Editing Time (section)

=== Arrow of time ===
{{Main|Arrow of time}}

Unlike space, where an object can travel in the opposite directions (and in 3 dimensions), time appears to have only one dimension and only one direction—the past lies behind, fixed and immutable, while the future lies ahead and is not necessarily fixed. Yet most laws of physics allow any process to proceed both forward and in reverse. There are only a few physical phenomena that violate the reversibility of time. This time directionality is known as the [[arrow of time]]. Acknowledged examples of the arrow of time are:<ref>{{Cite journal |last1=O’Byrne |first1=J. |last2=Kafri |first2=Y. |last3=Tailleur |first3=J. |last4=van Wijland |first4=F. |date=2022-01-25 |title=Time irreversibility in active matter, from micro to macro |url=https://doi.org/10.1038/s42254-021-00406-2 |journal=Nature Reviews Physics |volume=4 |issue=3 |pages=167–183 |doi=10.1038/s42254-021-00406-2 |issn=2522-5820|arxiv=2104.03030 |bibcode=2022NatRP...4..167O }}</ref><ref>{{Citation |last=Maxwell |first=Nicholas |title=Relativity Theory May not Have the Last Word on the Nature of Time: Quantum Theory and Probabilism |date=2017 |work=Space, Time and the Limits of Human Understanding |series=The Frontiers Collection |pages=109–124 |editor-last=Wuppuluri |editor-first=Shyam |url=http://link.springer.com/10.1007/978-3-319-44418-5_9 |access-date=2025-03-01 |place=Cham |publisher=Springer International Publishing |doi=10.1007/978-3-319-44418-5_9 |isbn=978-3-319-44417-8 |editor2-last=Ghirardi |editor2-first=Giancarlo}}</ref><ref>{{Cite book |url=https://www.cambridge.org/core/product/identifier/9781139225700/type/book |title=Complexity and the Arrow of Time |date=2013-08-08 |publisher=Cambridge University Press |isbn=978-1-139-22570-0 |editor-last=Lineweaver |editor-first=Charles H. |edition=1 |doi=10.1017/cbo9781139225700 |editor-last2=Davies |editor-first2=Paul C. W. |editor-last3=Ruse |editor-first3=Michael}}</ref>
# Radiative arrow of time, manifested in waves (e.g., light and sound) travelling only expanding (rather than focusing) in time (see [[light cone]]);
# [[Entropy (arrow of time)|Entropic arrow of time]]: according to the [[second law of thermodynamics]] an isolated system evolves toward a larger disorder rather than orders spontaneously;
# Quantum arrow time, which is related to irreversibility of [[measurement in quantum mechanics]] according to the [[Copenhagen interpretation]] of [[quantum mechanics]];
# Weak arrow of time: preference for a certain time direction of [[weak force]] in [[particle physics]] (see [[CP violation|violation of CP symmetry]]);
# [[Physical cosmology|Cosmological]] arrow of time, which follows the accelerated [[expansion of the Universe]] after the [[Big Bang]].

The relationships between these different arrows of time is a hotly debated topic in [[theoretical physics]].<ref>{{Cite book |last1=Coveney |first1=Peter |title=The arrow of time: a voyage through science to solve time's greatest mystery |last2=Highfield |first2=Roger |date=1991 |publisher=Fawcett Columbine |isbn=978-0-449-90630-9 |edition=1st |location=New York}} </ref>

The [[second law of thermodynamics]] states that [[entropy]] must increase over time. [[Brian Greene]] theorizes that, according to the equations, the change in entropy occurs symmetrically whether going forward or backward in time. So entropy tends to increase in either direction, and our current low-entropy universe is a statistical aberration, in a similar manner as tossing a coin often enough that eventually heads will result ten times in a row. However, this theory is not supported empirically in local experiment.<ref>{{cite book |last=Greene |first=Brian |author-link=Brian Greene |title=The Fabric of the Cosmos |title-link=The Fabric of the Cosmos |date=2005 |publisher=Penguin Books Limited |isbn=978-0-14-195995-5 |chapter=Chapter 6: Chance and the Arrow |access-date=16 September 2017 |chapter-url=https://books.google.com/books?id=yZujlUD1oAAC |archive-url=https://web.archive.org/web/20200820035526/https://books.google.com/books?id=yZujlUD1oAAC |archive-date=20 August 2020 |url-status=live}}</ref>