> You can't actually "make a mark" on a particle Yes, of course. That was loose wording for "affect a particle a way that is meaningful", i.e. prepare it. :)

> You can't actually "make a mark" on a particle Yes, of course. That was loose wording for "affect a particle a way that is meaningful", i.e. prepare it. :) > "But that's cheating!" you say. "I want the flip to be random." Actually, no, I'd rather the bit be useful! > If you flip a coin with enough precision you can make it come up reliably on one side or the other. Sure, if I know enough about the universe at some moment in time, by unitarity, I know its state at all other times. Having enough information to run the laws of physics forward to compute the outcome of the 2020 super bowl coin flip isn't much different, then, than running them backward to get the 2019 flip (and just as impractical). But what is interesting is that I could correlate (say) the spin of an electron with the result of the flip and read it later to tell what the flip was, but not earlier to tell what the flip will be. (Obviously information about the past coin flip is available in many more places than just in my prepared particle, but I don't need anything more than that one qubit to precisely know the result. That's a big difference from needing to know everything in the coin's past light cone! ) If we peel away the human-imposed notions of time and causality (as you do in your article), and see spacetime as a "block" with microscopic time symmetry, or perhaps even further dismantled into only basis states of Hilbert space, it's still obscure to me why particles-- even individual ones-- seem to "carry" information only from the past and not from the future. On an intuitive level it seems perfectly natural. On the level of (time-symmetrical) fundamental physics, I can't pin down why it would be.