Planck mass

Unlike the other Planck units, this one is not remote at all: 22 μg is a visible speck, roughly the mass of a flea's egg or a large grain of pollen. That everyday size is a genuine puzzle rather than a coincidence — it is the mass at which a particle's Compton wavelength equals its own Schwarzschild radius, so it marks the boundary of quantum gravity, and yet it is 10¹⁹ times heavier than a proton.

That ratio is the hierarchy problem: why is gravity so absurdly feeble compared with the other forces, or equivalently why are the particles we know about so much lighter than the Planck scale? Supersymmetry, extra dimensions and the multiverse are all attempts at an answer, none confirmed. In particle units m_P c² = 1.22 × 10¹⁹ GeV, roughly 10¹⁵ times the LHC's reach, which is why quantum gravity is so hard to test.