Category: philosophy

Quantum Optics course – thoughts and notes

Jan 2026 – Apr 2026 – I am teaching a course on Quantum Optics. Below you will find some random thoughts and notes related to my reading. I will be updating the list as I go along the semester. You can add your comments below.

  1. Anyone interested in physics should know a bit about renormalized QED and the efforts that went behind it… It still remains a benchmark of how experiments and theory work in elevating each other…
    • Hari Dass (erstwhile, IMSc) on FB made an interesting observation:it’s unfortunate that after all those and subsequent developments, a mystery is being built out of renormalisation..it was the price to pay for assuming, without any justification, that the microscopic description held to arbitrarily small distances..wilson,schwinger and even feynman have clarified that the right way to do physics is to start with an effective description with a cutoff, which can be fully quantum in nature, and keep extending it to higher and higher scales with the help of further data, as well as with better theoretical understanding..
  2. “The photon is the only particle that was known as a field before it was detected as a particle.” 
    • This is how Weinberg introduces the birth of quantum field theory. He further adds:  “Thus it is natural that the formalism of quantum field theory should have been developed in the first instance in connection with radiation and only later applied to other particles and fields.”Ref: S. Weinberg (in Quantum Theory of Fields, p.15,  1995)
      • Sudipta Sarkar (IIT G) made an interesting observation in facebook:
        • “In some sense, it did right! Dirac started QFT with the effort to quantise radiation! But formally, it is not easy to write down the quantum version of electrodynamics owing to gauge symmetry. It took quite a bit of time to understand how to manage a quantum theory with massless states!“
        • My reply: “indeed..the reconciliation of symmetry was a bottleneck. I am also amazed by the progress of thought, especially by Dirac, who took the harmonic oscillator problem and treated it the way he did. Historically, the question of quantization of particles was already an established programme, but to quantize the field was indeed a major challenge, and hence ‘second quantization’.“
        • The concept of creation and annihilation operators is an intriguing one because it brings in the thoughts from the commutation relationship that existed in classical physics and transfers that into quantum mechanics. This intellectual connection is mainly attributed to Dirac, and historically, this has been one of the most important connections to be made. The question of field quantization already existed in 1920s, but it is thanks to Dirac who really made this connection in a systematic and mathematically consistent way.
  3. In the context of the quantum harmonic oscillator model of electromagnetic radiation, the shift from canonical variables such as position and momentum to creation and annihilation operators is a fascinating one. Interestingly, this progression further leads to the so-called number operator. It is also a progression from Hermitian to non-Hermitian and again back to a Hermitian operator. In the process of understanding the number operators, one realizes that the ground-state results in the so-called zero-point energy. Taken further, the commutation of the number operator with the electric field of the electromagnetic radiation results in the number-amplitude uncertainty. This further gives an insight into why the field amplitude has a non-zero spread, even for the n = 0 state, and therefore results in the so-called vacuum fluctuations.
    • It can’t get more quantum than this…
  4. An essay on Quantum States in Argand Diagrams: https://historyofscience.in/2026/02/03/quantum-states-in-argand-diagrams-vacuum-coherent-and-squeezed/

On Criticism

How to criticize somebody’s work? This is a question we often ask in academia, especially while writing referee reports for articles in journals and theses submitted by students. It is important to learn constructive criticism of academic work, which makes criticism a tool that can lead to positive feedback. When we talk about positive feedback, it does not mean that you will have to applaud the work. It means that anybody who is receiving the feedback should be able to build on it and improve their work.

In this regard, the philosopher Daniel Dennett has come up with some thoughts on critical commentary of somebody’s work. One of the key points he notes: ‘You should attempt to re-express your target’s position so clearly, vividly, and fairly that your target says, “Thanks, I wish I’d thought of putting it that way”’.

This way of changing the perspective on a piece of work is one of the crucial aspects of constructive criticism. It helps you to understand the role as a reviewer in not only correcting somebody’s mistake but also helping them to build on their own thoughts. Many times, criticism is looked down on as a negative thing. But done this way, it is probably one of the most enriching processes, not only for the person who is receiving the feedback, but also for the person who is criticizing the work. The correct way to criticize is to think with different perspectives and add to the body of knowledge that the author has already presented. In that way, knowledge is progressed and corrected for mistakes, if any.

Teaching & Meaning

What adds meaning to my academic work?

Perhaps, an anonymous feedback on your teaching is one of them….

“very well taught course at a well defined pace. The interesting way various different aspects and fields in Optics was introduced was fascinating, made us so very keen on knowing more! The mind maps at the beginning of every topic, the indexes professor made was a great way to keep the bigger picture in mind and helped us glide through it. The assignment was also a great way to make us go through materials without feeling it it be imposing, rather finding it more interesting! Thank you so much Sir for this amazing course, the enthusiastic way in which you taught, all the great conversations you engaged in with us, and opened our eyes to explore so much more in this field! thank you!!“

I had a diverse class (BS-Physics majors, MS Quantum Tech, iPhD) with 110+ students, and I am glad a lot of students enjoyed the course this time.
I am a bit overwhelmed by the positive feedback I received on my teaching methods. For sure, I learnt about the subject as much as they did.

And as I always say: there is more to learn…for all of us..

Human interaction zindabad :-)

Create to Understand

Below are two quotes on the blackboard of Feynman’s office in Caltech which were found just after his death.

 
The first of these quotes by Feynman is a guiding principle for anyone who wants to learn. The second quote is an idealistic one, but a good approach to becoming a ‘problem-solving’ researcher. Feynman was a master of this approach.
 
From a philosophy of science perspective, researchers can be both ‘problem creators’ and ‘problem solvers’. The latter ones are usually famous.
 
Michael Nielsen, a pioneer of quantum computing and champion of open science movement, has an essay titled: Principles of Effective Research, in which he explicitly identifies these two categories of researchers, and mentions that “they’re not really disjoint or exclusive styles of working, but rather idealizations which are useful ways of thinking about how people go about creative work.”.
 
He defines problem solvers as those “who works intensively on well-posed technical problems, often problems known (and sometimes well-known) to the entire research community in which they work.” Interesting, he connects this to sociology of researchers, and mentions that they “often attach great social cache to the level of difficulty of the problem they solve.”
 
On the other hand, problem creators, as Nielsen indicates, “ask an interesting new question, or pose an old problem in a new way, or demonstrate a simple but fruitful connection that no-one previously realized existed.”
 
He acknowledges that such bifurcation of researchers is an idealization, but a good model to “clarify our thinking about the creative process.”
 
Central to both of these processes is the problem itself, and what is a good research problem depends both on the taste of an individual and the consensus of a research community. This is one of the main reasons why researchers emphasize defining a problem so much. A counterintuitive aspect of the definition of the problem is that one does not know how good the ‘question’ is until one tries to answer and communicate it to others. This means feedback plays an important role in pursuing the problem further, and this aptly circles back to Feynman’s quote: “What I cannot create, I do not understand”.
 
 
 
 
 
 
 

Physics Ideas for Entrepreneurs

Starting a new (ad)venture

A YouTube channel dedicated to discussing physics ideas for entrepreneurs

I bring ideas from an ocean of physics and present them to anyone interested in using them for business and entrepreneurship. These are not physics lectures, but discussions on ideas with a perspective of economic utility.

As with all my ventures, it is open source.

Join me in this journey, and please share and subscribe

The first video is out:

Raman essay and Open-Access

I see that the essay I wrote on CV Raman and made open access (thanks to Resonance, which published it) has been used by several educators on YouTube, including some in Indian languages. Also, the Google AI overview shows the published essay as the main reference for a search related to Raman’s science communication (see slideshow below).

I am glad to see that making one’s writing open to all has positive effects. Open-access, not just for readers, but also for authors, is beneficial. Especially in India, paywalls for science are a detriment.

My worry is that open-access publishing has been mainly driven by commercial publishers that extract huge funds from the publishing authors. This defeats the purpose of open science, especially when the research of an author is publicly funded. Added to that, Indian researchers and writers cannot afford to pay huge sums for publishing articles and books.

The publication landscape (including journals and books) across the world needs an introspection. Open-access model is effective only when the readers and authors have access to that model. Otherwise, the model becomes a paywall for authors.

Quantum Optics – teaching in Jan 2026

More than 22 years ago, I started my journey as a research student in theoretical physics – Quantum Electrodynamics (QED) + Radiative Transfer (MSc summer project at the Indian Institute of Astrophysics), and my special paper in the MSc final semester was QED. Later in my PhD, I branched into experiments on light scattering (Raman, Mie & Rayleigh).

Over the years, QED and quantum optics have always been at the back of my mind while studying, researching and teaching.

Come January, I will be teaching a course on Quantum Optics to MS(Quantum Tech), MS-PhDs, and 4th-year physics UGs

I designed the first course on this topic at IISER Pune about a decade ago with the able inputs from Prof. Rajaram Nityananda, and I have taught the course a few times. Now, after a few years, I will teach it again.

With the emergence of quantum sci & tech, there is a new impetus and excitement on this topic.

Having said that, the foundations of the topic remain the same, and Quantum Optics has a wonderful history and philosophy associated with it…and where better to start than Dirac’s classic (see below).

Look out for ‘quantum blogs’ in 2026…

Humanizing Science – A Conversation with a Student

Recently, I was talking to a college student who had read some of my blogs. He was interested in knowing what it means to humanize science. I told him that there are at least three aspects to it.

First is to bring out the wonder and curiosity in a human being in the pursuit of science. The second was to emphasize human qualities such as compassion, effort, mistakes, wrong directions, greed, competition and humour in the pursuit of science. The third thing was to bring out the utilitarian perspective.

The student was able to understand the first two points but wondered why utility was important in the pursuit of humanizing science. I mentioned that the origins of curiosity and various human tendencies can also be intertwined with the ability to use ideas. Some of the great discoveries and inventions, including those in the so-called “pure science” categories, have happened in the process of addressing a question that had its origin in some form of an application.

Some of the remarkable ideas in science have emerged in the process of applying another idea. Two great examples came into my mind: the invention of LASERs, and pasteurization.

I mentioned that economics has had a major role in influencing human ideas – directly or indirectly. As we conversed, I told the student that there is sometimes a tendency among young people who are motivated to do science to look down upon ideas that may have application and utility. I said that this needs a change in the mindset, and one way to do so is to study the history, philosophy and economics of science. I said that there are umpteen examples in history where applications have led to great ideas, both experimental and theoretical in nature, including mathematics.

Further, the student asked me for a few references, and I suggested a few sources. Specifically, I quoted to him what Einstein had said:

 “….So many people today—and even professional scientists—seem to me like someone who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is—in my opinion—the mark of distinction between a mere artisan or specialist and a real seeker after truth..”

The student was pleasantly surprised and asked me how this is connected to economics. I mentioned that physicists like Marie Curie, Einstein and Feynman did think of applications and referred to the famous lecture by Feynman titled “There is Plenty of Room at the Bottom” (1959).

To give a gist of his thinking, I showed what Feynman had to say on miniaturization:

“There may even be an economic point to this business of making things very small. Let me remind you of some of the problems of computing machines. In computers we have to store an enormous amount of information. The kind of writing that I was mentioning before, in which I had everything down as a distribution of metal, is permanent. Much more interesting to a computer is a way of writing, erasing, and writing something else. (This is usually because we don’t want to waste the material on which we have just written. Yet if we could write it in a very small space, it wouldn’t make any difference; it could just be thrown away after it was read. It doesn’t cost very much for the material).”

I mentioned that this line of thinking on minaturization is now a major area of physics and has reached the quantum limit. The student was excited and left after noting the references.

On reflecting on the conversation, now I think that there is plenty of room to humanize science.

Why is astronomy interesting? Chandra likes Wigner’s answer

The questions “Why is astronomy interesting; and what is the case for astronomy?” have intrigued me; I have often discussed these questions with my friends and associates. Granted that physical science, as a whole, is worth pursuing, the question is what the particular case for astronomy is? My own answer has been this: Physical science deals with the entire range of natural phenomena; and nature exhibits different patterns at different levels; and the patterns of the largest scales are those of astronomy. (Thus Jeans’ criterion of gravitational instability is something which we cannot experience except when the scale is astronomical.) Of the many other answers to my questions, I find the following of Wigner most profound: “The study of laboratory physics can only tell us what the basic laws of nature are; only astronomy can tell us what the initial conditions for those laws are.”

from A Scientific Autobiography: S. Chandrasekhar (2011) by edited by Kameshwar C. Wali