From Wikipedia, the free encyclopedia Causal contact
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 in space, for instance, exerts a field force on all
other objects of mass, according to Newton’s law of uni-
versal gravitation. Because this force exerted by one ob-
ject 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 10 billion light-years
away from the earth would not be in causal contact with
it. Anything less than 10 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:
The light cone is constructed as follows. Taking as
event p a flash of light (light pulse) at time t0, all events A worldline through a light cone in 2D space plus a time di-
mension.
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. • Events inside the past light cone of E are those that can
Given an event E, the light cone classifies all events in emit a beam of light and affect what is happening at
spacetime into 5 distinct categories: E.
• Events on the future light cone of E. • All other events are in the (absolute) elsewhere of E
• Events on the past light cone of E. and are those that will never affect and can never be
• Events inside the future light cone of E are those affected by E.
affected by the beam of light emitted at E. See the causal structure of Minkowski spacetime for a
more detailed discussion.
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Categories:
• Mechanics
• Theoretical physics
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