Time reversal and temporal asymmetry: SMM/DTT and Eternalism and the block universe

01The question behind the comparison

Reversing time in a physical theory means more than replacing a displayed t by −t. One must transform quantities such as velocities, momenta and angular momenta appropriately, and specify how the dynamics and boundary conditions behave. A process can obey a reversible law while occurring in an asymmetric state. DTT’s passage from inner generation to outer completion could represent a fundamental asymmetry, an effective loss of access, or a boundary-conditioned orientation. These are alternatives with different consequences, so a useful comparison does not treat the word irreversible as sufficient explanation.

A completed record is typically stable because many microscopic degrees of freedom support it. That stability need not make the total evolution noninvertible. If a system becomes correlated with an environment and the environment is ignored, the reduced description can lose visible coherence even when the joint dynamics preserves information. Reversing the reduced evolution is then difficult because the discarded correlations are needed. This offers a natural language for a completion interface, provided its environment and resource budget are included rather than absorbed into a metaphysical label.

The six nested levels can distinguish different forms of reversibility. An inner operation may be mathematically invertible, while an observer cannot access its inverse after completion. A higher level may retain enough information to reverse a lower operation, while the outer record omits that information. This hierarchy would support a precise account of apparent irreversibility. It would differ from a fundamental destructive update, which changes the theory’s allowed trajectories themselves. Both are intelligible, but only a specified model determines which description applies.

02The single-source and nested-time proposal

SMM begins with one nonspatial generative source. DTT distinguishes its inner production from the outer order in which completed results become accessible. The six inner levels are nested functional dependencies within that production; the seventh denotes outer completion. They are not six independent clocks, six spatial directions, or six simultaneously occupied temporal axes. This distinction makes the framework more specific than a general claim that everything changes: it asks how a world of persistent relations can be generated before its spatial representation is taken for granted. ^[2]

03The comparator’s explanatory structure

Eternalism treats events at different times as equally real. A block representation displays a history as a spacetime whole. Relativistic geometry provides powerful reasons to be careful about a universal present, but it does not by itself prove every metaphysical claim associated with eternalism. An eternalist can describe local causal processes, change and the internal experience of time without supposing that objects are motionless within each event.

A complete trajectory is a relation among positions and clock readings. Representing all its points together does not eliminate the difference between walking and standing: the trajectories remain different. The relevant issue for DTT is whether generation adds a physically consequential relation that a complete history omits, or instead offers an explanation of how such a history is constituted. A block description can contain all the records an observer ever accesses. It need not contain an additional moving spotlight. This makes completion, rather than the mere presence of change, the sharp point of comparison. ^[1]

04The precise agreement and difference

A block can contain both reversible microscopic laws and an irreversible-looking macroscopic history. The difference is encoded in states, correlations and boundary conditions across the history. DTT’s direction toward completion could explain these structures, but it does not contradict eternalism merely because observers cannot undo their records. The block description already permits asymmetric accessibility.

A physically significant DTT claim would specify a completion map that differs from ordinary reversible joint dynamics or from standard reduced-state evolution. Its predictions would then be compared at the level of observable probabilities. An ontological claim that each completed event becomes actual only once may remain compatible with the same probabilities and therefore be a different kind of explanation. This comparison helps avoid an overstatement: demonstrating decoherence is not demonstrating objective passage, and demonstrating a source-dependent arrow is not automatically refuting a complete-history ontology. DTT’s strongest contribution is to identify what the familiar block description leaves dynamically unexplained and supply a constrained mechanism for that feature. ^[1]

05Worked example: what follows and what does not

Start with a qubit in (|0⟩ + |1⟩)/√2 and an environment in |e⟩. Let an interaction produce (|0⟩|e₀⟩ + |1⟩|e₁⟩)/√2, where the environment states are orthogonal. The qubit alone is then described by a diagonal density matrix with probabilities one half, because tracing over the environment removes the off-diagonal terms. The joint state remains pure. An inverse joint unitary can in principle restore the original superposition, whereas operations on the qubit alone generally cannot. This is an example of lost local access, not a proof that the universe has erased information. A physical DTT completion law can be compared with this explicit distinction.

06What this comparison contributes

An account of temporal asymmetry is strongest when it identifies the operation whose inverse fails, the information needed for reversal, and the observer for whom that information is unavailable. These questions also keep metaphysical becoming distinct from laboratory reversibility. A theory may affirm genuinely new actual events while retaining reversible equations for measurable correlations; whether that combination is coherent depends on its ontology and its rule for records, not on an isolated slogan about quantum mechanics.

THE SAME QUESTION, DIFFERENT STARTING POINTS

The comparison at a glance

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