Comment by yk | Hacker News Reader

Comment by yk | Hacker News Reader

Someone should perhaps point out, that the article talks about quasiparticles. Quasiparticles are a bit similar to the particle aspect of a quantum mechanical wave. So for example deformations of the lattice of an crystal are sound waves, but it is also possible to look at the deformations as a field and then quantize the field. Excitations of this field look suspiciously similar to a particle, as in they are produced by the same mathematical formalism. The particles associated with excitations of the electro-magnetic field are photons, and the particles associated with excitations of this deformation field are called phonons. The difference is, that the electro magnetic field is a fundamental aspect of nature, while the deformation field only describes deformations of a given crystal and therefore photons are fundamental particles, while phonons are quasi particles.

Something similar happens in this discovery, there is some aspect of the dynamic in the crystal that can be described as a Weyl spinor. But that does not mean, that a new fundamental particle is discovered, it just means that a new dynamic in a crystal is observed. ( Which is of course cool in its own right, and probably technologically more relevant than a new fundamental particle.)

Thanks, great summary. To translate to non-physicists: It quacks like a particle, it walks like a particle but it's not really a particle. Like counting stones or apples, the both can be understood in terms of similar arithmetic. Same here, only more maths. And, those properties that were observed, may come in handy.

But what is a "real" particle anyway?

The difference between a bullet made of real particles and one made of math is that the real one can kill you.

That's true for values of "you" reading this in 2015, at least. :)

Quasi-particles are not just math. They have measurable properties that are like properties of real particles (just like "holes" in P-type semiconductors can be measured to behave like quasi-positrons). Or, replying to your proposition:

"With the holodeck safeties off, even holographic bullets can kill." - Picard, First Contact.

:)

> just like "holes" in P-type semiconductors can be measured to behave like quasi-positrons

I'm sorry to ask because I should actually know this, isn't the remaining positive charge accounted to the protons in the core? A moving "hole" is just a figure of speech.

Hmm, I think you got it in reverse. We are talking about of properties of real physical things all the time here. We just can't expect to instantiate any random piece of math in the real world - like take a specific vector subspace and try to use it as a katana, that would be silly.

We take a physical system and apply some mathematical properties to it. In the case of bullet I think what is interesting is to model it's energy, E = 1/2 mv*v. Now, if we take the kinetic energy of a bullet, and apply it to any other object, and then instantiate this object in the real world the lethality of the object depends on the nature of the object. A ball of hay of a diameter of 1 m in 1 atm 10 meters away with the kinetic energy of a bullet does not sound lethal. An object more like a bullet, like an iron nail, probably can be. Gummy bear? Who knows.

The point is, the maths here are about physical things, although it can take a lot of experimental work to manufacture a specific physical system that exhibits behaviour which is modeled with specific piece of math.

"real" particles are those that can leave the holodeck ([1]) "quasi-particles" vanish at the doorstep :)

[1] https://en.wikipedia.org/wiki/Holodeck

PS: replace "holodeck" with "tantalum arsenide crystal" to yield a less off-topic comment.

That's a really good explanation.

What about strongly interacting theories? Are quarks and gluons quasi-particles then too? They can't leave the holodeck aka. Hadron.

I think this could break your classification :)

Thank you for this explanation.

But hey, speaking of TNG... Can't you just stick on of those fancy crystals in your computer?

Wait, computers made out of crystals? Did that just happen? :)

Computers are already made out of fancy crystals. But these new crystals would be even fancier than the old ones.

One example happens in semiconductors

You have n-type material, where the charge carriers are electrons. So far so good (a real particle)

In p-type material, the charge carriers are the absence of an electron (called a hole).

It is of course, not a real particle, but it carries charge as well as a real particle.

> You have n-type material, where the charge carriers are electrons. So far so good (a real particle)

Even that: It's a quasi particle. It's an electron with an effective mass, which differs from the mass of an electron in free space.

The effective mass arises because of the collective electron interaction with the crystal atoms. At a range of high energies, the electron even exhibits the same behavior you'd expect from a relativistic particle (with something different than c for the ultimate velocity.)

That's just a figure of speech. The charge of an ion stems from the ratio of electrons to protons. To every "hole" there is a proton, which is a particle. I am repeating myself.

It's not just a figure of speech, it's a useful abstraction. The protons don't move, but the holes seem to do that. Not only that, if you pretend the holes have a certain amount of mass you can predict how they move, and they move similarly to how a positibly-charged particle would move in the same circumstances.

In this context, the particles that appear in the Standard Model. So a quasiparticle can be understood in terms of the underlying structure of matter ( a crystal in this case), a 'real' particle can not be understood in terms of some underlying mechanism. Granted, "can not be understood in those terms yet" is preliminary and a not really satisfying definition.

It is as real a particle as any other, no? If you look closely enough all particles are just waveforms in particular fields (e.g. an electron will "interfere with itself" in a double-slit experiment).

I suppose the word 'real' is kinda hard here. From classical experimental point of view electron is a particle that can be observed and has an identity and existence regardless of context, whereas quasiparticles such as phonons manifest only in mathematical models that 'can be instantiated' in specific experimental setup, or describe physics only in specific limited context, such as the experiment here.

I'm of the school though that adheres to the notion that maths can be used to model the world, and while it strongly suggests 'the world is made of math' this is not conclusively the case. So, to me, mathematical equivalence does not mean 'physically equal' since physical systems can exhibit many more degrees of freedom than just those captured in a specific model. Sometimes the maths and physics match astoundingly well.

Don't phonons manifest in sound, which is very audible?

Only in lattices I think - i.e. a specific substrate such as crystals:

https://en.wikipedia.org/wiki/Phonon

It' more complicated. The electrons are waves in a quantized field, so the math is more complicated. When you try to simplify the math, with some simplifications you get something that looks like a classical particle and with other simplifications you get something that looks like a classical wave. So in some experiments you get electrons that behave like a particle and in some experiments you get electrons that behave like a wave.

Anyway, it's difficult to say what a real particle is. The easiest example is the Z0 particle, because you can think it is a mix of the W0 particles with the B0 particles, https://en.wikipedia.org/wiki/W_and_Z_bosons#Z_bosons

Anyway, if we "agree" that a real particle is something that can travel in vacuum, then the electrons are real and the particles in this experiment are not real. But if you talk with the people in solid state physics, they are totally convinced that phonons are real particles in spite they can travel only inside a solid.

But from the point of view of a phonon, the solid is indistinguishable from a vacuum. What distinguishes the 'real' vacuum from what the phonon sees?

Why is what we call 'the universe' necessarily primal? Maybe it is 'vibrating solids' all the way down. If there is something primal, then what are its characteristics that allows what we see as the vacuum to emerge?

To explain this using human-sized things, let's think about centrifugal force and centripetal force.

The latter is an actual force. The former is a "fictitious force" resulting from the mathematical transformation between fixed and rotating reference frames. If you are in a rotating reference frame, the force feels real, along with Coriolis and Euler fictitious forces. But to an outside observer in the fixed reference frame, there is only centripetal force.

Similarly, the article is describing a quasi-particle within a meta-material crystal that behaves similarly to a magnetic monopole. Outside the crystal, the behavior may be explainable by other means, but electrons inside the crystal can't tell the difference between the quasi-particle and the real thing, in much the same way that you can feel a centrifugal force when you spin around.

So even if Weyls do not actually exist, we now have a metamaterial that can fake it well enough to fool an electron.

I read the article as saying that the discovery of the Weyl fermion, (albeit as a quasi-particle) provides a proof of concept that it actually can exist in nature (rather than being just an esoteric theoretical curiosity with no physical analogue) and that its manifestation in this form provides a strong indication that it also exists as a primary particle.

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