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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Arrow of time | 3/4 | https://en.wikipedia.org/wiki/Arrow_of_time | reference | science, encyclopedia | 2026-05-05T11:15:00.259019+00:00 | kb-cron |
=== Radiative arrow of time === Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves, so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it, making the latter contradictory to the second law of thermodynamics in usual circumstances.
=== Causal arrow of time === A cause precedes its effect: the causal event occurs before the event it causes or affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow. An epistemological problem with using causality as an arrow of time is that, as David Hume maintained, the causal relation per se cannot be perceived; one only perceives sequences of events. Furthermore, it is surprisingly difficult to provide a clear explanation of what the terms cause and effect really mean, or to define the events to which they refer. However, it does seem evident that dropping a cup of water is a cause while the cup subsequently shattering and spilling the water is the effect. Physically speaking, correlations between a system and its surrounding are thought to increase with entropy, and have been shown to be equivalent to it in a simplified case of a finite system interacting with the environment. The assumption of low initial entropy is indeed equivalent to assuming no initial correlations in the system; thus correlations can only be created as we move forward in time, not backwards. Controlling the future, or causing something to happen, creates correlations between the doer and the effect, and therefore the relation between cause and effect is a result of the thermodynamic arrow of time, a consequence of the second law of thermodynamics. Indeed, in the above example of the cup dropping, the initial conditions have high order and low entropy, while the final state has high correlations between relatively distant parts of the system – the shattered pieces of the cup, as well as the spilled water, and the object that caused the cup to drop.
=== Quantum arrow of time === Quantum evolution is governed by equations of motions that are time-symmetric (such as the Schrödinger equation in the non-relativistic approximation), and by wave function collapse, which is a time-irreversible process, and is only physically real in explicit collapse interpretations of quantum theory, such as the Diósi–Penrose model, the Ghirardi–Rimini–Weber theory, or the Transactional interpretation, which uses the direct-action or "absorber" theory of fields. The conventional approach is to assume that quantum decoherence explains irreversibility and the second law of thermodynamics, thus claiming to derive the quantum arrow of time from the thermodynamic arrow of time; however this is a matter of some debate, since the underlying dynamics is assumed to be unitary and thus reversible.
A conventional account of decoherence is to say that following any particle scattering or interaction between two larger systems, the relative phases of the two systems are at first orderly related, but subsequent interactions (with additional particles or systems) make them less so, so that the two systems become decoherent. Thus decoherence is a form of increase in microscopic disorder – in short, decoherence increases entropy. Two decoherent systems can no longer interact via quantum superposition, unless they become coherent again, which is normally impossible, by the second law of thermodynamics. In the language of relational quantum mechanics, the observer becomes entangled with the measured state, where this entanglement increases entropy. As stated by Seth Lloyd, "the arrow of time is an arrow of increasing correlations". However, under special circumstances, one can prepare initial conditions that will cause a decrease in decoherence and in entropy. This has been shown experimentally in 2019, when a team of Russian scientists reported the reversal of the quantum arrow of time on an IBM quantum computer, in an experiment supporting the understanding of the quantum arrow of time as emerging from the thermodynamic one. By observing the state of the quantum computer made of two and later three superconducting qubits, they found that in 85% of the cases, the two-qubit computer returned to the initial state. The state's reversal was made by a special program, similarly to the random microwave background fluctuation in the case of the electron. However, according to the estimations, throughout the age of the universe (around 14 billion years) such a reversal of the electron's state would only happen once, for 0.06 nanoseconds. The scientists' experiment led to the possibility of a quantum algorithm that reverses a given quantum state through complex conjugation of the state. Note that quantum decoherence merely allows the appearance of quantum wave collapse (based on the vanishing of diagonal elements of the density matrix); it is a matter of dispute whether the collapse itself actually takes place or is redundant and apparent only. While the theory of quantum decoherence is widely accepted and has been supported experimentally at the level of the applicable density matrix, the conventional theory's inability to predict actual measurement outcomes via non-unitary collapse remains. That is, the density matrix obtained from standard unitary-only decoherence (without actual collapse) is an improper mixture that cannot be interpreted as reflecting a determinate measurement outcome. Thus the arrow of time question continues to be addressed by way of explicit collapse approaches.
=== Particle physics (weak) arrow of time ===