How Einstein’s general relativity changed the understanding of the universe
Till the early 20th century, the concept of gravity was Newtonian. Which probably started with an apple falling onto him. This eventually led Newton to think that some kind of force is pulling Apple. According to him any particle of matter in the universe attracts any other with a force varying directly as the product of the masses and inversely as the square of the distance between them.
This theory was dominated till the early 20th century. After that Albert Einstein gave the theory of special relativity according to which “Gravity causes the curvature of space itself.” As per the Newtonian model — Time is fixed, space is fixed, and gravity is the mysterious force that attracts any element with mass. But according to Einstein - Gravity bends space-time itself.
John Wheeler — “Space-time tells matter how to move and matter tells space-time how to curve”
That’s general relativity.
Einstein started thinking by using light as a reference. By taking the speed of light as a constant he hypothesised what will happen when a light moves through a space impacted by a curvature by a mass object. In the example given below where two light rays DC and BA are traveling at the speed of light. One with the influence of gravity and the other without.
As the distance traveled by light DC which is under the influence of gravity is greater than light BA which isn’t. To keep the speed of light constant, time itself must pass slower in the gravitational field relative to the time in empty space.
This is precisely why time passes quickly in the international space station compared to the clock on Earth.
To prove this hypothesis, Arthur Eddington captured the position of stars in a full solar eclipse in 1919. He thought that if Einstein’s hypothesis was correct then the position of stars near the sun would appear different than the predicted location based on where they should be seen at night. This is because when light passes near the Sun it should be bent due to the curvature of space by gravity.
The experiment was performed in 1919 in Brazil, during a full solar eclipse.
If the theory of Einstein was correct then the position of stars near the sun should be changed during a full solar eclipse.
The shift in the position of stars was visible and the position was mostly accurate according to calculations.
The theory was proved.
Einstein was a celebrity now.
Sometimes space-time gets so distorted that nothing escapes from it, even light. This is called a black hole. A mass concentrated in an infinitely small point with infinite density. This is called a singularity.
This theory revolutionized the current understanding of the universe and paved the way for quantum physics.
