Throwing light on ‘Light’

We can do lots of things with light – it can be absorbed partially or fully,it can be simply transmitted,it can be reflected or totally scattered. But can one bend it also? The answer ,of course,is yes. The phenomenon of refraction is well known to us. But when it comes to celestial light,this phenomenon of bending of light carries a totally different significance. Let me elaborate a little bit.

Albert Einstein theorized that one of the important consequences of his famous theory of relativity is that light would bend in the space under certain conditions. The theory is quite complex,but I will try to convey the theory and its significance in simple terms,using a metaphor to convey the complex thoughts.
Einstein coined the expression ‘SPACE-TIME’ to describe the matrix which pervades entire universe. The simplest and well known metaphor for SPACE-TIME is a stretchy,flexible fabric. To begin with,when there is nothing placed on the fabric,the fabric is rather flat and wrinkle-free . On the other hand,the flexible fabric will develop depression or dent when some object is placed on it. This is also described as ‘warping'(Or curvature) of SPACE-TIME. The heavier the object,the deeper the depression because it is a flexible and elastic fabric. Let us now consider the Sun,the earth and the moon in the SPACE-TIME. Obviously,the most massive Sun will generate the biggest dent followed by a relatively smaller dent made by the less massive earth while the smallest dent is caused by the least massive moon. Einstein related the size of the depression with the strength of gravity pull exercized by the massive body. Obviously then,Sun will exercise the highest gravity followed by the earth and then the moon. The rotation of earth around the Sun is essentially due to the pull or attraction exercised by the ‘bigger dent’ or ‘hollow’ of the Sun .

This brings us to question of the bending of light mentioned earlier. Einstein predicted that this big ‘depression’ created by the Sun can bend light slightly if any light(say,from a distant Star) happens to pass by It. Or rather,the star light will bend when it experiences a massive curvature(or depression)generated due to Sun in SPACE TIME. In fact the final proof of Einstein’s theory critically depended on the actual measurement of the angle by which a Star light bends. This piece of vital data was provided by a British cosmologist Arthur Eddington,who made precise measurements on bending of light from a distant star. Eddington made this historic measurement in the year 1919 during a solar eclipse. The significance of conducting the experiment during a solar eclipse is that during a solar eclipse,the star light can be viewed clearly without being obscured by the brilliance of the Sun.
It turned out that the angle of deflection measured by Eddington matched very closely(within experimental error) with the value predicted by Einstein’s theory of relativity.That was the moment of truth which fundamentally changed the way we perceived gravity and the universe. I will explain in a subsequent post how this led to a number of theories on the creation of the universe including the famous Big Bang theory.

The theory of relativity led to very precise understanding of the effects of gravity. This enables us to predict whether an asteroid will collide with the earth or go past harmlessly. It also could explain the phenomenon of ‘black holes’.
In fact even GPS(global positioning systems)satellites,which help in Map-making, surveying lands,tracking and surveillance,synchronization source for cellular network protocols etc,use calculations based on relativistic corrections for precise time measurements.

Needless to say that the theory,though highly complex,is not merely of academic interest but of great practical significance too. Due to its complexity, it is not easy to grasp its full significance even for specialists, not to talk of the laymen.

There is an interesting anecdote to illustrate how complex the theory is. Eddington,who made the historic experiments to measure the angle of deflection of a Star light as it passes the Sun,was a man of great credentials. He had graduated from Cambridge university and had written a famous paper entitled Mathematical theory of relativity,which was acknowledged as a master piece,even by Einstein. A physicist by name Ludwig Silberstein who considered himself an authority on Einstein’s theory was believed to have remarked to Eddington that he(Eddington)must be one of the three guys in the world to understand general theory of relativity. As Eddington stared in silence,Silberstein told him no to be so modest. Eddington was believed to have quipped: ‘On the contrary,I wonder who is the third person’.
Contrast this with what Einstein himself had to say during an interview with the media. On being praised by the media for his phenomenal achievements which virtually changed the way we looked at the universe,he remarked: “I have no special talents – I am merely passionately curious”. This was,of course,the modest side of Einstein during the hey days of his celebrity status. In a subsequent post I will present a slightly uglier side of his personality.

Note: The comparison of SPACE-TIME with a stretchy fabric is rather crude. SPACE-TIME has 4 dimensions(3 of space and one of Time), while fabric is just 2 dimensional. But a metaphor is only a metaphor,serving a limited purpose of conveying a complex idea.

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Published in: on March 14, 2010 at 9:56 am  Comments (1)  

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One CommentLeave a comment

  1. Thanks sir! I learned a lot and that made me to seek more of it.


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