In Order to understand light cone, we need little perspective of what is space time.
Giving A little Background of Space Time before explaining Light Cone :
Before concept of space-time, as per Newtonian physics space could be easily defined on Euclid geometry which is x,y & z dimensions as we know.
So Any event happening in space could be pin-pointed on the (x,y,z) co-ordinate system & this would remain same for all all types of observer which are stationary, moving or in accelerated motion…i.e..irrespective of any inertial frame of references.
This remained same for 200 years until Maxwell came up with equations describing speed of electromagnetic waves which are only dependent on properties of medium (permeability & permittivity), predicting speed of EM waves as C, Since Light is also an Electromagnetic wave its speed is also same (C).
This Caused radical changes in understanding of whole concept of Space..now speed of light is limited which is C & not infinite , which meant any event happening at farthest distance would not be necessarily visible to us in a moment, light will take time to travel that distance to reach our eye so that we can register that certain event has happened.
For e.g..if a Star has exploded one light year distance from us in earth, light will take 1 year to reach us & then we would be able to see bright light in sky indicating star has exploded…point to be noted here is — event has happened one year in past & light took 1 year to travel & reach us & then we registered that event has occurred in space.
So we see above by simple explanation it is now not possible to define a Event by just Space Co-ordinates, we also need a reference of time to measure when this event happened in space.
Now if complicate things & add Different frames of references here, i.e.. what if we are traveling away from star at speed of c/2 will that time of star explosion remain same or change ? We are changing speeds (inertial frames) & since speed of light is not infinite, the transformation from one reference frame to another had to affect the time also ie.. in one co-ordinate system of space-time(observer in space ship) he observed explosion of star at time t1 at distance d1, it will be different for observer in another co-ordinate system of space time(observer on earth) with different time t2 at distance d2…where d1 & d2 are distances contracted or expanded as he moves faster or slower.
These questions led to Special theory of Relativity & thus concept of Space- time as a Coordinate system & Newtonian physics to explain motion of bodies in Euclid Co-ordinate system has to be abandoned while working over great distances as in Cosmos or in very higher velocities near speed of light.
Space- Time (Minkowski diagram):
A space-time diagram is nothing more than a graph showing the position of objects as a function of time which we discussed above.
For Simpler explanation Minkowski took one space dimension — x over horizontal X — axis & time dimension — t over vertical Y Axis.
Minkowski Diagrams
So X Axis would represent space & y axis would represent time. Because the speed of light is special, space-time diagrams are often drawn in units of seconds and light-seconds, or years and light-years, so a unit slope [45 degree angle] corresponds to the speed of light.
LIGHT CONE:
This gives concept of Light Cone, if a event is happening inside a light cone in positive time axis (frame having faster than speed of light) we say it is a event which will happen in future.
Event which is there on Cone on Negative time axis we say these are historical events & had already happened in past.
Events happening over Horizontal axis — x portrays events happening at present in real time — Here & Now !
