The Light Cone



The Light Cone
--------------

In special and general relativity, a light cone is the path 
that a flash of light, emanating from a single event (a single 
point in space and a single moment in time) and traveling in 
all directions, would take through spacetime.  Light cones
are always oriented in the time direction and light rays always
travel at 45° angles. 

Consider the (1,-1,-1,-1) form of the metric.

  

For the (1,-1,-1,-1) form of the metric:

Lightlike (LL):  c²dt² - dx² = 0  v = c

Timelike (TL):  dτ² = c²dt² - dx² > 0  v < c

Spacelike (SL):  ds² = -c²dt² + dx² < 0  v > c

For the (-1,1,1,1) form of the metric:

Lightlike (LL):  -c²dt² + dx² = 0  v = c

Timelike (TL):  dτ² = -c²dt² + dx² < 0  v < c

Spacelike (SL):  ds² = c²dt² - dx² > 0  v > c

Therefore,

ds² = -dτ²

or,

ds = idτ

In the timelike case, time is the bigger factor.  The 
distance light travels is greater than the spatial 
separation.  2 events can take place at the same position 
but not at the same time.  Therefore, a causal relation 
between the two events is possible, i.e. one event can 
cause or influence the other.  Timelike events are inside 
the light cone.

In the spacelike case, space is the bigger factor.  The 
spatial separation is greater than the distance light 
travels.  2 events can take place at the same time but 
not at the same position.  Therefore, any causal relation
between the two events is impossible, i.e. one cannot affect
the other.  Spacelike events are outside the light cone.

Simply put, they show how far something is apart compared 
to ct.  In the timelike case, if you are fast enough, you 
can be at event A and at event B, it is only a matter of 
time until you see the second event.  In the spacelike 
case, the events C and D are too far apart in space.  You 
cannot see both of them together, no matter how fast you 
travel.  As soon as event C happened and you go as fast 
as possible, event D will have happened before you arrive 
there.

Rindler Wedge
-------------



In region I the worldlines are timelike.  At first sight 
this would seem to be at odds with the definition of the 
light cone where region I is spacelike and region II is 
timelike.  However, this is misleading.  The light cone 
requires that the worldlines pass through the origin in 
the Minkowski frame (X = 0,T = 0), which they clearly do 
not.  If an observer did pass through the origin, then 
indeed region I would be spacelike and not reachable.  
What is really happening is that every point (event) along 
the hyperbolae will have their own past and future light 
cones.  All events from the perspective of the observer at 
a particular point are then either outside (spacelike), 
inside (timelike), or on (light like) the associated light 
cone.  Since observers traveling along the hyperbolae can 
never exceed the speed of light, these worldlines stay within 
the light cone and are, therefore, always timelike.  

In region II, space and time trade places.  The spatial 
coordinate in region I becomes timelike, and the time 
coordinate becomes spacelike, i.e.  

Schwarzchild metric:

dτ² = (1 - r_S/r)dt² - (1/(1 - r_S/r))dr² - (1/c²)r²Ω₂²

For r < r_S the metric becomes:

ds² = -(1 - r_S/r)dt² + (1/(1 - r_S/r))dr² + (1/c²)r²Ω₂²

Rindler metric:

dτ² = (Rα/c)²dω² - dR²

ds² = -(Rα/c)²dω² + dR² 

Consequently, the hyperbolas in region II are spacelike.

In these diagrams, light rays are constrained to travel at 45° 
and are shown in pink.  Understanding this fact allows any 
number of different scenarios to be investigated.  For example, 
consider Alice and Bob in a rocket.

Alice is ejected at point, P, and follows the blue trajectory.  
At Q Bob shines a light ray at Alice which she receives.  At
R Bob sends out another light ray which Alice receives just 
as she hits the singularity.  At point S Bob shines a third 
light ray at Alice but this doesn't reach her because she has 
already hit the singularity and no longer exists!

From Bob's perspective, he see's Alice falling towards the
event horizon, E, at a slower and slower rate, seemingly 
taking forever (ω = ∞) to get to the point where she gets
'stuck' on the horizon.

Once Alice is behind the event horizon she cannot escape.  In 
order to do so she would have to exceed the speed of light 
which is not possible.  Her fate is sealed and she is torn 
apart by the extreme gravitational tidal forces as she heads 
towards the singularity.

Redshift Academy