It's one of those enduring Zen koans of science that we've all grown up with: Light behaves as both a particle and a wave-at the same time. Einstein taught us that, so we're all generally on board, but to actually understand what it means would require several Ph.D.s and a thorough understanding of quantum physics.
What's more, scientists have never been able to devise an experiment that documents light behaving as both a wave and a particle simultaneously. Until now.
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That's the contention of a team of Swiss and American researchers, who say they've succeeded in capturing the first-ever snapshot of light's dual behavior. Using an advanced electron microscope – one of only two on the planet – at the EPFL labs in Switzerland, the team has generated a kind of quantum photograph of light behaving as both a particle and a wave.
The experiment involves firing laser light at a microscopic metallic nanowire, causing light to travel - as a wave - back and forth along the wire. When waves traveling in opposite directions meet, they form a "standing wave" that emits light itself - as particles. By shooting a stream of electrons close to the nanowire, the researchers were able to capture an image that simultaneously demonstrates both the wave-nature and particle-nature of light.
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"This experiment demonstrates that, for the first time ever, we can film quantum mechanics - and its paradoxical nature - directly," says lead researcher Fabrizio Carbone of EPFL, on the lab's project page. The study is to be officially published this week in the journal Nature Communications.
The image provided is shown above, issued with the following caption from EPFL: "Energy-space photography of light confined on a nanowire, simultaneously showing both spatial interference and energy quantization." If you find it all a little hard to unpack - believe me, I'm entirely sympathetic - the team has also released this rather friendly companion video: