Traditionally when we think of sound waves, we think of invisible vibrations moving weightless through the air - not carrying any mass.
That might be about to change. Physicists have just provided further evidence that particles of sound really can carry tiny amounts of mass. And that means they can produce their own gravitational fields - which could be a big deal for our understanding of space.
But let's back up for a second and go back to the basics. Kick a ball, and you put energy into it. Einstein would tell you you've also contributed a tiny bit of mass by making it accelerate.
If that ball is a tiny particle, and the kick is a wave of sound, you might imagine the same thing. Yet for decades, physicists have argued over whether the momentum within a surge of jiggling particles adds up to a net amount of mass.
Last year, physicist began
to investigate how
different waves decay and scatter in a super cold fluid of helium.
Not only did they show that sounds can actually generate a non-zero value for mass, but they might also weirdly 'float' along gravitational fields in an anti-gravity sense.