Cycle of Discovery: Atoms, Nuclei, Hadrons, and Standard-Model Particles

The discovery of the fundamental constituents of the Universe has gone in a cycle:

  1. Discovery of a few entities. Category may be poorly defined or not even properly recognized.
  2. Discovery of many entities. Well-defined category.
  3. Discovery of regularities among the entities.
  4. Discovery of underlying simplicity and the causes of the regularities.

These entities have gone through that cycle, with all but the Standard Model completing it:

  1. Atoms and chemical elements
  2. Atomic nuclei
  3. Hadrons
  4. Standard-Model particles

Atoms and chemical elements

Their first stage started at least two and a half millennia ago, with the first known speculations on the nature of matter dating back to then. Pre-Socratic Greeks proposed earth, water, air, fire, and sometimes aether as the fundamental constituents or elements, and Chinese proposed earth, wood, metal, water, and fire. Speculations on how divisible matter is also go back that far, with some Greeks and Indians proposing a limit: indivisible particles or atoms.

But alongside them were recognized different kinds of metals, with seven metals being recognized in Greco-Roman antiquity: gold, silver, copper, iron, tin, lead, and quicksilver. These got associated with the seven “planets”: the Sun, the Moon, Venus, Mars, Jupiter, Saturn, and Mercury, in that order. This is why quicksilver is more usually known to speakers of many languages as versions of “mercury”, though speakers of some languages use versions of “quicksilver” or “silverwater”.

The elements moved into the second stage with Antoine-Laurent de Lavoisier’s publication of his Elementary Treatise of Chemistry in 1789, which featured the first modern definition of elements, and also a list of them:

Light, heat, O, N, H, S, P, C, Cl, F, B, Sb, As, Bi, Co, Cu, Au, Fe, Pb, Mn, Hg, Mo, Ni, Pt, Ag, Sn, W, Zn, CaO, MgO, BaO, Al2O3, SiO2

Aside from light and heat, that is 31 presently-recognized chemical elements or oxides of them. Lavoisier himself only classified them into metals and nonmetals, but other chemists found additional regularities, and even tried to organize them into a Periodic Table of Elements.

In 1869, Dmitri Mendeleev announced his version, which included gaps for elements that he predicted. He likely felt very justified in doing so, because since Lavoisier’s time, some 31 more elements had been discovered, and he could easily have concluded that there may be more to discover. His predictions were successful; the missing elements, gallium and germanium, were discovered in 1875 and 1886, and their properties were close to his predictions.

This moved the chemical elements into the third stage, and while it was happening, scientists were making progress on their divisibility. Between 1798 and 1804, Joseph Proust did several experiments, and showed that some kinds of mixtures follow a Law of Definite Proportions, while some do not. The definite-proportion mixtures we now call compounds. John Dalton showed that atomism explained this law very well, and even estimated their relative weights. His successors expanded on his work, and showed how various properties of gases could be accounted for by supposing them to be swarms of atoms and molecules (groups of atoms stuck together) bouncing around while seldom colliding. These included the Ideal Gas Law:

(pressure) = (number density) * k * (temperature)

But toward the end of the 19th cy., physicists started discovering evidence that atoms were composite. In 1896, J.J. Thomson had showed that “cathode rays” are composed of “electrons”, particles with a charge-to-mass ratio about 1800 times that of the highest value for a charged atom. But what was the positively-charged part like? Distributed across the atom with the electrons residing in it like plums in a plum pudding, though many physicists.

In 1909, Ernest Rutherford, Hans Geiger and Ernest Marsden decided to test that hypothesis by shooting alpha particles from radium at some gold foil. Most of the alphas went through, but some were deflected, and a few of them bounced backward. This startling result was like an artillery shell bouncing off of tissue paper, wrote Rutherford. The positively-charged part was a “nucleus” much smaller than an atom, typically around 100,000 times smaller.

But why don’t electrons spiral in to nuclei? Solving that conundrum helped quantum mechanics develop. Physicists worked out that electrons are waves as well as particles, and and their wave nature means that if they are confined to close to a nucleus, then they must move fast, pushing up their total energy. So in an atom, the electrons have spiraled in as far as they could go.

Elements and atoms now moved into the fourth stage, with quantum chemists working out how their properties are derived from the behavior of their orbiting electrons. It takes a lot of computerized number-crunching to get good numbers, but quantum chemists have risen to that challenge, getting reasonable agreement for individual atoms and small molecules.

Atomic nuclei

They quickly skipped through the first stage of their discovery and entered their second stage, as Rutherford and others showed that having nuclei was not just a quirk of gold atoms. Each element had its own kind of atomic nucleus, and Rutherford discovered in 1913 that some elements have several several kinds or “isotopes”. Rutherford also discovered in 1921 that smashing alpha particles (helium-4 nuclei) into nitrogen makes hydrogen-1 nuclei, which he named protons.

It was quickly discovered that isotopes’ masses were approximately integral multiples of the hydrogen-1 mass, and in 1921 Ernest Rutherford speculated that most nuclei contain “neutral protons”. This sent nuclei into the third stage, and they moved into the fourth stage with the discovery of these neutral protons or neutrons in 1932 by Ernest Chadwick.

This was soon followed by Carl Friedrich von Weizsaecker’s semi-empirical mass formula, which treats nuclei as liquid drops, and which has been reasonably successful. Several physicists then developed a “shell model” for nuclear structure, in analogy with with electrons in atoms; it also has had a fair amount of success. Calculating nuclear structure from the interactions of individual protons and neutrons has been very difficult, requiring a lot of number crunching, but that also has been done.


Hadrons entered the first stage with the discovery of the proton. When neutrons were discovered, it was quickly recognized that they and protons are held together in nuclei by a force much stronger than the protons’ electromagnetic repulsion.

Starting in the late 1940’s, more and more strongly-interacting particles were discovered, and hadrons moved into the second stage. Many of them are so short-lived that they only show up as resonances or spikes in the parent particles’ reaction rates. Enrico Fermi famously complained about this particle zoo that “If I could remember the names of all these particles, I’d be a botanist.”

Hadrons entered the third stage with Murray Gell-Mann’s and George Zweig’s quark model of 1964, though for some years afterwards, a lot of physicists had questions about what kind of entity a quark was. Were quarks real particles or some sort of theoretical abstraction? But in 1968, particle-accelerator experiments showed that protons are made of “partons”, and further experiments on them showed that partons were quarks, thus moving hadrons into the fourth stage by 1973-74.

Calculating the structures and interactions of hadrons from first principles can be done, but it requires dividing space and time into a 4D lattice and then doing a LOT of number crunching. But it has recently been possible to predict the mass of the proton to within 2%.

Standard-Model particles

The Standard Model’s particles spent much longer in the first stage than nuclei or hadrons.

The first one discovered was the photon or electromagnetic field, and its discovery followed a sequence similar to my four stages. The first electromagnetic phenomenon to be discovered was visible light, a discovery that is likely as old as humanity. Electrostatic and magnetic effects were next; one of the first to notice them was Thales of Miletus around 600 BCE. But it was not until the 19th cy. that their interconnections were discovered and mathematical descriptions worked out. Electric currents are moving electric charges. Electric charges make electric fields and interact with them. Electric currents make magnetic fields and interact with them. A changing magnetic field makes an electric field around itself. Light is polarized, and when it travels through a material with a magnetic field applied, its polarization plane can rotate (Faraday rotation).

These descriptions were unified in 1873 by James Clerk Maxwell in his famous equations, which included an additional term, the “displacement current”, in which a changing electric field makes a magnetic field around it the way that an electric current does. He discovered wave solutions, with the waves having polarization and traveling at the speed of visible light in a vacuum.

Heinrich Hertz followed up by making electromagnetic waves with macroscopic currents: radio waves. Over the next half century or so, it was discovered that molecules, atoms, and nuclei can act like miniature antennas when they change state, emitting and absorbing infrared, visible, ultraviolet, X-ray, and gamma-ray spectral lines at strengths that can be predicted.

Electrons were the next Standard-Model particle discovered, in 1896. Protons were discovered in 1921 and neutrons in 1932, but they were not shown to be composite for nearly half a century, and the next Standard-Model particle discovered was the muon in 1936. Wolfgang Pauli speculated about neutrinos in 1930, noting the missing energy and angular momentum of beta decays, and they were discovered in 1956. With speculations about quarks and W particles and the like, the Standard Model entered the second stage in the 1960’s.

From there, it gradually moved into the third stage starting in the late 1960’s, with Sheldon Glashow, Steven Weinberg, and Abdus Salam proposing the electroweak theory in 1968 and quantum chromodynamics (QCD) being developed in the late 1960’s and early 1970’s.

The electroweak theory includes the photon, of course, and the W, to explain weak-interaction decays. It predicted a neutral version of the W, the Z, and the first evidence of the Z appeared in 1973. The W and the Z were discovered more directly in 1983, by seeing decays that fitted what was predicted for those particles.

QCD states that quarks are held together by gluons, which also interact with each other. Quarks and gluons cannot go more than about 10-15 m from each other (quark/gluon confinement), but smashing them at each other probes their behavior at smaller length scales, where they are more weakly coupled to each other. Energetic quarks and gluons make jets of hadrons as they separate, and quark-quark-gluon jet events were first observed in 1979.

Turning to quarks, the first “flavors” discovered were up, down, and strange. Protons are up-up-down and neutrons up-down-down, and the strange quark got its name because of how particles containing it decayed at weak-interaction rates rather than the much faster strong-interaction rates. The charm quark was proposed in 1965 by Sheldon Lee Glashow and James Bjorken to fit in with the weak interactions, and a particle containing it, the J/psi particle, was found in 1974. A year before, Makoto Kobayashi and Toshihide Maskawa proposed that weak-interaction CP violation implies the existence of at least six quark flavors; the bottom quark was found in 1977 and the top quark in 1995.

The Standard Model has been in the third stage since the mid to late 1970’s, but various physicists have been trying to take it into the fourth stage with Grand Unified Theories and Theories of Everything (GUT’s and TOE’s). The track record with atoms, nuclei, and hadrons suggests that there is likely such a theory, but the details are not very well constrained by the Standard Model, and it will be difficult to do particle-accelerator experiments at GUT and TOE energy scales (1016 and 1019 GeV — a proton’s mass is about 1 GeV).

That’s a big reason for interest in the Large Hadron Collider’s upcoming results; what particles it will make — or will not make. These particles include some missing pieces of the the Standard Model puzzle, like the Higgs particle and supersymmetry partners of the known particles. Finding these particles, if they exist, will improve our ability to extrapolate to GUT energies and test various unification hypotheses there, possibly bringing the Standard Model into the fourth stage.

I have not mentioned gravity, because it is hard to construct a self-consistent quantum theory of it, and because it has had an entirely separate discovery track. But quantum gravity is an essential part of a TOE, because nongravitational particles and interactions are successfully described by quantum field theories. The most successful TOE to date has been string theory, which incorporates gravity in a natural way, but which does not predict the Standard Model with even an extremely rough approximation of unambiguity — one can get oodles of other low-energy limits from it.

So while we may soon get closer to the fourth stage for the Standard Model, our efforts to include gravity may continue to be stuck in the third stage.


One Response

  1. […] Cycle of Discovery: Atoms, Nuclei, Hadrons, and Standard-Model … […]

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