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Why the night sky is dark: Olbers' paradox and the light that hasn't arrived yet Go outside and look between the stars: it's dark. That's strange. In an endless, unchanging universe, every line of sight would end on a star, and the whole… Earth & Space4 min read3 Nov Why a microwave leaves cold spots: a standing wave 12.2 cm long, and a chocolate bar that measures the speed of light Microwave ovens fill a metal box with radio waves 12.2 cm long. The waves bounce off the walls and add up into a standing wave, a frozen pattern of hot spots… Food Science5 min read1 Nov Why the sky is blue and the sunset red: Rayleigh scattering and the length of the air The sky is blue and the sunset is red, yet the sunlight is the same. Air scatters short blue waves about four times more strongly than long red ones, a… Physics4 min read25 Oct Why a siren drops in pitch as it passes: the Doppler effect An ambulance siren sounds high as it comes at you and drops the moment it passes, yet the driver hears one steady note. The trick is in how crests per second… Physics3 min read21 Oct Why two almost-equal notes make a wobble: beats and the piano tuner's trick Play two almost-equal notes together and you hear one pitch that swells and fades. Those swells are beats, and their rate is simply the difference in… Music & Sound4 min read20 Oct Why a boat's wake is always a 39° V: Kelvin's wedge Behind every boat spreads a V of ripples, and it is always about 39° wide, whether the boat crawls or speeds. Lord Kelvin worked out why in 1887: only waves… Physics4 min read14 Oct Why a flute and a clarinet sound different on the same note: harmonics and timbre Play the same note on a flute and a clarinet, at the same pitch and loudness, and you can still tell them apart at once. That difference is called timbre, and… Music & Sound4 min read13 Oct Why the bottom of a dropped Slinky hangs in mid-air: a wave has to bring the news Hold a Slinky by its top, let it stretch, and let go. The top rushes down while the bottom coil hovers, apparently ignoring gravity, until the collapsing coils… Physics5 min read11 Oct Why a tone can climb forever without getting higher: the Shepard illusion Play a tone that climbs and climbs, forever, and never gets any higher. It sounds impossible, but it only takes a few sine waves and one volume curve. A… Music & Sound3 min read6 Oct Why two almost-in-tune notes throb, and how tuners use the wobble Play two notes that are almost in tune and you hear one note that swells and fades: wah-wah-wah. Neither string is wobbling. The throb, called a beat, appears… Music & Sound5 min read27 Sep Why traffic jams appear out of nowhere Sometimes a motorway grinds to a halt and then clears up with no crash, roadworks or merge in sight. That's a phantom traffic jam, and it starts with one… Engineering4 min read26 Sep