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How Many Elementary Particles Are There, Really?

quantamagazine.org

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Re: How Many Elementary Particles Are There, Really?

#81

Post by antonvs »

dpark wrote: post #55
antonvs wrote: post #21

Earlier quoted context omitted.

Every particle type has its own field, but the OP article is counting a single particle type multiple times based on properties like spin and polarization. At one point the article reaches the number 118. That corresponds directly to 37 quantum fields once you take the "double counting" into account.

Where are you getting 37? The standard model has 17 fields. If you pick and choose which properties to select as unique fields, maybe you can get the number 37, but at that point why not 118 fields?

> The standard model has 17 fields.

Without qualification, that's false. 17 is a simplified or compressed view of what the Standard Model describes. I gave more detail in this comment:

https://news.ycombinator.com/item?id=48700610

37 is what you get from counting Dirac matter fields (24) plus gauge fields (12) plus the Higgs. That's post-symmetry-breaking, and doesn't account for chirality.

If you count fundamental field components in the electroweak-symmetric Lagrangian, you get 43. I broke down both of those numbers in my comment linked above.

> If you pick and choose which properties to select as unique fields, maybe you can get the number 37, but at that point why not 118 fields?

There's no picking and choosing involved - quite the opposite. It's counting what the QFT math specifies. Particles with e.g. different color charges can't share the same field. To get to 17 from either of the above, you have to ignore quark color charges and the different gluon types. It's essentially a classification of types of particles that combines field together, it's not a count of fields.

Re: How Many Elementary Particles Are There, Really?

#83

Post by d4ng »

gus_massa wrote: post #79
d4ng wrote: post #30

There are also 17 wallpaper groups. That always seemed like a funny number. I know it's a long shot, but is there a relation?

As the article explains, counting is very hard. IMHO, I like to count the x3 colors of quarks and the x2 chirality of bosons. So I get 16*3 fermions and 1+8?+3?+1 bosons, in total 61 but the number of bosons is not a hill I will do die on. On the other extreme, there are some proposal to reduce the number of particles, in particular it makes a lot of sense to consider the electron and neutrino as a single class of pa…

Very interesting. I imagine though that the 17 model is reasonably consistent. I wonder what parallels there are with other areas of mathematics.

Re: How Many Elementary Particles Are There, Really?

#84

Post by antonvs »

antonvs wrote: post #73
AnimalMuppet wrote: post #34

Earlier quoted context omitted.

You've said that "37 fields" at least twice. It doesn't seem to come from the arxiv article you linked, though. And it seems rather high to me. (Of course, 118 seems ridiculously high...) Anyway: Would you list them? Or supply a link to somewhere that does?

First, just to clarify - there are different ways to count the quantum fields, just as there are different ways to count particles, as the article points out. You really need to specify the premises you're using to count them. But either 17 or 37 are natural counts. 17 is a somewhat simplified version, which ignores quark color charges and groups the W and Z bosons together. Here's how the list of 37 typically breaks…

Here's a more exact breakdown of the 118 particle number from the article:

72 quarks: 6 flavors x 3 colors x 2 (particle/antiparticle) x 2 (spin up/down)

12 charged leptons: 3 flavors x 2 (particle/antiparticle) x 2 (spin up/down)

6 neutrinos: 3 flavors x 2 (particle/antiparticle)

2 photons: 1 photon field x 2 polarizations

16 gluons: 8 types x 2 polarizations

9 electroweak bosons: 3 types (Z0, W+, W-) x 3 polarizations

1 Higgs boson

That totals 118. Here's a summary of how those come from the 37 fields I listed:

4 x 18 quarks

4 x 3 charged leptons

2 x 3 neutrinos

2 x 1 photons

3 x 3 electroweak bosons

2 x 8 gluons

1 x Higgs boson

Re: How Many Elementary Particles Are There, Really?

#85

Post by ziofill »

gus_massa wrote: post #71
ziofill wrote: post #62

Physicist here. I don’t buy some of these distinctions, like the chirality. Chirality is an observable, it’s like saying there are two photons because they can come in two polarizations, but polarization is not an inherent property: it depends on how we measure it. So I could describe any photon in the left/right chiral basis just as well as in the vertical/horizontal basis or any two antipodal points in the Poincaré…

Chirality is a real property of (most) elementary particles. For example the electron with left chirality has a weak hypercharge of -1, but the electron with right chirality has a weak hypercharge of 0. https://en.wikipedia.org/wiki/Weak_hypercharge#Definition In some sense, they are very different particles. Also, only the left version interact with the weak interaction.

Ah, then I was thinking about helicity

Re: How Many Elementary Particles Are There, Really?

#86

Post by antonvs »

Noaidi wrote: post #78
antonvs wrote: post #22

Earlier quoted context omitted.

A wave is a phenomenon that propagates through a field - i.e. the field is what allows the wave to exist. (The philosophy of that admittedly gets messy, though, e.g. "are fields real objects?")

Yes, very messy and ultimately unknowable.

Sure. We can say the same thing about a "quantum wave", though.

Re: How Many Elementary Particles Are There, Really?

#87

Post by rolph »

Noaidi wrote: post #9
colejohnson66 wrote: post #5

Earlier quoted context omitted.

That's what the various string theory proponents start from. There's "too many" different subatomic particles, so there surely must be something smaller that they're composed of?

How long can you break something apart until you cannot any longer? The things we are breaking apart are illusions in a sense. There will always be a smaller particle because that is what we are looking for. When we understand that everything that we see is a manifestation of a probability wave, then we will understand everything is a wave and end these foolish experiments.

planck scale is the theoretical limit to disassembly.

Re: How Many Elementary Particles Are There, Really?

#89

Post by pdonis »

unholiness wrote: post #54
pdonis wrote: post #40

Earlier quoted context omitted.

> W and Z bosons, photons, etc have fixed masses, charges, interaction strengths with other particles. But you can form a continuous set of linear combinations of these things, just as you can with gluons. Indeed, what the article calls W and Z bosons (and photons) are just such linear combinations--the ones that appear in the low energy limit after the electroweak phase transition occurs. Before that phase transitio…

In a hypothetical scenario where we were inventing the standard model in the first 10^-11 seconds after the big bang, you're right there would be an analogy there. But in that scenario, our standard model would say there was one electroweak particle, not that there were 8 gluons. In our own universe, the fact that electroweak symmetry breaks ensures there are 4 electroweak particles and not other combinations. There'…

> in that scenario, our standard model would say there was one electroweak particle

No, it wouldn't. There would still be four; they would just be called W1, W2, W3, and B. The electroweak vector space doesn't change when the electroweak symmetry is broken; it has 4 basis vectors before, and 4 basis vectors after. All that changes is which basis is the most "natural" to use in describing physics at the given energy scale.

(And there would still be eight gluons as well--what I say below about those applies just as well above the electroweak symmetry breaking energy scale as below.)

> There's no corresponding thing to contain gluons to individual particles

If you mean that there is no "natural" choice of basis for the gluon vector space, that's not quite true either. The Gell-Mann matrices are a natural choice of basis for the adjoint representation of SU(3) (or, equivalently, the defining representation of the Lie Algebra of SU(3)), which is the gluon representation. Those eight matrices are what physicists typically are referring to when they refer to the eight gluons.

Re: How Many Elementary Particles Are There, Really?

#90

Post by HarHarVeryFunny »

behringer wrote: post #70
HarHarVeryFunny wrote: post #67

Earlier quoted context omitted.

Interesting, but (way out of my depth here) why do these symmetries have to exist? It would be much more satisfying (not that nature exists to be satisfying) if we could explain our universe starting from some universal constraints on things that must be true of any non-random mechanistic universe, plus some set of ( This seems about as close as we could get to explaining our universe... Basically saying that god fli…

You might not want to visit because it's probable you would explode or have some other horrific death due to incompatibility between your fields and theirs.

Yes, probably would not turn out well. Will put it on my bucket list next to "visit black hole event horizon".
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