Evidence record

Why does time only move one way?

Every sentence on the exploration belongs to at least one claim below. Each says what kind of statement it is, what it rests on, what the evidence does not establish, and how far it was checked. Nothing here is a score, and none of it is a verdict on a belief or a theory.

Last checked 24 September 2026.

Source checked: 38
A judgement, not a fact: 13
Presented as open: 3

If something here is wrong, misquoted, out of date or unfair, write to support@unknownoceans.com and give the claim number, which does not change.

What the question takes for granted

A question in physics and the philosophy of physics. The asymmetry itself is not in doubt; what is open is where it comes from, whether a low-entropy beginning answers the question or only moves it, and whether time is the kind of thing that flows at all.

The fundamental laws do not themselves pick a direction of time
Established
Classical mechanics, electromagnetism, gravitation and the unitary quantum evolution are invariant under time reversal. Certain weak interactions are the known exception, and they do not appear to be what drives thermodynamic behaviour.
A low-entropy beginning needs an explanation of its own
Contested
Huw Price argues that it does; Craig Callender doubts that explaining it is demanded. The page reports both.
Time flows
Contested
On the static view, passage is a feature of how we experience the world; on the dynamic view it is real and mind-independent. The page reports the dispute and takes no side.
Time is fundamental
Open
Canonical quantum gravity arrives at an equation with no time evolution in it, and whether that means time is absent at the fundamental level is argued over.

How the question is labelled

Claim: The arrow of time

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Arrow of time

Kind: Framing. Signposting. It asserts nothing a source could check.

The question

Claim: Almost every fundamental law works equally well backwards.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

"Almost" is the weak-interaction exception the first section describes.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The puzzle is usually said to arise due to fundamental physics being time symmetric, or more precisely, time reversal invariant.
Supports
Press release: The Nobel Prize in Physics 1980"Symmetry by time reversal" and "Consequences of the prize-winning discovery"
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: The symmetry is also valid for processes controlled by gravitational and strong forces.

Later work looked for

Crossref, 2013 on, "time reversal violation observation": BaBar’s direct observation in B mesons (2012) and later reviews confirm the weak-interaction exception; nothing adds another.

Claim 3 · checked 24 September 2026 · AI-assisted audit

Claim: Nothing else does.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

A figure for the everyday, macroscopic processes the page goes on to describe, which run one way only. The SEP entry: "Temporal asymmetries are everywhere."

Supports
Thermodynamic Asymmetry in Timeopening
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Heat flows from hot to cold. The smell of coffee spreads throughout its available volume. ... The temporally reversed order of these processes never happen.

Claim 4 · checked 24 September 2026 · AI-assisted audit

Claim: The asymmetry of time is the most obvious fact about experience and one of the least explained facts in physics.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s summary. The SEP entry calls the asymmetry "one of the most salient" features of the universe and treats its grounding as an open problem.

Supports
Thermodynamic Asymmetry in Timeopening
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The thermodynamic time asymmetry is one of the most salient and consequential features of the physical universe.
Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: just as work on the first problem of the origins of the thermodynamic arrow remains active, so too does research on the second.

Claim 5 · checked 24 September 2026 · AI-assisted audit

Claim: The standard account pushes the question back to the beginning of the universe and leaves it there.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Summarises the known and uncertain sections: the statistical account plus a low-entropy beginning.

Supports
Thermodynamic Asymmetry in Timesection 3.9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Without some new physics that eliminates or explains the Past Hypothesis, or some satisfactory “third way”, it seems we are left with a bald posit of special initial conditions.

Claim 6 · checked 24 September 2026 · AI-assisted audit

Claim: Drop a glass and it shatters.

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Film it and run the film backwards and you immediately know something is wrong.

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: But the laws governing every molecule in that glass are, with one small exception, indifferent to the direction of time.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The puzzle is usually said to arise due to fundamental physics being time symmetric, or more precisely, time reversal invariant.
Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: weak interactions between quarks and leptons can violate time symmetry yet these violations don’t appear to be responsible for thermodynamic behavior.

What this does not establish

Time-reversal invariance of quantum mechanics holds for the unitary evolution of the state; collapse interpretations are not invariant (SEP, section 3.8).

Later work looked for

Crossref, 2013 on, "time reversal violation observation": the weak interaction remains the only known exception.

Claim 9 · checked 24 September 2026 · AI-assisted audit

Claim: Run them backwards and they are equally valid.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: time reversal means that for every entropy-increasing solution to the classical equations there is a mirror entropy-decreasing solution

What this does not establish

What the equations allow, not what is seen: reversed processes are allowed and almost never occur, which is the puzzle the page goes on to describe.

Claim 10 · checked 24 September 2026 · AI-assisted audit

Claim: The asymmetry you just watched is not in the laws.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from claim 9: the dynamical laws are time-symmetric apart from an exception that does not appear to drive thermodynamic behaviour, so the asymmetry has to come from somewhere else. The page names the candidate, a low-entropy beginning, in its next sections.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Thermodynamics is not a fundamental physical science. Hence it must inherit its massive time asymmetry from the microworld. But from where?

Claim 11 · checked 24 September 2026 · AI-assisted audit

What the laws actually say

Claim: What the laws actually say

Kind: Framing. Signposting. It asserts nothing a source could check.

The microscopic laws are nearly time-symmetric

Claim: The microscopic laws are nearly time-symmetric

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The puzzle is usually said to arise due to fundamental physics being time symmetric, or more precisely, time reversal invariant.
Supports
Press release: The Nobel Prize in Physics 1980"Consequences of the prize-winning discovery"
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: All attempts have been unsuccessful to avoid such a radically new conclusion as that which says that perfect symmetry by time reversal is not always true.

Later work looked for

Crossref, 2013 on, "time reversal violation observation": nothing that changes it.

Claim 13 · checked 24 September 2026 · AI-assisted audit

Claim: Classical mechanics, electromagnetism, general relativity and the Schrödinger equation of quantum mechanics all work the same run forwards or backwards.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: because the classical equations of motion are time reversal invariant, nothing in the original explanation necessarily referred to the direction of time
Supports
Press release: The Nobel Prize in Physics 1980"Symmetry by time reversal"
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: Such symmetry by time reversal is in fact valid for all processes governed by electromagnetic forces. It is therefore, a cornerstone in physics and chemistry. The symmetry is also valid for processes controlled by gravitational and strong forces.
Supports
Thermodynamic Asymmetry in Timesection 3.8
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Modulo some philosophical concerns about the meaning of time reversal ... the equation governing the unitary evolution of the quantum state is time reversal invariant.

What this does not establish

For quantum mechanics this is the unitary evolution only. Collapse interpretations are not time-reversal invariant, and the SEP entry notes philosophical disagreement over what time reversal means.

Later work looked for

Crossref, 2013 on, "time reversal violation observation": nothing that changes it.

Corrected

24 September 2026. Previously: “Classical mechanics, electromagnetism, general relativity and quantum mechanics all work the same run forwards or backwards.” The time-reversal invariance of quantum mechanics is that of its unitary evolution. Collapse interpretations are not invariant (SEP, section 3.8), so the claim is narrowed to the equation that carries it.

Claim 14 · checked 24 September 2026 · AI-assisted audit

Claim: There is one known exception: certain weak-force interactions violate time-reversal symmetry,

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: weak interactions between quarks and leptons can violate time symmetry yet these violations don’t appear to be responsible for thermodynamic behavior.
Supports
Press release: The Nobel Prize in Physics 1980"Consequences of the prize-winning discovery"
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: All attempts have been unsuccessful to avoid such a radically new conclusion as that which says that perfect symmetry by time reversal is not always true.

Later work looked for

Crossref, 2013 on, "time reversal violation observation": BaBar’s direct observation (2012) is added as claim 17; nothing that adds a second exception.

Claim 15 · checked 24 September 2026 · AI-assisted audit

Claim: first evidenced indirectly through the CP violation observed in kaon decay in 1964

Kind: Historical claim. A statement about the past that historical evidence can test.
Review: Source checked. The cited passage was read, and it says this.

Supports
Press release: The Nobel Prize in Physics 1980opening paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: for the discovery of violations of fundamental symmetry principles in the decay of neutral K-mesons.
Supports
Press release: The Nobel Prize in Physics 1980"Consequences of the prize-winning discovery"
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: The result of the prize-winning work showed for the first, time that the left-right asymmetry is not always completely compensated by transforming from matter to antimatter. ... a radically new conclusion as that which says that perfect symmetry by time reversal is not always true.
Context
Evidence for the 2π Decay of the K₂⁰ MesonCrossref record
Cited · Link answered, 19 October 2026 · Catalogue record only, 24 September 2026
The source reads: Evidence for the 2π Decay of the K₂⁰ Meson
The record gives the four authors, Physical Review Letters 13, 138, and a publication date of 27 July 1964.
Supports
Observation of Time-Reversal Violation in the B0 Meson Systemfirst paragraph, and reference 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026
The source reads: The observations of CP -symmetry breaking, first in neutral K decays [1] ... Hence, it is expected that the CP -violating weak interaction also violates time reversal invariance. ... [1] J.H. Christenson et al., Phys. Rev. Lett. 13, 138 (1964).
Carries the year and the inference from CP violation to time-reversal violation, which is why the page says “indirectly”.

What this does not establish

The 1964 paper was not readable here. The finding is from the Nobel press release; the year and the step from CP violation to time-reversal violation are from BaBar 2012, which cites the 1964 paper as the first observation. The step is an inference, which is why the page says "indirectly".

Corrected

24 September 2026. Previously: “first evidenced through the CP violation observed in kaon decay in 1964.” The 1964 result is CP violation, from which time-reversal violation is inferred; a direct observation was reported by BaBar in 2012 (abstract read). The page now says both.

Claim 16 · checked 24 September 2026 · AI-assisted audit

Claim: and reported directly by the BaBar experiment in 2012.

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Observation of Time-Reversal Violation in the B0 Meson Systemabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Physical systems · Experiment
The source reads: These nonzero results represent the first direct observation of T violation through the exchange of initial and final states in transitions that can only be connected by a T -symmetry transformation.
Supports
Observation of Time-Reversal Violation in the B0 Meson Systemresults, and closing paragraph
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Physical systems · Experiment
The source reads: this corresponds to a significance equivalent to 14 standard deviations (σ), and thus constitutes direct observation of T violation.

What this does not establish

The result is in neutral B mesons, from 468 million B-meson pairs, at a significance equivalent to 14 standard deviations; "first direct observation" is BaBar’s own description, and the paper notes earlier evidence related to T violation in neutral kaons.

Later work looked for

Crossref, 2013 on, "time reversal violation observation": conference reports of the same result and later reviews; nothing that contradicts it.

Claim 17 · checked 24 September 2026 · AI-assisted audit

Claim: That violation is real but specialised, and it does not appear to be what makes coffee cool and never spontaneously reheat.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: weak interactions between quarks and leptons can violate time symmetry yet these violations don’t appear to be responsible for thermodynamic behavior. But see Roberts (2022).

Other readings

The SEP entry points to Bryan Roberts’s Reversing the Arrow of Time (2022) as a place where this is questioned.

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": titles only; nothing read that ties thermodynamic behaviour to weak-interaction violation.

Corrected

24 September 2026. Previously: “That violation is real but far too small and too specialised to explain why coffee cools and never spontaneously reheats.” The page gave a reason ("far too small") and a certainty that the cited reference work does not: it says the violations "don’t appear to be responsible for thermodynamic behavior" and points to a dissent.

Claim 18 · checked 24 September 2026 · AI-assisted audit

The statistical account works

Claim: The statistical account works

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: the Boltzmannian explanation seems to offer a plausible and powerful framework for understanding why the entropy of systems tends to increase with time.

What this does not establish

The SEP entry notes substantial open questions about its details, including what justifies the probability measure, and that the derivations are rigorous only in ideal cases.

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": toy models of an arrow without a past hypothesis (2024) and a 2025 companion chapter; titles only, nothing that changes the standing of the framework.

Claim 19 · checked 24 September 2026 · AI-assisted audit

Claim: The second law of thermodynamics says entropy (roughly, a measure of how many microscopic arrangements are consistent with what you can see) tends to increase.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Boltzmann’s entropy is the logarithm of the volume of states that look the same from outside, so it measures the number of arrangements rather than being it.

Supports
Thermodynamic Asymmetry in Timesection 2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Loosely put, the Second Law is understood as the claim that entropy spontaneously increases with time.
Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The key ingredient is partitioning Γ into compartments, such that all of the microstates X in a compartment are macroscopically (and thus thermodynamically) indistinguishable.
Notation restored: MathJax markup rendered as the SEP page renders it (Γ is the state space), checked against the page’s source, read 2026-09-24. The text as extracted reads: The key ingredient is partitioning \(\Gamma\) into compartments, such that all of the microstates \(X\) in a compartment are macroscopically (and thus thermodynamically) indistinguishable.

Later work looked for

A definition and a statement of the law; not a finding later work revises.

Corrected

24 September 2026. Previously: “(roughly, the number of microscopic arrangements consistent with what you can see)” Boltzmann entropy is k log of the phase-space volume of a macrostate (SEP, section 3), a measure of the number of arrangements rather than the number itself.

Claim 20 · checked 24 September 2026 · AI-assisted audit

Claim: There are vastly more ways for a glass to be broken than intact, so the transition runs one way overwhelmingly often.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: There are simply many more ways to be distributed with uniform temperature and pressure than ways to be distributed with nonuniform temperature and pressure.
Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: almost all microstates realizing non-equilibrium macrostates are such that their entropy value is overwhelmingly likely to increase with time.

What this does not establish

The glass is an illustration; the entry’s examples are gases and heat. By the same reasoning run backwards, a present state is also likely to have come from a higher-entropy past, which is why the account needs the low-entropy starting point the next sentence names.

Claim 21 · checked 24 September 2026 · AI-assisted audit

Claim: This explains the arrow given a low-entropy starting point.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The former method works if we assume that earlier states of the universe are of comparatively low-entropy and that (relatively) later states are not also low-entropy states.

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": proposals for an arrow without a past hypothesis (2024, titles only); none read that displaces the standard account.

Claim 22 · checked 24 September 2026 · AI-assisted audit

Claim: It does not explain the starting point.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Without some new physics that eliminates or explains the Past Hypothesis, or some satisfactory “third way”, it seems we are left with a bald posit of special initial conditions.

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": nothing that changes it.

Claim 23 · checked 24 September 2026 · AI-assisted audit

The universe began in an extraordinarily low-entropy state

Claim: The universe began in an extraordinarily low-entropy state

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Earlier states do not have higher entropy than present states because we make the cosmological posit that the universe began in an extremely tiny section of its available phase space.
Supports
Spontaneous Inflation and the Origin of the Arrow of Timesection 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: The origin of the Second Law is traced back to initial conditions: the early universe had an extremely low entropy, allowing it to continue to increase thereafter.

What this does not establish

It is inferred from the present universe and the early smoothness, not measured; Earman argues the past hypothesis cannot yet be stated in the language of general relativity (SEP, section 3.1).

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": nothing that changes it.

Claim 24 · checked 24 September 2026 · AI-assisted audit

Claim: The early universe was hot and remarkably uniform: the radiation left over from it varies by about one part in 100,000 across the sky.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Planck and the cosmic microwave backgroundsection "What is the cosmic microwave background?"
Cited · Link refuses automated checks, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Reference work
The source reads: When the Universe was born, nearly 14 billion years ago, it was filled with hot plasma of particles (mostly protons, neutrons, and electrons) and photons (light).
Supports
Planck and the cosmic microwave backgroundsection "How many space missions have studied the cosmic microwave background?"
Cited · Link refuses automated checks, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement
The source reads: it also shows very small temperature fluctuations on the order of 1 part in 100,000 across the sky.

Later work looked for

Crossref, 2019 on, "cosmic microwave background temperature anisotropy Planck": later analyses study anomalies at large scales; nothing that changes the size of the fluctuations.

Corrected

24 September 2026. Previously: “The early universe was hot, dense and remarkably uniform.” The sentence cited only a philosophy reference that does not describe the early universe. It now rests on ESA’s account of the cosmic microwave background, and gives the measured uniformity.

Claim 25 · checked 24 September 2026 · AI-assisted audit

Claim: That uniformity, in a universe with gravity, is a very low-entropy condition,

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Loosely associating gravitational homogeneity with low-entropy and inhomogeneity with higher entropy, inflation is arguably a source of a low entropy “initial” condition.
Supports
Spontaneous Inflation and the Origin of the Arrow of Timesection 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: The low entropy of our current universe is clearly related to its homogeneity and isotropy

What this does not establish

Entropy for gravitating systems is not well defined: Carroll and Chen write that "we do not have a reliable understanding of entropy in the context of quantum gravity", and the SEP entry calls the association loose.

Claim 26 · checked 24 September 2026 · AI-assisted audit

Claim: which is where the arrow of time ultimately comes from on the standard account.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Boltzmann, as well as many of the twentieth century’s greatest scientists, e.g., Einstein, Richard Feynman, and Erwin Schroedinger, saw that this hypothesis is necessary given our (mostly) time asymmetric laws.

Other readings

Other proposals the SEP entry sets out: a direction in time itself, time-asymmetric collapse laws, and models such as Carroll and Chen’s that do without a past hypothesis.

Later work looked for

Crossref, 2023 on, "past hypothesis arrow of time": nothing that displaces it as the standard account.

Claim 27 · checked 24 September 2026 · AI-assisted audit

Whether the past hypothesis explains anything

Claim: Whether the past hypothesis explains anything

Kind: Framing. Signposting. It asserts nothing a source could check.

Whether the past hypothesis is an explanation

Claim: Whether the past hypothesis is an explanation

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Assume a low-entropy beginning and the statistical account delivers the thermodynamic arrow,

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This package, they say, implies the thermodynamic arrow.
Qualifies
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: they do so rigorously only in ideal cases and much remains controversial. Still, it strikes many as physically plausible.

What this does not establish

The derivations are rigorous only in ideal cases.

Corrected

24 September 2026. Previously: “Assuming a low-entropy beginning reproduces every thermodynamic arrow we observe.” "Every" overstated it. The SEP entry records the "subsystem" worry, that the past hypothesis is consistent with no subsystem on Earth behaving thermodynamically, and notes the derivations are rigorous only in ideal cases.

Claim 30 · checked 24 September 2026 · AI-assisted audit

Claim: though how a condition on the whole universe reaches each glass and cup is still argued over.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: How exactly does the global entropy increase of the universe imply local entropy increase among the subsystems (which, after all, is what causes us to posit the Second Law in the first place)?

Claim 31 · checked 24 September 2026 · AI-assisted audit

Claim: The assumption is doing all the work and is itself unexplained.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from claims 22 and 23: the statistical account delivers the arrow only with the low-entropy start, and nothing accepted explains the start.

Supports
Thermodynamic Asymmetry in Timesection 3.9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: it seems we are left with a bald posit of special initial conditions.

Claim 32 · checked 24 September 2026 · AI-assisted audit

Claim: Whether it needs explaining is disputed:

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: there is the question of whether the Past Hypothesis itself demands explanation. In physics it is often claimed that explaining the Past Hypothesis is a central problem. But why?

Claim 33 · checked 24 September 2026 · AI-assisted audit

Claim: Huw Price argues that it does, Craig Callender doubts it.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: See Callender 2004a,b for some skepticism that explaining the Past Hypothesis is demanded and Price 1996, 2004 for the contrary position
Supports
Thermodynamic Asymmetry in TimeBibliography
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Price, Huw, 1995
Full names from the entry’s bibliography, which also lists Callender, Craig.

Corrected

24 September 2026. Previously: “Critics point out that this assumption is doing all the work and is itself unexplained.” The page gave one side of a named disagreement as what "critics point out". The SEP entry sets out both positions and who holds them.

Claim 34 · checked 24 September 2026 · AI-assisted audit

Claim: We have traded a puzzle about time for a puzzle about initial conditions.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s reading of the past hypothesis, which the SEP entry frames in the same terms.

Supports
Thermodynamic Asymmetry in Timesection 3.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: We want to know whether our question has merely been shifted back a step, whether the explanation is a disguised appeal to special initial conditions.

Claim 35 · checked 24 September 2026 · AI-assisted audit

How many arrows there are

Claim: How many arrows there are

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: The thermodynamic arrow, the cosmological arrow of expansion, the arrow of radiation, and the psychological arrow of memory all point the same way.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

The "arrow of memory" is the entry’s epistemological arrow, that there are records of the past and not of the future, together with its psychological arrow.

Supports
Thermodynamic Asymmetry in Timesection 3.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The most obvious one is the inexorable expansion of the universe.
Supports
Thermodynamic Asymmetry in Timesection 3.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: It is therefore tempting to think that there is a relationship between the thermodynamic and electromagnetic arrows of time.
Supports
Thermodynamic Asymmetry in Timesection 4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The epistemological arrow. Roughly put, we know more about the past than the future.

Later work looked for

Which arrows the literature names and that they share a direction; not a finding later work revises.

Claim 37 · checked 24 September 2026 · AI-assisted audit

Claim: Whether they are one phenomenon or several that happen to align is not settled.

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
Thermodynamic Asymmetry in Timesection 3.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: it’s premature to say that the thermodynamic arrow is the one arrow to rule them all. Most agree that the wave asymmetry is ultimately “thermodynamic” but after that matters are contested.
Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: But is the arrow of time a fundamental feature of the world? Or can it be reduced to some other feature, thus simplifying our metaphysics as a whole?

Claim 38 · checked 24 September 2026 · AI-assisted audit

Why the beginning was the way it was

Claim: Why the beginning was the way it was

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: You remember yesterday and not tomorrow.

Kind: Observation. Something measured or seen, as the named source reports it.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The epistemological arrow. Roughly put, we know more about the past than the future.

Later work looked for

An everyday observation the entry describes; not a finding later work revises.

Claim 40 · checked 24 September 2026 · AI-assisted audit

Claim: Nothing in the equations requires that.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

On the account the SEP entry sets out, the asymmetry of records comes from the low-entropy boundary condition, not from the time-symmetric dynamical equations.

Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: the Past Hypothesis effectively is the world’s Ready State. This very special state is what causes there to be macroscopic traces of the past in the present but not macroscopic traces of the future in the present.

Claim 41 · checked 24 September 2026 · AI-assisted audit

Why the initial conditions were what they were

Claim: Why the initial conditions were what they were

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: No accepted theory explains why the universe began in a state of such low entropy.

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
Thermodynamic Asymmetry in Timesection 3.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The cosmological hypotheses may be true, and in some sense, they may even explain the low-entropy initial state. But they do not appear to provide an explanation of the thermodynamic asymmetry that makes it nomologically necessary or even likely.
Supports
Thermodynamic Asymmetry in Timesection 3.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Although quantum cosmology is currently very speculative, there has been a substantial amount of debate on this issue

Other readings

Some doubt that the initial state needs an explanation at all (Callender, in the SEP entry, section 3.1).

Claim 43 · checked 24 September 2026 · AI-assisted audit

Claim: Answering it appears to require understanding physics at the beginning, which requires a theory of quantum gravity,

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 3.4
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Quantum cosmology, it is often said, is the theory of the universe’s initial conditions.
Supports
Spontaneous Inflation and the Origin of the Arrow of Timesection 2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: Unfortunately, we do not have a reliable understanding of entropy in the context of quantum gravity

Claim 44 · checked 24 September 2026 · AI-assisted audit

Claim: and no such theory has yet been tested by experiment.

Kind: What the field holds. The settled view of a field, as a review, a consensus statement or a reference work gives it. Not one study.
Review: Source checked. The cited passage was read, and it says this.

Supports
Quantum Gravity, in the Stanford Encyclopedia of Philosophysection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: there are no experiments in quantum gravity, and little in the way of observations that might qualify as direct or indirect data or empirical evidence.
Qualifies
Quantum Gravity, in the Stanford Encyclopedia of Philosophyopening
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This scale is so remote from current experimental capabilities that the empirical testing of quantum gravity proposals along standard lines is rendered near-impossible, though there have been some recent developments that suggest the outlook might be more optimistic than previously surmised

What this does not establish

The entry notes recent proposals for laboratory tests.

Later work looked for

Crossref, 2023 on, "gravitationally induced entanglement experiment quantum gravity": proposals and models for tabletop tests (titles only); none reports a completed test.

Corrected

24 September 2026. Previously: “which requires quantum gravity, which does not exist in testable form.” The candidate theories exist and some make predictions; what is missing is an experiment. The SEP entry also records recent proposals that may make tests possible.

Claim 45 · checked 24 September 2026 · AI-assisted audit

Whether time is fundamental

Claim: Whether time is fundamental

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: The canonical approach to quantum gravity arrives at a central equation, the Wheeler-DeWitt equation, in which the state of the universe does not evolve: there is no time variable in it.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Quantum Gravity, in the Stanford Encyclopedia of Philosophysection 3.2.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: its counterpart in the constraint-quantized theory, Hψ = 0, would prima facie seem to indicate that the true physical states of the system do not evolve at all: there is no t.
Notation restored: MathJax markup rendered as the SEP page renders it, checked against the page’s source, read 2026-09-24. The text as extracted reads: its counterpart in the constraint-quantized theory, \(H \psi = 0\), would prima facie seem to indicate that the true physical states of the system do not evolve at all: there is no \(t\).
Supports
Time from quantum entanglement: an experimental illustrationintroduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: a canonical quantization of general relativity yields the Wheeler-De Witt equation [7, 8] predicting a static state of the universe, contrary to obvious everyday evidence.

What this does not establish

The SEP entry says "prima facie", and that the difficulty generalises to many other approaches; what it means is the next claim.

Corrected

24 September 2026. Previously: “Several approaches to quantum gravity produce equations with no time variable at all, with time emerging as an approximation.” The page stated one reading of the problem of time as the outcome. The SEP entry names the equation and the approach, and reports two readings: Barbour’s, that time is illusory, and the argument that true Hamiltonians generate the evolution.

Claim 47 · checked 24 September 2026 · AI-assisted audit

Claim: Some, Julian Barbour among them, read this as a sign that time is illusory; others argue that the evolution is there and harder to find.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Quantum Gravity, in the Stanford Encyclopedia of Philosophysection 5.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Julian Barbour (2001a,b), for one, thinks that it tells us that time is illusory
Supports
Quantum Gravity, in the Stanford Encyclopedia of Philosophysection 5.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: the super-Hamiltonian itself should not be expected to generate time-evolution; rather, one or more “true” Hamiltonians should play this role, though uncovering such Hamiltonians is no easy matter.

Claim 48 · checked 24 September 2026 · AI-assisted audit

Claim: If the first reading is right, the question is not why time flows one way but why anything like flow appears in a description that does not contain it.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Conditional on the first reading in claim 48; the SEP entry on time poses the same question.

Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: What would it mean for something temporal to be grounded in something atemporal and what sort of grounding relation would be involved?

Claim 49 · checked 24 September 2026 · AI-assisted audit

Why we experience a present

Claim: Why we experience a present

Kind: Framing. Signposting. It asserts nothing a source could check.

Claim: Relativity describes a four-dimensional spacetime in which there is no fact about which distant events are happening now.

Kind: What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review: Source checked. The cited passage was read, and it says this.

Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: According to standard presentations of special relativity, there is no fact of the matter as to whether two spatially separated events happen at the same time.

What this does not establish

That is the standard presentation. The SEP entry notes that general relativity sometimes allows a preferred slicing into instants, and that some interpretations of quantum mechanics seem to need absolute simultaneity.

Corrected

24 September 2026. Previously: “Physics describes a four-dimensional structure in which all times are equally real. It contains nothing corresponding to now moving.” The page stated one side of a live dispute, the static view of time, as what physics says. The SEP entry on time separates the physics (the relativity of simultaneity) from the philosophical readings, and records that presentists and other A-theorists can reject the reading without conflict with any experiment.

Claim 51 · checked 24 September 2026 · AI-assisted audit

Claim: Many philosophers read that as all times being equally real, with nothing corresponding to now moving;

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Timesection 8, The Static Theory of Time
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The correct ontology does not change over time, and it always includes objects from every region of spacetime. ... There is no dynamic aspect to time; time does not pass.
Supports
Timesection 8
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: they correspond to the two most popular combinations of views about time to be found in the literature

Claim 52 · checked 24 September 2026 · AI-assisted audit

Claim: defenders of a real present reply that the physics does not force the reading.

Kind: What scholars argue. A report of what specialists argue, or where they disagree.
Review: Source checked. The cited passage was read, and it says this.

Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: This means that presentists and other A-theorists have a way of avoiding the worry from relativity without any conflict with empirical results

Claim 53 · checked 24 September 2026 · AI-assisted audit

Claim: Whether the felt passage of time is a fact about reality or a fact about the kind of observer we are is genuinely unresolved,

Kind: Open question. Something the page says is not known.
Review: Presented as open. The page says this is unresolved, and the review found that it is.

Supports
Timesection 8
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Static Theorists of course admit that time seems special to us, and that it seems to pass. But they insist that this is just a feature of consciousness
Supports
Timesection 8, The Dynamic Theory of Time
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The passage of time is a real and mind-independent phenomenon.

Claim 54 · checked 24 September 2026 · AI-assisted audit

Claim: and sits on the boundary between physics and the study of mind.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The dispute draws on physics (relativity, thermodynamics) and on psychology, as the SEP entry sets out.

Supports
Timesection 12
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Another interesting line of research uses empirical work in psychology to better understand what is happening cognitively when we judge time as passing.

Claim 55 · checked 24 September 2026 · AI-assisted audit

Where the arrow might come from

Claim: Where the arrow might come from

Kind: Framing. Signposting. It asserts nothing a source could check.

Time’s arrow is entirely inherited from cosmology

Claim: Time’s arrow is entirely inherited from cosmology

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Albert (2000, 2015) and Loewer (2012) instead trace them to a package dubbed the Mentaculus

Other readings

Others hold that time itself is directed (SEP, section 3.6), and Hemmo and Shenker argue that the program will not succeed for the psychological arrow.

Claim 57 · checked 24 September 2026 · AI-assisted audit

Claim: On this view there is nothing to explain about time itself: it is symmetric, and every asymmetry we notice traces back to the universe having started in an unusual state.

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: Let’s briefly consider how Albert and Loewer propose to derive the thermodynamic arrow, the epistemic arrow, and the causal arrow all from the Mentaculus.
Supports
Thermodynamic Asymmetry in Timesection 4.2
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: the past hypothesis (that the entropy of initial macrostate M(0) is extremely low), a uniform probability distribution over the microstates that realize M(0), the present macrostate M(t), and the dynamical laws of the microlevel.
Notation restored: MathJax markup rendered as the SEP page renders it, checked against the page’s source, read 2026-09-24. The text as extracted reads: the past hypothesis (that the entropy of initial macrostate \(M(0)\) is extremely low), a uniform probability distribution over the microstates that realize \(M(0)\), the present macrostate \(M(t)\), and the dynamical laws of the microlevel.

What this does not establish

A program, not a result: the SEP entry records vigorous criticism of its reach, and "every asymmetry" is its ambition rather than something shown.

Other readings

Hemmo and Shenker argue it will not succeed for the psychological arrow; others propose that time itself is directed.

Claim 58 · checked 24 September 2026 · AI-assisted audit

Claim: The question then becomes a question in cosmology.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from claim 58: if every arrow traces to the initial state, what is left to explain is that state.

Supports
Thermodynamic Asymmetry in Timesection 3.1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: A cosmological hypothesis claiming that in the very distant past entropy was much lower will work.

Claim 59 · checked 24 September 2026 · AI-assisted audit

The arrow points away from a low-entropy moment in both directions

Claim: The arrow points away from a low-entropy moment in both directions

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Spontaneous Inflation and the Origin of the Arrow of Timeabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: An important consequence of this picture is that inflation occurs asymptotically both forwards and backwards in time, implying a universe that is (statistically) time-symmetric on ultra-large scales.

What this does not establish

A proposal; nothing has tested it.

Claim 60 · checked 24 September 2026 · AI-assisted audit

Claim: Some proposals suggest that if the universe’s entropy were lowest in the middle of its history rather than at its beginning, and could grow without limit in both directions, observers on either side would each find entropy increasing away from that moment, so the arrow would be local rather than global.

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Spontaneous Inflation and the Origin of the Arrow of Timeabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: Starting from de Sitter, inflation can increase the total entropy of the universe without bound, creating universes similar to ours in the process.
Supports
Spontaneous Inflation and the Origin of the Arrow of Timesection 1
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model
The source reads: We also predict that this structure should be recovered infinitely far into the past, with a reversed thermodynamic arrow of time. ... about some Cauchy surface of minimum entropy.
Supports
Thermodynamic Asymmetry in Timesection 3.3
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The crucial assumption in this model – one also found in the gravitational theory of Barbour, Koslowski, and Mercati (2014) – is that entropy is unbound.

What this does not establish

Speculative models resting on the assumption that entropy has no maximum. Carroll and Chen’s also depends on spontaneous eternal inflation.

Corrected

24 September 2026. Previously: “Some proposals suggest that if the universe had a low-entropy state in its middle rather than its beginning, observers on either side would each experience entropy as increasing away from it, meaning the arrow is local to observers rather than global.” The cited model rests on entropy being unbounded, which the SEP entry names as its crucial assumption; the page left it out. Carroll and Chen’s middle state is a minimum rather than a specially low-entropy state.

Claim 61 · checked 24 September 2026 · AI-assisted audit

Claim: These are speculative models.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The authors present the model as a suggestion, with its assumptions stated.

Supports
Spontaneous Inflation and the Origin of the Arrow of Timeabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: We suggest that spontaneous eternal inflation can provide a natural explanation for the thermodynamic arrow of time, and discuss the underlying assumptions and consequences of this view.

Claim 62 · checked 24 September 2026 · AI-assisted audit

Time emerges from something else

Claim: Time emerges from something else

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: The second question mentioned above is a question about whether time itself
Supports
Timesection 11
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Reference work
The source reads: might merely be an emergent feature of the world. This question has become especially pressing as philosophers of physics have turned their attention to theories of quantum gravity

What this does not establish

The SEP entry calls work in this area nascent.

Claim 63 · checked 24 September 2026 · AI-assisted audit

Claim: If time is not fundamental, it may emerge from entanglement between parts of the universe: in a mechanism Page and Wootters proposed in 1983, a universe that is static as a whole looks as if it evolves to an observer inside it who reads one part as a clock.

Kind: Hypothesis. A proposed explanation, not an established one.
Review: Source checked. The cited passage was read, and it says this.

Supports
Time from quantum entanglement: an experimental illustrationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems.
Supports
Time from quantum entanglement: an experimental illustrationintroduction and reference 9
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: Thus, time would be an emergent property of subsystems of the universe deriving from their entangled nature ... D.N. Page and W.K. Wootters, Phys. Rev. D, 27, 2885 (1983).

What this does not establish

A proposal about how time could appear; Moreva and others list two main objections to it, including doubt that quantum mechanics can describe the whole universe with its observers inside it.

Corrected

24 September 2026. Previously: “If time is not fundamental, it may emerge from entanglement structure or from the relationships between subsystems. Work in this direction is mathematically serious and experimentally untested.” The passage cited only the SEP quantum gravity entry, which does not discuss entanglement as a source of time. It now names the mechanism from a paper that describes it, and "experimentally untested" is made exact: the mechanism has been reproduced in a laboratory, which is an illustration and not a test of where our time comes from.

Claim 64 · checked 24 September 2026 · AI-assisted audit

Claim: The idea is mathematically serious and still controversial.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Moreva and others call the idea "extremely elegant but controversial".

Supports
Time from quantum entanglement: an experimental illustrationintroduction
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory
The source reads: Thus, time would be an emergent property of subsystems of the universe deriving from their entangled nature: an extremely elegant but controversial idea

Claim 65 · checked 24 September 2026 · AI-assisted audit

Claim: A 2014 experiment reproduced the mechanism with two entangled photons,

Kind: Experimental result. What an experiment or trial found, under its own conditions. A result in animals, cells or a simulation is not a result in people.
Review: Source checked. The cited passage was read, and it says this.

Supports
Time from quantum entanglement: an experimental illustrationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Physical systems · Experiment
The source reads: We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second

What this does not establish

A two-photon laboratory system, not the universe. Posted in 2013 and published in 2014.

Later work looked for

Crossref, 2015 on, "Page Wootters time entanglement experiment": later theoretical extensions (2019, 2021) and theses (titles only); nothing that changes what the experiment shows.

Claim 66 · checked 24 September 2026 · AI-assisted audit

Claim: which illustrates it without testing whether our own time arises this way.

Kind: This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review: A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The authors describe their experiment as an illustration of the mechanism in a small subsystem.

Supports
Time from quantum entanglement: an experimental illustrationabstract
Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Physical systems
The source reads: Here, to provide an insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters’ mechanism of “static” time

Claim 67 · checked 24 September 2026 · AI-assisted audit

What was read

Evidence for the 2π Decay of the K₂⁰ MesonPhysical Review Letters · Crossref record: Physical Review Letters 13(4), 138 to 140, published 27 July 1964; J. H. Christenson, J. W. Cronin, V. L. Fitch, R. TurlayCatalogue record only, 24 September 2026 · CopyrightThe APS full text refused this environment (403), Crossref and OpenAlex carry no abstract, and Unpaywall lists no repository copy. The finding is checked against the Nobel press release for Cronin and Fitch.
Thermodynamic Asymmetry in TimeStanford Encyclopedia of Philosophy · Stanford Encyclopedia of Philosophy, "Thermodynamic Asymmetry in Time", substantive revision 12 June 2026; read in fullRead in full, 24 September 2026 · Copyright; quoted briefly
Quantum Gravity, in the Stanford Encyclopedia of PhilosophyStanford University · Stanford Encyclopedia of Philosophy, Weinstein and Rickles, substantive revision 26 February 2024; read the opening and sections 3.2, 4.2 and 5.1Read in full, 24 September 2026 · Copyright; quoted briefly
Spontaneous Inflation and the Origin of the Arrow of TimeSean M. Carroll and Jennifer Chen · arXiv:hep-th/0410270v1 (27 October 2004); read the abstract, sections 1 and 2 and the opening of section 3Read in full, 24 September 2026 · Copyright; quoted brieflyPDF text breaks some words; excerpts avoid those runs.
Press release: The Nobel Prize in Physics 1980The Royal Swedish Academy of Sciences · NobelPrize.org press release, 14 October 1980; read in fullRead in full, 24 September 2026 · Nobel Prize Outreach; quoted briefly
Observation of Time-Reversal Violation in the B0 Meson SystemBABAR Collaboration, Physical Review Letters · arXiv:1207.5832v4 (2 January 2013), PDF read as text (pdftotext); published as Physical Review Letters 109, 211801 (19 November 2012) per CrossrefRead in full, 24 September 2026 · arXiv non-exclusive distribution licence; quoted briefly
Planck and the cosmic microwave backgroundEuropean Space Agency · ESA web page, read in fullRead in full, 24 September 2026 · ESA; quoted briefly
Time from quantum entanglement: an experimental illustrationEkaterina Moreva and others, Physical Review A · arXiv:1310.4691v1 (17 October 2013); published as Physical Review A 89, 052122 (May 2014) per Crossref; read the abstract and introductionRead in full, 24 September 2026 · Copyright; quoted briefly
TimeStanford Encyclopedia of Philosophy · Stanford Encyclopedia of Philosophy, "Time", substantive revision 4 December 2025; read sections 3, 5, 8, 11 and 12Read in full, 24 September 2026 · Copyright; quoted briefly