One of the most fundamental questions in physics is regarding time and whether we live in a block universe (where past, future and present exist simultaneously and we merely sample moments) or whether the universe "grows" with time (and creates new information). As you can see, it's a fundamentally question of whether new information can be created and if so, where does it come from. An alternative way of putting the question is if the future is pre-determined and our inability to predict is merely our lack of knowledge about initial conditions of the universe and/or its dynamical laws. I think quantum mechanics forces us to believe in indeterminism. Properties of quantum systems do not have definite values until we measure them and upon measurement, what values we get cannot be said with certainty. This indeterminism doesn't mean that reality is "lawless" and anything can happen. This merely says that there are some things that we cannot fundamentally predict because such information doesn't yet exist. Note that even many-world interpretation doesn't fully resolve this indeterminism because even though it suggests all possibilities actually happen, it doesn't explain in what sense measurement outcomes are probabilistic if observer finds itself in all branches. The same problem of indeterminism exists in classical physics also which assumes real numbers exist and that we can assign precise (infinitely dense) numbers to physical properties like momentum or position. The trajectories in classical physics can be chaotic which means extremely small deviations can lead to large deviations and since we can't specify position of particles with infinite precision, fundamental uncertainty about future outcomes exist. Einstein popularized the idea of block universe, which the following video explains intuitively. <iframe width="560" height="315" src="https://www.youtube.com/embed/EagNUvNfsUI" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe> I think the fundamental assumption here is the plane of simultaneity. Just because something is in my plane of simultaneity doesn't mean I can know the information that exists there. In practice, I can only know information that propagates to me and until then all I have is various regularities and correlations to predict what information might reach me (and there's always uncertainty in such predictions). So, unlike what I wrote in [[Demystifying the arrow of time]], we can model the passage of time in two ways: - Epistemological gap: I don't know precise microstate of the universe and that lack of knowledge leads me to be uncertain about future. Although I'm more certain about the past because entropy was low then. - Or, Ontological gap: The amount of information in universe grows in a particular direction and that direction I identify as time. Importantly, this growth of information is not merely epistemological (that it shows lack of knowledge). But such information truly doesn't exist yet. Ultimately, these are metaphysical positions. Perhaps there's an experiment to distinguish them but trivially indeterministic theories can be made deterministic by bunching all new generated information into "initial conditions". In fact, that's the whole basis of the [super-determinism](https://en.wikipedia.org/wiki/Superdeterminism) interpretation of quantum mechanics. So, it boils down to metaphysical preference. References: - [[Reading notes/@delsantoPhysicsDeterminismAlternative2019]] - [[Reading notes/@gisinClassicalIntuitionisticMathematical2020]] - [[Reading notes/@dowekRealNumbersChaos2013]] - [[@delsantoRelativityIndeterminacy2021]]