Practically as a matter of human intuition we understand that an event from the future cannot _cause_ an event in the past. In addition, this fact is true in every experimentally verified theory in physics. [Special relativity](Special%20Relativity%20(Index).md) adds additional constraints to this notion with what's called a _[Light cone](Causality.md#Light%20cone)_. This means that we additionally have to take into account how far objects are from each other in order to determine if it is possible for one to impart an effect on the other within a a certain time-frame. # Light cones In addition to defining time as dimension in [Spacetime](Spacetime.md) we include the speed of light as a constraint that forms a _light cone_ that can be associated with each event in spacetime. The light-cone at every point in spacetime constrains the possible trajectories starting from that point.  (image adapted from Carroll S., _Spacetime and Geometry_) # Causality in quantum mechanics ## Misconceptions surrounding the delayed-choice quantum eraser The [[Delayed Choice Quantum Eraser]] is occasionally presented as experimental evidence for _retrocausality_ but this is not the case. %%So far the idea that it isn't tied to causality is based on what Harald Weinfurter personally told me. Do other physicists agree?%% #Mechanics #Mechanics/SpecialRelativity