Why does light travel in a straight line – quora electricity 2015


Consider you want to go to the North Pole of Earth. But you don’t have a straight road till North Pole. The road you have many turns. Hence while travelling towards North Pole, you won’t be able to spend your entire momentum towards North. Some of your momentum also gets distributed among East or West as you move towards North.

We live in a 4 dimensional (4D) fabric which Einstein called as Spacetime. It consist of 3 dimensions of Space and one Time dimension. Everything from the smallest subatomic particle to the largest star exists inside this 4D fabric. This fabric is the stage in which the drama of our universe takes place.

Einstein discovered that speed and time are interlinked. Faster you move, slower you experience time. Movement is defined in the 3 space dimensions (left right, top bottom, front back). When you move around in this 3 dimensions, your time slows down. I have written in detail here: Jeevan Rex’s answer to What is the relationship between time and space, and can space exist without time, or vice-versa?

Everything in the universe travels in the speed of light. Even if you sit idle, you are traveling in the speed of light in the dimension of time towards the direction of future. If you move around, some of your momentum is spend in the other 3 space dimensions. Hence you escape a little from traveling in the dimension of time. If you move in one direction at the speed of light, your entire momentum is used to travel in the space dimension. You may not get aged as none of your momentum is used to travel in time dimension.

Light has to travel in straight line so that none of its momentum goes into time dimension. For light, time doesn’t move. It never travels in time dimension. None of its momentum enters time dimension. The speed of light is the fastest moving thing in the universe. It is the cosmic speed limit. Nothing can travel faster.

In a dusty atmosphere it is sometimes possible to see light travelling and it does appear to be moving in a straight line. The fact that light travels can be demonstrated by putting an object in its path. If the object is opaque the result is a degree of blackness on the other side of it which is due to the absence of the light. The dark area is a shadow.

As light does appear to travel in straight lines then light is usually modeled as straight lines in drawings. This can sometimes be very confusing because to make the explanatory diagrams easier to understand the light from source is usually depicted as just coming from the top and bottom of the source when it is in fact coming from the whole source. For example in the diagram below the photograph in the pinhole camera is shown by just two lines when of course it is generated by billions of lines of light coming from the object.

Shadows can provide evidence that light travels in straight line. Think of the shadows that are formed on a cloudless sunny day, they are very well defined shadows. Supposing at this point that light did not travel in straight lines what would the shadows look like? They would probably be very fuzzy just like you would get on a sunny day with lots of clouds in the sky. When the sky is cloudy the sunlight is reflected of the clouds and they become secondary sources of light so on such a day the shadow is formed from lots of different light sources coming from a variety of directions.