We use optical illumination to generate thermal fields, creating non-reciprocal interactions between passive and active colloids. Active colloids absorb light and produce thermal gradients, driving thermo-osmotic forces that induce propulsion and chiral motion. Our Langevin simulations, backed by experimental observation, reveal how to control colloidal behavior. May have implications in light-driven chiral motion and nonlinear dynamics.
Super effort by Rahul, Ashutosh & Sneha from our group, who combined numerical simulations, analytical theory, with experimental observations.
The 2 anonymous reviewers made us think and work hard, and we thank them!
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.
New ideas are often created by the merging of two old ideas. How often is this true, and how often do we tend to forget this?
Today I visited the Institute of Science Tokyo, formerly known as Tokyo Tech. This is a new avatar of a very interesting institution funded by the government of Japan. By merging the Tokyo Institute of Technology with the Tokyo Medical and Dental University, a very interesting concept has emerged: the Institute of Science Tokyo. These two institutions have been important pillars of the research and educational landscape of Tokyo, and I had the privilege of visiting this new place, which is a result of a new merger.
Thanks to the invitation and fantastic hospitality of Prof. Daiki Nishiguchi, a faculty member in the Physics Department of the Institute of Science Tokyo, I had a memorable experience. I met Daiki a couple of years ago at the University of Tokyo, where he previously held a faculty position. Recently, he has moved to the Institute of Science Tokyo to establish his independent research group as an Associate Professor.
I gave a physics seminar on some of our work on structured light and confinement of soft matter, especially thermally active colloidal matter in optothermal potentials. Since Daiki and his group (see image below) have expertise in topological soft matter, my seminar emphasized structured topological beams, including ring optical beams and optical vortices. I gave an overview of our experimental results and highlighted the prospect of utilizing the topology of light to interact with topological soft matter.
There is much to explore at this interface, and again, it brings me back to the point that new ideas often emerge from the merging of evolving old ideas, such as topological light and topological soft matter.
This is my third visit to Japan, and I always find their calm, focused, and deeply committed research environment inspiring. There is much to learn from their approach to science and technology, and my visit to the Institute of Science Tokyo reinforced this thought.
I thank Daiki and his research group for the wonderful time I had at their laboratory and offer my best wishes to him in his new explorations.
Whereas Sunday was bright, sunny, and clear for outdoor activities, Monday started cloudy with a forecast of rain. I started from my living place to Kyoto University around 10 in the morning. I took the city bus, which shuttles people from the city centre to the university. Within half an hour, I was in a serene, green, and beautiful campus, typical of a Japanese university. Kyoto University has a rich blend of modern and ancient architecture, and I was not surprised to see a large maroon-coloured ark at the entrance of the university.
With Prof. Tetsuro, who hosted me at the Graduate School of Informatics at Kyoto University.
I met Tesuji Tetsuro upon arrival (our previous in-person meeting was in the 2023 Optics & Photonics Congress on optical manipulation at Yokohama). He had just arrived from his run (he is a regular marathon runner), and we had a brief chat. He had arranged an office for me to occupy for the day. We had a short discussion and thereafter went for lunch. Prof. Kazuo Aoki (Tetsuro’s erstwhile advisor at Kyoto University) accompanied us, and I was delighted to meet him. We had a delicious lunch at a small Italian restaurant.
Around 3 pm, we met at the seminar hall where I gave my talk titled Hot Brownian Dynamics Driven by Structured Light. One of the key points I emphasized in my talk was the relevance of structured light in driving Brownian dynamics of colloids. I spoke about various parts of the stochastic differential equation (see equation 1 below) that represent the dynamics of a colloidal system interacting with an external force.
A key element of my discussion was the generalized driving force on the right-hand side of the equation, where the conventional restoring force in an optical trap can be generalized to an external driving force due to structured light. This versatile force is a result of a large set of linear and angular momentum states of structured light. These states can drive soft matter, further resulting in unconventional assembly and dynamics. Furthermore, the generalized driving force can include not only the optical force but also the thermal and hydrodynamic effects initiated by optical illumination. The combination of these forces culminates in a resultant force, offering an unconventional driving mechanism to drive the structure, assembly, and dynamics of colloids and other kinds of soft matter systems, including droplets and fluids. I showed some of our experimental results related to the above-mentioned concepts with emphasis on rotational and orbital degrees of freedom. I also presented our recent results on synchronization in an optothermal trap.
campus mapnear the entrance of Kyoto University at a Japanese izakayaWith Tetsuro and some PhD students
We had a long discussion on how to measure fluid dynamic properties around such colloids, especially when there is an external perturbation force, such as a laser beam, which can itself influence the colloidal dynamics. Tetsuro also mentioned his protocols and certain simulation strategies utilized to study thermo-osmotic flows in such situations. I learned about interesting methods they have been developing to numerically simulate the interactions using differential temperatures. The strategy is interesting and deserves further attention by the community. He also showed his experimental setup and gave a tour of his laboratory facilities.
Overall, it was a long, thoughtful day with wonderful discussions on topics of common scientific interest. We ended with a delicious dinner at a Japanese izakaya, and I thank Tetsuro for his invitation and hospitality. Kyoto University has a wonderful atmosphere for research, and I hope to visit again.
The who’s who of Japan’s theoretical physics (and future Nobels) in 1951. They were meeting at Kyoto to establish an inter-university research institute.
This photo was further reproduced at :
Takaiwa, Yoshinobu, Masako Bando, Haruyoshi Gotoh, Hisao Hayakawa, Kohji Hirata, Kazuyuki Ito, Kenji Ito, et al. 2014. “Memorial Archival Libraries of Yukawa, Tomonaga, and Sakata.” In Proceedings of the 12th Asia Pacific Physics Conference (APPC12). Vol. 1. JPS Conference Proceedings 1. Journal of the Physical Society of Japan. https://doi.org/10.7566/JPSCP.1.019005.
Duff’s famous physics textbook from 1900 (5th edition) owned by Yukawa
Yukawa’s name on the book
Hideki Yukawa’s picture on the Nobel website
Apart from sipping the wonderful Japanese coffee and exploring the streets of Kyoto on foot, I have been looking into the archives of Kyoto University. I am mainly searching for records and books related to their physics department, and obviously, one of the names that pops out very often is Hideki Yukawa.
Yukawa was one of the Nobel laureates from this university. He obtained his Nobel Prize in Physics in 1949 for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces. He is a big name in physics, and there is a physical potential named after him, which means one can understand the intellectual heft he carries as a physicist. Yukawa spent most of his scientific career at Kyoto, specifically at the Kyoto Imperial University (now, no more imperial :-) ), and is regarded as one of the inspirations for a battery of many excellent theoretical physicists to have emerged out of not only Kyoto but also Japan, and perhaps many parts of the world. While looking through the archival records, I came across one of the textbooks owned by Yukawa, which has his signature on it. It made my day !
The textbook titled “A Text-Book of Physics,” edited by A. Wilmer Duff, is a classic. Yukawa had the 5th edition (1921), and this book went on to have 3 more editions. I hope to write more about this particular textbook because the author, Wilmer Duff, had a connection to Madras University (as a Professor) in India and was also on the faculty of my post-doc alma mater – Purdue University !
The scientific world is a small place with unanticipated, wonderful connections :-)
When we study the history of science, specifically physics, we find that a good idea simultaneously existed in various places. This suggests it may be better not to overemphasize a person for the origin of an idea. If we focus on the context, utility, and exchange of ideas, we get a broader picture of scientific ideas. This approach creates a spatio-temporal network, an interesting way to view the historical evolution of ideas across the humanities, space, and time. People, ideas, and technologies collectively progress the frontiers of science in various places at different times. In that sense, science, including physics, is a global human endeavor. This is evident when we look into the history of mechanics from ancient times until now. Mechanics is a fundamental sub-discipline of physics and has a strong connection to mathematics and engineering. It has evolved with logical reasoning and understanding of natural phenomena.
Engineering structures, which humanity has long been interested in, have played a significant role. Mechanics has been the playground of philosophers, scientists, and engineers. The questions it raised led to new thinking and new technologies. Mechanics offers a way to understand and engineer the universe. In this episode, we explore its links to mathematics, engineering, and key thinkers.
The importance of counting.
Counting has played a critical role in human life for a long time. In fact, we use fingers as a counting device, and this has been a very powerful tool for a significant period. So much so that it has been utilized to keep a tally of small numbers, which can be counted and analyzed in everyday life. Interestingly, as the numbers became larger, one had to externalize the counting process, and in ancient times, people had very creative methods to count objects. One of the fascinating aspects of counting was to use bones. Yes, bones were used as platforms on which marks were created, and these marks were utilized as the counts in a tally.
Images of Ishango bones with periodic marks on its surface.
One of the pieces of evidence for such behavior was found in the Congo Basin in a place called Ishango. The bones that were found are dated around 9000 BCE to 6500 BCE (although a big debate is going on regarding the dates, with some putting it beyond 20,000 BCE), on which marks have been identified that look like counts registered on the platform. Indeed, it is quite fascinating to see how people used various devices to enumerate objects. If you ask any child what is 1 plus 1, they will be able to say that it is 2. This may sound trivial, but the concept of addition itself was not in the historical context. To consider two numbers and add them together needs a certain degree of abstraction, which has been part of the evolution of mathematics since ancient times. A variety of counting methods have been devised in different civilizations, which have added a kind of flavour to the history of mathematics.
Importantly, language has played a critical role in facilitating a vocabulary for counting. Depending upon the syntax and the order of letters, numbers have been represented in a variety of ways across space and time of human history.
In this context, let me emphasize two important aspects of numbers and their representation. The first aspect is related to the positional notation. What is it? Let me give you an example. If you take numbers 24 and 42, the position of the number 4 is different. In 24, the number 4 is in the unit’s place, and in the number 42, it is in the tens place. So, the position of the number determines its value, and this is an important concept that civilizations have thought about and utilized in their counting systems. The second aspect is the concept of zero. Let me give an example. If you consider number 007 and compare it to 700, obviously, depending on the location of the zero, the value of the number drastically changes. But what is intriguing is that various civilizations used a variety of symbols, such as dots, circles, and empty spaces, to represent nothingness. What is not trivial is the recognition of zero as a number by itself. This needs a leap of thought because it must be an abstraction of a concept where the nothingness has to be associated with a number, and hence the association to zero. It was Brahmagupta around the year 628 CE in his Brahma Sputa Siddhanta that we first encountered the concept of zero as a number. This is indeed one of the great achievements because, without zero as a number, one cannot build mathematical concepts. It is both fundamental and profound. It has further played a critical role in laying the foundation of mathematics as we know it today.
02 Geometry
Now, let’s look at the connection between geometry and physics. Across various civilizations, the size and shapes of objects were curiosities, and understanding them was an important necessity for everyday life. Given that objects in the natural and artificial world come in various sizes and shapes, it was necessary to understand them for further utilization.
In ancient Greece, Thales of Miletus was one of the earliest to use a mathematical way of thinking and to formulate a framework to understand nature through logical analysis and not based on faith or myths. This thinking further percolated to all the subsequent philosophers, and that included a person named Pythagoras. Pythagoras’ life and times are not as well documented as those of other Greek philosophers, but by some estimates, he was supposed to have lived around 570 BCE to 495 BCE.
During that time, the Greeks had colonized various parts of Europe, and this included some parts of Italy. Pythagoras remained in that colonized part of Greece, where he had established a school, which was also interestingly a mythical cult. His school hardly shared any information with the outside world, and this is probably one of the reasons why there is very little known about Pythagoras’ life and times.
In fact, none of his writing has survived to date, and most of the information that we get is from indirect sources. However, the attribution of some scholars to Pythagoras’ work needs attention. Interestingly, Pythagoras did some experiments and tried to understand the production of sound.
He made an interesting connection to the strings and the pleasant sound that they produce. He hypothesized that there is a rational number of steps in strings that led to the pleasant sound. This thinking was further extrapolated to rational numbers, and that became an interesting connection. Pythagoras has also been attributed to have thought about astronomical objects.
Earth being spherical is one of the concepts that he had thought about and played a role in rationalizing the distances of objects such as the Sun, Moon, and planets. This kind of methodical thinking further influenced many Greek schools of thought, and this included the famous Plato’s Academy. Plato himself was a renowned philosopher, but he had a very strong inclination towards mathematics.
He also came up with the five solids and the four elements, which played a critical role in his interpreting of the natural world based on them. But it is in 300 BCE that we see an epoch in geometry in the form of Euclid’s Elements. Euclid of Alexandria was one of the great mathematicians whose work is still of significant relevance today.
Euclid, like many of his predecessors, had a life immersed in the ancient university system—in his case, the University of Alexandria. Again, not much is known about Euclid’s life and times, except for the fact that he wrote 13 volumes of his magnificent book titled Elements.
This book has turned out to be the foundation of mathematics and has played a critical role in creating a new worldview both for natural scientists and abstract mathematicians. Most of what we know today about Euclid is thanks to a Greek commentator, Theon of Alexandria, who lived roughly 700 years after Euclid. He played a critical role in interpreting and highlighting the works of Euclid, and going forward in time, Arabs took a keen interest in Euclid’s geometry and incorporated it into their education and research.
Euclid’s work on geometry is a masterpiece, which has 13 books in a series and contains 465 theorems. Each of them contains foundational knowledge about geometrical entities, including lines, angles, shapes, and solid geometries that past people had discussed. It is a tribute to his knowledge that Euclid’s Elements is still in print, and this shows how much the impact of Euclid has been over the centuries.
Importantly, the geometrical way of thinking has deeply influenced physics, along with the principle of counting and geometry. Physics, armed with mathematics, became an important way of looking at natural life in ancient times. This way of thinking further influenced another remarkable thinker named Archimedes of Syracuse.
If you want to think about a remarkable person who has deeply contributed to science and mathematics from ancient times, there is nobody better than Archimedes. Born in 287 BCE, Archimedes had a remarkable life because the number of things that are associated with him related to science, mathematics, and technology is probably unsurpassed compared to anybody else across the ages. Generally, when we talk about Archimedes, we associate him with the famous Eureka, where he probably ran naked in the excitement of discovering a specific concept related to buoyancy.
Of course, this might be altogether a myth, but the science that Archimedes did was indeed real and outstanding. He contributed to various areas in science, including mechanics, hydrodynamics, optics, engineering, and mathematics in both the pure and applied forms. We know about his achievements thanks to nine ancient Greek treatises, which give us a glimpse of his work.
An important aspect related to mechanics is the fact that in the ancient age, one can divide the contributions in terms of statics and dynamics. The dynamics aspect was mainly related to thinking driven by Aristotle and his school, which has turned out to be kind of incorrect from the modern viewpoint. But when it comes to statics, Archimedes had a very important role to play, and many of the discoveries he made have turned out to be correct and highly useful.
Related to statics, he wrote many interesting treatises, one of them being On the Equilibrium of Planes. In this book, he talks about the concept of the lever and utilizes the concept of the center of gravity. It is in this treatise where the concept of the center of gravity is used to understand various geometries, and he discusses the center of gravity of different geometrical objects.
Another important book related to Archimedes is On Floating Bodies. In this book, he discusses buoyancy and gives an important hypothesis to understand bodies immersed in a fluid. His discoveries were very critical in naval architecture.
Archimedes was not only an outstanding scientist but also an excellent engineer. He designed a water pump in which a hollow cylinder had a rotating helical shaft, which could pump water efficiently. This is usually called the Archimedes pump, and it has been used even to date.
Archimedes also contributed to the development of mathematics. He wrote On the Sphere and Cylinder and The Method, used for mechanical analysis.
One has to wonder whether he was one single person or many. Steven Strogatz’s book related to calculus, titled Infinite Powers: How Calculus Reveals the Secrets of the Universe, has a beautiful description of Archimedes’ contribution to understanding curves, including the circle and the determination of pi in an ingenious way. The logical process Archimedes used was unsurpassed for his time, and his contributions to science are among the most important from the ancient age.
There are also interesting stories related to his work, and one of them is called the Archimedes Palimpsest. A palimpsest is a technique in which one writes something, erases it, and rewrites on the same surface. In 1906, a Danish professor, Johan Heiberg, visited Constantinople to examine documents related to prayers, dated from the 13th century. To his surprise, it turned out that there was an underlying document beneath that prayer text, which was Archimedes’ writing.
It is truly outstanding that someone could discover such an important document after such a long time, and that’s another reason why one should do archival work—because you never know what kind of jewels one can discover. Archimedes contributed to various aspects of science, mathematics, and technology, but it is also vital to appreciate that he used a logical way of thinking. Such thinking had a deep influence on people who followed him, and even today, the process of his analysis stands up to scholarly scrutiny. It’s critical for us to realize that such people play a key role in spreading important ideas in science, in physics, and, in this case, mechanics.
Archimedes will surely be remembered as one of the greatest human beings who propelled human scientific thought. The legacy of Archimedes has been kept alive by introducing his figure on the Fields Medal, a major prize in mathematics. It’s considered the Nobel Prize equivalent in mathematics.
On the medal, there is an engraving with the quote: “Rise above oneself and grasp the world.” It is a great quotation to not only engrave on a medal but also to follow in letter and spirit.
With the same spirit to rise and grasp the world, we will explore the physics of mechanics going forward.
Padmanabhan, Thanu, and Vasanthi Padmanabhan. 2019. The Dawn of Science: Glimpses from History for the Curious Mind. Springer.
Stein, Sherman. 1999. Archimedes: What Did He Do Beside Cry Eureka?
Strogatz, Steven. 2019. Infinite Powers: How Calculus Reveals the Secrets of the Universe. Boston New York: Mariner Books.
Wu, Shiyue, and Francesco Perono Cacciafoco. 2024. “Understanding through the Numbers: Number Systems, Their Evolution, and Their Perception among Kula People from Alor Island, Southeastern Indonesia.” Humans 4 (1): 34–49. https://doi.org/10.3390/humans4010003.
K. Sridhar is a theoretical physicist and author currently at Azim Premji University, Bengaluru. Formerly at TIFR Mumbai, his research spans high-energy physics, including extra dimensions and supersymmetry. Sridhar also engages deeply with philosophy, literature, and education. He is the author of Particle Physics of Brane Worlds and Extra Dimensions (Cambridge University Press) and co-editor of Breaking the Silo: Integrated Science Education in India. In this conversation, we discuss his intellectual pursuits, including his recent novel Ajita.
In the 1960s, C. V. Raman wrote a series of papers on floral colors and the physiology of vision. In there, he was very interested in the origin of colors from various different flowers. This was also motivated by his fascination with optics and natural colors in vegetation. Specifically, during that era, he had a large garden at his institution and he was deeply immersed in understanding the origin of the colors from these wonderful living creatures.
By using his knowledge of spectroscopy and the chemistry of pigments, he was able to explore some of the spectral features of the floral colors. The diagrams that you are seeing are illustrations from his paper published in 1963.
As you can observe, these illustrations are beautifully created. I don’t know whether Raman himself drew these pictures, but one should really appreciate the artist who has created them.
In a broader sense, it also indicates two important aspects. The first is that Raman was deeply motivated by natural phenomena. His intuition of optics helped him to understand the origins of a variety of natural optical processes. Spectroscopy was a crucial element in all the things that he did. The second aspect is that, in a deeper sense, aesthetics is interwoven with the pursuit of science and Raman’s work, especially towards the later part of his life, showcased it.
There is a fascinating video conversation with Richard Feynman where he describes the appreciation of the beauty of flowers by a scientist. Raman’s appreciation of beauty is close to what Feynman is describing in the video.
C. V. Raman was a curious person. He had a deep inclination to explore natural phenomena, using the knowledge and tools he had accumulated over several decades. In that sense, he was a scientist driven by curiosity before and after his Nobel prize.
Next time when you see a flower, remember that it is a creature of beauty and science merged together.
“How might optical computers beat electronic computers? …….. There are three main metrics of computing performance for which we might aim to achieve an advantage: latency, throughput and energy efficiency…”
In the immediate future, designing energy-efficient computational platforms will be a necessity. Electronic transport is noisy and dissipative. Optical alternatives can be important, but challenges remain…
Given that the speed of light is the upper limit of information transport and processing, optics will be a vital ingredient in computation. In hindsight, it has already been. But there is more to it than just the speed, as the review article explains elaborately..
VISMAYA - History & Philosophy of Physics 