Why We Remember the Past and Not the Future
When I was a student at Warwick University, this was a topic in my philosophy class. But it's not a question for philosophers; it's physics. Like this.
We should avoid time-laden assumptions. The universe displays an asymmetry between lower-entropy states near the Big Bang and those farther from it. From any particular state, call the past those slices of spacetime nearer to the Big Bang, and the future those farther away. Time, in this geometric sense, is simply the ordering of these slices along the expanding fabric (really the positive entropy gradient) of the universe.
Our question is: why records exist of events closer to the Big Bang but not of those farther from it.
Imagine that yesterday a supernova became visible in the sky. The light from that explosion reached Earth, interacted with our atmosphere and the Earth's surface, and altered the states of countless atoms. Those microscopic changes are, in principle, correlated with the original event, yet they rapidly disperse through further interactions, merging into general thermal motion. The information is effectively lost in the wider thermal randomisation of energy.
When you saw the supernova, however, something different occurred. The arriving photons triggered a cascade of interactions within your sensory system. Neural structures in your brain formed a configuration that encoded features of that event. This configuration can persist through subsequent physical states and can later be retrieved. A memory, in this physical sense, is a subsystem capable of forming and maintaining a structured correlation with some external interaction, preserving that correlation across later states, and retrieving it when required. That low entropy persistent record was created by you generating greater entropy in your environment, of course.
Now consider that tomorrow another supernova will appear in the sky. At this present slice of spacetime, the photons from that explosion have not yet entered your light cone. They have not interacted with you or with the matter surrounding you. The relevant regions of the universe have not yet exchanged information. No physical trace of that event exists in your current environment-state, and therefore no memory could correspond to it.
The difference between what we call past and future arises from this incomplete communication within the universe. Because the cosmos began in a low-entropy condition, its regions are still in the process of exchanging interactions. Not all parts of the universe have yet influenced one another. The network of interactions—propagating within light cones—links some regions while leaving others still disconnected. What we call the past consists of those regions that have already communicated with our present state; the future consists of those that have not.
Memory, therefore, is a local expression of this broader asymmetry. It depends on structured systems—brains, instruments, records—that can retain correlations once interactions occur. We remember the past because its signals have already reached us and have been encoded. We do not remember the future because its signals have not yet arrived.
If, in some remote epoch, the universe reaches thermodynamic equilibrium - its state of maximum entropy - and every region has exchanged all information accessible to it, then no new records can be formed. Patterns will cease to emerge, distinctions will dissolve, and the measure of change we call time will have lost operational meaning in a state of thermalised uniformity. The spacetime metric continues to exist but the possibility of registering the passage of time has disappeared.
