Quantum Relativity

General Relativity

Gravity is a force really different from other forces. So there are four main forces: Electromagnetic, Strong, Weak and Gravity. Out of these four forces Electromagnetic, Strong and Weak forces are to be universally accepted as they hold their properties true at the level of quantum particle dimensions. While we can't say the same for Gravity.

Newton in his book ,'Philosophiæ Naturalis Principia Mathematica' first put forward the idea of Gravity. According to his theory, an object with more mass attracts objects with less mass. And another principle of gravity was that stars remained fixed. This didn't make sense because according to his theory, the stars with less mass should revolve around the stars with a larger mass and the stars with the same mass should fall into each other. But Newton defended his theory by stating that there were infinite stars in the universe, so there was no exact centre for the stars to fall into or revolve.

So Albert Einstein, proposed the theory of general relativity about 200 years after Newton proposed his theory. Just imagine you are in free fall. You would not feel your weight. If you weighed yourself 50kgs standing still you would weigh 0kg in the free fall. So he realised that gravity is acceleration. 

Now imagine you are travelling in a rocket at the acceleration of 9.8 m/s² holding a flashlight. The light would curve. But why? Doesn't light always travel straight which is the shortest path to reach? What if the curved path was the shortest path? So this thought experiment of Einstein led to the idea that gravity was a curvature of spacetime, not a force. The greater the mass of an object, the more the curvature.

Now, this might sound bizarre and you might have questions like what makes the planets revolve around stars. So imagine that the curvature of the Sun extends to 1km on each side and all the planets of the solar system are inside this 1km radius. So as the planets are inside the curvature, they start to revolve around the sun, Just like if you place a basketball on the centre of the trampoline and roll some tennis balls on the trampoline, they would start revolving the basket ball. So here the basketball would be the sun and the tennis ball are planets. 

Now, what makes the moon revolve around the earth instead of the sun? As every object with mass has its own curvature, suppose that each planet has a curvature of 10m radius. So the moons or asteroids are inside the radius of curvature of planets revolving planets. You can fact-check this by placing a basketball in the centre and roll a tennis ball and a paddle ball close to each other on the trampoline. You would observe that the tennis ball will revolve around the basketball while the paddle ball revolves around the tennis ball.

Now this theory was just more than planets revolving around stars. So if small stars like the sun had a curvature large enough for eight planets to revolve there must be something larger which made the stars revolve in it's curvature. They are what we call black holes. Although Newton had disapproved of the movement of stars it was much later that it was realised that stars actually revolved.

The trampoline that was curved by the balls, is the space-time fabric of the space. This space-time fabric is like a surface for the stars and planets. So what we refer to as time on earth is the bending/curvature of this space-time fabric. This can be rephrased as that: Gravity control time. So if a star/planet has more mass it has more gravity, and when it has more gravity the time runs slower. 

We can say that time does not run the same everywhere. This doesn't refer to the time difference between the United States and Australia, because 1 hour is 60 minutes in both in United states and Australia. But 1 second on Jupiter would be 20 nanoseconds slower than earth. So time would pass faster on Earth than on Jupiter because the gravity on Earth is less than on Jupiter. This concept of general relativity is called Time Dilation.

The Time Dilation was actually proved by placing an atomic clock on a mountain and another on the surface of earth. The time of the clock on the Mountain was ahead than the time of the clock on the surface because the gravitational effect is less the further you're away from the surface of the earth. However, the difference in time was in nanoseconds. But this technically means that your head ages faster than your feet. 

And we can also say that if there were two twins, of which one twin lived on a mountain and the other twin lived on sea level and after some years if they met again on surface, the twin who returned from the mountain would be older than the twin on sea level because of the time dilation effect.

Now that we understand that the curvature of space is the curvature of time, can we say that space and time are the same things? But they are not! We know that when there is more mass there is more gravity, when there is more gravity space and time run slower, but when something travels faster through space they travel slower through time. So the relation between space and time is like heads and tails, the two sides of the same coin.

To Be Continued