Does Measurement Secretly Break Time Symmetry?

Added: 2026-05-05

[00:00:00]classical physics all of quantum mechanics leaving aside weak interactions which isn't really fundamental to quantum mechanics is time reversal invariant meaning it is just as easy or difficult to predict the future from my present observations as to retrodict the past.

[00:00:17]And there's no mathematical difference between the two.

[00:00:21]Except we're also taught in quantum mechanics that there's measurement and that measurement is this mystical event that we never define really clearly but that breaks the time symmetry because after measurement the state is completely reset. There is no reason when I find a particle at some position that I have to use that new state to calculate the future wave function but something different to calculate the past and yet that's what we were all taught.

[00:00:47]And Yakir has been struggling for decades with the question of how to make sense of the symmetry of quantum mechanics. So he really wanted to say if I know what a system was doing at t equals zero and I know what a system is doing at t equals one >> Mhm. both of those pieces of information should be equally useful to tell me what was going on at t equals point five.

[00:01:09]And it's weak measurement that allowed him to mathematically show that that was true. And there are as I said many people who are very very seriously thinking about this idea of retrocausality that the past affects the present.

[00:01:22]>> going to say then just connecting back to our earlier part do you then imagine that there are several arrows of time that time sub one time sub two and that some could go forward some could go backward? I certainly think that's possible. Um in a laboratory scale setting. Um I think for the universe as far as we can tell there's one arrow of time but there are different ways to define it. Oh you know there's the second law of thermodynamics when saying which direction does entropy increase? Is that a coincidence or can you prove that once you have the second law the psychological arrow of time must follow from it. Um we don't currently believe that the universe is supposed to stop expanding and collapse on itself but we can imagine universes that were going to do that and there were speculations that when the universe turned around that cosmological arrow of time reversing would also reverse the entropy [clears throat] and psychological arrows. So you could imagine saying oh it'll look really weird when the universe turns around and people all realize they're heading towards the big crunch and someone else saying they're never going to realize that because for them time will be running backwards too.

[00:02:28]So we all think the universe is expanding and there's a law of physics that shows that what feels future to you is always the expanding universe. We don't know that that's true.