View source for Arrow of Time

The '''arrow of time''' is the problem that the fundamental laws of physics are, with one small exception, symmetric under reversal of the time coordinate, while almost everything we observe is not. Cups break and do not unbreak; heat flows from hot to cold; we remember the past. Nothing in Newton's equations, Maxwell's equations, general relativity or the Schrödinger equation distinguishes one direction of time from the other, yet the world plainly does.

==The several arrows==

It is usual, following Eddington, who coined the phrase in 1927, to distinguish:

* The '''thermodynamic arrow''' — [[Entropy|entropy]] of an isolated system increases toward the future.
* The '''cosmological arrow''' — the universe is observed to be expanding, in one direction of time.
* The '''radiative arrow''' — Maxwell's equations admit both retarded and advanced solutions, but we observe outgoing waves from sources and never converging ones.
* The '''psychological arrow''' — we have records of the past and not of the future.
* The '''causal arrow''' — effects follow causes.

The one genuine asymmetry in the microscopic laws is in the weak interaction. CP violation was discovered in neutral kaon decays by Christenson, Cronin, Fitch and Turlay in 1964; direct time-reversal violation was demonstrated by CPLEAR in 1998 and by BaBar in the B-meson system in 2012. This asymmetry is real, but it is far too feeble and far too specific to explain why coffee cools.

The standard resolution of the thermodynamic arrow does not locate it in the dynamics at all. Boltzmann's statistical account explains why entropy increases toward the future ''given'' that the system is now in a low-entropy state, but the same argument run backwards would predict higher entropy in the past, which is false. The repair — the '''past hypothesis''' — is to postulate that the universe began in a state of extraordinarily low entropy. That converts the arrow of time into a question about initial conditions, and why the initial state was so special is an acknowledged open problem in cosmology, not a solved one. Penrose's Weyl curvature hypothesis, inflationary accounts, and various cyclic and bouncing proposals are attempts at it, and none commands consensus.

==On this wiki==

The arrow of time connects directly to the entropy and heat-death disputes catalogued here, which are set out at [[Entropy]]. The objection made most often on this wiki is not to the second law but to the extrapolation of it to the universe as a whole.

* [[Glenn Borchardt]] argues in "[[Resolution of the SLT-Order Paradox]]" that the paradox of locally increasing order dissolves in an infinite universe, in which the second law is a law of divergence with a complementary law of convergence, and in which "isolated system" has no physical instances. See [[Infinite Universe Theory]].
* [[Martin Kokus]], "[[Order versus Chaos in a Steady-State Cosmology]]", takes up the same question from the [[Steady State Theory|steady-state]] side. If the universe has no beginning, the past hypothesis has nothing to attach to, and the arrow has to be explained some other way — which is a real burden on those models and should be stated as such.
* [[Jean E Burns]], "[[Vacuum Radiation, Entropy and the Arrow of time, Momentum]]", looks for the origin of the asymmetry in vacuum fluctuations rather than in initial conditions.
* [[Robert M Kiehn]] develops a topological, non-equilibrium thermodynamics in which irreversibility is built into the geometry of the process rather than added statistically; see "[[Non-Equilibrium Systems and Irreversible Processes, Vol 2: Falaco Solitons, Cosmology and the Arrow of Time]]".
* [[Pharis E Williams]], "[[Mechanical Entropy and its Implications]]" and "[[Energy and Entropy as the Fundaments of Theoretical Physics Entropy]]", makes entropy a fundamental rather than derived quantity.
* [[David Sands]] re-examines Clausius' original formulation in "[[Clausius' Concepts of "Aequivalenzwerth" and Entropy; A Critical Appraisal]]", and [[Yi-Fang Chang]] argues for conditions under which entropy can decrease in "[["Negative Temperature", a Necessary and Sufficient Condition for Entropy Decrease in an Isolated System]]".
* [[Greg Volk]]'s "[[The Nature of Eynptor (Entropy)]]" and [[Valery Chalidze]]'s "[[Entropy Demystified: Potential Order, Life and Money]]" attack the concept itself.
* On the relativistic side, [[Thomas P Morton]]'s "[[SRT Requires Time Reversal]]" and [[Sadanand S Savarkar]]'s "[[A Heat-Death for Relativity]]" argue that the temporal structure of [[Special relativity|special relativity]] carries consequences its advocates have not accepted.

The wider treatment of time on this wiki — whether it is a substance, a dimension or a measure of change — is at [[Time]], and it bears on the arrow directly: if time is a relational bookkeeping device rather than a thing, the question of its "direction" changes character, becoming a question about the direction of physical processes rather than about time itself. That is a position several contributors here hold, and it is a respectable one.

==See also==

* [[Entropy]]
* [[Time]]
* [[Big Bang]]
* [[Steady State Theory]]
* [[Infinite Universe Theory]]
* [[Energy]]

[[Category:Time]]
[[Category:Philosophy]]
[[Category:Cosmology]]