Not a backdrop, an illusion, or an emergent phenomenon — time has a physical size that can be measured in laboratories
A new framework called assembly theory proposes that time is not a backdrop, an illusion, or a mere emergent side-effect of entropy — it is a fundamental, measurable physical property of matter itself. Complex objects that evolution and selection have produced — from proteins to computers — cannot exist outside of time, because time is the very stuff they are made from.
Physics has never had a theory in which time's movement and directionality are considered fundamental. Newton treated time as an external stage. Einstein folded it into a static four-dimensional block where all moments — past, present, future — coexist equally. Thermodynamics introduced time's arrow but only as an emergent consequence of heat and entropy, not as a basic feature of reality.
This leaves a serious explanatory gap: neither quantum mechanics nor thermodynamics can account for the existence of highly complex, evolved objects — DNA, proteins, cities, computers — that clearly cannot spring into being spontaneously. If time is not real and directional at a fundamental level, the origin and growth of complexity in the Universe remains physically unexplained.
The theory's predictions were tested by dissolving solid samples — stone, bone, flesh, and fossilised material from across the solar system — in solvent and running them through high-resolution mass spectrometry, which "weighs" molecular fragments after breaking them apart. Counting those fragments maps directly onto the theoretical assembly index, providing a lab-measurable confirmation of the model's core claim.
The boundary between abiotic and biotic chemistry is experimentally sharp, not gradual — molecules produced by random processes appear unable to cross the 13-step mark, while living systems routinely generate molecular structures above 15 steps. This sharpness is interpreted as a genuine phase transition in the underlying physics, not measurement noise.
The framework is still developing its relationship with quantum mechanics — assembly theory currently operates at the molecular scale and must be reconciled with the timeless wave-function descriptions that underpin quantum theory, which remain incompatible with time as a fundamental feature.
The precise mechanism by which the first self-replicating chemical networks "discovered" their assembly pathway — crossing the phase transition from abiotic to biotic chemistry — is unaddressed; assembly theory describes the transition's existence but not yet the physical dynamics of how it first occurs.
Whether space itself is emergent from time (as some quantum gravity proposals suggest) is a possibility the framework opens but does not resolve; how assembly theory integrates with existing approaches to quantum gravity remains an open frontier.
If assembly theory holds, the passing of time is not a human perception to be explained away — it is the physical material from which the most complex structures in the Universe are built. Every evolved object, from a protein to a city, is simultaneously a record of the past and a constraint on what futures are reachable, which means the Universe is not expanding through space as much as it is expanding through time.