Time is a strange concept according to several strains of science and related philosophical concerns. We have this everyday medium-macroscopic set of ideas about how there is an undiscovered country of the future, a now we are experiencing, and a past that we remember or model based on accumulated historical facts. When we venture into extensions of conceptual ideas like an infinite past or sequenced events we deploy reasoning about what their properties might be by excluding contradictory compositions of properties and using other kinds of limiting semantics to constrain a mental model of those concepts.
But that isn’t the weirder stuff. The weirder stuff is the result of a collision of measurement and scientific theory.
Take, for instance, the oft-described reversibility of Newtonian physics. We have an equation for an object’s motion that can be run backward in time. But entropy in large ensembles of things in motion is not reversible because of some odd property of energy dissipation into the environment that arises because of micro-interactions. Some say this creates an “arrow of time” in the face of these reversible equations.
But this is an odd way of characterizing mathematical statements that represent the uniformity of physical interactions. The idea of “reversibility” is just a matter of a computational representation of processes that do always flow forward in time. Running t from 0 to -∞ in an equation has no real relationship to any physical phenomena. So the reversibility of mathematical forms is just an interesting fact.
We can bind up space and time, as well, which also provokes feelings of incongruity when we start to talk about gravitational effects on relative elapsed time, or relative speed effects. Mostly we rely on the equations to produce an outcome that is verified by experiments or observations, though this does have important consequences as we will see further along.
Recent work in quantum physics has also shown that time and causality might be quite unlike our ordinary experiences of time at all. The experiments involve routing photons through beam splitters, polarizers, and then recombining them. The results are conclusive that the ordering of the operations doesn’t matter to the observed polarization and supports the notion that the time spread of the quantum wave function applies to ordering and causality as well.
Are there any applications of these time concepts that are meaningful to us? Perhaps the most important one is adjusting clocks for satellites moving in orbit. It’s a small but important adjustment that is critical to navigation via GPS signals. The quantum causality people have also shown that quantum communications systems might be improved by applying different paths for communicated bits.
There is also a speculative application among religious philosophers who like to state that infinite cosmological pasts are impossible. By doing so they think they are proving or justifying their other religious claims. I always come away questioning the notion that logical extensions of our everyday experience of time can have much bearing on the facts of the universe at hand. It might be fun to think about, but it is doesn’t move forward the more interesting work of actually figuring out how time works.