Editing Time (section)

=== 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>