![]() ![]() But the wildness truly began when Max Planck and Albert Einstein laid the foundations for quantum mechanics in the early twentieth century. It’s hard to overstate how wild this discovery was to physicists in Young’s time. Where a crest meets a trough, you get destructive interference and darkness. Where the crest of one wave overlaps with the crest of the other, you get constructive interference and a patch of light. Interference is possible only if light behaves as a wave that strikes both slits at once and diffracts through each, creating two sets of waves on the other side of the slits that propagate towards the screen. Instead, you see many bands of light and dark, strung out in stripes like a barcode: an interference pattern (see ‘Wave–particle weirdness’). If light were made of streams of particles, as Newton conjectured, you would expect to see two distinct strips of light on the screen, where the particles pile up after travelling through one slit or the other. ![]() “The experiments I am about to relate”, he told the Royal Society of London 1 on 24 November 1803, “may be repeated with great ease, whenever the sun shines.” In a simple, modern form, Young’s ‘double-slit’ experiment involves shining light of a single frequency (say, from a red laser) through two fine, parallel openings in an opaque sheet, onto a screen beyond. But perhaps his most enduring legacy is proving Isaac Newton wrong about light - and igniting a debate about the nature of reality that still persists. ![]() Thomas Young, born 250 years ago this week, was a polymath who made seminal contributions in fields from physics to Egyptology. The double-slit experiment’s interference patterns suggest something is in two places at once. ![]()
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