No, they haven’t, and no, that’s not what they found, and no, that doesn’t make sense.
Quantum field theory is how we understand particle physics. Each fundamental particle comes from a quantum field, a law of nature in its own right extending across space and time. That’s why it’s so momentous when we detect a fundamental particle, like the Higgs, for the first time, why it’s not just like discovering a new species of plant.
That’s not the only thing quantum field theory is used for, though. Quantum field theory is also enormously important in condensed matter and solid state physics, the study of properties of materials.
When studying materials, you generally don’t want to start with fundamental particles. Instead, you usually want to think about overall properties, ways the whole material can move and change overall. If you want to understand the quantum properties of these changes, you end up describing them the same way particle physicists talk about fundamental fields: you use quantum field theory.
In particle physics, particles come from vibrations in fields. In condensed matter, your fields are general properties of the material, but they can also vibrate, and these vibrations give rise to quasiparticles.
Probably the simplest examples of quasiparticles are the “holes” in semiconductors. Semiconductors are materials used to make transistors. They can be “doped” with extra slots for electrons. Electrons in the semiconductor will move around from slot to slot. When an electron moves, though, you can just as easily think about it as a “hole”, an empty slot, that “moved” backwards. As it turns out, thinking about electrons and holes independently makes understanding semiconductors a lot easier, and the same applies to other types of quasiparticles in other materials.
Unfortunately, the article I linked above is pretty impressively terrible, and communicates precisely none of that.
The problem starts in the headline:
Scientists have finally discovered massless particles, and they could revolutionise electronics
Scientists have finally discovered massless particles, eh? So we haven’t seen any massless particles before? You can’t think of even one?
After 85 years of searching, researchers have confirmed the existence of a massless particle called the Weyl fermion for the first time ever. With the unique ability to behave as both matter and anti-matter inside a crystal, this strange particle can create electrons that have no mass.
Ah, so it’s a massless fermion, I see. Well indeed, there are no known fundamental massless fermions, not since we discovered neutrinos have mass anyway. The statement that these things “create electrons” of any sort is utter nonsense, however, let alone that they create electrons that themselves have no mass.
Electrons are the backbone of today’s electronics, and while they carry charge pretty well, they also have the tendency to bounce into each other and scatter, losing energy and producing heat. But back in 1929, a German physicist called Hermann Weyl theorised that a massless fermion must exist, that could carry charge far more efficiently than regular electrons.
Ok, no. Just no.
The problem here is that this particular journalist doesn’t understand the difference between pure theory and phenomenology. Weyl didn’t theorize that a massless fermion “must exist”, nor did he say anything about their ability to carry charge. Weyl described, mathematically, how a massless fermion could behave. Weyl fermions aren’t some proposed new fundamental particle, like the Higgs boson: they’re a general type of particle. For a while, people thought that neutrinos were Weyl fermions, before it was discovered that they had mass. What we’re seeing here isn’t some ultimate experimental vindication of Weyl, it’s just an old mathematical structure that’s been duplicated in a new material.
What’s particularly cool about the discovery is that the researchers found the Weyl fermion in a synthetic crystal in the lab, unlike most other particle discoveries, such as the famous Higgs boson, which are only observed in the aftermath of particle collisions. This means that the research is easily reproducible, and scientists will be able to immediately begin figuring out how to use the Weyl fermion in electronics.
Fundamental particles from particle physics, like the Higgs boson, and quasiparticles, like this particular Weyl fermion, are completely different things! Comparing them like this, as if this is some new efficient trick that could have been used to discover the Higgs, just needlessly confuses people.
Weyl fermions are what’s known as quasiparticles, which means they can only exist in a solid such as a crystal, and not as standalone particles. But further research will help scientists work out just how useful they could be. “The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we’re just not capable of imagining now,” said Hasan.
In the very last paragraph, the author finally mentions quasiparticles. There’s no mention of the fact that they’re more like waves in the material than like fundamental particles, though. From this description, it makes it sound like they’re just particles that happen to chill inside crystals, like they’re agoraphobic or something.
What the scientists involved here actually discovered is probably quite interesting. They’ve discovered a new sort of ripple in the material they studied. The ripple can carry charge, and because it can behave like a massless particle it can carry charge much faster than electrons can. (To get a basic idea as to how this works, think about waves in the ocean. You can have a wave that goes much faster than the ocean’s current. As the wave travels, no actual water molecules travel from one side to the other. Instead, it is the motion that travels, the energy pushing the wave up and down being transferred along.)
There’s no reason to compare this to particle physics, to make it sound like another Higgs boson. This sort of thing dilutes the excitement of actual particle discoveries, perpetuating the misconception of particles as just more species to find and catalog. Furthermore, it’s just completely unnecessary: condensed matter is a very exciting field, one that the majority of physicists work on. It doesn’t need to ride on the coat-tails of particle physics rhetoric in order to capture peoples’ attention. I’ve seen journalists do this kind of thing before, comparing new quasiparticles and composite particles with fundamental particles like the Higgs, and every time I cringe. Don’t you have any respect for the subject you’re writing about?
“The ripple can carry charge…”
Wow, that’s kind of cool. By faster, I assume not FTL. (I seem to recall that electrons in copper move about 1/3 c? Certainly a lot of room for faster there.)
Right, not FTL. The article makes it sound like it’s the speed of light, but I don’t know if that’s another inaccuracy.
LikeLiked by 1 person
Excellent post. Unfortunately physicists themselves tend to encourage this type of misleading and confused journalism with the hope that it will generate more hype. Also, many of the concepts and terminology in solid state physics (such as effective mass) use looser analogies than the highly precise language that physics is typically known for. I think physicists sometimes think that because quasiparticles are a quantum phenomena, that justifies using ‘hype’ terms and not bothering to explain things in terms of classical phenomena. However, most quasiparticles have a classical analog. A classical phonon is just a sound wave or lattice vibration. A classical plasmon is just the entire free electron gas shifting relative to the position of the nuclei (or a wave in that gas). A ‘hole’ is just a missing electron. In my view, it is easier (and more physically accurate) to start by explaining the classical analog and then say that because of quantum mechanics, the excitations are quantisized, so they exist only only with discrete amounts of energy. The quantum mechanics is undoubtedly hard to convey, but the classical picture is easy. The only example I know of where this is not possible is the fractional quantum hall states. The ‘Weyl fermion’ may fall into a similar category, but in that case it’s still possible to relate it to classical / everyday physics, for instance, by mentioning that they only exist at very low temperatures and exist as a type of wave, etc.
I agree the article is a disaster. But, I also get that lots of educated laypersons who have taken a physics class or two and read books on physics may be much more familiar with classical mechanics, classical electromagnetism, nuclear physics, and particle physics, than they are with condensed matter physics which starts to overlap heavily with what is usually taught in an undergraduate college curriculum in inorganic chemistry, in geology, and in engineering classes. Most K-12 science teachers will have never taken a class in condensed matter physics. And, I’ve never seen a college offer a stand alone undergraduate condensed matter physics course in a college catalog (I’m sure there is one somewhere, but its not very common). Given how many physicists are specialists in the field of condensed matter physics, that is a real disservice.
I disagree that there is much of a physical difference between Higgs particle and say a quasiparticle like phonon. Sociological difference, yes , difference in technology required to produce them, yes – but no physical difference.
A particle is rigorously defined as an irreducible representation of relevant space-time symmetries. Both of them satisfy that definition. You can define them as poles in a Green function which also works for both. Experiments one uses to detect either of them
differ in energies, but both of them involve looking at some kind of scattering and decoding the result.
Usually, the word
elementary' andfundamental’ are bandied about in this context- but these are strictly classical notions which do not have any precise non-perturbative mathematical definition in a quantum field theory. This is especially clear in dualities where `what is elementary’ does not even have a duality invariant answer (is type IIA F1 string more elementary than type IIB F1 string ?). That shows that asking what is elementary is mathematically same as asking a non-gauge invariant question. Such a question has no physical meaning in a quantum theory.
But classical intuitions are hard to overcome (and might even be a useful way to think about things in a perturbative regime). So they persist.
In the underlying theory of the universe, both Higgs and phonons are quasiparticles. You can
easily imagine an alien civilisation where both are studied under the same field and called by the same name. One can also easily imagine a place where Higgs mechanism and superconductivity are taught always in the same course and thought of in same terms. Thus, all distinctions between them say more about history/sociology of human scientific endeavour, post world war II US/European funding structure, the resultant sociological emergence of the word `condensed matter’ etc. than about the particles themselves.
I don’t think that the notion of a “fundamental particle” is purely classical. I agree that, fundamentally, the Standard Model and CMT QFTs are attempting to do the same sort of thing, modeling some part of the universe using a low-energy effective field theory. But the part of the universe modeled by the Standard Model is substantially larger than that modeled by any particular CMT system. Neither is “fundamental”, in the sense that both are low-energy consequences of some high energy, nonperturbative, duality-ridden theory…but one describes essentially everything we have access to, and the other describes a few blocks of material in specialized labs.
I do agree that CMT and HEP-theory are much closer kin than most people realize, and probably will get closer in future.