First page of Einstein's manuscript, explaining the idea of general relativity and gravity.
(Public Domain: US)
The problem was planet Mercury. By the late 19th century, astronomy has developed to measure the orbit of mercury so precise that they realise the orbit did not follow Newton’s law of gravitation. Instead of a steady elliptical orbit, it went through another movement we call precession. The elliptical orbit of mercury didn’t stay in place but changes it position where the long axis of the ellipse will rotate the sun at about one degree in ten thousand years. That means the elliptical orbit changes position to form a huge circle that completes a turn in 3.6 million years.
Here comes a man to rescue – Albert Einstein. Einstein published his paper called “General theory of Relativity” in 1915 that radically redefined gravity. He made the revolutionary suggestion that the very fabric of space and time itself is “bent” or “curved” in the presence of a massive object. He proposed that the physical quantity of space and time are intimately connected. The idea that space and time itself are interconnected is actually not so strange when you think about this: When you have a date with someone, you must know about the location and time. There is simply no point telling your date about the place to meet up but not the time. So, one can now think that space and time is “woven” like a fabric because each event is linked to a specific time and space.
To demonstrate how General Relativity works require a little imagination. Here is an analogy - Picture the fabric of space-time as a thin sheet of rubber. Now we pull the four edges of the sheet and lift it up from the ground. What you see is a “flat” space where space-time is uniformly spread. Now, throw a bowling ball onto that rubber sheet and you’ll notice the ball sinks into the sheet of rubber, curving the surface of the rubber sheet. Now, roll a marble towards the bowling ball and you’ll see the marble traveling in a straight line and as it approaches the bowling ball, its path will curve towards the bowling ball, circulating it before it finally sticks to the bowling ball and stop.
What you’ve just pictured was the analogy of how gravity works in General Relativity. The bowling ball represents a massive object (like the sun) and the marble on the other hand, is a less massive object (earth) orbiting it on the curvature of space-time. Now you must be thinking “How is the marble orbiting the bowling ball? Because the marble will roll closer to the bowling ball and eventually stops.” The reason is because there is the presence of frictional force between the rubber sheet and the marble. Theoretically, if we remove the friction factor (which means we are having perfect lubrication between the marble and the rubber sheet), inserting the marble in a correct angle and we will observe the marble orbiting the bowling ball forever and ever in a circular orbit. So, essentially, mass is able to bend the structure of space and time itself and the consequences are the effects of gravity. Now with Einstein’s view of gravity, the problem in physics specifically the analogous orbit of Mercury can be explained.