Author Archives: Anshul Kogar

List of actions

In the post before the previous one, I advocated for faculty and personnel in physics departments to reflect the make-up of the populace. Here is an excellent concrete list by Professor of Chemistry Kensha Marie Clark of the University of Memphis of actions that your department can undertake (click on the tweet for the whole list or on the unrolled tweet at the link). A lot of this list applies to other underrepresented folks as well:

Meissner effect as amplified atomic diamagnetism

As you can probably tell from my previous post, I have found the recent activism inspiring and genuinely hopeful of it translating into some form of justice and meaningful action. At the end of this post I share a few videos that I found particularly poignant.

It’s hard to imagine the history of condensed matter physics without both the discovery and theory of superconductivity. Superconductivity has played and continues to play an outsized role in our field, and it is quite easy to understand why this is the case. All one has to do is to imagine what our world would look like with room temperature superconductivity. Besides the potential technological implications, it has also garnered attention because of the wealth of stunning effects associated with it. A few examples include the Josephson effect, flux quantization, persistent superconducting currents, vortex lattices and the Meissner effect.

Now, these effects occur for various reasons, but there are a couple of them that can be viewed to some extent as a microscopic effect on a macroscopic scale. To show what I mean by that, I am going to focus on the Meissner effect and talk about how we can view it as an amplification of atomic diamagnetism. One could also extend the this microscopic to macroscopic amplification picture to the relationship between a Josephson junction in a superconducting ring and the Aharonov-Bohm effect, but I’ll leave that discussion to another day.

To understand what I mean by amplification, let’s first look at atomic diamagnetism. Here we can use a similar logic that led to the Bohr model of the atom. Two conditions are important here — (i) the de Broglie relation \lambda = h/p and (ii) the Bohr quantization condition n\lambda = 2\pi r which states that only integer wavelengths are allowed in a closed loop (such as an atomic orbit). See the image below for a simple picture (click the image for the source).

We can use the classical relation for the momentum p=mv in addition to equations (i) and (ii) above to get mvr = n\hbar, which is what Bohr got in his atomic model. It’s worth noting here that when the atom is in its ground state (i.e. n=0), there is no “atomic current”, meaning that j = ev = 0. Without this current, however, it is not possible to have a diamagnetic response.

So how do we understand atomic diamagnetism? To do so, we need to incorporate the applied field into the deBroglie relation by using the canonical momentum. By making the “Peierls substitution”, we can write that p = mv+eA. Using the same logic as above, our quantization condition is now mvr = n\hbar - eAr. Now, however, something has changed; we do get a non-zero current in the ground state (i.e. j = ev = -e^2A/m for n=0). Qualitatively, this current circulates to screen out the field that is trying to “mess up” the integer-number-of-wavelengths-around-the-loop condition. Note also that we have a response that is strictly quantum mechanical in nature; the current is responding to the vector potential. (I realize that the relation is not strictly gauge invariant, but it makes sense in the “Coulomb gauge”, i.e. when \nabla\cdot A=0 or when the vector potential is strictly transverse). In some sense, we already knew that our answer must look obviously quantum mechanical because of the Bohr-van Leeuwen theorem.

If we examine the equation for the electromagnetic response to a superconductor, i.e. the London equation, we obtain a similar equation j = n_sev = -n_se^2A/m, where n_s is the superfluid density. The resemblance between the two equations is far from superficial. It is this London equation which allows us to understand the origin of the Meissner effect and the associated spectacular diamagnetism. Roughly speaking then, we can understand the Meissner effect as an amplification of an atomic effect that results in a non-zero ground state “screening” current.

I would also like to add that the Meissner effect is also visible in a multiply connected geometry (see below). This time, the magnetic field (for sufficiently small magnetic fields) is forbidden from going through the center of the ring.

What is particularly illuminating about this ring geometry is that you don’t have to have a magnetic field like in the image above. In fact, it is totally possible to have a superconducting ring under so-called Aharonov-Bohm conditions, where a solenoid passes through the center but the ring never sees the magnetic field. Instead, the superconducting ring “feels the vector potential”. In some sense, this latter experiment emphasizes the equation above where the current really responds (in a gauge-invariant way) to a vector potential and not just the magnetic field.

Understanding the Meissner effect in this way helps us divorce the Meissner effect from the at-first-sight similar effect of persistent currents in a superconducting ring. In the Meissner effect, as soon as the magnetic field is turned off, the current dies and goes back to zero. This is because through this entire process, the superconductor remains in its ground state. Only when the superconductor is excited to higher states (i.e. n=1,2,3…) does the current persist in a metastable fashion for a quasi-infinitely long time.

To me, understanding the Meissner effect in this way, which exposes the connection of the microscopic to the macroscopic, harks back to an old post I made about Frank Wilczek’s concept of upward inheritence. The Meissner effect somehow seems clearer through his lens.

Now as promised, here are the couple videos (if the videos don’t play, click on the panel to take you to the twitter website because these videos are worth watching!):


I am not Black. I am not American. I do not understand the many nuances of American and African-American culture. I do not understand the extra struggle African-American people have to go through each day. But there are some things that are easy to understand. It is easy to understand that the killing of yet another unarmed Black man is due to structural racism. It is easy to understand that the involved police officers did not view George Floyd as a man that was their equal. And it is easy to understand why people are incensed about this.

A lot needs fixing here, and it’s going to take a while for that to happen. But as I write this in Los Angeles with the sound of sirens going by my apartment every few minutes, it is hard not to think of the 1992 L.A. riots. It is hard not to think about what happened in the wake of the acquittal of the officers involved in the brutal beating of Rodney King. When looking at police violence against the Black community, it is easy to feel like very little has changed since then.

This time calls for some reflection about how all of us, in the institutions where we work or participate, can enact some change.

A few months ago, the physics and astronomy department at my new institution, UCLA, invited Sherard Robbins to come and speak about the demographics and minority representation in our department. He asked us to take a look around the room and to see if the representation in the room reflected that of the general population in Los Angeles. This was an embarrassing and shameful exercise. It is shameful because we do not have a single Black faculty member. It is also shameful because women are hugely underrepresented.

Representation matters. It particularly matters in positions of power. It matters because when you see people that look like you and are culturally similar to you in a position you thought was unattainable, you start to believe you can do it. It also matters because people tell stories, and stories are mediators of humanization. When you hear about your culturally different colleague’s weekend with their family at the beach, you see them as a parent, spouse, and human.

I am currently in a position of power. I have been an assistant professor now for almost a year, and because it is so new and fresh, it contrasts strongly with my previous position as a postdoc. I went from having almost no power and social responsibility to being thrust into a position where my words and actions do have an affect on undergraduates, graduate students, postdocs and other faculty members. I know that many of you who read this blog are or will be in similar positions in the future. So when you are afforded the privilege of such a position, it is your responsibility (just as it is now mine) to make sure that conduct in your department changes. It is your responsibility to make sure that the make-up of the department starts to reflect that of the greater population. It is your responsibility to ensure that traditionally underrepresented groups make it into positions of power. And it is your responsibility because if you don’t do it, no one else will.

The activists in the streets deserve tremendous credit for making their voices and anger heard. And it’s important that those in positions of power take actions that say “we hear you”.

I sign off with a rather profound quote adapted from the Talmud for the film Schindler’s List:

Whoever saves one life saves the world entire.

Slight detour

I am still planning to follow up my previous post on environmental negligence and will write a post about CFCs in the near future. However, I saw this YouTube video recently and found it harrowing. The British government had known about the consequences of acute radiation poisoning, but chose to perform these tests anyhow. In addition to the lives irreversibly changed, there is also the remarkable fact that these people were able to see live images of bones and blood vessels with their eyes. Does anyone have a good explanation as to how this would even be possible?


Environmental negligence part 1: Leaded gasoline

Before I start this post, I just want to say that I hope you are all doing okay with regard to the spread of coronavirus. It is important that we take this situation seriously in effort to minimize the risk to yourselves and others.

As a professional scientist, I have to admit that I sometimes struggle with technology’s dual nature. I realize that studying physics and being able to come up with ideas to test in the laboratory is an enormous privilege. But scientific knowledge also comes with substantial weight.

Probably the biggest challenges facing humanity today, environmental damage and climate change, have partly arisen due to the outgrowth of technologies related to the study of electromagnetism and thermodynamics in the 19th century. Eliminating greenhouse gas emissions is a monumental task that is nuanced with all sorts of moral issues that relate to the developing world. In particular, the developing world has not been primarily responsible for much of the anthropogenic greenhouse gasses we see in our atmosphere today, but will likely end up seeing their development stunted as the world attempts to cut emissions.

In this series of posts, I raise some questions pertaining to how we as a society here in the US have dealt with environmental issues in the past and identify a few patterns of behavior. Specifically related to this, I ask whether there exist deep-rooted structural problems and whether there exist options to fix them. I focus on three particular examples (among many!), that of leaded gasoline, chloroflourocarbons (CFCs) and PFOA (Teflon) to illustrate the difficulties in fighting environmental damage, but also how there is some hope in doing so.

For thousands of years, lead has been used for many purposes, most famously for pipes and aqueducts in the Roman empire. Although even the Romans were aware of the toxicity of lead, they continued to use it. Thus, lead poisoning has been in the public consciousness for at least a couple thousand years. In the US, it was thought that low levels of lead exposure did not pose a serious health risk, however. For example, lead had been used in paint for centuries. In the early 1920s, though, something changed. Lead made its way into the air we breathe through the automotive industry as an additive to gasoline due to its anti-knock properties. It was this use of lead, as a gasoline additive, that put lead basically everywhere in the atmosphere and surface ocean water. Despite the well-known health risks of lead to the general public, there were no studies conducted by any government agency or by any of the companies before sales of leaded gasoline were permitted in the marketplace.

Just a couple years after tetraethyl lead (TEL) was included in gasoline, workers at the company producing leaded gasoline (Ethyl Gasoline Corporation, a joint venture between General Motors, DuPont and Standard Oil) started suffering consequences. Some died, some “went crazy” because lead is a neurotoxin and others showed significant mental deterioration. Alexander Gettler and Charles Norris, a toxicologist and the Chief Medical Examiner of New York, were tasked with performing autopsies on four of the workers that had died in relation to work at the company. Their report, published in 1925, showed significant levels of lead in the brain tissue of the victims, more so than in patients that exhibited conventional lead poisoning (i.e. not from TEL). They speculated that the lead in TEL was somehow attracted to brain tissue more than regular lead.

Around the time of their report, New York, New Jersey and the city of Philadelphia all banned the sale of leaded gasoline. Also in 1925, the U.S. Surgeon General formed a task force with the intention of performing a more thorough investigation of the effects of lead on the population, though excluded Gettler and Norris from the committee. Around the same time, Thomas Midgley Jr., one of the inventors of TEL and a scientist at the Ethyl Gasoline Corporation, published a paper on the lack of hazards posed by leaded gasoline to the general public. I hesitate to even mention that this was a massive conflict of interest. In addition to this paper, there were heavy propaganda efforts aimed at the public to make it seem like leaded gasoline was a huge step forward in the automotive industry. Below are a couple examples of advertisements from 1927 and 1953 respectively (notice how lead is never mentioned):

The U.S. Surgeon General ultimately sided with Midgley and industry insiders, citing a single short seven-month study that showed a lack of evidence that TEL was causing harm. The federal government lifted all bans on the sale of leaded gasoline. In a rather foreboding gesture, the task force did acknowledge the possibility that with more cars on the road in the future, the issue would have to be re-visited. This kicking of the can down the lead-coated road would last about 60 years. More about the Surgeon General’s report can be read here (PDF!).

One thing I should mention about the U.S. Surgeon General’s task force is that it abided by what became known as the Kehoe Rule, which puts the burden of proof on showing that leaded gasoline is unsafe. This is in contrast to the precautionary rule, which puts the burden of proof on showing that leaded gasoline is safe if introduced into the public arena.

How did leaded gasoline ultimately get banned from use? This is where the story takes an unlikely turn. Enter Clair Patterson, a geochemist working on trying to date the age of the Earth. At the recommendation of his PhD advisor, Patterson started working on trying to figure out the ratio of lead to uranium in old rocks, as uranium-238 would eventually decay into lead after 4.5 billion years. What Patterson found was startling. He was basically finding lead contaminants everywhere. Whatever rock he looked at, no matter how clean his laboratories were, would always be contaminated. He started to figure out that everything in his lab was contaminated with lead. Distilled water, glassware, you name it, was contaminated. This prevented him from obtaining the correct ratio.

Because of this contamination, Patterson spent years building the world’s first “cleanroom”, that would be lead-free. Below is a rather inspiring image of Clair Patterson scrubbing the lab floor (taken from here):


With his massive effort to create a lead-free zone, Clair Patterson was ultimately able to obtain to the age of the earth: 4.5 billion years. But this isn’t what this story is about.

After going to such lengths to fight off lead contamination, Patterson realized where the lead was coming from. In 1965, Patterson tried to convince the public that leaded gasoline was a major health hazard by publishing Contaminated and Natural Lead Environments of Man. Even though he was the world’s foremost expert on the topic at the time, he was left off a National Research Council research effort to study lead in the atmosphere in 1971. See a pattern (see above about Gettler and Norris)? Once Patterson turned his studies toward lead contamination in food, it became abundantly clear that lead was present in every facet of life on earth.

For his efforts, Patterson was hounded by industrial insiders and refused contracts with many research organizations. But ultimately, he did win his long-fought battle. He was massively helped in this battle by Herbert Needleman, who performed research showing that long exposure to lead in children likely resulted in a lower mental capacity. In 1986, the US phased out leaded gasoline, more than 65 years after the first warnings were put out by scientific watchdogs.

There is much to learn from this particular story, but before I go onto conclude, I would like to recap a couple more historical anecdotes in the days to come that I think we can learn from. More to follow…


*Much of this post was learned through the following references:

Looney Gas and Lead Poisoning: A Short, Sad History