Bending of light? How is that possible? We all know that light always travels in straight line. In fact light is an electromagnetic radiation that is responsible for the sense of our sight. Without light we would blinded forever.
If suppose light was able to bend in one particular medium, then we all would be seeing each other in different places which results in a messed up optical illusion. This optical illusion doesn't occur to anyone of us because light always travels in straight lines in a particular medium. Obviously the degree of bending of light differs in various mediums. Say if suppose you half dip a pencil inside a Water (Another Medium) you will notice that the pencil would look slightly bent inside the water than in air. This is because due to the deflection of light in that medium it makes it look like its bent inside the water. But what is it that it has to do with gravity? Honestly nothing. I've introduced you to one of the properties of light to prove that the relative index( i.e the number that describe how light propagates through a medium; in this case: how the light bends in different medium.) of air and vacuum are approximately similar i.e The Reflective Index of vacuum is 1 and the reflective index of air is 1.000293 approx. 1. So in Space (Vacuum) light behaves the same as how it behaves in earth i.e it travels at straight lines. Now if there is no bending of light in vacuum then why would I mention the topic as Gravitational bending of light?
You see, from the title itself you would have guessed that the bending of light is somehow related to gravitational field of an object. In order to elaborate this idea with simplicity I have to introduce you to another abstract concept called General Relativity.
General Relativity
General Relativity is the curved geometric theory of gravity. As I've already mentioned it before in few articles, Newton's gravitational theory explained everything from the falling of an apple to the earth and other planets revolving around the sun in their own orbit. But it failed in one major thing, it couldn't explain how this gravitational force worked or what it is? Newton himself knew that he couldn't understand it but he left it to god. He said that some mystical force was responsible for gravity. Newton's laws was helpful in various fields but it failed to explain Mercury.
It always failed when computed, many physicists even tried to solve it by introducing many mathematical methods but it didn't work for mercury. But Einstein's General Relativity theory worked really well for it. In fact Newton's Theory of Gravitation is just an approximate theory. General Relativity is known to be the accurate account for gravity. Even though the mathematics of General Relativity are too complicated, its equations are elegant and this one inch equations explains almost everything in our universe. Einstein knew that this theory was correct before even they tested it. Avoiding all the mathematical concepts I'll try to explain it to you in short terms in order to explain how the light bends due to gravity.
Space-Time
Space as we all know has three dimensions, the one everything lies. Up-down, right-left, front-back.
These three dimensions are all familiar to us. But Einstein thought of another dimension he called it the time dimension. Special Relativity his earlier theory published in 1905 tells us that time is not the same for everyone. Unlike Newton who thought of time as absolute, Einstein knew that time is another dimension. The faster you go, the slower your clock ticks. That is why when you move at the speed of light your time relativity ticks even more slower, than the person who is standing still, and after few hours of your trip you will notice that you are much younger than your friend who was standing still. This might be complicated, take a look at this article Special Relativity for more information on it.
You know what? Lets just forget about the notion of this abstract Special Relativity all you need to know is that time is another dimension. Extract from The Elegant Universe - Brian Greene
You would have to know four things in order to meet your friend.
First: Which street?
Second: Which Building?
Third: Which floor?
fourth: What time?
The first three all are three spatial dimensions and the fourth one is the time dimension.
With this idea bare with me for a few moments. Now that you are aware that there are four dimensions. Space-Time is a four dimensional mathematical representation of our reality.
This Space-time is like a fabric, it can be bent, curved, twisted, flexible and many other properties that are almost similar to a bed cloth.
Thought Experiment:
Call your four of your friends and tell them to hold a bed cloth stretched wide. Now take a football and place it in the center of the cloth. You will notice that the mass of the ball creates a curvature around it. Now roll a tennis ball with an initial speed you will notice that the ball will try to take the circular path around the curvature of the football. This what exactly happens in space. Our earth revolves around the sun because of the curvature made by the mass of the sun in the fabric of space-time.
The main idea here is that: Anything that has mass and energy will distort space-time. Since light has energy it is certainly possible for light to bend due to the curvature of a massive object, just like how the moon takes the path of the earth's curvature.
Wait, we haven't actually come to the problem yet? But the main problem here is that how can a light(collection of photons) have no mass at all but still have energy?
In order to understand this we need to indulge into a bit of Special Relativity and Quantum Mechanics. I will continue this in the next Article series.
If suppose light was able to bend in one particular medium, then we all would be seeing each other in different places which results in a messed up optical illusion. This optical illusion doesn't occur to anyone of us because light always travels in straight lines in a particular medium. Obviously the degree of bending of light differs in various mediums. Say if suppose you half dip a pencil inside a Water (Another Medium) you will notice that the pencil would look slightly bent inside the water than in air. This is because due to the deflection of light in that medium it makes it look like its bent inside the water. But what is it that it has to do with gravity? Honestly nothing. I've introduced you to one of the properties of light to prove that the relative index( i.e the number that describe how light propagates through a medium; in this case: how the light bends in different medium.) of air and vacuum are approximately similar i.e The Reflective Index of vacuum is 1 and the reflective index of air is 1.000293 approx. 1. So in Space (Vacuum) light behaves the same as how it behaves in earth i.e it travels at straight lines. Now if there is no bending of light in vacuum then why would I mention the topic as Gravitational bending of light?
You see, from the title itself you would have guessed that the bending of light is somehow related to gravitational field of an object. In order to elaborate this idea with simplicity I have to introduce you to another abstract concept called General Relativity.
General Relativity
General Relativity is the curved geometric theory of gravity. As I've already mentioned it before in few articles, Newton's gravitational theory explained everything from the falling of an apple to the earth and other planets revolving around the sun in their own orbit. But it failed in one major thing, it couldn't explain how this gravitational force worked or what it is? Newton himself knew that he couldn't understand it but he left it to god. He said that some mystical force was responsible for gravity. Newton's laws was helpful in various fields but it failed to explain Mercury.
It always failed when computed, many physicists even tried to solve it by introducing many mathematical methods but it didn't work for mercury. But Einstein's General Relativity theory worked really well for it. In fact Newton's Theory of Gravitation is just an approximate theory. General Relativity is known to be the accurate account for gravity. Even though the mathematics of General Relativity are too complicated, its equations are elegant and this one inch equations explains almost everything in our universe. Einstein knew that this theory was correct before even they tested it. Avoiding all the mathematical concepts I'll try to explain it to you in short terms in order to explain how the light bends due to gravity.
Space-Time
Space as we all know has three dimensions, the one everything lies. Up-down, right-left, front-back.
These three dimensions are all familiar to us. But Einstein thought of another dimension he called it the time dimension. Special Relativity his earlier theory published in 1905 tells us that time is not the same for everyone. Unlike Newton who thought of time as absolute, Einstein knew that time is another dimension. The faster you go, the slower your clock ticks. That is why when you move at the speed of light your time relativity ticks even more slower, than the person who is standing still, and after few hours of your trip you will notice that you are much younger than your friend who was standing still. This might be complicated, take a look at this article Special Relativity for more information on it.
You know what? Lets just forget about the notion of this abstract Special Relativity all you need to know is that time is another dimension. Extract from The Elegant Universe - Brian Greene
You would have to know four things in order to meet your friend.
First: Which street?
Second: Which Building?
Third: Which floor?
fourth: What time?
The first three all are three spatial dimensions and the fourth one is the time dimension.
With this idea bare with me for a few moments. Now that you are aware that there are four dimensions. Space-Time is a four dimensional mathematical representation of our reality.
This Space-time is like a fabric, it can be bent, curved, twisted, flexible and many other properties that are almost similar to a bed cloth.
Thought Experiment:
Call your four of your friends and tell them to hold a bed cloth stretched wide. Now take a football and place it in the center of the cloth. You will notice that the mass of the ball creates a curvature around it. Now roll a tennis ball with an initial speed you will notice that the ball will try to take the circular path around the curvature of the football. This what exactly happens in space. Our earth revolves around the sun because of the curvature made by the mass of the sun in the fabric of space-time.
The main idea here is that: Anything that has mass and energy will distort space-time. Since light has energy it is certainly possible for light to bend due to the curvature of a massive object, just like how the moon takes the path of the earth's curvature.
Wait, we haven't actually come to the problem yet? But the main problem here is that how can a light(collection of photons) have no mass at all but still have energy?
In order to understand this we need to indulge into a bit of Special Relativity and Quantum Mechanics. I will continue this in the next Article series.
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