> Is information ever lost? From a practical perspective, yes; but I get the feeling you're not asking that ;). Let's say that we know everything possible abo

> Is information ever lost? From a practical perspective, yes; but I get the feeling you're not asking that ;). Let's say that we know everything possible about the universe at a time t (in some reference frame). Can I reconstruct what happened at any time in the past? If I can, then it seems reasonable to say that 'no information has been lost' (going the other way suggests that no information gets created either). A lot of Physicists seem to think the answer is yes, we can construct the past if we actually knew everything. The evolution of the wavefunction is unitary, so you should be able to run the universe backwards to find out what happened (unless wavefunction collapse is real, in which case, we're losing information all the time). Note that in order to do this, you need to know the entire wave function; which isn't measurable. However, under our current understanding of black holes, the answer seems to be "Looks like it?" https://en.wikipedia.org/wiki/Black_hole_information_paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox Since we don't know how to resolve General Relativity with Quantum Mechanics, we can't really say with confidence whether the universe can 'be played backwards' or not.