Quantum alchemy

At Cern, lead turns to gold, for an instant

An ancient dream takes shape among particles: when light transforms matter

by Francesca Cerati

13 May 2025

3' min read

Key points

A modern echo of Chrysopoeia
A dream that doesn't pay, but shines
Gold shining in the darkness of the cosmos

3' min read

No philosopher's stone, no secret formula. Just the cold precision of modern physics, a few billion euros in infrastructure, and a 27-kilometre-long particle accelerator. At Cern, at the heart of Europe's particle physics laboratory, scientists witnessed a scene that would have put 17th century alchemists to shame: lead atoms that, following almost imperceptible but extremely violent collisions, are transformed into gold atoms. Real gold atoms, albeit tiny and destined to disappear in less than a millionth of a second.

The star of the experiment is the Lhc, the Large hadron collider, the most powerful particle accelerator in the world. During a series of tests conducted between 2015 and 2018, physicists collided beams of lead ions launched at almost the speed of light. And while most of the collisions generated storms of subatomic particles, in some rare instances the nuclei did not collide head-on but brushed against each other. It was in those moments that the impossible happened.

Thanks to the intense electromagnetic field surrounding each lead ion, a pulse of photons - particles of light - is released, capable of penetrating a nearby nucleus and changing its structure. If all goes according to theory (and the detectors confirm that it does indeed sometimes happen), lead loses three protons, slipping to atomic number 79. Gold.

A modern echo of the Chrysopoeia

"It is a real transmutation, but on a subatomic scale and for a very short time," explains Uliana Dmitrieva, a physicist at the Institute for Nuclear Research in Moscow and part of the Alice collaboration, the sophisticated detector that has recorded these rare events. "Thanks to our instruments, we have succeeded for the first time in systematically identifying and analysing gold production at Lhc.

The quantity? About 29 picograms, or trillionths of a gram. Nothing to polish or melt: we are talking about 86 billion gold nuclei that materialise and disintegrate in the space of a microsecond, swallowed up by the structure of the collider itself or by new reactions.

Yet gold is not the only protagonist in this particle game. The experiment has also produced thallium and mercury atoms, in greater quantities than gold, revealing that every collision touched upon is a kind of miniaturised nuclear workshop. However, gold, as in mythology, remains the symbolic metal, the most evocative.

A dream that does not pay, but illuminates

Of course, it is not an efficient process. Creating gold in this way is the opposite of cost-effective: it requires huge amounts of energy, highly advanced machinery and the result is measured in atoms, not bars. 'It is the most expensive and inefficient method of producing gold,' jokes Jiangyong Jia, a physicist at Stony Brook University. 'But for us, it is the side effect of much more interesting physics.

Because the point is not the gold itself, but the understanding of what happens when the heaviest particles in the universe brush against each other at extreme speeds. How photons interact with nuclei, for example, can affect the stability of the beams inside the accelerator and open up new avenues for studying matter under extreme conditions - the same as in the first moments of the universe.

Gold shining in the darkness of the cosmos

'It is amazing to think that our detectors can capture events that produce thousands of particles, but also isolate very rare cases where only a handful are formed,' says Marco van Leeuwen, physicist at Utrecht University. In those few moments, we really are on the borderline between science and dreams'.

A dream as old as human civilisation, resurfaced in the laboratories of the most advanced physics. No longer to get rich, but to understand. To look at matter in a new light. There, between tunnels of magnets and invisible beams, lead becomes gold. If only for an instant.