How close would a supernova have had to be for its shock wave to affect planet formation?
The supernova shock probably started the collapse of the whole solar nebula, from which both stars and planets formed. I do not know precisely how close it would have to be, but my guess is that a few hundreds of light-years would do it. In the pepper demonstration: I don’t understand why everything eventually begins rotating with the same average velocity. The pepper is mostly a marker of the water velocity. Because water molecules are constantly colliding with each other, they tend to end up bumping each other in the general direction most of them are going. Consider a particular molecule: if it happens to be going opposite to direction of the average flow, it’s more likely than not to hit something that’s going in the average flow direction, which will nudge it towards the average direction. Something moving much faster than average will typically smack into slower moving things and slow down; something moving much slower than average will tend to get kicked to a higher speed. After
