What is the most beautiful concept in physics? Answers to that question will be many and diverse — depending on whom you ask, their background and 'soft spots', on their teachers, and on personal 'light bulb' moments. There are ideas in physics that strike us as simply beautiful from the first moment we encounter them: their full implications and significance still far from apparent, they seem to have an innate and inevitable elegance.

For many, and certainly for those who have a weakness for quantum mechanics, the Aharonov–Bohm effect is among those 'beautiful things'. The very idea is intriguing: that a solenoid can have a measurable effect on a charged particle even if the particle never passes through a region where either the solenoid's electric or magnetic fields are non-zero. But the implications are of course much broader.

Credit: M. BERRY

Last year marked 50 years since its discovery. Writing on page 160 of this issue, Murray Peshkin and Lev Vaidman survey an “intellectually delicious” meeting that took place, in December 2009, at the birthplace of the effect, the H. H. Wills Physics Laboratory in Bristol. The event celebrated the fiftieth anniversary of the discovery of the Aharonov–Bohm effect, and also 25 years since the discovery of Berry's phase, another (and not unrelated) concept that, for all its many ramifications, also charms physicists with its inherent beauty. On page 148, Sir Michael Berry recollects the events surrounding his famous 1984 paper, discussing both the work that followed shortly after and the anticipations of the geometric phase that preceded his own. “In retrospect what we call 'discovery' sometimes looks more like emergence into the air from subterranean intellectual currents”, says Sir Michael.

There is much more still to emerge from the 50-year-old Aharonov–Bohm effect, which may well have impact on the very foundations of quantum physics. Indeed, on page 151 Sandu Popescu argues that the deepest implication of the effect has been widely ignored so far — and that, without it, a real understanding of the nature of quantum mechanics is impossible.

When John Keats wrote his 1818 poem Endymion it wasn't physics he had in mind, but his famous opening line rings true in this case too: “A thing of beauty is a joy for ever”.