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| Image: wikimedia commons |
Now, assume you are traveling in a vehicle at a speed of 30 km/h. In front of you, another vehicle passes you at a speed of 30 km/h. In your view, the speed of the vehicle is 60 km/s. Now if the same vehicle is traveling close to you at 30 km/h in your direction, it will not appear to move in your view.
Now, you will feel that the velocity of the vehicle is zero. This velocity is called the relative velocity. That is, the speed of an object can vary according to the speed of the observer.
Now let's come to this story.
All waves travel through some medium. For example, sound waves travel through air, and water waves travel through water. So, our scientists in the early 1800s guessed that light waves from the sun must have reached Earth through some medium. The medium was called ethers. But they did not prove this. This was just a hypothesis. They also believed that these ethers were filled with the universe.
For this, they formulated some characteristics. According to them, these ethers are much less dense. Hence, planets can easily penetrate through them. And because of their high elasticity, light waves can travel very quickly through them.
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| Distribution of light in the universe Image: Wikimedia commons |
In the 1860s, physicist "James Clerk Maxwell" published his study of the electromagnetic wave properties of matter. In this study, he proposed that"the speed of light varies with the electrical and magnetic properties of the medium."
But the scientists of that time had more faith in classical physics. So Maxwell's ideas remained unaccepted until 1880. His views were accepted only through some studies done after that.
Now in the universe, let us assume that an object is moving with velocity v, and a light ray is emitted from the object. The speed of light as seen from that object appears to be c.
(Here c refers to the speed of light in a vacuum of 3×10⁸ m/s.)
But from our earth, if we calculate the speed of light emitted by the object, its speed will be c+v. That is, the speed of light would have increased.
Maxwell's equations do not consider this variation. According to him, the speed of light varied only with the electromagnetic properties of its medium.
At the end of the 19th century, our scientists put forward some ideas about the speed of light. According to them, there is some kind of fixed structure throughout this universe. They are ethers. And in this system, the speed of light is c.
At a given speed, the speed of light emitted from another moving system can increase or decrease depending on the direction in which the system is moving.
But to cause a change in the speed of light, the object that emits the light must move at a very high directional speed. None of the experiments conducted so far have affected the speed of light at all.
It was also proved in the future that the velocity of a light-emitting object (whether moving or not) in a vacuum is only 3×10⁸ m/s.
Because light has no mass. For example, a person now fires a gun in a direction from a vehicle that is moving at speed 'v'. Let 'u' be the velocity of the bullet exiting the gun.
When we measure the velocity of this bullet while standing at a fixed place, its velocity is v+u. Here, the increased velocity of this bullet is due to its mass. But light has no mass. Therefore, the speed of light in a vacuum is always constant. Its velocity varies only when passing through any medium.
Michelson and Morley's experiments:
To prove the idea that the universe is filled with ethers, two physicists named Michelson and Morley did some experiments.
The experiment was completed between April and July 1887.
According to their thinking, ethers are constant in this universe. But our earth is still functioning. So in our view, the ethers are also moving. They decided to do an experiment using this movement of ethers.
That is, in a river with a certain velocity of current, one should not swim straight towards the end that is directly opposite to it. Because he will be pushed by the current and reach the shore a little away from the end. But if we swim in the opposite direction from that current, we can reach the opposing point on a straight path.
Ethers continue to operate on our earth like this current. Therefore, when the light is directed from the earth in a direction perpendicular to the movement of the ethers, it should not go in a straight direction. The path of this light should be slightly curved. If it is not so bent, we can assume that there is no such thing as ether in this universe.
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| Image: wikimedia commons |
Michelson, in his experiment, first a straight, light beam on a mirror, inclined at 45° to the direction of the ethers. The back of the glass is lightly coated with aluminum. So now that mirror can both reflect and refract the light. Therefore, the transmitted light is bisected in perpendicular directions.
(Here, there is a path L2 in the direction in which the ethers are considered to be moving. And there is still a path L1 perpendicular to the movement of the ethers.)
These two light rays are reflected off a mirror placed across their path. The two reflected light rays again the inclined mirror at an angle of 45°. These two light rays overlap each other and are directed towards the screen.
Here, there is no problem with the light beam traveling in the same direction as the light emitted (In L2). Because it goes in the same direction as the ether is moving.
But the ray of light which is perpendicular to the direction of the ether is to be looked at here. This ray of light travels, as we have said before, like passing across a river with a current.
That is, if there is an ether, light cannot travel straight across it. It must have deviated by a certain amount towards the direction in which the ether could travel.
But, no such deviation took place. Physicist Morley repeated the same experiment by making the light reflect almost 18 times in the same experiment.
In his experiment also, the light beam, which is perpendicular to the ether, was not deflected.
Therefore, these two physicists came to the conclusion that 'there is no such thing as ether in this universe'.