Thought Experiment that made Einstein a Celebrity
Equivalence Principle explained...
Imagination is everything. It’s a preview of life’s coming attractions. - Albert Einstein
Greetings, fellow Bohron.
In 1666, Isaac Newton formulated the Universal Law of Gravitation that allows us to calculate the amount of gravitational force acting between different bodies. But he was unable to explain what caused the bodies to exert such a force in the first place.
Why would any two masses simply attract each other? There ought to be a reason for that.
Our quest to understand how gravity works culminated in the early 20th century, through the work of Albert Einstein. After 1905, Einstein was riding high on the success of his four incredible papers including Special Relativity. In 1915, Einstein came up with his General Theory of Relativity, a revolutionary solution that earned him global recognition.
Hold on, we are not gonna dive straight into the mind-boggling notions of this theory. Yes, general relativity is difficult, very difficult. Have a look at the equations below.
We start by venturing into Einstein’s deepest imaginations and reliving his thought experiments that changed our view of the universe forever.
The Falling Guy
One day, while observing a window washer on a ladder, Einstein imagined what would happen if the guy fell down. In this state of free fall, there will be no normal force of the ground acting on him but only the gravitational force downwards. With nothing to push him up, he would experience weightlessness.
Elevator Thought Experiment
Imagine that you are standing on a weighing scale in an elevator, at rest on earth. The scale simply reads your weight. If the elevator is moving upwards in space with an acceleration g = 9.8 m/s², you would still weigh the same, since there is nothing to differentiate between Earth’s gravity and an equivalent accelerated frame.
But when the acceleration is exceedingly high, you will find yourself glued to the floor. Since the scale will be pushing up against you with an enormous amount of force, you would weigh several times higher.
What if the elevator is rapidly accelerating downwards? Yeah, I know, you guessed it.
Bending of Light
Imagine yourself again in the elevator moving up with the acceleration, 9.8 m/s². This time you are holding a torch that emits a beam that strikes the other side of the elevator. You would expect the height where the beam strikes to be the same as the one from which it is emitted.
But since the elevator is moving upwards very rapidly, the light beam strikes at a lower height. If the same experiment is repeated on the Earth, the result will be the same.
Einstein realized that gravity and acceleration are equivalent and how we observe a situation just depends upon the reference frames we wish to choose. This is known as the Equivalence Principle.
Einstein described this idea as the happiest thought of his life. Imagine how amazing he would have felt to have discovered this simple yet so powerful notion of relativity.
This observation raises a very obvious question. Since light always takes the shortest path to travel, it must move in a straight line. Then how did it curve?
This was where Einstein had his Eureka! moment. He realized that the light was indeed taking the shortest path, but the shortest path wasn’t a straight line but a curve. How does this happen?
We will investigate this idea next time.
As the rumour has it, an apple fell and we got a formula for the gravitational force. A person fell, and we realized how gravity works. I hope you too are falling for physics enjoying our awesome articles ; )
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