Charles Kuen Kao (1933–2018) is the reason your internet is made of glass. In 1966, a 32-year-old
engineer at a research lab in Harlow, Essex, he published a paper making a claim that sounded, to
put it kindly, optimistic: glass fibre could carry light-borne telephone calls for kilometres —
if only someone would make glass about fifty times clearer than the clearest glass on
Earth. The best optical glass of the day lost light at roughly 1000 dB/km, meaning half the
light vanished every three metres; a window pane is gloom by the far end of a corridor. Kao and his
colleague George Hockham had done the unfashionable thing and asked why. Their answer:
the murk wasn't the glass — it was the dirt in the glass, stray iron and other impurities
at parts-per-million levels. Purify it, they calculated, and
What makes Kao's story delicious is what he did after the paper. A theorist might have waited to be proven right; Kao went door to door. For years he criss-crossed the world's glassworks and labs — Britain, the US, Japan, Germany — armed with measurements and patience, politely insisting to companies whose business was windows and bottles that they could, and should, make the purest solid material ever manufactured. He built his own instruments to measure transparency no existing instrument could certify, showed that fused silica was intrinsically clear enough, and kept up a drumbeat of talks with an engineer's cheerful stubbornness. (Colleagues recalled that the response ranged from scepticism to pity.) In 1970 the glassmaker Corning — one of the doors he had knocked on — announced a fibre at 17 dB/km, beating Kao's threshold. Within a decade fibre was in the ground; within three, it had crossed the oceans, and the price of a long-distance phone call quietly collapsed. He then spent the 1970s and 80s doing the unglamorous work that makes revolutions stick — pushing standards toward the single-mode fibre the world now uses, and later serving as Vice-Chancellor of the Chinese University of Hong Kong. Obituarists reached for the same phrase: the man who changed the world's wiring.
In 1966 a transatlantic call travelled through copper coaxial cable laid on the seabed — TAT-1, the first such cable, carried all of 36 simultaneous calls between two continents, which is why calling New York from London was a budgeted event. Copper's problem is bandwidth: high-frequency signals die quickly in metal, so capacity crawled upward cable by expensive cable. Light waves oscillate hundreds of thousands of times faster than the microwaves in coax, so a single guided beam could in principle carry the traffic of a million copper pairs — if it could survive the trip. That "if" was Kao's life's work. A modern fibre pair carries tens of terabits per second: every second, more information than TAT-1 could have moved in years. When you video-call another continent without thinking about the cost, you are spending Kao's dividend.
The Nobel Prize in Physics arrived in 2009, forty-three years after the paper — half to Kao "for groundbreaking achievements concerning the transmission of light in fibers for optical communication", the other half to the inventors of the CCD sensor (a tidy pairing: the prize year honoured both the pipes of the information age and its eyes). By then Kao could not deliver his own Nobel lecture. He had been diagnosed with Alzheimer's disease in 2004, and the illness had advanced; his wife Gwen Kao — an engineer herself, whom he had met when they were colleagues in London in the 1950s — stood beside him throughout, delivered the lecture "Sand from centuries past: Send future voices fast" on his behalf, and the Swedish court gently bent protocol so that the King stepped down to Kao rather than asking him to walk to the King. The couple spent his final years founding and running the Charles K. Kao Foundation for Alzheimer's Disease in Hong Kong, redirecting his fame at the disease that was erasing him. There is a hard, luminous irony here that none of his obituarists could resist, and neither can we: the man who taught the world's memories to travel as light spent his last decade losing his own.
Kao's fingerprints are all over the photonics you can learn here: the