Posts by: Philip Ball

Another brilliant idea: laser beams illuminate fog. (CC BY 2.0/ Jeff Keyzer)

By Philip Ball 

I just got back from having my broken wrist X-rayed (it’s doing fine, thanks), and noticed that a laser beam was used to position and align the X-ray source. Hardly the most sophisticated use of these optical devices, it’s true, but a little reminder that there’s probably hardly a day goes by in the life of an average urbanite without the laser’s beam of coherent photons impinging on it. From supermarket barcode scanning to broadband fibre-optic telecommunications, lasers are everywhere.

The fundamental idea behind this mainstay of modern life was published one hundred years ago by Albert Einstein. But blink and you’ll miss it in his seminal paper, “The quantum theory of radiation”, published in German in Physikalische Zeitschrift 18 121 (English translation here). Einstein is trying to work out what Max Planck’s “quantum hypothesis” – that the energy of an oscillator must take discrete values equal to some integer multiple of the oscillation frequency times a constant h – implies for the way light interacts with matter.

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