Gravity

What is so special about  Gravity?

Does know about gravity explains everything in the universe? 

I  ain't going to explain to you the textbook description as it is boring and absurd. So, the special things about gravity are; it is somewhat strong and somewhat weak (Gravity is strong on Earth in comparison to on Moon, you might have heard or read that people float on Moon. I won't be giving you an example of Blackhole unless I explained you.)

Newton's Idea of Gravity:

He described gravity as a force of attraction between two objects as we are attached to Earth. He says if we know Speed and time, we can predict the future positions of any particle or object. So, his theory explains the motion of Earth, but later they found Uranus. Scientist recorded some deviation in the trajectory of Uranus around the Sun. So, here Newton's laws were failing, but they thought Newton could not be wrong. They recalculate the Uranus orbit by assuming a planet X which balance Newton's Gravity around the Uranus, and later on, we got Neptune. 

Precession of the perihelion of Mercury's orbit

In 1965, Mercury showed some deviation from its predicted trajectory, and Newton's laws failed to describe Mercury's motion. So, again it was recalculated assuming the new planet which is still undiscovered, this hypothetical planet is called "Vulcan", then Einstein came and said as Mercury is so close to the Sun, it will not follow Newton's law and the proposed his own 'Theory of General Relativity' which described almost everything astronomically. 

If Newton is wrong, then how come it explains 90% of the world that we see even cars and building are building according to Newton's laws. When we put small numbers in Einstein's theory of general relativity we get Newton's laws. 

 So, how Einstein solved the problem of Mercury’s motion? Well, according to him; gravity is not a force (yes schools don't teach us this) in fact gravity is acceleration.

 Acceleration really!!!!!!!!!

(Acceleration: Change in direction of speed)

Imagine you are in a lift and whenever the light goes down you feel lighter, what if the lift is going down with some speed (accelerating say 9.8 m/s^2) then you would be able to stand on the ceiling of that lift (if you are worried it will crash than for the safe side imagine it in the outer space). Before it gets the acceleration, the gravity was downward, and afterwards, gravity is upward, so gravity changed because of speed? NOOO!! Gravity is acceleration.

Imagine a large sheet of rubber-like trampoline and put a blowing ball (or heavy ball) at the centre then put tennis ball (lighter ball) anywhere on that sheet with some push, now, the tennis ball will go around in circular motion (like a spiral) and will end up at the centre. Similarly, Earth is that tennis ball and Sun is that blowing ball. The lighter ball was moving because of acceleration caused on the sheet (from the heavier ball) not because of the force, and this acceleration is “gravity”. And that rubber sheet is space-time.

In 3 dimensional view

And Einstein did the calculation for the same concept and solved the motion of Mercury. It explains mostly everything except the things which are very very very and very small for example electrons & quarks. So, here another pillar of physics Quantum Mechanics was born to explain atomic structures.

3D view of an Earth moving in
around the Sun in space-time

Maybe you guys have heard/ read about space-time, 4-dimensions in articles or sci-fi movies. So, what exactly is space-time? Space includes length, width and height (3 dimensions) normally any object can be explained in these 3 dimensions like the screen is 7 inches long, 3.4 inches wide and 0.5 mm thick. The most important thing to notice the length, width and height all are just lengths (different names because of different angles). And time, so what is time? 3 hours 10 minutes, a day, year etc, all these examples are of time. 

4D view of an object moving in space-time

How time is related to space?

Answer this question how far the railway station from your home is? And without looking into GPS or maps, you can quickly answer it like 1hr 20 minutes far by bus (or whatsoever). Take another example; how far is Chennai from Delhi? The answers could be: 2200 kilometre, 3 hours air travel (plane), 1d 18hr by car (road travel), so we are stating the length in units of time so this how time is related to space.

Now space-time is used to describe anything in the universe as a map. But as it is nearly impossible to predict the exact space length of any (suppose) star as the universe is eternally expanding, and nothing is static in it. So, we mostly say in time coordinated (time dimension) like Sagittarius A* Black Hole is 25,640 light-years away from us, it simply means it takes 25,640 years for the light to reach us. But there is an object which predicts very precisely like satellite uses space-time to coordinates, and thus GPS is also based on space-time. 

Sun is  151.11 million km away from us, and likewise, it takes approximately 8 min for the light to reach us, so we can also say that Sun is 8 min far. 

As we get to know, that in general relativity heavier object makes the space-time bends (curved to be specific), so if an object is very very very very like infinite times heavy then it does creates a black hole in space-time.

Black Hole