that persisted until Kepler’s elliptical model. He discusses why celestial objects move in circular paths, distinguishing their motion from that of objects on Earth.
Expanding on his theory of the five fundamental elements—earth, water, air, fire, and ether—Aristotle’s ideas parallel similar concepts in other ancient civilizations, such as those in India and Mesopotamia.
Aristotle also studied natural phenomena through observations, and a related 4 book series was titledMeteorology. These books addressed:
Weather phenomena (clouds, wind, rain, lightning)
Natural bodies (lakes, rivers, seas, and geological formations)
Dynamic processes such as volcanoes and the transformation of matter
Another interesting book that Aristotle wrote about was based on his views on matter. The title of the work is unconventional and reads: On Generation and Corruption. This work, consisting of two books, discusses:
The four elements and their combinations in forming matter
The processes of transformation, mixing, and decomposition of substances
One may wonder how we know so much about Aristotle, even though he lived around 350 BCE. This is one of the remarkable features of human beings. They carry forward ideas from one generation to another.
Of course, this transmission across ages may cause errors in translation, but at least we get a gist of the idea through percolation. The records that we obtain through transmission will depend on the preservation of the sources of ideas. This is where the written text becomes so important because a physical text can be preserved, transported and reproduced with relative ease compared to oral records from the past.
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Most of the information about Aristotle comes from Corpus Aristotelisium(4). This is a multinational project about preserving Aristotle’s work. Around 200 texts are associated with Aristotle, out of which 31 have survived as the ancient textual evidence. These texts are from later eras across different countries, but one can make a connection to the original source through educated guesses and cross-connections.
So, I hope you have got a glimpse of and range of topics Aristotle was interested in from a physics viewpoint. Aristotle’s broad approach to physics covered motion, change, matter, and the cosmos. While many of his explanations were later refuted, his method of questioning and systematic exploration laid the groundwork for future scientific inquiry(5). His emphasis on philosophical reasoning remains an essential part of the intellectual history of physics.
In this essay, we discussed the origins of physics from Aristotle’s philosophical viewpoint. We learnt about his life and remarkable scholarly output. Going forward, we will explore other ideas and thinkers from the ancient age whose work you will generally find in physics textbooks. Can you guess what ideas they are and whom I am referring to? Think about it for a while…
References:
1. Adamson P. Classical Philosophy: A history of philosophy without any gaps, Volume 1. Oxford, New York: Oxford University Press; 2014. 368 p. (A History of Philosophy).
2. Shields C. Aristotle. In: Zalta EN, Nodelman U, editors. The Stanford Encyclopedia of Philosophy [Internet]. Winter 2023. Metaphysics Research Lab, Stanford University; 2023 [cited 2025 Mar 27]. Available from: https://plato.stanford.edu/archives/win2023/entries/aristotle/
3. Bodnar I. Aristotle’s Natural Philosophy. In: Zalta EN, Nodelman U, editors. The Stanford Encyclopedia of Philosophy [Internet]. Spring 2025. Metaphysics Research Lab, Stanford University; 2025 [cited 2025 Mar 27]. Available from: https://plato.stanford.edu/archives/spr2025/entries/aristotle-natphil/
4. Winzenrieth J. The Textual Transmission of the Aristotelian Corpus. In: Zalta EN, Nodelman U, editors. The Stanford Encyclopedia of Philosophy [Internet]. Spring 2025. Metaphysics Research Lab, Stanford University; 2025 [cited 2025 Apr 2]. Available from: https://plato.stanford.edu/archives/spr2025/entries/aristotle-text/
Many of the Nobel winners in science deserve the prize they get, but there are many deserving who do not make the list for various reasons, including sociology, geography & financial support. Using the Nobel prize as a benchmark of progress may lead to errors in the judgment of a country.
A better way to judge is to ask: how are science and technology in a country making lives better for the people of the country & the world? A better life includes both intellectual & material aspects. We, in India & the global south, can make progress if scientific thinking becomes prevalent in the everyday discourse of society, from family conversations to political debates. And prizes, by design, are exclusive. It is easier to exclude a country that is not scientific as no one cares in such a situation.
Scientific progress in a society is generally bottom-up. We have a large population of young people who should be more scientific in their worldview. If we have a large, scientifically oriented population, science and technological achievements become proportional.
If science-based intellectual & material progress is achieved, prizes will follow. But a Nobel should not be our primary goal. Better lives & better minds should be.
The above picture is from Friedrich, Bretislav, and Dudley Herschbach. “Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics.” Physics Today 56, no. 12 (December 1, 2003): 53–59. https://doi.org/10.1063/1.1650229.
During the formative years of quantum mechanics (early 1900s), the spin and orbital angular momentum of atoms were found to be quantized by theoretical arguments. Experimental proof was lacking.
Stern-Gerlach experiment provided the first experimental proof in 1922. They took a beam of neutral silver atoms and deflected them through an inhomogeneous magnetic field.
Silver atoms have an unpaired electron in their outermost orbit. If they were to obey quantum mechanics, they should exhibit a spin of +1/2 or -1/2. When subjected to an external magnetic field, the electrons with +1/2 or -1/2 should spatially split into two. That is exactly what Stern and Gerlach observed, and below is the first picture of the same.
To quote the authors:
Gerlach’s postcard, dated 8 February 1922, to Niels Bohr. It shows a photograph of the beam splitting, with the message, in translation: “Attached [is] the experimental proof of directional quantization. We congratulate [you] on the confirmation of your theory.” (Courtesy AIP Emilio Segrè Visual Archives.)
This experiment was one of the most important observations in quantum mechanics and further confirmed the quantization of spins, which is now common knowledge in physics.
Superstition is a belief system or behavior of an individual that cannot be justified with evidence and logic. It is usually associated with people who do not (or do not want to) think critically. From the history and philosophy of science, we learn that a few famous thinkers of the past had some form of belief that can be termed superstitious. Of course, they were products of their times and environments, but it is always interesting to learn about the contradictions.
Take, for example, Kepler, Galileo and Newton. They were 3 important figures who laid the foundation of classical mechanics (along with many other things). But they also had their pet beliefs that were neither logical nor scientific.
In the preface of his book, Karl Popper has to say about the superstition of the 3 individuals mentioned:
Each of the three intellectual giants was, in his own way, caught up in a superstition. (‘Superstition’ is a word we should use only with the greatest caution, knowing how little we know and how certain it is that we too, without realizing it, are caught up in various forms of superstition.) Galileo most deeply believed in a natural circular motion – the very belief that Kepler, after lengthy struggles, conquered both in himself and in astronomy. Newton wrote a long book on the traditional (mainly biblical) history of mankind, whose dates he adjusted in accordance with principles quite clearly derived from superstition. And Kepler was not only an astronomer but also an astrologer; he was for this reason dismissed by Galileo and many others.
Of course, I am bringing this up not to justify any superstition. But to highlight the fact that people whom we call ‘heroes’ are humans and have their beliefs and flaws. We may derive inspiration from their work, but not all aspects of their character may be suitable for emulation.
We will have to adapt what is good and discard what is not. You may ask: what is the definition of ‘good’? Well, that is a topic for a different debate, but in this context, I would say ‘good’ are the ideas and methods developed by the abovementioned that are testable and falsifiable. Karl Popper may be happy with that definition.
There is a new book (88 pages) on the philosophy of science that discusses the demarcation problem between science and pseudoscience. The topics look interesting, and have relevance in a day and age where science has been appropriated for various purposes, including spirituality.
One will have to ask how to differentiate science from something that may sound like science but, with further exploration, turns out to be a hoax?
This book tries to address this issue from a philosophical viewpoint.
Our world is a place with complex ideas superimposed on people with ever-changing attention. Complex ideas are complex because they depend on multiple parameters. If something changes in the world, then that change can occur due to multiple reasons.
Unlike a carefully designed physics experiment, there are too many ‘hidden variables’ in human life and behavior, especially when they act collectively. In such a situation, it is pertinent to search for models to understand the complex world. Models, by definition, capture the essence of a problem and do not represent the complete system. They are like maps, zoomed out, but very useful if you know their limitations. I keep searching for mental models that will help me understand the complex world in which I live, interact, and comprehend.
Among many models, one of them that I use extensively is the follow-the-money model. This model explains some complex processes in a world where one does not have complete information about a problem.
Take, for example, the incentives to choose a research project. This is a task that as scientists, we need to do very often. In the process of choosing a project to work on, researchers have to factor in the possibility of that research being funded prior to the start of the project. This is critical for scientific research that is dependent on infrastructure, such as experimental sciences, including physics, chemistry, and biology. Inherently, as researchers, we tend to pick a topic that is at the interface of personal interest, competence, relevance, and financial viability.
The viability is an important element because sustained funding plays a critical role in our ability to address all the contours of a research project. Thus, as scientists, we need to follow the money and ask ourselves how our research can be adapted to the financial incentives that a society creates. A case in point is research areas such as AI, where many people are aware of its potential and, hence, support from society and an opportunity to utilize the available incentive.
It is important for the public to be aware of this aspect of research where the financial incentive to execute a project plays a role in the choice of the project itself. The downstream of this incentive is the opportunity to employ more people. This means large funding projects and programs attract more researchers. More people in the research area generate more data, and more data, hopefully, will result in more knowledge in the chosen research area. This shows how financial incentives play a critical role in propelling a research area. In that sense, the ‘follow the money’ model has a direct correlation with more researchers flocking towards a research area.
The downside of this way of functioning is that it skews people towards certain areas of research at the cost of another research area which may not find financial support from the society. This is a topic that is generally not discussed in science classes, especially at the undergraduate and research level but I think we should discuss with students about this asymmetry as their futures are dependent on financial support that they can garner.
Broadening the scope further, the ‘follow the money’ model is useful to understand why a certain global trend rises or falls. A contemporary global upheaval is the situation of war in Ukraine and Gaza. At first sight, it looks like these wars are based on ideologies, but a closer look reveals that these wars cannot be fought without financial support. Such underpinning of the money running the war reveals patterns in geopolitics that are otherwise not easy to grasp.
Ideologies have the power to act as vehicles of human change, but these vehicles cannot be propelled without the metaphorical fuel – that is, money. The ‘follow-the-money’ model can show some implicit motivation and showcase how ideologies can be used as trojan horses to gain financial superiority either through captured resources or through showcasing the ability to capture that resource. Following money is also a very powerful and useful model for understanding many cultural, sociological and political evolution, even in a complex country like India and other South Asian countries. I leave it as an intellectual assignment for people who want to explore it 😊. You will be surprised how effective it can be in explaining many complex issues, provided we know the limitations of the model.
As I mentioned earlier, a model is like a map. It is limited by resolution, the dimension and the viewpoint. But they are useful for navigating a complex world.
VISMAYA - History & Philosophy of Physics 