The Alchemists’ Dream

by Nicholas Mee on September 23, 2017

The alchemists’ dream was to understand and control the structure of matter and to turn base metal into gold. Modern physicists are equally curious, but hopefully rather less avaricious.

Four Forces

The Cockcroft-Walton machine.

The 20th century saw the development of machines to delve deep into the structure of matter. John Cockcroft and Ernest Walton constructed the first particle accelerator that could interrogate the atomic nucleus in 1932. It is now in the Science Museum in London and looks like a machine that Dr Frankenstein would have been proud of.

By the middle of the century all known phenomena could be described in terms of just four forces. Gravity controls the large-scale structure of the universe, but the intrinsic weakness of gravity means its effect is utterly negligible between pairs of fundamental particles, such as electrons in atoms. By contrast, the other three forces all play important roles in particle physics. The electromagnetic force holds atoms together. It gives us the electrical industry and the whole science of chemistry. The strong force binds the nucleus of an atom together. The weak force is responsible for some types of radioactivity and plays a critical role in synthesizing atoms in the stars.

A Modern Alchemist

There has been an equally impressive consolidation in understanding the particles on which these few forces act. During the 1950s and 1960s particle accelerators revealed new particles at a disconcerting rate. Murray Gell-Mann was the leading architect of new ideas that would bring order to the particle mayhem. Known for his broad spectrum of interests, which range from linguistics to ornithology, he is shown here playing an arithmetical African game known as oware. The game is said to have a symbolic significance. The board is sometimes set east to west to align with the rising and setting sun. If the board is the world, the stones are the stars and the cups are the months of the year. Moving the stones is said to mimic the gods moving through space and time.

Gell-Mann playing oware.

Gell-Mann discerned a deeper substructure to the multitude of particles that feel the strong force. He proposed in 1964 that these particles, which include protons, neutrons and their more exotic relatives, are formed from a new class of fundamental particles called quarks—a nonsense word Gell-Mann borrowed from a line in James Joyce’s mind-bending novel Finnegans Wake.

Initially Gell-Mann deduced the existence of three types of quark known as up, down and strange. Three more quarks have since been discovered: the charm, bottom and top quarks, making a total of six. A similar model was proposed by George Zweig who referred to these sub-components as aces. A key feature of these models is that protons contain two up quarks and one down quark, while neutrons are formed of two down quarks and one up quark. Particles composed of quarks are known collectively as hadrons.

Left: proton, Right: neutron, where d labels the down quarks and u labels the up quarks.


Richard Feynman with some of his particle interaction diagrams.

In the late 1940s Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga and Freeman Dyson devised a quantum theory of electromagnetism. Quantum electrodynamics, or QED as it is usually known, provides an incredibly precise explanation of the electromagnetic force at the level of particle interactions. According to QED, electrons, protons and other charged particles interact by passing photons—the fundamental particles of light—back and forth. Feynman is celebrated on the stamp shown here, where he is depicted along with the diagrams he invented to represent particle interactions. QED has also proved to be the perfect model for constructing theories of the other forces.

Gluing the Quarks Together

The strong force is described by a theory called quantum chromodynamics (QCD) in which quarks interact through the exchange of particles known as gluons. Why are they called gluons? Because they provide the glue to hold quarks together, of course. QCD is like QED on steroids. It is the same sort of theory, but while there is just one photon, there are eight types of gluon. Also, photons are uncharged and therefore don’t interact with other photons, whereas gluons feel the strong force, so they do interact with other gluons. This makes QCD calculations more complicated, but the theory works incredibly well and provides a wonderful description of the workings of Nature at very short distances, such as within the nucleus of an atom.


The weak force has its own idiosyncracies. It operates through the exchange of three particles known as the W+ (W-plus), W (W-minus) and Z bosons. Unlike photons, which are massless, this trio are heavyweights. It is as though the weak force operates by lobbing cannonballs between particles, rather than ping pong balls, which accounts for its weakness and short range. Peter Higgs and two other teams of physicists independently invented a new quantum field to give mass to the weak force exchange particles. If the theory was correct, the electromagnetic and weak forces would now be seen as two aspects of a single unified electroweak force, despite their obvious differences. A critical prediction of the theory is the existence of a new fundamental particle—the Higgs boson.

The Large Hadron Collider (LHC) is the modern descendent of early atom smashers such as the Cockcroft-Walton machine. It is the highest energy accelerator of all time. In the LHC the projectiles are protons, which are accelerated in two beams that intersect at four points around the machine where head-on collisions occur. Hence the name—it is a large machine, 27 kilometres in circumference, in which protons, which are hadrons, are smashed into each other.

Almost all particles created in collider experiments are unstable. Such particles rapidly transform into two or more lighter particles that carry away the energy released. Eventually the only particles left are members of a small collection of stable particles. These include the protons and electrons, photons and neutrinos. If these particles were not stable there would be no atoms and no light.

On 4th July 2012 CERN announced the LHC had found the Higgs boson. This completes the unification of the electromagnetic and weak forces—the greatest unification of forces since Faraday and Maxwell 150 years ago.

The beam pipe of the LHC during construction.

A Triumph of Modern Physics

The combination of the electroweak force and the strong force is known as the Standard Model. This incredibly successful theory is one of the great triumphs of modern physics. It explains the structure of matter in terms of a handful of fundamental particles and a couple of forces.

Fundamental particles divide naturally into two classes: fermions and bosons. Fermions are the particles from which matter is composed. They are named after Enrico Fermi. Bosons are particles such as photons that are exchanged between other particles to produce forces. They are named after Satyendra Nath Bose.

There are just 12 fundamental fermions along with their antiparticles. These fermions form three generations of four particles. The first generation consists of the up and down quarks, the electron and the electron neutrino shown in the first column below. Ordinary matter is formed from the first three of these particles. Each particle in the second and third generations carries the same charges and seems to be just a heavier replica of the corresponding particle in the first generation. These particles are shown in the next two columns.

The right-hand part of the table shows the fundamental bosons, responsible for producing the strong, electromagnetic and weak forces. The discovery of the Higgs boson filled the last empty slot in the table.

The Standard Model Table of Particles.

It is a remarkable fact that the Standard Model is consistent with every particle physics experiment ever performed. For instance, LHC researchers released data in 2016 of a particle composed of a strange quark and a bottom anti-quark decaying into a pair of muons. The Standard Model predicts this is an extremely rare event occurring just three times in every billion decays of the particle, and this matches exactly what the researchers observed, making it the rarest decay process ever measured. It highlights the precision with which particle physics predictions are being tested, and, so far, the Standard Model agrees with the experimental results every time.

Where Do We Go From Here?

Despite the incredible success of the Standard Model it cannot be the end of the story. The Standard Model provides a very concise account of the structure of matter, but it has various loose ends and unexplained features. There is no explanation of why there are three generations of matter particles. There is no explanation of why the masses of the fundamental particles vary so greatly, as illustrated below.

The masses of the fundamental particles vary greatly. The volume of each sphere shown here is proportional to the mass of the corresponding particle. The masses of the neutrinos are far too small to be visible.

There is also the question of further unification. The Standard Model comes close, but it does not unify the electroweak force with the strong force. And although there are not many fundamental particles, the number is far more than we might expect from a truly fundamental theory.

Most serious, perhaps, is a rather more disturbing issue. About 85% of the matter in the universe exists in some mysterious form that does not fit into the Standard Model Table of Particles. It has been dubbed dark matter.

Further Information

There is a lot more about the structure of matter and how the Standard Model came about in my book Higgs Force: Cosmic Symmetry Shattered.

There is more information about dark matter here: Most of the Universe is Missing!

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