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Monday, September 15, 2014

Nifty papers I wrote that nobody knows about (Part I: Solitons)

I suppose this happens even to the best of us: you write a paper that you think is really cool and has an important insight in it, but nobody ever reads it. Or if they read it, they don't cite it. I was influenced here by the blog post by Claus Wilke, who argues that you should continue writing papers even if nobody reads them. I'm happy to do that, but I also crave attention. If I have a good idea, I want people to notice. 

The truth is, there are plenty of papers out there that are true gems and that should be read by everybody in the field, but are completely obscure for one reason or another. I know this to be true but I have little statistical evidence because, well, the papers I am talking about are obscure. You can actually use algorithms to detect these gems, but they usually only find papers that are already fairly well known. 

In fact, this is just common sense: once in a while a paper just "slips by". You have a bad title, you submitted to the wrong journal, you wrote in a convoluted manner. But you had something of value. Something that is now, perhaps, lost. One of my favorite examples of this sort of overlooked insight is physicist Rafael Sorkin's article: "A Simple Derivation of Stimulated Emission by Black Holes", familiar to those of you who follow my efforts in this area. The article has 10 citations. In my view, it is brilliant and ground-breaking in more than one way. But it was summarily ignored. It still is, despite my advocacy.

I was curious how often this had happened to me. In the end the answer is: not so much, actually. I counted five four papers that I can say have been "overlooked". I figured I would write a little vignette about each of them, why I like them (as opposed to the rest of the world), and what may have gone wrong--meaning--why nobody else likes them.

Here are my criteria for a paper to be included into the list:

1.) Must be older than ten years. Obviously, papers written within the last decade may not have had a significant amount of time to "test the waters". (But truthfully, if a paper does not get some citations within the first 5, it probably never will. )

2.) Must have had fewer than 10 citations on Google Scholar (excluding self-citations).

3.) Must not be a re-hash of an idea published somewhere else (by me) where it did get at least some attention.

4.) Must not be a commentary about somebody else's work (obvious, this one). 

5.) Must be work that I'm actually proud of. 

When going through my Google Scholar list, I found exactly four papers that meet these criteria. 

(Without taking into account criterion 5, the list is perhaps twice as long, mind you. But some of my work is just not that interesting in hindsight. Go figure.)

These are the four papers in the final list:

1. Soliton quantization in chiral models with vector mesons, C Adami, I Zahed (1988)
2. Charmonium disintegration by field-ionization, C Adami, M Prakash, I Zahed (1989)
3. Prolegomena to a non-equilibrium quantum statistical mechanics, C Adami, NJ Cerf (1999)
4. Complex Langevin equation and the many-fermion problem, C Adami, SE Koonin (2001).

I will publish a blog post about one of these each of the coming weeks.

I'll start in reverse chronological order:

Physics Letters B 215 (1988) 387-391. Number of citations: 10 

This is actually my first paper ever, written at the tender age of 25. But it didn't get cited nearly as much as the follow-up paper, which was published a few months earlier: Physics Letters B 213 (1988) 373-375. 

How is this possible, you ask? 

Well, the editors at Physics Letters lost my manuscript after it was accepted, is how it happened! 

You have to remember that this was "the olden days". We had computers all right. But we used them to make plots, and send Bitnet messages. You did not send electronic manuscripts to publishers. These were sent around in large manila envelopes.  And one day I get the message (after the paper was accepted): "Please send another copy, we lost ours". Our triplicates, actually, because each reviewer gets a copy that you send in, of course. I used to keep all the correspondence about manuscripts from these days, but I guess after moving offices so many times, at one point stuff gets lost. So I can't show you the actual letter that said this (I looked for it).  Of course, after that mishap the editorial office used a new "received" date, just so that it doesn't look so embarrassing. And arxiv wouldn't exist for another 4 years to prove my point.

So that's probably the reason why the paper didn't get cited: people cited the second one that was published first, instead. But what is this paper all about?

It is about solitons, and how to quantize them. Solitons were my first exposure to theoretical physics in a way, because I had to give a talk about topological solitons called "Skyrmions" in a theoretical physics seminar at Bonn University in, oh, 1983. Solitons are pretty cool things: they are really waves that behave like particles. You can read a description of how they were discovered by John Scott Russell riding his horse alongside a canal in Scotland, and noticing this wave that just... wouldn't... dissipate, here

Now, there is a non-linear field theory due to T.H.R. Skyrme that has such soliton solutions, and people suggested that maybe these Skyrmions could describe a nucleon. You know, the thing you are made of, mostly? A nucleon is a proton or a neutron, depending on charge. Nuclei are are made from them. Your are all nucleons and electrons really. Deal with it. 

Skyrme incidentally is the one who died just days after I submitted the very manuscript I'm writing about, which started the rumour that my publications are lethal. Irrelevant fact, here. 

Skyrme's theory was a classical one, and so the question arose what happens if you quantize that theory. This is an interesting question because usually, if you quantize a field you create fluctuations of that field, and if these fluctuations were of the right kind, they should (if they fluctuate around a nucleon) describe pions. And voilà: we would have a theory that describes how nucleons have to interact with pions. 

What are pions, you ask? Go read the Wiki page about them. But really, they are the stuff you get if you bang a nucleon and an anti-nucleon together. They have a quark and an anti-quark in them, as opposed to the nucleons, that have three quarks: Three quarks for Muster Mark

Now, people actually already knew at the time what such an interaction term was supposed to look like: the so-called pion-nucleon coupling. But if the term that comes out of quantizing Skyrme's theory did not look like this, well then you could safely forget about that theory being a candidate to describe nucleons. Water waves maybe, just not the stuff we are made out of.  

So I started working this out, using the theory of quantization under constraints that Paul Dirac developed, because we (my thesis advisor Ismail Zahed and I) had stabilized the Skyrmion using another meson, namely the ω-meson. You don't have to know what this is, but what is important here is that the components of the ω field are not independent, and therefore you have to quantize under that constraint.

You very quickly run into a problem: you can't quantize the field because there are fluctuation modes that have zero energy. Indeed, because in order to do the quantization you have to take the inverse of the matrix of fluctuations, these zero modes create a matrix that cannot be inverted (its determinant vanishes). What to do?

The answer is: you find out what those zero modes are, and quantize them independently. It turns out that those zero modes were really rotations in "isospin-space", and they naturally have zero energy because you can rotate that soliton in iso-space and it costs you nothing. I figured out how to quantize those modes by themselves (you just get the Hamiltonian for a spinning top out of that), then project out these zero modes from the Skyrmion fluctuations, and quantize only those modes that are orthogonal to the zero modes. And that's what I proceeded to do. Easy as pie.

And the result is fun too, because the resulting interaction term looks almost like the one we should have gotten, and then we realized that the "standard" term of chiral theory comes out in a particular limit, known as the "strong coupling" limit. Even better, using this interaction I could calculate the mass of the first excitation of the nucleon, the so-called Δ resonance. That would be the content of the second paper, which you now know actually got published first, and stole the thunder of this pretty calculation.  

So what did we learn in this paper in hindsight? Skyrmions are actually very nice field-theoretic objects, and the effective theory (while obviously not the full underlying theory that should describe you, namely the theory of quarks and gluons called Quantum Chromodynamics, or QCD), this approximate theory can give you very nice predictions about low energy hadronic physics, where QCD actually is not at all predictive. That's because we can only calculate QCD in the high-energy limit (for example what happens when you shoot quarks at quarks with lots of energy, for example). Research on Skyrmions (and low-energy effective theories in general) is still going on strong, it turns out. And perhaps even more surprising is this: there is now a connection (uncovered by my former advisor), between these Skyrmions and the holographic principle

So even old things turn out to be new sometimes, and old calculations can still teach you something today. Also we learn: electronic submissions aren't as easily lost behind file cabinets. So there is that.

Next up:  Charmonium Disintegration by Field-Ionization [Physics Letters B 217 (1988), 5-8]. A story involving the quark-gluon plasma, and how an old calculation by Cornel Lanczos from 1930 can shed light on what happens to the J/$\psi$, when suitably modernized. All of 5 citations on Google Scholar this one got. But what a fun calculation! Read on here

2 comments:

  1. Damn, I was looking forward to part 5 about quantum measurement :(

    ReplyDelete