## [The Real Meaning of E=mc²](https://www.youtube.com/watch?v=Xo232kyTsO0)
- photon does not have a rest mass, but it has the speed of light
## What is the meaning of $E=m c^2_{sound}$ for the sound?
- perhaps, there is a meaning of mass, which is not total mass but partial sound-susceptibility related mass?
- (strange Q) - does the phonon have a mass, because phonon has the velocity much smaller than the light velocity?
