The so-called breakdown of spacetime
An assertion which is often made is that quantum gravity effects will become important at the Planck length — about meters — and the notion of spacetime will break down at that length. People like to wax lyrical about spacetime turning into some kind of “quantum foam” at that level.
This bugs me. If it really is the case, then why doesn’t the notion of mass doesn’t break down at the Planck mass, which has the rather hefty value of about kilograms? What’s the critical difference between mass and length?