This might sound strange… BahadirTanriover/Getty

Heavy metal music isn’t heavy after all – it is actually the opposite. Sound waves have mass and can interact via gravity, but that mass is negative. In other words, sound floats upwards.

Angelo Esposito at Columbia University in New York and his colleagues calculated the relationship between sound and gravity, taking into account complicated particle interactions that had previously been ignored. They found that, although the effect is small, sound waves should have negative gravitational mass.

“It’s almost like antigravity,” says Ira Rothstein at Carnegie Mellon University in Pennsylvania. “The atoms that are …