Illustrating the Higgs mechanism, Part 1

Most people are probably aware of the Higgs boson, long predicted, and recently observed by the Large Hadron Collider in 2012. The Higgs boson is was predicted by the Higgs mechanism, which is a theory that endows certain particles with mass.

Now, you might be wondering why I, a researcher in superconductivity, am talking about particle physics. In fact, the Higgs mechanism started out as a theory of superconductivity, and was soon imported to particle physics. I leave the historical details to your encyclopedia of choice.

To understand the superconductor/particle physics connection, consider the question: For which particles does the Higgs mechanism generate mass?

In particle physics, the Higgs mechanism generates mass for the W and Z particles, the mediators of the Weak force. The reason it’s called the Weak force is because it’s so short range, which is a consequence of the W and Z having mass. To my understanding, other elementary particles derive their mass from their interaction with the Weak force. (not quite correct, although link is correct.  See comments for details)

Transcript: There are four fundamental forces between particles: (1) Gravity, which obeys this inverse square law: F= G m1 m2 over d squared. (2) Electromagnetism, which obeys this inverse-square law: F = k q1 q2 over d squared and also Maxwell's equations (3) The Strong nuclear force, which obeys, uh... well, umm... it holds protons and neutrons together. It's strong. And (4) the Weak force. It [mumble mumble] radioactive decay [mumble mumble]--And those are the four fundamental forces!
From xkcd. Hover text: “Of these four forces, there’s one we don’t really understand.” “Is it the weak force or the strong–” “It’s gravity.” Accurate.

In superconductors, the Higgs mechanism generates mass for the electromagnetic field. This causes the electromagnetic field to be short range. Thus, if you apply an external electric or magnetic field, it will not penetrate into a superconductor of any appreciable size. The zero magnetic field is called the Meissner effect; the zero electric field leads to zero resistance current.

That’s the short explanation of what the Higgs mechanism does, but I wanted to explain how it works. Also, I wanted to try out this new doodling software…
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What are topological defects?

This is a repost of an article I wrote in 2013, back when I was willing to take the time to explain physics stuff, and generate lots of images too.

Donut and Coffee
(image source)

It’s often said that topology is the branch of mathematics where they can’t tell the difference between a donut and a coffee mug. They each have a single hole (the mug’s handle and the donut hole), and that’s all that matters. If I may overanalyze this joke, the point seems to be that topology is so disconnected from our everyday experience. How is this useful?

I wish to explain one particular use of topology in physics: topological defects.

A topological defect is a sort of knot that exists in the microscopic structure of a material.* You can move the knot around from atom to atom, but you can’t untie it. We’ll get into how that works soon enough.

*Material is a vague term for “stuff”. Later I’ll discuss a few different materials including magnets, liquid crystals, and superconductors [Read more…]

Paper: Do memes really go viral?

When a meme becomes popular, we often say that it has gone “viral”. This word suggests that memes become popular by being particularly infectious. In analogy to epidemics, a particularly viral meme hits a critical threshold where it just won’t die, because each new infection spreads it to even more people.  But is this epidemic model actually true?

A paper titled “The Structural Virality of Online Diffusion” puts into question the very idea that popular memes are viral. They point out another mechanism by which memes can become popular: the broadcast. Rather than infecting multiple generations of followers, a meme may become popular simply by infecting one person with a lot of followers.

Transcript: two tree structures are shown. The first is a single root node with many children. The second is a root node with two children, each of which have two more children, and so on.
Figure 1 from the paper. The tree on the left illustrates a broadcast, while the one on the right illustrates viral spreading.

The paper considers a massive amount of Twitter data to determine whether the most popular links are broadcast-like or virus-like. On average, they more resemble broadcasts, but there is a huge amount of variation.
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Ribbon physics

At March Meeting, there was a fascinating talk about the phase diagram of ribbons.  Yeah, just like the solid/liquid/gas phase diagram of water!  Their paper is called Helicoids, Wrinkles, and Loops in Twisted Ribbons (see on PRL if you have journal access).  Here’s the first figure:

Ribbon phase diagram

Left side: many possible responses of a stretched and twisted ribbon.  Right side: a phase diagram of the ribbon as a function of stretching force (T) and twisting angle (θ).

The researchers claim, “Our results can be used to develop functional structures”.  I don’t know what that means, but I’m already sold!  I am going to replicate this experiment!

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Not every scientist is above average

My invitation to FTB caught me at a busy time. Last week, I was at the APS March Meeting, which is the largest physics conference in the world of the year. There were about 10,000 attendees, and about 50 simultaneous sessions throughout the week, with no lunch breaks.

Of course, I manage to find break time anyway. Quite frequently, it seems that there is not one single session going on that would be of interest to me. I just wouldn’t understand them.

In previous years, I used to find this very depressing. On my conference app, I’d bookmark all the sessions on superconductivity, which is my area of study. Then I’d sit in on a session, and find that I understood not a single talk, not even a little. And every 12 minutes there would be another talk, and another, and another, for hours. Then I’d try going to a different room focused on superconductivity, and the same thing would happen again.

It’s no wonder that impostor syndrome is so common among physicists.
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