Category: History

28. Words of Berry

Michael V. Berry is a distinguished theoretical physicist. He has made outstanding contribution towards classical and quantum physics, including optics (Pancharatnam-Berry phase, caustics, etc.). Berry is also a prolific writer and commentator on science and its pursuit. Recently, I came across a foreword published on his webpage, that I think is provocative but worth reading..here is a part of it :

“At a meeting in Bangalore in 1988, marking the birth centenary of the Nobel Laureate C V Raman, I was asked to give several additional lectures in place of overseas speakers who had cancelled. During one of those talks, I suddenly realised that underlying each of them was one or more contributions by Sir George Gabriel Stokes. Understanding divergent series, phenomena involving polarized light, fluid motion, refraction and diffraction by sound and of sound, Stokes theorem (I didn’t know then that he learned it from Kelvin)…the list seemed endless.

My enthusiasm thus ignited, I acquired Stokes’s collected works and explored the vast range and originality of his physics and mathematics (separately and in combination). Paul Dirac was certainly wrong in his uncharacteristically ungenerous assessment (reported by John Polkinghorne), dismissing Stokes as “… a second-rate Lucasian Professor”. On the contrary, in every subject he touched his contributions were definitive, and influenced all who followed. Perhaps Dirac failed to understand, as we do now, that discovering new laws of nature is not the only fundamental science: equally fundamental is discovering and understanding phenomena hidden in the laws we already know…………..”

An important takeaway is that fundamental science can also evolve as a consequence of existing laws applied to new boundary conditions or systems. In an essence, Berry’s comment also resonates with PW Anderson’s argument on emergence, which laid a philosophical foundation and integrated science of condensed matter. Undoubtedly, Stokes made some profound discoveries in physics, and a recent book illustrates his science and life (Berry’s foreword is from the same book).

post script: in the year 2000, Berry shared the IgNobel prize, with Andre Geim, for magnetic levitation of frogs. As you may know, Geim went on to win the Nobel prize in Physics (2010) for his groundbreaking work on graphene.

Will Berry get a Nobel prize in 2020 ? He is certainly a deserving candidate…we will see on 6th Oct…

October 2, 2020
26. Trinocular View of Science
When I come across any book, I do two things : first, I take a glimpse at table of contents, and second, I read the preface/foreword to the book. The second part is generally revealing in its own way, as I get to learn not only about the content of the book, but also about the human side of the topic under study. Recently, I was reading a technical book. In there, I came across a foreword written by Jacques Friedel, in which he quotes his grandfather Georges Friedel, and a part of the quoted text is reproduced below :

…none of the three approaches – the naturalist, the physicist, and the mathematician – should be neglected and that a healthy balance must be preserved amongst them !……

The text in bold is my emphasis. This quote resonates with what I think is a good way of doing science. Let me elaborate a bit on this “trinocular” view of science.

Image courtesy : Pexels – Creative Commons License
- Naturalist: In this approach, one can cater to the curiosity of the self by absorbing and observing nature. In a way, this approach helps you to connect with a phenomenon at a personal level with a touch of imagination of ones own. The feeling of wonder is what plays a critical role to be a naturalist, and a naturalist approach is to take this grasp seriously, and wonder about why nature behaves the way it does. In a way, most of the children are naturalist. Also, this approach, in my view, is one of the fundamental aspect of what makes us human : the ability to wonder and question.
- Physicist (scientist to be more general) : Once you observe a phenomenon or intrigued by a fact, the questions to ask are: why and how such a thing happens? To answer these questions, you need to bring in the existing knowledge of science and look into the problem at hand through this metaphorical lens. You will have to ask to what framework of concepts does your observation belong to, and try to cast your naturalist observation in this light. This helps you to identify the scientific parameters of the problem, i.e., the dependent and independent variables. With this knowledge about parameters, you can not only probe the system under study, but also control it in a systematic way (first step to engineering). Such a control gives us an intellectual platform to construct hierarchical structures, which can further serve as foundation to new phenomena and structures.
- Mathematician : This viewpoint brings in the analytical framework to the observations at hand. From the scientific thought – via hypothesis, experiments and models, we would have obtained some insight into a problem. These building blocks can be further refined and articulated in a precise language, such that we can generalize the problem to a larger set of questions which can go beyond the system under study. This transfer of real to abstract picture is what make mathematics so powerful. It catches the essentials of the problem, and facilitates a framework for generalization, which can be further applied to a new problem.
What I have discussed above is a way (not the only way) to approach research in natural science. Interestingly, the above 3 approaches need not be considered in chronological order. The inspiration to study a natural phenomenon or anything for that matter can be initiated from any of the 3 approaches. A question or an observation in any one framework can be cast as a query in any other framework, and that is what makes pursuit of science so wonderful.

Perhaps, the most important lesson from the Friedel’s quote is to keep a healthy balance of all the three approaches while studying a natural process. Importantly, this triangulation and extrapolation of approaches is how you build knowledge : be it engineering, medicine, public policy or any facet of epistemology. At the heart of all these approaches is to look at a problem from multiple viewpoints and be open to adaptation, criticism, and revision.

After all, depth in view needs more than one cue !
July 12, 2020
Science + Poetry –> Quo Vadis ?

Image: Pixabay (creative common license)

Recently, I read an article titled The Quantum Poet. It is about Amy Catanzano, an academic poet amalgamating poetry with quantum physics. What is impressive is that she is trying to create a platform to communicate emerging trends in quantum world through poetry. She thinks poetry can bring something unique in terms of presentation which may help us understand science in a better way. In her own words she describes the power of poetic presentation :

“Poetry is a nuanced and complex form of language that goes beyond simple dictionary definitions of individual words. Poems use rhythm, visual structure, line breaks, word order, and other devices to explore invisible worlds, alter the flow of time, and depict the otherwise unimaginable”

Attempts to bring science and poetry together is an active effort now, as evidenced by projects such as “The Universe in Verse”, which is an emerging platform where scientist and poets not only exchange ideas but also get together to create something new. An early proponent of this philosophy is the poet Ursula K. Le Guin, who describes beautifully why science and poetry are necessary to understand the world that is overloaded with information :

“Science describes accurately from outside, poetry describes accurately from inside. Science explicates, poetry implicates. Both celebrate what they describe. We need the languages of both science and poetry to save us from merely stockpiling endless “information” that fails to inform our ignorance or our irresponsibility.”

Whereas the above examples show how poets are embracing science, I should mention that scientist too have been active in this endeavor. Roald Hoffmann, the Nobel prize winning chemist is one of the great examples of this.

The combination of science and poetry has interesting connection in ancient Indian tradition too. Specifically, many of the Sanskrit surtras essentially do this as evidenced in some old Indian texts. If you want to know more, I suggest you read this article by Roddam Narasimha. His work, in my opinion, is a reliable source on topics related to science in ancient India. Interestingly, many languages in India do combine poetry with puzzles. One example that immediately comes to my mind is a lyrical puzzle in Kannada by Purandara Dasa called Mullu koneya mele.

A famous essay by C.P. Snow titled “Two Cultures” observed that arts and science, which are two endeavors of human activities, have to come together for a richer intellectual human experience. A lot has been debated on this topic. Perhaps, the above examples show that the two cultures indeed can inspire each other to create something neither of them can create individually. Of course, there is still a lot to achieve in this direction.

Science, arts and sports are three pursuits of human beings which are integral parts of our lives. Personally, I cannot imagine a world devoid of them. Let me conclude with a small poem I wrote sometime ago (this is a modified version that I had posted on facebook) :

Cycles of thought set question into motion,

it pours meaning into life as a cerebral conception.

Fathering an idea: a borrowed perception;

no endeavor is original, everything an inception.

Science, Arts and Sports are facets of inspiration;

after all, what is life without their juxtaposition.

ps : Disheartening to know the passing away of Indian actors Irrfan Khan and Rishi Kapoor. A lot of people are sad… reinforces the importance of art and artists in human society.

April 30, 2020
Raman’s Nephews
28th Feb of every year is celebrated as National Science Day in India. I have previously written about the science behind the National Science Day. This day is associated with the discovery of Raman effect. Raman had a great legacy and influence on Indian science. In addition to being a great scientist, CV Raman encouraged the pursuit of science (with exceptions).

One of his legacies was the impression and influence he had on some close members of his family. Below is a small list of his illustrious nephews who made significant contributions in science.

Raman’s younger sister, Sitalakshmi, had 5 sons.

3 brothers : Pancharatnam, Ramseshan, Chandrasekhar. Image courtesy : Indian Academy of Sciences

Among them 3 were scientists :

Sivaraj Ramseshan (10 October 1923 – 29 December 2003)
- A renowned crystallographer
- played a key role in fostering institutions such as IISc and Indian Academy of science.
- INSA has a nice biographical note on Ramseshan
Sivaramakrishna Chandrasekhar (6 August 1930 – 8 March 2004)
- Made outstanding contributions to the science of liquid crystals
- Was elected as Fellow of Royal Society for his work on liquid crystals
- A biographical note , compiled by Royal Society, is a worthy read.
- He is the other Chandrasekhar :)
Shivaramakrishnan Pancharatnam (1934–1969)
- The Pancharatnam of the fame of Panchratnam-Berry phase
- Made ground-breaking contributions in optics
- Unfortunately, died very young (35 yrs)
- Prof. Rajaram Nityananda has compiled an excellent biographical article on him.
Raman’s brother was C.S. Iyer, He had a son:

Subrahmanyan Chandrasekhar. Image courtesy : University of Chicago

Subrahmanyan Chandrasekhar (19 October 1910 – 21 August 1995)
- Perhaps the most celebrated of the lot
- S. Chandrasekhar was an astrophysicist who went to win the Nobel Prize in Physics
The real impact of science and technology, is not only in the materialistic gains of a society but also in the way it elevates the thought process of a society. Science as a pursuit of human knowledge influences thinking of human beings, and hence plays a vital role in shaping the character and culture of any individual, family, community, country and the world.

We should also remind ourselves that “impact of a scientist” cannot be judged merely by counting the number of papers/patents they publish nor by the high-office they hold in corridors of (scientific and political) power. If anything, such a judgment of impact should be left to the posterity.

On a related note, Kameshwar Wali, physicist and biographer of Subrahmanyan Chandrasekhar writes :

Chandra often quoted from a letter of his friend Edward A Milne during his Cambridge years:

“Posterity, in time will give us our true measure and assign to each of us our due measure and humble place; and in the end it is the judgement of posterity that really matters. He really succeeds who preserves accordingly to his lights, unaffected by fortune, good or bad. And it is well to remember there is no correlation between posterity and the judgement of contemporaries.”

Science zindabad !
February 28, 2020
Importance of a failed experiment

India’s recent mission to reach the moon, Chandrayaan 2, has spurred a lot of interest, and I am glad that it is getting the attention it deserves. As we know, the space vehicle was supposed to land on the south pole of the moon but lost communication with earth just before the touchdown. The initial goal of landing the spacecraft was not achieved as per the expectation. The efforts of the people involved in this mission are indeed laudable. Given the drive, commitment and financial support that ISRO has, I am sure they will achieve greater things in the future.

This event is also a good occasion to talk about the importance of failed experiments in science, and below is my take:

This semester I have been teaching an advanced physics lab course to the 4^th year BS-MS students. There are about 23 students, and we have been performing some experiments on concepts such as Thermionic emission, statistics in radioactive decay, electron spin resonance, Zeeman Effect, etc. As you may guess, all these experiments have deep connection to quantum mechanics, and its manifestation is evidenced in the lab. These experiments are designed such that we can test some hypothesis by formulating them as a question, and the experiments aim to reveal an answer to the posed question. As part of the process, the students explore the basic theory behind the experiment, understand the rationale behind the instrumentation utilized, and perform measurement and analyse the error in them. They are expected to record their observations, and finally submit a report in the form of a small research paper.

Many a times, the experiment that the students perform do not work according to the plan. So they need to troubleshoot the problem, and understand why things are failing. This stage of troubleshooting is where one LEARNS about how to do an experiment. After a careful analysis, they figure out where the problem was, and rectify it to proceed further. This whole process requires attention to details, better understanding of the instruments under use, and importantly a lot of patience. In a way, a lab course, if done in the right spirit, is one of the most fulfilling aspects of science education because it interfaces the abstract knowledge with the real world. So our understanding of the physical world is not only enriched but also we gain some degree of control over it, which is kind of empowering, so to speak.

Now what about experiments in a research lab? Well, the story is even more interesting in this situation. A majority of the times, the experiments that we design in a research lab DOES NOT work. In fact, we will not even know whether the direction we are taking is indeed the most accurate and appropriate one. Therefore, a careful design of experiments guided by hypothesis, and an educated “guesstimation” plays a vital role. Even with all precautions, we may fail to perform the experiments according to the plan. So the question is: how does one react during such a situation?

This is where the training we get in the laboratory courses is very vital. We need to fine tune our thinking to know what it is to do an experiment. Given the high probability of failure, we need to consider every experiment as a path to learn something new. This means that the negative result what we get should be considered as a feedback to our thought process.

With this new information from the failed experiment, there are at least two important prospects: First is that it will improve our understanding about the current situation, and throw some light on corrections that we need incorporate in our experiments. The second and more interesting aspect is that it can lead to a completely new direction of research which we may have otherwise ignored. This emergence of new direction is what makes experimentation very interesting. The new, uncharted path that a failed experiment can take us may result in some major discovery or inventions. History of science has a few examples of experiment with negative results that have led to major breakthroughs (for example Michelson-Morley experiment). A caveat to add is that not all negative experiment may result in a breakthrough. Generally speaking, paying attention to the failure is imperative to learn something new, and the same goes with experimentation. In an essence, true progress in experiments (and science in general) can be achieved only by revising it further. Let me conclude by quoting Peter Medawar (Advice to a Young Scientist (1979), 94):

“All experimentation is criticism. If an experiment does not hold out the possibility of causing one to revise one’s views, it is hard to see why it should be done at all.”

September 8, 2019
Expression as Exploration

“How often I found where I should be going only by setting out for somewhere else.”
R. Buckminster Fuller

About a month ago, I had an opportunity to interact with school students who were on the verge of transitioning from 10^th and 11^th grade. This event was part of a tech-fest organized by College of Engineering, Pune. The topic of discussion was “what scientist does in everyday life?” The students were very communicative (surprise!) and asked many questions (another surprise!), which was heartening. During the interaction, one of the issues we discussed was the importance of note-taking, as part of any serious observation in science, art or any other creative pursuit.

One of the curious questions asked by a student was the following: “If there are so many technological tools that are available to us today, why should we at all write by hand? Why don’t we directly learn typing on a computer instead of handwriting?”

This was an important question, and I did mention that writing by hand has not only the benefit of processing thoughts more effectively, but also provides a sense of creation that may be lost while typing a text. Furthermore, symbolic representation, manipulation and thought processing – as done in mathematical thinking or calligraphy – is more conducive and convenient in the hand written form.

I also pointed out that there is some scientific evidence which indicates that handwritten notes have greater impact on processing the information in our brain, than when the same notes are typed on a device. I told that there is a form of elegance and individuality that a handwritten displays, which may not be represented in a text that is typed. I mentioned that writing in general and handwriting in particular, was not only a form expression but also as form of exploration. I indicated that just like music, writing has a psychological benefit of its own. It helps you to explore your thoughts and creates a sense of connection with oneself. Interestingly, it will also take you on a journey which you may not anticipate. The quote at the beginning of this blog sums it up nicely. Writing is a form of exploration, and by merely writing, we are taken to new worlds which we had not envisaged or planned to go.

In this blog I give 2 examples of a scientist and a writer, who have effectively used handwritten text in their work and have deeply impacted their respective fields. The choice is purely personal, as they are inspirational to me. Here we go….

Marie Curie

Image Credit : India Today

Cutting-edge science in early 1900s, especially in experimental physics and chemistry has had a great impact on modern society. Among the many who thought deeply about the nature of matter, Marie Curie’s contribution stood out. As a dedicated researcher, she not only developed elaborate experimental methods by herself to unveil the secrets of radioactivity, but also silently built a school of thought where dodgy, experimental exploration motivated new questions and directions in natural science. Below text is a snapshot from Marie Curie’s notes which describes the sample preparation in her lab. Interestingly, the mentioned texts of Marie Curie are still radioactive (and kept under isolation), and will remain radioactive for another 1500 year!

Image credit : Wellcome Images

Virginia Woolf

Image Credit : Getty Images

A literary giant who is surely one of the pioneers of modernist thought process, kept a diary for herself all throughout her life. In my opinion she was a great humanist who redefined the art of narrative from a modern perspective. What’s more, her texts are so quotable that anybody who reads them will get a new viewpoint of the world which we had never seen. Below I reproduce a copy of her handwritten page of her famous book “A room of one’s own”. In this text, the story is still in the making, but you can see how a cluttered text at that time has evolved into a masterpiece now.

Image Credit : Cambridge University

Well….preaching without practice is always hollow. When I was interacting with the students regarding handwritten text, they asked me whether I do write by hand. And my answer was yes, and below is a small handwritten note from my own notebook:

Snapshot of text from my notebook

Handwritten text has its own aesthetic value, and I believe it should be retained as long as human expression exists.

Virginia Woolf once famously wrote,

“Thoughts without words… Can that be?”

November 11, 2018
Physics Nobel….via Geetanjali

Every year the Science club at IISER-Pune organizes “Nobel evening – an event that has been a part of IISER since its inception and has thus become tradition”. This is essentially a gathering of IISER community where public lectures are delivered on the latest Nobel prizes.

On 22nd Oct 2018, we had this year’s Nobel evening, and in there I gave a talk on 2018 Nobel Prize in Physics. It was indeed an honour to talk about the work which has created such a deep impact on both science and technology.

While I was preparing for the talk over the weekend (talk was on a monday), I took a break and casually looked into my collection of books. I happened to skim through Tagore’s Geetanjali. As I went through the beautiful verses of this epic poem, what immediately struck me was its 44th verse. I could see an interesting analogy between the 44th verse, and the 2018 Nobel prize in physics. So I decide to use this analogy in closing of my talk. The whole talk had 25 slide. Below I show the 3 slides from my talk : the first one being the opening slide and last two are my closing slides that contain the analogy I mentioned about….

October 22, 2018
A Random Walk in Edinburgh
Diffusion is a simple yet fascinating physical phenomenon. By merely observing how an object moves around in a medium as a function of time, there is a lot of stuff one can learn about the environment, about the diffuser and about the interaction between diffuser and its environment. Over the last few days, I have been studying some papers related to trajectory of individual nanostructures in liquid environment, and have learnt some interesting aspects such as sub-diffusion and super-diffusion.

Concomitantly, I came across one of the better poems I have read in recent times on travelling: Childe Harold’s Pilgrimage by Lord Byron George. This is a long poem, but a couple of stanzas are worth a read:

There is a pleasure in the pathless woods,
There is a rapture on the lonely shore,
There is society where none intrudes,
By the deep Sea, and music in its roar:
I love not Man the less, but Nature more,
From these our interviews, in which I steal
From all I may be, or have been before,
To mingle with the Universe, and feel
What I can ne’er express, yet cannot all conceal.

Roll on, thou deep and dark blue Ocean—roll!
Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin—his control
Stops with the shore;—upon the watery plain
The wrecks are all thy deed, nor doth remain
A shadow of man’s ravage, save his own,
When for a moment, like a drop of rain,
He sinks into thy depths with bubbling groan,
Without a grave, unknelled, uncoffined, and unknown

These two readings (of the paper and the poem) were on the same day, and I felt a connection between diffusion and travelling. It kept lingering on my mind for the next few days, and I felt going deeper and exploring it further. So, I went back to my archived files on my laptop and started exploring some photographs I have taken over the years of travel. During this exploration, I found some of my travelogue related to Scotland when I visited that beautiful country in June 2013. During that trip, I and some of my colleagues were mainly visiting Glasgow University. During the last leg of the trip, we visited the University of Edinburgh and Edinburgh city for a day.

It was a bright, sunny day on 21 June 2013 (generally 21 June is the longest day of the year). We took an early morning train from Glasgow to Edinburgh. Around 9am in the morning, we were at University of Edinburgh, and we visited the departments of physics and chemistry. This was followed by talks by us and a few researchers at the university. After our interaction and lunch, we had about 5 to 6 hours to spend in the city of Edinburgh before we could take our train back to Glasgow. So, we went to the city centre, and I decided to visit the travel information desk. I wanted to know if I could see around the city within 5 hours or so, and what places could I visit on feet. One of my favourite activities, especially when I am travelling alone, is to take a random walk around the city (of course with a map), and explore the places on feet. I have found these “on-feet” explorations can closely connect you to the place, and importantly slows one down so that one can pause, observe and grasp the local environment in its details. In an essence, this “confined Brownian motion” can lead to some interesting insight and thoughts.

Coming back to Edinburgh, I gathered all the information of possible sites I could visit on feet within 4 to 5 hours, and here are a few things I explored during the walk:
1. Statue of Sherlock Holmes:
Although a fiction-character, Sherlock Holmes has a real statue in Edinburgh! Anybody who has read Sherlock Holmes also knows its author Aurthur Conan Doyle cannot miss this place. Close by to the Holmes’ statue is a pub named The Conan Doyle (see below)
1. Bronze statue of Adam Smith:
A 10 feet long monument of Adam Smith cannot be ignored. The celebrated economist, philosopher and author of “The Wealth of Nation”, is one of jewels in the crown of Scotland.
1. Statue of James Clerk Maxwell:
Interestingly, this was the hardest thing to find in the city. It was at a remote corner of the town, and very few people knew that there is indeed a statue of Maxwell in Edinburgh city. It took me almost 45 to 60 minutes to explore the statue. I was almost about to give up, but somehow I did not want to….so I went ahead, and found this statue of the celebrated physicist. It was a happy moment!
1. The famous Scotch Wishky trail:
How could one miss this! This was one of the easiest things to find on my path, and what I found inside this trail was nothing short of breath-taking variety of Scotch.
1. The cliff edge:
This was strictly-speaking not during the walk in the city, but just before that, and is perhaps the picture that has stuck in my mind all the while when I think about Scotland. Unconsciously, when I think of diffusion in space and time, this is the same picture that comes back to my mind. There is something unique about a person at cliff edge, all by himself exploring his universe…I find it kind of philosophical and fascinating…..and also goes well with abovementioned poem.

Brain is a strange thing. It forces us to connect the unconnected, and the above content is just an example. Towards the end of the trip, I sat on the train back to Glasgow. I started listening to the music on my headphone, and the song on my playlist was Hotel California. As I relaxed back in the seat, I was struck by these lyrics :

Last thing I remember, I was
Running for the door
I had to find the passage back to the place I was before
‘Relax’ said the night man,
‘We are programmed to receive.
You can check out any time you like,
But you can never leave!’

I had checked out of Edinburgh, but my mind has never left that random walk…..
September 23, 2018
My Metaphoric Oxygen
There is no Frigate like a Book
To take us Lands away
Nor any Coursers like a Page
Of prancing Poetry –
This Traverse may the poorest take
Without oppress of Toll –
How frugal is the Chariot
That bears the Human Soul –

BY EMILY DICKINSON

Generally speaking, scientists are natural philosophers: they observe nature, ask questions, hypothesize an answer, test them through experiments and extend this exploration by escaping into the universe of ideas in books and journals. New ideas emerge from this exploration and join the chorus, and the intellectual journey continues. In my own research on light scattering, I have been deeply influenced by ideas of various fellow-explorers. For me, journal papers and books encompass the “metaphorical oxygen” for creativity and knowledge. Below I introduce you to some classic books which keep my research alive.
1. Absorption and Scattering of Light by Small Particles
  - Author(s): Craig F. Bohren and Donald R. Huffman
    
    Comments: There are two kinds of authors who write textbooks. One is the ‘boring kind’ and the other is the ‘Bohren kind’. If you want to fall in love with light scattering (and science in general), read books and articles by Craig Bohren. It will not only deeply influence your thinking, but also will show how a textbook can, and should, evolve a subject systematically. This particular classic has some of the most important ideas related to how light behaves when it interacts with matter comparable to the wavelength of light, and forms the bedrock on which a lot of contemporary research, including nanophotonics and plasmonics, is pursued. This book has wit, humour and a touch of poetry jumbled up together as flowing river of knowledge. To give you a spirit of their writings, let me reproduce the first paragraph of their introduction
1. Light Scatteing by Small Particles
  - Author(s): H.C. van de Hulst
    
    Comments: The first edition of this book was published in 1957, by the author was a legendary astronomer. This book has a beautiful description of single and multiple-scattering phenomenon, and describes specific situations where they apply. Written with an astrophysical viewpoint, it elegantly combines depth and breadth in a lucid way. This book has perhaps served as inspiration to most of the books written on light scattering.
1. The scattering of light and other electromagnetic radiation
  - Author(s): Milton Kerker
  - Comments: Some researchers have remarkable ability to choose problems that have far reaching consequences beyond the next research paper. Milton Kerker was one such legend. His research papers and this book has not only influenced the way physics of light scattering is studied, but has had deep impact on utilization of light scattering in various branches of science and technology. This 600 odd page book is indeed a masterpiece, and in a unique way caters to almost all kinds of researchers who are interested in light scattering.
2. Dynamic Light Scattering with applications to chemistry, biology and physics
  - Author(s): Bruce J. Berne and Robert Pecora
    
    Comments: A majority of the matter in biology and chemistry are suspended in a fluid. When an object in a medium undergoes Brownian motion, it influences the way a light beam scatters and traverses through that medium. This book explain the how and why of this fascinating topic. Written by experts in chemical physics, this classic serves as the foundation for light scattering in soft-condensed matter physics.
1. Molecular Light Scattering and Optical Activity
  - Author(s): Laurence Barron
    
    Comments: Historically, light scattering by molecules has been studied by legends such as Rayleigh, Raman and many more. Interestingly, all these legends emphasized the connection between polarization of scattered light and structure of matter. In this book, Barron puts together these ideas in a very elegant way, and motivates and develops the phenomenon of optical activity from a molecular physics viewpoint. Given that a majority of biomolecules are chiral in nature, the insight that one obtains by reading this book has direct implication in understanding the structure and dynamics of biomolecules such as amino acids, proteins and DNA.
1. Scattering, Absorption, and Emission of Light by Small Particles
  - Author(s): MI Mishchenko, LD Travis, AA Lacis
    
    Comments: Mischchenko is a scientist at NASA, and his books on light scattering have had great influence in aerosol science, radar technology and many more. The T-matrix codes based on this book forms a very important tool across the research community that works on weather prediction and pollution monitoring.
1. Wave Propagation and Scattering in Random Media (Vol 1 and 2)
  - Author(s): Akira Ishimaru
    
    Comments: This classic from late 1970s was one of the elaborate attempts to put together wave propagation and scattering in a random media on a rigorous mathematical foundation. This 2 volume book has solutions to various mathematical problems that one encounters in light scattering physics, and makes an important connection to transport theory of light in a medium.
1. Optical Scattering Measurement and Analysis
  - Author(s): John C. Stover
    
    Comments: If you are interested in experimental aspect of light scattering, this is one of the best books. It is essentially a field guide, which tells you how to quantitatively make a light scattering measurement, and what aspects to look-out for. This is a very good book for students who want to get a hands-on experience in light scattering.
1. LASER LIGHT SCATTERING, Basic Principles and Practice
  - Author(s): Benjamin Chu
    
    Comments: Chu’s book develops the topic of laser light scattering in terms of both experimental aspect and theoretical foundations. Importantly, it connects the topics of light scattering to optical spectroscopy, and shows how one can obtain meaningful information about light-matter interaction.
1. Mesoscopic Physics of Electrons and Photons
  - Author(s): E. Akkermans and G. Montambaux
    
    Comments: Quantum mechanical entities such as electrons and photons can be confined in space and time. Depending on the geometry of confinement, very interesting physics such as weak and strong localization can emerge. This book looks at the physics of confined electron and photon from a unified viewpoint. It highlights similarities and difference between the electrons (fermions) and photons (bosons).
1. The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules
  - Author(s): Derek A. Long
    
    Comments: Written by a pioneer in the field, this book till date remains the most rigorous treatment on Raman scattering of light from a theoretical viewpoint. Based on quantum mechanical arguments, this book relies on perturbation theory, and clearly shows the connection between structure of molecules and how they influence the scattered light.
1. Principles of Surface Enhanced Raman Spectroscopy and Other Plasmonic Effect
  - Author(s): Eric C Le Ru and Pablo G. Etchegoin
    
    Comments: The most definitive book written on surface enhanced Raman scattering by two physicists whom I greatly admire. This book gives unified treatment of plasmonics and surface enhanced inelastic light scattering, and is written in a style catering to physics audience. The book has a lot of details and explanations, and also serves as excellent introduction to plasmonics and vibrational spectroscopy. Given that the authors themselves are pioneers in single-molecule Raman scattering, their insight into single molecule optics in plasmonic field is fascinating. Unfortunately, Etchegoin succumbed to cancer, and I could never meet him. However his great ideas and thoughts stay on…
1. Introduction to Wave Scattering, Localization and Mesoscopic Phenomena
  - Author(s): Ping Sheng
    
    Comments: Random lasing is an emerging topic of research in nanophotonics. The fact that one can have random structures assembled in space and time, and yet achieve spatial and temporal coherence is quite remarkable. This book brings together insights from wave scattering and mesoscopic physics to show how light behaves when confined to small volumes compared to wavelength of light. The insights obtained from this book are heavily used in the literature on random lasers.
1. Fundamentals of Atmospheric Radiation
  - Author(s): Craig F. Bohren and Eugene E. Clothiaux
    
    Comments: Bohren weaves his magic…..again. Although the title of this book indicates atmospheric radiation, the way the authors treat the topic of absorption, emission and scattering of light is fascinating. This book gives a broad viewpoint of interaction of light with matter, and shows one can and should treat the subject coherently. The references and problems are very relevant and interesting, and I have found some gems while reading through this text.
August 26, 2018

Trapping Questions and Evolving Answers

A voice said, Look me in the stars
And tell me truly, men of earth,
If all the soul-and-body scars
Were not too much to pay for birth.

—- “A Question” By Robert Frost

In research, as in life, humble questions can sometimes lead to profound answers. A curious question flying as a passing thought in the mind of a researcher can equally lead to some important discoveries and inventions. Furthermore, what starts as a simple question, evolves into a creature that the questioners themselves would have not envisaged. This evolution of thought in various directions is fascinating to say the least, and history of science is dotted with such examples.

Take for example Arthur Ashkin of Bell Labs, who in late 1960s, asked the following question:

“is it possible to observe significant motion of small particles using the forces of radiation pressure from laser light?”

Note- at that point of time, lasers were still a relatively new invention, and people were looking for an application. In that context, it was indeed an interesting question to ask about the effect of laser beam on a small particle which may be immersed in fluid or in vacuum. After all, radiation pressure should have some effect on the motion of particles, as evidenced in the case of comet tails.

With this question, Ashkin embarked on a journey that conceptually and literally pushed and revolutionized a large part of our science and technology based on lasers. Ashkin’s question led to the realization of laser-based optical trap of microscopic objects, which further evolved into a major experimental tool not only in physics but also in biology and chemistry.

Below figure shows the conceptual schematic of Ashkin’s experiment, in which he introduced two counter-propagating laser beam which created an optical potential to stably trap an object in space and time. The physics of optical trapping itself in intriguing, in which, the compelling battle between forces due to in-line pushing and orthogonal pulling will be eventually won by the pulling component. A stable energy minimum is achieved at the center of the focused laser beam, in which the object of interest happily resides. Of course, parameters such as refractive index of the object and the medium play a critical role, so does the alignment of laser beam and its wavelength.

trap Ash — Optical schematic of the first optical trap created by Arthur Ashkin. Adapted from the original paper [1]*.

There are two important aspects to Ashkin’s work. One is that he pursued on a simple question that lead to an important observation, which has had far researching consequences not only in physics but also in biology and allied research areas, and the second point is that a few people, in his own lab felt that the discovery was not important. In the first chapter of his book, Ashkin describes a very interesting situation after he had performed this seminal work:

It may be interesting and instructive to recall the initial reactions of other scientists to paper [1]*, which described the earliest trapping work. At Bell Labs., before a manuscript could be sent out to a journal it had to undergo an internal review to make sure it would not tarnish the laboratory’s excellent reputation in research. Since paper [1]*was intended for Physical Review Letters, it was sent to the theoretical physics department for comment. The Bell Labs, internal reviewer made only four points: (i) there was no new physics here, (ii) the reviewer could not actually find anything wrong with the work (this is a reminiscent of the famous Pauli insult, when he commented on some work he thought worthless that “it is not even wrong!”), (iii) the work could probably be published somewhere, and (iv) but not in Phys. Rev. Lett.This four-point internal referee report from the theoretical group greatly distressed me, and so I went to my boss, Rudi Kompfner, inventor of the traveling wave tube, whom I greatly admired. Rudi, a man usually slow to anger, simply said, “Hell, just send it in!” As it turned out, I had no problem whatever with the Physical Review Letters reviewers. In 1999, paper [1]* had the honor of being selected as one of the 23 seminal papers on atomic physics reprinted in the compilation, “The Physical Review — The First Hundred Years”, edited by Henry Stroke, American Institute of Physics Press and Springer Verlag (1999) on the occasion of the centennial of the American Physical Society.

There are at least two important lessons in this story: a) not always one can instantaneously judge the importance of a research work and b) the notion of “new physics” depends on how you look at a topic and judge its implication. To see how a new result can connect to something else requires a kind of broad view of science well beyond the boundaries of the “known unknowns”.

Going further, Ashkin did not stop his train of questions. He writes that he was intrigued by the observations which further motivated him to explore on the following topics:

Could traps be observed for macroscopic particles in other media such as air or even in a vacuum? Could optical manipulation be used as a practical tool for studying light scattering, for example, and other properties of small macroscopic particles?

Evolution of Ideas

After some resistance, slowly the physics community started taking notice of Ashkin’s experiments, and paid more attention towards the simple yet powerful methods he was developing. What followed was indeed a revolution. The methods he developed immediately caught the attention of two very diverse research communities – one was of atomic physicists and other one was of biologists. Whereas the former were interested in trapping and cooling atoms, the later were in desperate search for non-invasive optical tools that could trap and manipulate cellular and sub-cellular objects. Optical trapping indeed catered enormously towards these research efforts. It not only led to “new and interesting physics”, but also some wonderful experiments in soft-matter and biological sciences. In order to give you a gist of the way Ashkin’s work evolved, below I give a table of interesting research results. As you will see, the papers themselves discuss topics and problems that were not envisaged by Ashkin, but the influence of his ideas percolated deep and wide.

Year	Link to the relevant papers and my comments
1982	Electromagnetic mirrors for neutral atomsThis paper theoretically proposed use of evanescent optical fields at dielectric-vaccum interface to reflect neutral atoms. The concept of radiation pressure at an interface was emphasized.
1986	· Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure· Experimental observation of optically trapped atoms These were the foundational experiments on laser cooling and trapping of atoms, which went on to win the 1997 Nobel Prize in physics. Note that Ashkin missed out on the prize!
1989	Optical Binding This introduced a fascinating concept of binding microscopic objects with long range optical forces facilitated by electromagnetic fields. This topic is still of great interest, and still inspires a variety of experiments.
1992	Movement of micrometer-sized particles in the evanescent field of a laser beam This paper was a pioneering contribution towards movement of particles in fluids using an evanescent wave of laser beam.
1993	Direct observation of kinesin stepping by optical trapping interferometry The abstract of this paper is worth a read and tells a compelling story :“Do biological motors move with regular steps? To address this question, we constructed instrumentation with the spatial and temporal sensitivity to resolve movement on a molecular scale. We deposited silica beads carrying single molecules of the motor protein kinesin on microtubules using optical tweezers and analysed their motion under controlled loads by interferometry. We find that kinesin moves with 8-nm steps.”
1996	Optical vortex trapping of particles This was one of the first experiments to use vortex beams to trap objects. In conclusion of the paper, the authors envisage trapping application based on holograms, which were created soon after the proposal.
1997	Theory of nanometric tweezerA first significant jump towards extrapolating optical trapping to sub-wavelength scales. The idea of utilizing a metal nano-tip to trap dielectric objects was proposed. This paper laid an excellent foundation for optical manipulation at nanometer scale.
1998	Optical tweezer arrays and optical substrates created with diffractive optics This literally added new dimensions to optical trapping, where a diffractive optical element, a static hologram in this case, was introduced in the optical scheme. This laid the foundation towards parallel trapping on conventional set-up, and has turned out to be extremely useful for applications in soft-matter physics and biological applications.
2001	Force of surface plasmon-coupled evanescent fields on Mie particles This theoretical paper compares how evanescently-excited surface plasmon polaritons at metal-dielectric interface can exert more force on Mie particle compared to a dielectric-dielectric interface, thus creating a platform for film-based plasmonic manipulation of micro-objects.
2006	Surface Plasmon Radiation Forces This was the first report that experimentally showed how surface plasmon from a metal interface exerted about 40 times more force on a micron sized particles compared to a dielectric interface. Importantly, this paper measure the trapping potential depths created by surface plasmon on a metal-film.
2007	Parallel and selective trapping in a patterned plasmonic landscape This was perhaps THE BREAKTHROUGH experiment in plasmonic trapping, that showed how gold nano-disc could create parallel traps of micron scale object at significantly lower power compared to optical trapping. A majority of plasmon trapping experiments nowadays derive their inspiration from this paper
2009	Self-induced back-action optical trapping of dielectric nanoparticles This experimental paper is one of the first reports which harness the feedback from the trapped 50 nm object to improve the performance of the trap. This significantly reduces the power of laser one needs to use for trapping experiments and represents truly a nanometric optical trap.
2010	Laser Printing Single Gold NanoparticlesOptical trapping forces are harnessed to printing individual gold nanoparticles on glass substrates. This has opened up new opportunities to directly fabricate nanostructure from colloidal phase onto a surface of interest.
2012	Subkelvin Parametric Feedback Cooling of a Laser-Trapped Nanoparticle To quote the authors “Using a single laser beam for both trapping and cooling we demonstrate a temperature compression ratio of four orders of magnitude”. This opens a new avenue to perform optical tests of quantum mechanics using isolated nanoparticles.
2014	Plasmofluidic Single-Molecule Surface Enhanced Raman Scattering from Dynamic Assembly of Plasmonic NanoparticlesThis is an experiment report from my group where we showed that one could not only create a large scale plasmonic trap of multiple nanoparticles, but also one can utilize it to perform single-molecule spectroscopy.
2016	Direct Measurement of Photon Recoil from a Levitated Nanoparticle Another experimental breakthrough where the photon recoil from a single nanoparticle is measured.
2018	Opto-thermoelectric nanotweezers This experimental paper shows how optical, thermo-plasmonic and electric fields can be combined to trap and manipulate nano-object in fluids.

To conclude I will again quote Ashkin, who makes an important observation in an editorial he wrote on the occasion of commemorating 50 years after the discovery of laser:

As we look to the future, what can we anticipate? Certainly much more of the present hot fields such as: single atom studies; properties and behavior of single biological molecules such as mechanoenzymes and nucleic acids; mechanical properties of single molecules and tissue; studies of particle arrays; and particle separation schemes. Of course, we cannot anticipate serendipitous discoveries. We can only hope to recognize them when they occur.

After all, one question leads to another….and the rest is evolution…you see!

August 12, 2018