Editing Spacetime (section)

=== Light cone ===
{{Main|Light cone}}
{{anchor|Figure 2-4}}
[[File:ModernPhysicsSpaceTimeA.png|thumb|Figure 2–4. The light cone centered on an event divides the rest of spacetime into the future, the past, and "elsewhere"]]

In Fig. 2–4, event&nbsp;O is at the origin of a spacetime diagram, and the two diagonal lines represent all events that have zero spacetime interval with respect to the origin event. These two lines form what is called the ''light cone'' of the event&nbsp;O, since adding a second spatial dimension (Fig.&nbsp;2-5) makes the appearance that of two [[cone|right circular cones]] meeting with their apices at&nbsp;O. One cone extends into the [[future]] (t>0), the other into the [[past]] (t<0).

[[File:World line.png|thumb|Figure 2–5. Light cone in 2D space plus a time dimension]]
A light (double) cone divides spacetime into separate regions with respect to its apex. The interior of the future light cone consists of all events that are separated from the apex by more ''time'' (temporal distance) than necessary to cross their ''spatial distance'' at lightspeed; these events comprise the ''timelike future'' of the event&nbsp;O. Likewise, the ''timelike past'' comprises the interior events of the past light cone. So in ''timelike intervals'' &Delta;''ct'' is greater than &Delta;''x'', making timelike intervals positive.<ref name="Schutz" />{{rp|220}}

The region exterior to the light cone consists of events that are separated from the event&nbsp;O by more ''space'' than can be crossed at lightspeed in the given ''time''. These events comprise the so-called ''spacelike'' region of the event&nbsp;O, denoted "Elsewhere" in Fig.&nbsp;2-4. Events on the light cone itself are said to be ''lightlike'' (or ''null separated'') from&nbsp;O. Because of the invariance of the spacetime interval, all observers will assign the same light cone to any given event, and thus will agree on this division of spacetime.<ref name="Schutz" />{{rp|220}}

The light cone has an essential role within the concept of [[causality]]. It is possible for a not-faster-than-light-speed signal to travel from the position and time of&nbsp;O to the position and time of D (Fig.&nbsp;2-4). It is hence possible for event&nbsp;O to have a causal influence on event&nbsp;D. The future light cone contains all the events that could be causally influenced by O. Likewise, it is possible for a not-faster-than-light-speed signal to travel from the position and time of A, to the position and time of O. The past light cone contains all the events that could have a causal influence on O. In contrast, assuming that signals cannot travel faster than the speed of light, any event, like e.g.&nbsp;B or&nbsp;C, in the spacelike region (Elsewhere), cannot either affect event&nbsp;O, nor can they be affected by event&nbsp;O employing such signalling. Under this assumption any causal relationship between event&nbsp;O and any events in the spacelike region of a light cone is excluded.<ref>{{cite web|last1=Curiel|first1=Erik|last2=Bokulich|first2=Peter|title=Lightcones and Causal Structure|url=https://plato.stanford.edu/entries/spacetime-singularities/lightcone.html|website=Stanford Encyclopedia of Philosophy|publisher=Metaphysics Research Lab, Stanford University|access-date=26 March 2017|archive-date=17 May 2019|archive-url=https://web.archive.org/web/20190517122738/https://plato.stanford.edu/entries/spacetime-singularities/lightcone.html|url-status=live}}</ref>

{{anchor|Relativity of simultaneity}}