We've come a long way since the fifth century B.C.E., when Greek philosophers, Democritus and Leucippus, expounded the not-unreasonable theory that everything is formed of indivisible, indestructible constantly moving atoms, separated by empty space. Well this year, 2024, sees the 70th anniversary of the founding of the European Council for Nuclear Research, better known as CERN, a facility dedicated to improving our understanding of matter at its most fundamental level.

We all know of CERN as home to the Large Hadron Collider, one of the most sophisticated machines ever built by humankind, which is the latest in a series of increasingly impressive particle accelerators that have been evolving in design since the inauguration of Europe's foremost particle research facility based near Geneva in Switzerland, whose underground facility straddles the border with France.

The name CERN actually started out as an acronym of the Copenhagen-based committee set up to establish the research facility in the aftermath of the Second World War, however, the now iconic title stuck as the laboratory opened for business in 1954. Proposed by the likes of Niels Bohr and Louis de Broglie, as a center for collaborative particle and nuclear research, the laboratory’s raison d’etre was to understand conditions such as existed a fraction of a second after the “big bang”.

In order to explore the interactions at suitably high energies, physicists using particle accelerators exploit the interchangeability of energy and matter, inherent in that most famous of equations (E = mc²) whereby they not only defy those ancient Greek philosophers by destroying the constituent particles of matter but indeed create new ones! This is most effectively achieved by crashing particles into one another at extremely high speeds. Earlier accelerators probed matter by firing beams of particles, often alpha and beta, at stationary targets. This was fine if the purpose of the accelerator was to utilise the short de Broglie wavelengths of the incident beam particles to examine a target at an extremely microscopic level of detail. However, if one is hoping to create new matter by “repurposing” the energy from a collision, in order to maximise the residual energy, it is most desirable to end up with zero net momentum. This can be achieved by colliding particles in beams that are travelling in opposite directions so that, before and after the experiment, in accordance with the conservation of momentum, the total momentum sums to zero. This means that the maximum possible kinetic energy in the tightly focused beams of particles can go into creating any resultant matter - or even antimatter!

At CERN bunches of particles are injected into the ultra-high vacuum of the accelerator ring (currently 100 m underground with a circumference of 27 km) where they reach increasingly high velocities by passing through a series of resonant radio frequency cavities which essentially transfer energy from an electromagnetic field to the particle bunches; analogous to the way a surfer gains energy from a surface gravity wave in the ocean. The beams of accelerated particles are forced into, and held in, their circular paths by electromagnets at regular intervals around the ring, whose field magnitude increases commensurately with the speed of the particles as they continue their orbit. As we know, accelerated charged particles like protons or electrons lose electromagnetic radiation, in the form of synchrotron radiation (a term derived from the synchronisation of the electromagnetic fields providing the energy) and whilst synchrotron radiation can be useful in certain other situations, in a colliding-beam accelerator its production is minimised by making the radius of the storage ring as great as possible thus reducing the centripetal acceleration (v²/r).

The products of the beam collisions are picked up by a range of detectors, where the resultant particles are identified by their measuring characteristics of mass, charge and so on. High energy particles resulting from the collisions travel through powerful magnetic fields leaving ionisation tracks in the respective medium of each detector, which will have been chosen with particular regard to the attributes of the sought-after particle. The most impressive experiment at CERN must surely be ATLAS, the largest detector ever built for a particle accelerator, with a mass comparable to the Eiffel Tower, in which billions of collisions can occur every second, between particles that have been accelerated almost to the speed of light at energies up to 7 TeV.

Highlights of the last seven decades, at CERN, include the first observations of anti-nuclei and the later creation of anti-atoms, the 1983 discovery by Carlo Rubbia and Simon van der Meer (et al.) of the W and Z bosons which mediate the weak force, and perhaps most iconically, certainly in the eyes of the public, the detection in 2012 of the enigmatic Higgs boson! In many ways, the progression of discoveries at CERN is the story of particle physics throughout the latter part of the 20th century and into the 21st century.

The vast amount of data produced in the experiments at CERN have prompted the facility to lead the way in computational power and indeed it was the need to share this information quickly and efficiently between collaborating scientists and engineers that spawned the now ubiquitous worldwide web. Even the touchscreen technology that we all enjoy today was pioneered at CERN. The facility has always captured the public imagination not least through the creative licence of Dan Brown's novel, Angels and Demons, and subsequent movie. And of course teachers will remember the mild outbreak of somewhat irrational panic that ensued with the inauguration of the LHC when a minority of the populace became convinced that it would produce an Earth-swallowing black hole.

Particle physics, like most sciences, is a truly internationally collaborative endeavour - the recent accession of Estonia to membership of the CERN organisation bringing the total number of full member states to 24. And please bear in mind: whilst CERN may be celebrating this significant anniversary, other amazing particle accelerators are available!

Images

• CERN photo by Aurélien Clément Ducret on Unsplash
• Magnet under construction photo by Aurélien Clément Ducret on Unsplash
• LHC photo by Antonio Vivace on Unsplash