>[!citation] >Carroll, S. (Host). (2025, January 6). Does time exist? (No. 300) [Audio podcast episode]. In *Mindscape*. https://www.preposterousuniverse.com/podcast/2025/01/06/300-solo-does-time-exist/ >[!abstract] >It is unknown whether time is an *emergent* or *fundamental* property of the universe. > [!note] Notes > Is time real? This is not the same as asking whether time is real or illusory; just because something is emergent, like a man-made table, does not mean it isn't real. > > The nature of time has long been a consideration for human thinkers. In pre-Newtonian physics, space was understood to be this three-dimensional manifold in which all things *existed*; and time was merely what *happened* to those things. Time was classically about *change* from one state to another. > > Then Newton came along, and with him, [[Newton's laws of motion|rigid laws]] that clarified *how* things change over time. That [[deterministic]] world view culminated in the [[Laplace's demon]] [[gedanken]] (which posited —incorrectly— that it is possible to calculate past and future states of the universe based only on classical laws and a perfect knowledge of the current state). Time was then seen as a *location in space*, as if referring to a specific frame within the movie of the universe. > > Then came Einstein, who further combined space and time into a four-dimensional manifold, though with the added twist of rendering time no longer universal, but instead *velocity-dependent* (special relativity) and *mass-dependent* (general relativity). > > Both Newton's classical mechanics and Einstein's relativity belong to the *eternalist* viewpoint that the past, present and future form one real continuum; as opposed to the presentist viewpoint that only *now* is real. > > As a short digression, in the mid-19th century, the Second Law of Thermodynamics introduced the idea that entropy only ever increases with time (or remains constant in closed systems). From that idea emerged clear evidence for an *arrow of time* in physics, which did not exist in newtonian mechanics. Suddenly, there was a physical foundation for why, from the perspective of the present, the past is not equivalent to the future; there is only one way forward in time. There exist other arrows of time; such as how we remember the past but not the future, etc. Though they might just be emerging from the same Second Law of Thermodynamics and the ever-increasing entropy. > > It is possible, however, that the concept of the arrow of time is immaterial to the question of the emergence of time. Carroll (2025) uses the analogy of a frictionless pendulum swinging back and forth indefinitely: no arrow can be determined, and yet time is objectively passing. So, we must go back to this central question of *what is time*. Even in general relativity where there is no universal frame of reference, time remains some form of coordinate (in spacetime), which is not that different from Newton's classical view (note: Julian Barbour disagrees with that). > > Quantum mechanics, however, departed from that concept. Using the Schrödinger equation, it is possible to input the current state of a quantum system and output its rate of change. That function is linear, which means that it solves individual quantum states evolving over time, but it also solves how *their sum* evolves over time. > > This is important, because according to the Schrödinger equation, time does not end. There is always a quantifiable output from that function for any given quantum state. This is unlike classical or relativistic mechanics in which singularities appear; the Big Bang was one such singularity at which time began, and there could be another in our future where time ends, such as a Big Crunch. The linearity of the Schrödinger equation, on the other hand, rules out any infinite result and predicts an eternal universe. > > As an aside, this is different from chaos theory, or non-linear systems in which minute differences in the initial conditions will propagate and grow exponentially later on. In linear systems, minute differences remain so. There is no chaos in the Schrödinger equation, nor is there the possibility of a singularity.