Here's the arxiv link, for those who want to read the original article paywall-free: https://arxiv.org/abs/2004.04818 https://arxiv.org/abs/2004.04818. To be c

Here's the arxiv link, for those who want to read the original article paywall-free: https://arxiv.org/abs/2004.04818 https://arxiv.org/abs/2004.04818. To be clear, this isn't an unconditional speed limit. It appears to be something like "the maximum speed of sound in matter which consists of atoms" (maybe I've missed a few conditions). In particular, this speed limit is known to be violated inside of neutron stars (where the speed of sound is near 1/sqrt(3) times the speed of light, and may even exceed that). The paper also mentions a proposed lower bound on viscosity. This is the KSS bound (https://arxiv.org/abs/hep-th/0405231 https://arxiv.org/abs/hep-th/0405231), and various theories violating it have been constructed. The violations, of course, don't get mentioned nearly as much as the proposed (and all-but-debunked) bound.