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The Ancient Greeks had a name for the smallest particle: the ‘atom’, meaning ‘not cuttable’. But ever since Ernest Rutherford famously split the atom in experiments at Cambridge University around a century ago, it’s been clear that the name is a misnomer. He found that atoms contain a central nucleus around 10,000 times smaller than the atom itself.

By the early 1960s, researchers firing electrons at atoms were finding hints that even the protons and neutrons making up atomic nuclei contain some kind of structure – now known to be quarks – trapped inside them. In the last few years, experiments at the Large Hadron Collider particle accelerator have suggested that quarks may themselves be at least 10,000 times smaller than protons and neutrons.

But there’s one subatomic particle that’s far smaller still, and not even the most powerful particle accelerator has come close to pinning down its size: the electron.

Physics textbooks sometimes mention the so-called ‘classical radius of the electron’, which – at around three-million-billionths of a metre – is similar to that of a proton. But this comes from a theory that assumes the electron is just a ball of electromagnetic energy, which isn’t really true.

A more reliable insight into its true size comes from using quantum theory, which ties the size of subatomic particles to their angular momentum (‘spin’) and their magnetic properties. In the case of the electron, measurements of these properties suggest that the electron is at least 1,000 times smaller even than quarks.

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