Editing Time (section)

== Physical definition ==
{{Classical mechanics|cTopic=Fundamental concepts}}
{{Main|Time in physics}}

Until [[Albert Einstein|Einstein's]] reinterpretation of the physical concepts associated with time and space in 1907, time was considered to be the same everywhere in the universe, with all observers measuring the same time interval for any event.<ref>Herman M. Schwartz, ''Introduction to Special Relativity'', McGraw-Hill Book Company, 1968, hardcover 442 pages, see {{isbn|0-88275-478-5}} (1977 edition), pp. 10–13
</ref> Non-relativistic [[classical mechanics]] is based on this Newtonian idea of time. Einstein, in his [[Special relativity|special theory of relativity]],<ref>A. Einstein, H. A. Lorentz, H. Weyl, H. Minkowski, ''The Principle of Relativity'', Dover Publications, Inc, 2000, softcover, 216 pages, {{isbn|0-486-60081-5}}, See pp. 37–65 for an English translation of Einstein's original 1905 paper.
</ref> postulated the constancy and finiteness of the speed of light for all observers. He showed that this postulate, together with a reasonable definition for what it means for two events to be simultaneous, requires that distances appear compressed and time intervals appear lengthened for events associated with objects in motion relative to an inertial observer.

The theory of special relativity finds a convenient formulation in [[Minkowski space]]time, a mathematical structure that combines three dimensions of space with a single dimension of time. In this formalism, distances in space can be measured by how long light takes to travel that distance, e.g., a [[light-year]] is a measure of distance, and a meter is now defined in terms of how far light travels in a certain amount of time. Two [[Event (relativity)|events]] in Minkowski spacetime are separated by an ''[[Spacetime interval|invariant interval]]'', which can be either [[space-like]], [[light-like]], or [[time-like]]. Events that have a time-like separation cannot be simultaneous in any [[frame of reference]], there must be a temporal component (and possibly a spatial one) to their separation. Events that have a space-like separation will be simultaneous in some frame of reference, and there is no frame of reference in which they do not have a spatial separation. Different observers may calculate different distances and different time intervals between two events, but the ''invariant interval'' between the events is independent of the observer and their velocity.

=== 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|url-access=subscription }}</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>

=== Classical mechanics ===
In non-relativistic [[classical mechanics]], Newton's concept of "relative, apparent, and common time" can be used in the formulation of a prescription for the synchronization of clocks. Events seen by two different observers in motion relative to each other produce a mathematical concept of time that works sufficiently well for describing the everyday phenomena of most people's experience. In the late nineteenth century, physicists encountered problems with the classical understanding of time, in connection with the behavior of electricity and magnetism. The 1860s [[Maxwell's equations]] described that light always travels at a [[Speed of light|constant speed]] (in a vacuum).<ref>{{Cite journal |date=2013 |title=Deriving 0 and 0 from First Principles and Defining the Fundamental Electromagnetic Equations Set |url=https://ijerd.com/paper/vol7-issue4/G0704032039.pdf |journal=International Journal of Engineering Research and Development |volume=7 |issue=4 |pages=33 |issn=2278-067X}}</ref> However, classical mechanics assumed that motion was measured relative to a fixed reference frame. The [[Michelson–Morley experiment]] contradicted the assumption. Einstein later proposed a method of synchronizing clocks using the constant, finite speed of light as the maximum signal velocity. This led directly to the conclusion that observers in motion relative to one another measure different elapsed times for the same event.

[[File:World line.svg|upright=1.2|thumb|Two-dimensional space depicted in three-dimensional spacetime. The past and future [[light cone]]s are absolute, the "present" is a relative concept different for observers in relative motion.|left]]

=== Spacetime ===
{{Main|Spacetime}}
Time has historically been closely related with space, the two together merging into spacetime in [[Albert Einstein|Einstein's]] [[special relativity]] and [[general relativity]]. According to these theories, the concept of time depends on the [[inertial frame of reference|spatial reference frame of the observer]], and the human perception, as well as the measurement by instruments such as clocks, are different for observers in relative motion. For example, if a spaceship carrying a clock flies through space at (very nearly) the speed of light, its crew does not notice a change in the speed of time on board their vessel because everything traveling at the same speed slows down at the same rate (including the clock, the crew's thought processes, and the functions of their bodies). However, to a stationary observer watching the spaceship fly by, the spaceship appears flattened in the direction it is traveling and the clock on board the spaceship appears to move very slowly.

On the other hand, the crew on board the spaceship also perceives the observer as slowed down and flattened along the spaceship's direction of travel, because both are moving at very nearly the speed of light relative to each other. Because the outside universe appears flattened to the spaceship, the crew perceives themselves as quickly traveling between regions of space that (to the stationary observer) are many light years apart. This is reconciled by the fact that the crew's perception of time is different from the stationary observer's; what seems like seconds to the crew might be hundreds of years to the stationary observer. In either case, however, causality remains unchanged: the [[past]] is the set of events that can send light signals to an entity and the [[future]] is the set of events to which an entity can send light signals.<ref>{{cite web |url=https://www.youtube.com/watch?v=ev9zrt__lec |title=Albert Einstein's Theory of Relativity |publisher=YouTube |date=30 November 2011 |access-date=24 September 2013 |url-status=live |archive-url=https://web.archive.org/web/20131017182611/http://www.youtube.com/watch?v=ev9zrt__lec |archive-date=17 October 2013  }}</ref><ref>{{cite web |url=https://www.youtube.com/watch?v=V7vpw4AH8QQ |title=Time Travel: Einstein's big idea (Theory of Relativity) |publisher=YouTube |date=9 January 2007 |access-date=24 September 2013 |url-status=live |archive-url=https://web.archive.org/web/20131017182714/http://www.youtube.com/watch?v=V7vpw4AH8QQ |archive-date=17 October 2013  }}</ref>

{{Clear left}}

=== Dilation ===
{{Main|Time dilation}}
[[File:Relativity of Simultaneity.svg|thumb|[[Relativity of simultaneity]]: Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and occurs later in the red frame.]]

[[Albert Einstein|Einstein]] showed in his thought experiments that people travelling at different speeds, while agreeing on [[Causality (physics)|cause and effect]], measure different time separations between events, and can even observe different chronological orderings between non-causally related events. Though these effects are typically minute in the human experience, the effect becomes much more pronounced for objects moving at speeds approaching the speed of light. [[Subatomic particle]]s exist for a well-known average fraction of a second in a lab relatively at rest, but when travelling close to the speed of light they are measured to travel farther and exist for much longer than when at rest.

According to the [[Special relativity|special theory of relativity]], in the high-speed particle's [[Inertial frame of reference|frame of reference]], it exists, on the average, for a standard amount of time known as its [[mean lifetime]], and the distance it travels in that time is zero, because its velocity is zero. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seem to shorten. Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.

Einstein (''[[The Meaning of Relativity]]''): "Two [[Event (relativity)|events]] taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relative to K, which register the same simultaneously." Einstein wrote in his book, ''Relativity'', that [[Relativity of simultaneity|simultaneity is also relative]], i.e., two events that appear simultaneous to an observer in a particular inertial reference frame need not be judged as simultaneous by a second observer in a different inertial frame of reference.

According to general relativity, time also runs slower in stronger [[Gravitational field|gravitational fields]]; this is [[gravitational time dilation]].<ref>{{Cite journal |last=Barukčića |first=Ilija |date=2011 |title=The Equivalence of Time and Gravitational Field |journal=Physics Procedia |language=en |volume=22 |pages=56–62 |doi=10.1016/j.phpro.2011.11.008|doi-access=free |bibcode=2011PhPro..22...56B }}</ref> The effect of the dilation becomes more noticeable in a mass-dense object. A famous example of time dilation is a thought experiment of a subject approaching the [[event horizon]] of a [[black hole]]. As a consequence of how gravitational fields warp spacetime, the subject will experience gravitational time dilation. From the perspective of the subject itself, they will experience time normally. Meanwhile, an observer from the outside will see the subject move closer to the black hole until the extreme, in which the subject appears 'frozen' in time and eventually fades to nothingness due to the diminishing amount of light returning.<ref>{{Cite web |last=Gohd |first=Chelsea |date=2023-05-03 |title=What Happens When Something Gets 'Too Close' to a Black Hole? |url=https://science.nasa.gov/universe/what-happens-when-something-gets-too-close-to-a-black-hole/ |access-date=2025-03-01 |website=NASA Science |language=en-US}}</ref><ref>{{Cite web |last=Gunn |first=Alastair |date=2025-01-12 |title=The (very strange) reason black holes are secret time machines |url=https://www.sciencefocus.com/space/black-hole-time-machine |access-date=2025-03-01 |website=BBC Science Focus Magazine |language=en}}</ref>

=== Relativistic versus Newtonian ===
[[File:Lorentz transform of world line.gif|Views of spacetime along the [[world line]] of a rapidly accelerating observer in a relativistic universe. The events ("dots") that pass the two diagonal lines in the bottom half of the image (the past [[light cone]] of the observer in the origin) are the events visible to the observer.|thumb]]

The animations visualise the different treatments of time in the Newtonian and the relativistic descriptions. At the heart of these differences are the [[Galilean transformation|Galilean]] and [[Lorentz transformation]]s applicable in the Newtonian and relativistic theories, respectively. In the figures, the vertical direction indicates time. The horizontal direction indicates distance (only one spatial dimension is taken into account), and the thick dashed curve is the spacetime trajectory ("[[world line]]") of the observer. The small dots indicate specific (past and future) events in spacetime. The slope of the world line (deviation from being vertical) gives the relative velocity to the observer.

In the Newtonian description these changes are such that ''time'' is absolute:<ref>{{Cite book |last1=Knudsen |first1=Jens M. |url=https://books.google.com/books?id=rkP1CAAAQBAJ&pg=PA30 |title=Elements of Newtonian Mechanics |last2=Hjorth |first2=Poul G. |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-3-642-97599-8 |page=30 |language=en}}</ref> the movements of the observer do not influence whether an event occurs in the 'now' (i.e., whether an event passes the horizontal line through the observer). However, in the relativistic description the ''observability of events'' is absolute: the movements of the observer do not influence whether an event passes the "[[light cone]]" of the observer. Notice that with the change from a Newtonian to a relativistic description, the concept of ''absolute time'' is no longer applicable: events move up and down in the figure depending on the acceleration of the observer.

=== Quantization ===
{{See also|Chronon}}

Time quantization refers to the theory that time has a smallest possible unit. Time quantization is a hypothetical concept. In the modern established physical theories like the [[Standard Model]] of particle physics and [[general relativity]] time is not quantized. [[Planck time]] (~ 5.4 × 10<sup>−44</sup> seconds) is the unit of time in the system of [[natural units]] known as [[Planck units]]. Current established physical theories are believed to fail at this time scale, and many physicists expect that the Planck time might be the smallest unit of time that could ever be measured, even in principle. Though tentative physical theories that attempt to describe phenomena at this scale exist; an example is [[loop quantum gravity]]. Loop quantum gravity suggests that time is quantized; if gravity is quantized, spacetime is also quantized.<ref>{{Cite web |last=Lewton |first=Thomas |date=2023-07-10 |title=The Physicist Who Bets That Gravity Can't Be Quantized |url=https://www.quantamagazine.org/the-physicist-who-bets-that-gravity-cant-be-quantized-20230710/ |access-date=2025-03-02 |website=Quanta Magazine |language=en |archive-date=13 February 2025 |archive-url=https://web.archive.org/web/20250213110413/https://www.quantamagazine.org/the-physicist-who-bets-that-gravity-cant-be-quantized-20230710/ |url-status=live }}</ref>