Light Cones

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A better version of this article is — THEORY OF RELATIVITY IN BRIEF which is also a featured article on our blog. So, we recommend you to read that article instead of this, and, also that article describes RELATIVITY completely.


We should discuss a quite interesting topic of LIGHT CONES. To understand this, we should find out what PHYSICS tells us about EVENTS and time.An event is a point in space-time whose position can be defined using 4 numbers (3 numbers of three space dimensions and one of time dimension).So, an event is a point in space-time whose position can be described with maximum of 4 numbers. When an event occurs it emits LIGHT and light is a WAVE. A wave is a disturbance in a medium or field. For example, when you drop a stone in water, water ripples are created like this:
In the above image, the source of the wave is the point where stone is dropped, that is, point A. But when this image is represented with time, it will look something like this:
In the above image, time is represented on the vertical axis (Y – Axis) and space is represented on the horizontal axes (X and Z Axes) and a cone is formed. The source of wave is represented by P. Similarily, when light is emitted from an event, we can construct two ‘light cones’ like this:
The Future Light Cone is in the future of Event P (i.e. all the events in future light cone occur after the event P) and The Past Light Cone is in the Past of Event P (i.e. all the events in the past light cone had already occurred before event P).The Event P is affected by the events in its Past light cone (i.e. Present is affected by Past) and the Event P affects all the events in its Future Light cone (i.e. Future is affected by Present).For example, all the events occurring in the universe lie in the light cone of the Big bang Singularity (Big Bang Singularity is the point from which the whole universe emerged). Take a look at the light cone of Big Bang:
In the above light cone of Big Bang, time is represented on X – axis(Horizontal Axis), instead of Y – Axis (Vertical Axis) So, all the events which are occurring today are directly (or indirectly) affected by The Big Bang.

About the Author – Madhur Sorout

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Sorout is currently a last third year high school student, living in India. His main fascination lies in Physics mainly in the field of general theory of relativity and topics related to it like Big Bang, Black Holes and Evolution of Universe. He likes to make out the meaning of what he see in this universe. He loves to read books by Stephen Hawking, Neil deGrasse Tyson and other authors (and Physicists).  He is an atheist and believes that Physics completely rejects the idea of a god. He likes to play cricket also and he wants to continue down the route of research in Theoretical Astrophysics.

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