Because of their quantum nature, atoms (like the particles they are made of) act like waves. The slower they move, the more stretched-out they become, dropping in pitch like a musical note sliding down the scale.
Why do I always find crap like that in articles written for a popular audience. If it isn't crap, I'm really missing something.
An example of outcome-based journalism, I guess.
"Physicist" has already, and thankfully, spared my from trying to explain what the author really meant. Funny, isn't it, that his "explanations" are invariably more succinct and precise than the original "quote" that he is explaining.
Now, if I only knew what a "Compton wavelength" is, I'd be all set .... I guess looking that up is a reasonable exercise for the student.
The Music of the Spheres?