Arrow of time
This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").
[1] The arrow of time paradox was originally recognized in the 1800s for gases (and other substances) as a discrepancy between microscopic and macroscopic description of thermodynamics / statistical physics: at the microscopic level physical processes are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true.
A ball that is tossed up, slows to a stop, and falls is a case where recordings would look equally realistic forwards and backwards.
According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy.
[3] In his book The Big Picture, physicist Sean M. Carroll compares the asymmetry of time to the asymmetry of space: While physical laws are in general isotropic, near Earth there is an obvious distinction between "up" and "down", due to proximity to this huge body, which breaks the symmetry of space.
(Strictly speaking, the weak interactions are asymmetric to both spatial reflection and to flipping of the time direction.
)[citation needed] In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated: Let us draw an arrow arbitrarily.
In June 2022, researchers reported[15] in Physical Review Letters finding that salamanders were demonstrating counter-intuitive responses to the arrow of time in how their eyes perceived different stimuli.
An interesting thought experiment would be to ask: "if entropy was increased in an open system, would the arrow of time flip in polarity and point towards the past."
"[18] Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the earth and its thermodynamic processes.
Alternatively, it may be an artifact of our place in the universe's evolution (see the Anthropic bias), with this arrow reversing as gravity pulls everything back into a Big Crunch.
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.
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.
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.
[26][27] However, under special circumstances, one can prepare initial conditions that will cause a decrease in decoherence and in entropy.
[29] The state's reversal was made by a special program, similarly to the random microwave background fluctuation in the case of the electron.
[29] However, according to the estimations, throughout the age of the universe (13.7 billion years) such a reversal of the electron's state would only happen once, for 0.06 nanoseconds.
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.
[30] Certain subatomic interactions involving the weak nuclear force violate the conservation of both parity and charge conjugation, but only very rarely.
This arrow had not been linked to any large-scale temporal behaviour until the work of Joan Vaccaro, who showed that T violation could be responsible for conservation laws and dynamics.
