Researchers see signature of “Majorana particles” inside superconducting iron | Hacker News Reader

Researchers see signature of “Majorana particles” inside superconducting iron

scientificamerican.com

120 points4k52 comments

It's an interesting result for solid-state physicists, but the title is very confusing to the layman. The finding is about quasi particles that have the same properties as a Majorana fermion (a Majorana bound state), due to how electrons behave in a superconductor (https://en.wikipedia.org/wiki/Majorana_fermion#Majorana_boun...). They did not detect a Majorana fermion itself. This is satisfactorily explained in the article, but the title is sensationalist.

> ... but the title is very confusing to the layman

Given the density of knowledge in your comment, I'm not sure the title could be much different to aid in the understanding of the layman.

"New Emergent Particle Acts as its Own Anti-Particle".

A layman will not know what "emergent particle" means. (I did not.) But they will at least know that the presence of an adjective implies it's not quite "a particle", and the adjective itself gives a hint to the meaning. If the layman is then piqued, they will get clarification in the article itself.

You wrote the style guide for Wikipedia's mathematics articles, didn't you? ;)

(By which I mean, a balance between immediate understanding vs. links for detail >> primarily links for detail without immediate understanding)

Would "Virtual Particle" possibly be a more correct and explanatory term?

Sadly, no, as "virtual particle" has a different meaning. See http://en.wikipedia.org/wiki/Virtual_particle. The technical term for these, I believe is "quasiparticle": http://en.wikipedia.org/wiki/Quasiparticle

So, for example, a "hole" in a semiconductor lattice is emergent, but a phonon is quasi?

I suppose we would call an L4 Lagrangian point an emergent phenomenon and not a virtual mass (just because it can be orbited). For one thing, if we did a surface integral around the L4 point, of the gravitational field, we would find that there is no mass there; the net flux is zero.

I do not know, as I am not a particle physicist. I got "emergent" from the popular press article. The terms "virtual" and "quasi" already have meanings in the field, so my point was it would be further confusing to use their non-technical meanings when trying to explain it to a layman.

Trust me, your description is orders of magnitude more confusing to the layman.

I disagree. The title implies a state of affairs which is not true; a reader is likely to take a solid understanding away from it, but that understanding is wrong.

chton's description will leave such a reader with a greater degree of confusion, but a higher overall level of correct understanding.

Sure, but that means that the title is more misleading, while the other explanation is more confusing. Confusing and misleading aren't the same thing.

I agree. I think, however, we can forgive chton for not parsing out the best word for "kind of badness" prior to our semantic discussion. (If we are to be charitable in understanding his/her comment, then we should assume "kind of badness" was the intent.)

For the record, I greatly prefer confusing to misleading. I would rather people have an incomplete understanding than an incorrect one.

> For the record, I greatly prefer confusing to misleading.

On this much, we agree -- in general at least.

OTOH, often, there are good reasons for prefering particular misleading descriptions over more confusing but less misleading ones (pedagogically, for instance, sequences of progressively-less-misleading explanations are often used, each of which is designed to limit how confusing it is to the target audience, to develop progressively better understanding.)

Agreed again. I often quote my intro to computer engineering professor who said, "Education is a series of small lies" when told us some circuits have ternary logic, not just binary. But I feel that this headline is not in that group.

I think that's true most of the time, but in this particular case, it probably isn't doing anyone much good (aside from the beneficiaries of that clickbait admoney)

Here's the layman's version:

Some particles are actual particles - like photons, electrons, protons, etc. Think of them as a droplet of water.

Some particles are not really particles, but kind of behave like a particle in some ways. Think of them as bubbles inside a liquid.

If you ignore the air inside, the bubbles don't really exist - they are just the absence of some liquid, that happens to look sort of like a particle with funky properties. Similarly, the particles described in this article don't really exist - they are "holes" in a lattice of other particles, that can mathematically be treated as if they were particles, but aren't actually real in a non-mathematical way.

That said, if Quantum Mechanics has taught us anything, it's that those "purely mathematical" concepts can have very real impacts on the world, so in a sense, it is exciting that these "pseudo-particles" have been found to exist, because the maths might reveal all sorts of funky shit we can do with them that would be impossible with regular particles, and the maths doesn't care that they're not regular particles - it works with either kind of particle.

Beautiful description.

This should be a top-level post.

Does this also allow us to assume that any unique properties that we observe from this quasi-particle will have a good chance of being applicable to an actual particle, and thus help us discover the first actual Majorana particle?

Good try, and less confusing (or confusing on a much higher level) than some alternatives, but the use of the word "really" and the implication that bubbles (and therefore quasi-particles) "don't exist" in this description is problematic because it is false: bubbles and quasi-particles exist and are as "really real" as electrons and water-droplets.

Nothing but metaphysical confusion is added by asserting that some things "really exist" while other perfectly ordinary things--things that can be created, manipulated, and destroyed--somehow "don't really exist".

A less metaphysically loaded description would be:

The mathematics that Majorana worked out was intended to describe elementary particles, which as the name suggests can't be divided into their component parts. It turns out that inside a superconductor, the motion of groups of electrons, all moving together thanks to the special properties of superconductors, can be described by the same math. These groups of electrons can be considered entities in their own right, and are called "quasi-particles". They are perfectly real: they just aren't elementary.

Furthermore, for quasi-particles the atomic lattice of the superconductor acts in the same way as empty space does for elementary particles: it gives them a place to exist and has properties that allow them to move around and interact with each other. Majorana's equations describes how they do this, so they are mathematically equivalent to elementary Majorana particles moving around in empty space.

/End of metaphysical pedantry.

That isn't pedantry, that's a fantastic description. Thanks for posting it.

>Some particles are actual particles - like photons, electrons, protons, etc. Think of them as a droplet of water.

>Some particles are not really particles, but kind of behave like a particle in some ways. Think of them as bubbles inside a liquid.

only at the currently explored quantum scales. Digging deeper the "actual particles" might as well happen to be just such bubbles.

>the bubbles don't really exist

probing an electron to some fermis has found so far no "liquid" only "air".

One can start suspecting that "real particles" are something like "air bubbles" too when one looks at the fact that the farther you try to separate the quarks of a particle the stronger the opposing force - pretty much like trying to expand the size of an "air bubble" inside the "liquid"

Here's a simpler version:

What these people have done is a way of arranging some electrons such that they behave like Majorana fermions (a fermion [particle] whose anti-particle is itself).

> Trust me, your description is orders of magnitude more confusing to the layman.

Fundamental physics tends to do that to laymen. A lot.

Regardless of what other folks are pointing out, maybe, confusing or not confusing, some layman child reads the article (or your comment) and somehow decides to go into Physics.

That would be great.

(facepalm) Another day, another "popularized" article that confuses condensed-matter quasiparticles with real particles...

Honest question:

If what we think of as real particles are really just useful abstractions over a more complicated reality, but that underlying reality is basically the same thing mathetmatically that exists in condensed-matter, is there a significant difference? Where does the analogy break down?

Yes, there is a significant difference: although the quasiparticle shows the same behavior as the 'real' particle in some respects, it shows different behavior in other respects.

For instance, the quasiparticle can be destroyed by the addition of some heat to the system, while a 'bare' Majorana fermion would not cease to exist in the presence of that amount of energy.

Analogy: in certain measurements (e.g. distribution of reflected light frequencies), a red circle is indistinguishable from a red sphere. However, in other respects (e.g. distribution of reflected light intensity), they are quite different.

From a theorists perspective there is not much difference, they are both modeled by Quantum Field Theories. However condensed matter theory deals mostly with non-relativistic phenomena. The idea of quasiparticles, like the one they have discovered is also present in particle physics, they are called "resonances". Depending on the energy scale you can integrate out the higher energy modes of your theory to get an effective theory, in which those resonances are now the "fundamental particles", examples include pions, Kaons etc. This is analogous to how you describe quasi-particles in condensed matter theory.

In contrast to condensed matter theory which is able to observe electrons on their own, the fundamental constituents in high energy particle physics have not all been observed on their own. So called quarks, the building blocks of protons and neutrons among other things, ordinarily never occur alone, due to something called confinement. This is analogous to how at low temperature in super conductors electrons appear as so called cooper pairs coupled by phonons, here quarks are in a "cosmic superconductor" coupled by gluons. One of the aims of the LHC experiment is to go to high enough energy to induce a phase transition to a quark gluon plasma, which would be analogous to the state electrons are normally in a metal.

So in conclusion, it's not a coincidence that both the renormalization group by wilson and the idea for the Higgs mechanism, which also has an analogue in the theory of high temperature superconductivity and was originally proposed by Anderson in the context of condensed matter theory, were discovered by theorists working in condensed matter theory.

There is a pretty strong overlap between material science and quantum field theory. To name just one example, the idea of the Higg's particle actually has it's genesis in theoretical solid state physics [1].

Most material solids can be described as a lattice, where there is some unit cell of a given size, say L, which is repeated periodically in all directions.

There are various types "quasi-particles" that can move through a lattice, examples include phonon's and poloron's. The thing that makes the quasi-particle concept useful is that it is greatly simplifies the description of the collective motion of a large number of particles which are all interacting.

An electromagnetic field in a region of space can (sort of) be described as a lattice, and this result is one of the deepest and most profound results in theoretical physics, imo. The basic idea is the EM field can be thought of in the following way: every point in space can be treated mathematically as a simple vibrating spring (harmonic oscillator).

In other words, the analogy between fundamental particles and quasi-particles breaks down because in a material solid there is a unit cell of size L, but in the vacuum this lattice size is 0.

The idea that every point in space is a harmonic oscillator works in the sense that it makes predictions that agree with experiment; however theoretically it has an extremely severe flaw which has motivated a large amount of research on the quantum vacuum. The problem is that the energy of a region of space with zero EM field (i.e. a vacuum) comes out to be infinite. There are various tricks to avoid this infinity, but the simplest one is to just use some non-zero value for the lattice spacing of the vacuum.

[1] http://en.wikipedia.org/wiki/Philip_Warren_Anderson

Here's the press announcement in 2012 of the preceeding work done in Delft: http://www.tudelft.nl/en/current/latest-news/article/detail/.... And here's a link to the related paper on ArXiV: http://arxiv.org/abs/1204.2792.

I know a few of the people working on the experimental setup within the Kavli Institute. Insanely complex setup! As an aerospace engineer, most of it goes well over my head, but it's interesting nonetheless!

> As opposed to particles found in a vacuum, unattached to other matter, these Majoranas are what’s called “emergent particles.” They emerge from the collective properties of the surrounding matter and could not exist outside the superconductor

Sounds a lot like some of the magnetic monopole announcements. It is always more of a situation than an actual thing.

This article does demonstrate the principle that virtually every area of active research in material science, no matter how obscure, will one day have a Very Important Application in Quantum Computers. sigh

Both matter and antimatter? You mean like the photon and, iirc, all the other neutrally charged elementary particles?

New quasi-particle is Majorana. :b

There is more to being antimatter than just having the opposite charge. The spin of the particle also matters. For a particle to be its own antiparticle, it would have to have spin 1/2. All elementary fermions have that property, but not much else.

Of the 2 classes, fermions and bosons, only fermions can be their own antiparticles. Bosons are defined with having an integer spin, so they can never have spin 1/2. Of the fermions, none are known with neutral charge except for neutrinos, and we're not sure if those are Majorana particles or not.

Photons, as you mention, are bosons, with spin 1, so they can't be their own antiparticle.

Photons are generally considered to be their own antiparticles: http://van.physics.illinois.edu/qa/listing.php?id=27107

Photons are something of a special case because they are massless. Gravitons, too. As bosons, their interactions are not limited by the Pauli exclusion principle, so they can not annihilate each other. They interact through different means (electromagnetic). They're both because they only have the common properties of particles and their antiparticles.

It's basically like saying "the number 0 is its own negative number". It's correct according to some definitions, but not useful.

Physicist here. You are confusing things. Having rest mass or Pauli-exclusion principle has nothing to do with qualification of being an anti-particle.

Z boson, for instance, does have mass and is its own anti-particle.

> It's basically like saying "the number 0 is its own negative number". It's correct according to some definitions, but not useful.

Photons have zero charge; an anti-particle has negative of the particle's charge (and at the same time, same rest mass and spin).

Aha, thanks for the correction, I must have picked up some bad info somewhere. I'll do my research better next time.

I'd say that zero being it's own negative number is an important thing not to forget. :)

Neutrons are fermions with neutral charge.

Exactly, and they also have spin 1/2, but we're unsure of their other properties that might make them Majorana particles.

They are composite particles and we know exactly how they behave: like fermions. They are not Majorana fermions, and nothing about them is mystery.

I explained how you're totally confusing things somewhere else in the thread.

My brain added an 'I' in the middle of "neutron" in the parent comment. Probably because I was in the middle of reading something else about neutrinos :) I definitely agree that neutrons are familiar territory.

There is no rule requiring antiparticles to also have spin 1/2, nor that they in general be n+1/2 spin particles (fermions).

All lepton/quarks observed have half half-integer spin so there are no examples there. The SUSY sleptons/squarks would have anti-particles but integer spin if they exist, though. The W+/W- have spin 1 and are eachother's antiparticle. Z0 and the photon are their own anti particles, also spin 1. Gluons (spin 1) have anti-particles that are all another types of gluon. For composite particles, anti-deuterium and anti-helium both have integer spin.

For what anti-particles actually are, I suggest looking up both C and CP conjugation.

> For a particle to be its own antiparticle, it would have to have spin 1/2

This is wrong. Being anti-particle has nothing to do with a particular spin. Photon is anti-photon (which by the way has spin 1).

There's still a distinction between matter and anti-matter for neutral particles. For instance, the neutron and the anti-neutron are distinct, despite being neutral. They have neutral charge, but opposite baryon number. The neutron will decay into a proton by emitting and electron, while the anti-neutron will decay into an anti-proton while emitting a positron. Conservation of baryon number prevents the neutron from decaying into an anti-proton, which would otherwise make neutron sources a cheap and convenient way of producing anti-protons.

That's because neutrons aren't fundamental particles. They are made up of (charged) quarks.

I literally just wanted to write the same.

> You mean like the photon and, iirc, all the other neutrally charged elementary particles?

You are missing the point. Photon (and every such other elementary particle that annihilates itself we know) is a boson.

Majorana fermion is a fermion whose anti-particle is itself. No such elementary particle exists (so far).

What these people have done is a way of arranging some electrons such that they behave like Majorana fermions.

As has been pointed out to me: ignore my previous comment. I got some fundamentals wrong and should leave the technical explanations to the real physicists :)

Is this a marijuana joke?

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