A Neutral Party

“Hi, folks, sorry I’m late to the party. What are we arguing about and which side am I on?”

“Hi, Vinnie. We started out talking about neutrality and Jim proved that we’re electrically neutral otherwise we’d spray ourselves apart because of like‑charge repulsons.”

“Yeah, an’ then we got into the Standard Module picture here and how it’s weird that the electron charge exactly cancels out the quark mixture in a proton even though electrons don’t have quarks and quarks don’t have exact charges.”

Jim’s on it. “Almost, Eddie. Quarks have exact charges, but they’re exact fractions. They just add up when you mix three of them to make a particle. Two of them, sometimes. Up‑quark, up‑quark and down‑quark is two‑thirds plus two‑thirds minus one‑third equals one. That’s one proton, exactly opposing one electron’s charge.”

Vinnie’s good at mental math. “What happens when you mix one‑third plus one‑third minus two‑thirds which is zero?”

“Two downs and an up. That’s a neutron.”

“Ups, downs, electrons, protons, neutrons — except for the neutrino the first column’s pretty much atoms, right? What’s with those other boxes?”

“We only see evidence for the other purple‑box quarks in collider records or nuclear reactions. Same for the muon and tau. They’re all way too unstable to contribute much to anything that hangs around. The guys in the red and gold boxes aren’t building blocks, they’re more like glue that holds everything else together. The green‑box neutrinos at the bottom are just weird and we’ll probably be a long time figuring them out.”

“Says here that neutrinos have zero charge, and so do most of the force thingies. Is that really zero or is it just too small to measure?”

“A true Chemistry‑style question, Susan. Charges we can count but you’re right, energy exchanges in a process have to be measured. The zero charges are really zero. For example, Pauli dreamed up the neutrino as an energy‑accounting trick for a nuclear process where all the charges went to known products but there was energy left over. If they existed at all, neutrinos could carry away that energy but they had to have zero charge. A quarter‑century later we detected some and they fit all the requirements.”

Vinnie perks up. “Zero charge so they doesn’t interact with light, teeny mass per each but there’s a hyper‑gazillion of them out there which oughtta add up to a lot of mass. Could neutrinos be what dark matter is?”

“Some researchers thought that for a while but the idea hasn’t held up to inspection. The neutrinos we know about come to about 1% of dark matter’s mass. Some people think there may be a really heavy fourth kind of neutrino that would make up the difference, but it’s a long shot and there’s no firm evidence for it so far. Dark matter doesn’t interact with photons, photons interact with electric charge, quarks have electric charge. If you’ve got quarks you’re not dark matter.”

“How about neutrons floating around?”
 ”Those molecular clouds I’ve read about Aren’t they neutral? Are there neutral stars?”
  ”How about neutron stars and black holes?”
   ”What’s a neutron star?”

“All good questions. Free neutrons are a bad bet, Vinnie — unless they’re bound with protons they usually emit an electron and become a proton within an hour. Susan, electrostatic forces would overwhelm gravity so we believe stars and molecular clouds must be electrically neutral or close to it. Anyway, stars and clouds can’t be dark matter because they’ve got quarks. Eddie, what do you suppose happens when a star uses up the fuel that keeps it big?”

“Since you ask it that way, I suppose it caves in.”

“Got it in one. If the star’s too big to collapse to be a white dwarf but too small to collapse to be a black hole, it collapses to be a neutron star. Really weird objects — a star‑and‑a‑half of of mass packed into a 10‑kilometer sphere, probably spinning super‑fast and possessing a huge magnetic field. From a ‘what is dark matter?‘ perspective, though, collapsed stars of any sort are still made of quarks and can’t qualify.”

“So what is dark matter then?”

“Good question.”

~~ Rich Olcott

  • Thanks to Alex, who asked a question.

Quarkery

Susan, aghast. “But I thought the Standard Model was supposed to be the Theory of Everything.”

Jim, abashed. “A lot of us wish that phrase had never been invented. Against the mass of the Universe it’s barely the theory of anything.”

Me, typecast. “That’s a heavy claim, Jim. Big Physics has put many dollars and fifty years of head time into filling out that elegant table of elementary particles. I remember the celebration when the LHC finally found the Higgs boson in 2012. I’ve read that the Higgs field is responsible for the mass of the Universe.”

“A little bit true, Sy, sort of. We think it’s responsible for about 1% of the mass of all the matter we understand. There’s another mechanism that accounts for the other 99%.”

Eddie, downcast. “I’m lost, guys. What Standard Module are you talking about?”

“Do you remember the Periodic Table of the chemical elements?”

“A little. Science class had big poster up on the wall. Had all kinds of atoms in it, right?”

“Yup. Scientists spent centuries breaking down minerals and compounds to find substances that chemical methods couldn’t break down any further. Those were the chemical elements, things like iron and carbon and oxygen. The Periodic Table arranges elements so as to highlight similarities in how they’ll interact. The Standard Model carries that idea down to the sub‑subatomic level.”

“Wait, sub‑subatomic level?”

“Mm-hm. Chemists would say that ‘subatomic‘ is about electrons, protons and neutrons. Count an atom’s electrons. That and some fairly simple rules can tell you what structure types it prefers to participate in and what it reacts with. Count the protons and neutrons in its nucleus. That gives you its atomic weight and starts you on the road to figuring reaction quantities. That’s all that the chemists need to know about atoms. All due respect, Susan, but physicists want to dig deeper. That’s what the Standard Model is all about.”

“So you’re saying that the protons and neutrons are made of these … quarks and things? Is that what comes out of those collider experiments?”

“No on both, Eddie. You ever whack a light pole with a baseball bat?”

“Sure, who hasn’t?”

“The sounds that came out, do you think the pole was made of them?”

“Course not, and I never bought the Brooklyn Bridge, neither.”

“Calm down, Eddie, just making a point. Suppose before you whacked that pole you’d attached a whole string of sensitive microphones all up and down it, and then when you whacked it you recorded all the vibrations your whack set off. Do you think with the recorded frequencies and a lot of math a good audio engineer could tell you what the pole is made of and how thick the casing is?”

“Maybe.”

“That’s what’s going on with the colliders. They whack particles with other particles, record everything that comes out and use math to work out what must have happened to make that event happen. Theory together with data from a huge number of whacks let people like Heisenberg, Gell‑Mann, Ne’eman and Nishijima to the seventeen boxes in that table.”

“‘Splain those particles to me.”

“Don’t think particles, think collections of properties. The Periodic Table’s ‘iron‘ box is about having 26 electrons and combining with 24 grams of oxygen to form 80 grams of Fe2O3. In the Standard Model table, the boxes are about energy, charge, lifetime, some technical properties, and rules for which can interact with what. We’ve never seen a free‑standing quark particle and there’s good reason to think we never will. We mostly see only two‑ or three‑quark mixtures. Some of the properties, like charge, simply add together. It takes a mixture to make a particle.”

“Then how did they figure what goes into a box?”

“Theoreticians worked to find the minimum set of independent properties that could still describe observations. Different mixtures of up and down quarks, for instance, account for protons, neutrons and many mesons.”

Vinnie, at last. “Hi, folks, sorry I’m late to the party. What are we arguing about and which side am I on?”

Higgs candidate LHC event trace
Electrons (green) and muons (red) exiting the event

~~ Rich Olcott

Neutral

It’s that kind of an afternoon. Finished up one project, don’t feel much like starting another. Spring rain outside so instead of walking to Al’s for coffee I take the elevator down to Pizza Eddie’s on 2. Looks like other folks have the same feeling. “Afternoon, all. What’s the current topic of conversation?”

“Well, Sy, it started out as Star Wars versus Star Trek but then Jim said he could care less and Susan said that meant he did care and he said no, he’s ambivalent and she said that still meant he cared, and—”

“I get it, Eddie. Susan, why does ‘ambivalent‘ mean Jim cares?”

“Chemistry, Sy. ‘Valence‘ means ‘bonding‘ and ‘ambi-‘ means ‘both‘ so ‘ambi‑valent‘ means ‘bonded to both‘.”

“But Susan, ambidextrous means able to use both hands, not unable to use either hand. I want to say I don’t particularly like or dislike either one.”

“It’s like trying to decide between fire ants or hornets. You could say ‘No‑win,’ right?”

“No, that’s not it, either, Eddie. That’s ‘everybody loses.’ I’m smack in the middle.”

“Sounds like absolute neutrality. Hard to get there.”

“Don’t look at Chemistry. If I take an acid solution and add just enough base to get to neutral pH, there’s still tenth‑micromolar concentrations of acid and base in there. I guess we could call that ambivalent.”

“Neutrality’s hard for humans and chemicals, yeah, but that’s where the Universe is.”

“Why do you say that, Jim?”

“Because we’ve got proof right in front of us. Look, planets and stars and people exist as distinct objects, right? They’re not a finely-divided mist.”

“So?”

“So if the Universe were not exactly electrically neutral, then opposite charges repelling would split everything apart.”
 ”Wait, nothing would have a chance to form in the first place.”
   ”Wait, couldn’t you have lumps of like 99 positives and 100 negatives or whatever that just cancel out?”

“Eddie, when you say ‘cancel out’ you’re still talking about being absolutely neutral at the lump level. It’s like this table salt that has positive sodium ions and negative chlorides but the crystals are neutral or we’d get sparks when I pour some out like this.”
 ”Hey, don’t waste the salt. Costs money.”

“I still think it’s weird how all electrons have the same charge and it’s exactly the same as the proton charge. Protons are made of quarks, right, and electrons aren’t. So how can you take three of something and have that add up to exactly one of something different?”

“I can give you Feynman and Wheeler’s answer to part of that, Susan. The electron has an anti‑partner, the positron, which is exactly like the electron in every way except it has the opposite charge. When electron and positron meet they annihilate to produce a burst of high‑energy photons. But there’s a flip side — high‑energy photons sometimes interact to make an electron‑positron pair. Feynman and Wheeler were both jokers. They suggested that a positron could be an electron traveling backward in time. Wheeler said, ‘Maybe they’re all the same electron,’ zig‑zagging across eternity. But that doesn’t account for the quarks. A proton has two up‑quarks, each with a charge of negative 2/3 electron, and one down‑quark with a charge of positive 1/3 electron. Add ’em up — you exactly neutralize one electron. Fun, huh?”

“Fun, Jim, but I’m a chemist. On a two-pan balance I can weigh out equal quantities of molasses and rock dust but I don’t expect them to interact with any simple mathematical relationship. Why should the quark’s charge be any exact multiple or divisor of the electron’s? And why is the electron charge the size it is instead of some other number?”

“Well, there you’ve got me. The quantum chromodynamics Standard Model has been amazingly successful for quantitative predictions, but not so good for explaining things outside of its own terms. The math lays out the relationship between quark and electron charge, but doesn’t give us a physical ‘why.’ The theory has 19 ‘adjustable constants’ but no particular reason why they should have the specific values that fit the observations. Also, the theory doesn’t include gravity. It’s a little embarrassing.”

“Sounds like you’re ambivalent about the theory.”

~~ Rich Olcott