Causal Contact

Two entities are in causal contact if there may be an event that has affected both in a causal way. Every object of

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...

, for instance, exerts a field

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...

on all other objects of mass, according to Newton's

law of universal gravitation Newton's law of universal gravitation is usually stated as that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the distanc ...

. Because this force exerted by one object affects the motion of the other, it can be said that these two objects are in causal contact. The only objects not in causal contact are those for which there is no event in the history of the universe that could have sent a beam of light to both. For example, if the universe were not expanding and had existed for 10 billion years, anything more than 20 billion light-years away from the earth would not be in causal contact with it. Anything less than 20 billion light-years away ''would'' because an event occurring 10 billion years in the past that was 10 billion light-years away from both the earth and the object under question could have affected both. A good illustration of this principle is the

light cone In special and general relativity, a light cone (or "null cone") is the path that a flash of light, emanating from a single event (localized to a single point in space and a single moment in time) and traveling in all directions, would take thro ...

, which is constructed as follows. Taking as event

p

a flash of light (light pulse) at time

t_0

, all events that can be reached by this pulse from

p

form the future light cone of

p

, whilst those events that can send a light pulse to

p

form the past light cone of

p

. Given an event

E

, the light cone classifies all events in spacetime into 5 distinct categories: * Events ''on the future light cone'' of

E

. * Events ''on the past light cone'' of

E

. * Events ''inside the future light cone'' of

E

are those affected by the beam of light emitted at

E

. * Events ''inside the past light cone'' of

E

are those that can emit a beam of light and affect what is happening at

E

. * All other events are in the ''(absolute) elsewhere'' of

E

and are those that will never affect and can never be affected by

E

. See the causal structure of Minkowski spacetime for a more detailed discussion. Mechanics Theoretical physics {{theoretical-physics-stub