I read popular physics: Quantum Leap

This is an entry in my series where I read physics articles in Scientific American, under the theory that having a physics PhD will save me.

This month’s article is “Quantum Leap” about the theory behind the quantum hall effect. Out of all the articles in this series so far, this is the closest to my actual field of study (I was a condensed matter experimentalist who studied superconductors). But I positively groaned when I saw it. It’s bad.

But before I get to the main attraction, I have some general commentary on the August issue. After two months of putting the coronavirus on the cover, SciAm’s cover has finally moved on, now featuring a story about oak trees. The columnists are all still talking about the virus, one about racial health disparities, one about masks, one about science denial. I think these articles are written 1-2 months in advance, so they’re a bit of a time capsule. Wait, aren’t we still talking about all the same things?

There’s also an interesting article about how Galileo lived through the plague of the 1630s—overlapping with the time he was subject to the inquisition. Funny, I don’t remember anyone mentioning that part while claiming to be modern Galileos. This article is of historic interest but I think it suffers for trying to suggest that Galileo is a model for responding to pandemics.

Next, I’d like to mention a couple letters in response to the article I covered in April, about mapping the Milky Way. I learned about the “winding problem”, about galactic spiral arms. If spiral arms follow the orbits of stars then they would become more and more tightly wound. It’s unknown whether spiral arms are destroyed and reformed, or if spiral arms are persistent patterns that do not follow the orbits of stars within them.

So, “Quantum Leap”… The good news is that it’s not paywalled, so you too can bang your head on this one.

Anticipating that the article would be rough, I first took a look at the images. They were not up to standard, being impossible to understand without the accompanying text (and as I found out, not any better with the text).

The article begins with a barely related anecdote whose main purpose is to name drop a famous actor and say he was a science advisor for some movie. This sets the stage for an article where the science is occasionally interrupted with personal narratives heavy on references to other physicists. I am not interested in hearing about physicists’ old boys club!

I think the only good bit of the article is where he explains the Quantum Hall Effect, although I already knew about it. The Hall Effect occurs when you run current through a material in one direction and apply a magnetic field in another direction. The magnetic force pushes electrons1 to the side, which induces a voltage. When you apply a sufficiently strong magnetic field, the Quantum Hall Effect emerges—the magnitude of the Hall Effect becomes quantized, jumping between discrete values.

The author claims that the Quantum Hall Effect was previously unsolved, which was news to me. I learned about the mechanism for the Quantum Hall Effect in undergraduate (just don’t ask me to explain it).2 But, I suppose there are multiple levels of unsolved-ness, and the mechanism I learned in undergraduate was likely too simplistic.

To explain the Quantum Hall Effect, the author begins with a thought experiment called Laughlin’s Quantum Pump. Although it seems simple enough, I did not understand the arrangement of objects or the logic of the argument. I’m also really thrown off by the reference to “fictional magnetic flux”… what makes it fictional? It’s a thought experiment so everything in it is fictional.

It only gets worse from there. I began to skim the article when it became clear that reading carefully would give me no value.

This article’s approach to science popularization is to leave in all the technical stuff but sprinkle in personal narrative. I honestly can’t tell whether the approach is fundamentally flawed, or the execution is just very poor.

One nice thing about having a PhD is that I can externalize any problems I have reading these articles. It’s definitely not my fault that I don’t get it, it’s the author’s fault. Gosh, I hope that my science writing is more effective than this.


1. More generally, the Hall effect applies to whatever particles are carrying electric current. For reasons I won’t get into, this does not always mean electrons. (return)

2. What I’m calling the Quantum Hall Effect is also known as the Integer Quantum Hall Effect, to contrast with the Fractional Quantum Hall Effect, which is more poorly understood and not taught to undergraduates. There is also the poorly understood Quantum Anomalous Hall Effect. The present article is only about the Integer Quantum Hall Effect. (return)


  1. Rob Grigjanis says

    I’m also really thrown off by the reference to “fictional magnetic flux”

    Without reading the article, maybe related to:
    The auxiliary magnetic field H in a magnetizable medium is defined as

    H = (1/μ₀)BM

    where M is the magnetization field.

    So, from Maxwell’s equations,

    ∇·H = −∇·M

    By analogy with the corresponding equation for the electric field, the RHS could be called the ‘fictional magnetic charge density”.

  2. says

    From what I can tell, no magnetizable medium is involved.

    I believe the thought experiment involves modifying quantum phase with the vector potential, but this barely gets referenced at all, I think it’s taken for granted that we just know how that works.

  3. Rob Grigjanis says

    Siggy @2: I did a quick skim of Laughlin’s 1998 Nobel Lecture. He talks about his pump starting around equation 12. Shades of the Aharonov-Bohm effect, but the stuff preceding it is well outside my comfort zone.

  4. Allison says

    I first took a look at the images. They were not up to standard, being impossible to understand without the accompanying text (and as I found out, not any better with the text).

    This applies to pretty much all Sci Am articles. 35 years ago, when I was a grad student, I remember people making the same complaint, and when I have seen articles about things I know anything about, the only reason I could understand them was I was already knew more about the topic that the article.

    I also knew someone who had written an article for Sci Am. She did her best to make the article clear and understandable, and she said that when the article came out, the editors had changed it to make far less understandable.

    My theory is that Sci Am’s target audience is people who have no interest in actually understanding anything but want to be able to regurgitate buzzwords and statements with which they can impress people at cocktail parties.

  5. says

    @Rob #3,
    Yes, I was also thinking of the Aharanov Bohm effect. It also makes me think of SQUIDs–that’s a superconducting thing. I think the thought experiment is within my grasp, but I’d have to read some other resource, and I guess I’m not willing to spend the time.

    @Allison #4,
    When I started out this series I had low expectations, but was pleasantly surprised on most months. Not this month.

    I’m reluctant to blame only the editors. I’ve always known condensed matter physicists to be really poor at explaining themselves. I do think the editors deserve part of the blame here, though, for approving the article in its current state.

  6. bmiller says

    Have to admit this whole issue is landing in the recycling bin. As a Californian, I was very interested in oak trees, but the whole article focuses on endlessly discussing how different sub-sub-sub-groups of species interbreed,etc. etc. etc.

    I have no physics PhD, so the article on the Quantum Hall Effect left me rather confused. But I am used to it. I agree the tone and non sequiturs were off putting!

    What do you think of Sabine’s blog? THAT is fun to read even when I am utterly lost. The snark and battles are amusing from the sidelines. I also sympathize, to the extent I can understand it, with her skepticism towards big money investments in “toys”.

  7. says

    @bmiller #6,
    I don’t know who that is. The premise of this series is that I have a dead-tree subscription. I don’t read the sciam blogs (and I gently assert a lack of interest).

  8. Rob Grigjanis says

    Siggy @8: It’s not a sciam blog. Sabine Hossenfelder (I assume that’s who bmiller is talking about) is a quantum gravity theorist whose blog is one of the better ones by physicists, IMO.

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