## History Of Gravity

Gravity, one of the four main fundamental forces in nature. It is one of the most commonly observed forces in nature and yet it is as still one of the least understood of all forces.

Gravity was brought into fame by this famous apple shown above when it garnered enough strength and weight to detach itself from the tree and land on Newton’s head. This triggered his brain cells into thinking what made the apple fall down and thus Newton penned the theory of gravity.
The formula of gravity as given by him was:

And although he made the theory of gravity in a real short while it took him
twenty years to publish it in his book.Why? Because he had to prove that the mass of an object lies at it’s centre and for that he had to develop calculus to such a point that this could be proved. So in a way that apple was responsible not only for the discovery of the theory of gravity but also the the development in the field of calculus.
But why did newton have to prove that the mass lies at it’s centre?

Take a look at the above picture. Now if you see the formula you would see that given no action of any external forces the two bodies would attract each other with the force that is inversely proportional to the square of the distance between them. But what is the exact distance between them. Because mass can be distributed over a large volume. So if the two bodies touched each other the force of gravity is supposed to be infinite right? Because the surface has mass and the mass is at zero distance from each other. Well that’s what the mass being concentrated at it’s centre means. The force of gravity is taken by calculating  the distance between the centres of gravity of two continuous volumes of masses. That way no matter how close or how small or heavy the objects are they would never make the distance zero. Think about it!!

This theory was an absolute success as it was a unifying concept that not only explained the free fall of apples towards the earth but also the movement of the planets around the sun and the movement of galaxies and stars!

Of course the problem was that nobody including Newton actually understood what gravity was!They knew it was a force but the origin or the behaviour of this force still remains unknown.That’s when Einstein made his epoch making theory of special relativity and also the general theory of relativity.

Einstein made a simple observation. He already found that nothing can exceed the speed of light from all frames of reference via his theory of special relativity. Then he put up a thought experiment. He supposed that the sun would suddenly disappear. In that case the force of gravity keeping the earth in it’s orbit around the sun would just disappear too. But the light from the sun takes 8 minutes to reach the earth. So in that case we would still see the sun and yet we would not feel it’s gravity. That leads to the conclusion that the effect of gravity reached the earth faster than the speed of light! But nothing can travel faster than the speed of light. This apparent contradiction led him to modify the theory of gravity a bit. While there was nothing wrong with the magnitude of gravitational force it was the way the gravitational force lines acted that was changed.
Einstein said that gravity was the effect of bending the geometry of the space around it that led to the observation of a force acting.

Look at the above diagram. Suppose the the black line represents the path along which it is possible to move also called a dimension.
If we move both one will travel in a straight line while the other will travel in a bent line. If the lines become invisible it will seem as if the the object on the straight line is under no force but the object moving on the curved line is being pulled by a force that is changing it’s direction.

This is exactly what einstein called it. He said gravity bends space in such a way that it seems like a force.His theories were tested and proven multiple times and they were almost always found to be true. He was truly a remarkable genius since he explained  a fundamental force through geometry!

Now fast forward to the present we have new theories called standard model, string theory etc. However they still find explaining gravity a mystery! Yes what is gravity has been found but what explains it’s origin is still a mystery! While the world might be rejoicing at finding the higgs boson the boson does not explain gravity. And though string theory does predict it , the theory itself is yet to be verified by the scientific community.

Of course nowadays newer more expensive equipment are being created which sought to find the elusive particle of gravity called gravitons. And hopefully we will fully gain an in-depth understanding of gravity one day. Maybe one day the unknown apple shall fall on someone’s head again and we shall be saved from the ignorance of that special force called gravity!

## Black holes-Not exactly Black!!

Black holes-Not exactly Black!!
Now we have known black holes as giant swirling  holes present in the universe which are known to swallow anything that comes close to it. Yes we have had countless rumors and fantasies about black holes being warps in time , a portal to another dimension and the doorway to a mirror world. Of course black holes don’t really do something like that but that doesn’t mean they are not fascinating by themselves.
The mystery of black holes has fascinated physicists, scientists and certainly many others alike with their mysterious behavior. Their birth, evolution etc. are still under study and are  the objects of obsession of most astrophysicists.

So If it’s not a portal to the next dimension then what is it and where do the objects that it pulls inwards disappear to?
Well think of it this way. You may have heard of how  when a body of mass m and another mass M attract each other with a force given by
F=G.M.m
r2
And the fact because of this force the potential energy that exists between them is
P.E. = G.M.m
r
And also the fact that escape velocity can be found by equating it with kinetic energy of the object to be equal to it’s potential energy
K.E.= 0.5 m.v2
Now equating the kinetic energy with the potential energy we get
v2 =G. m
r
Now imagine a mass with gigantic (and i really mean it)  mass and really tiny radius
such that                                G.M      =c2
r
where c is the speed of light.

Yes that’s what black holes exactly are. They are objects having mass and sometimes having charge as well as angular momentum such that any object on it will have to travel at the speed of light just to escape.  So that explains why all objects cannot escape the gravitational pull of a black hole.
Is it that strong?
Well it is so strong that it is known to even absorb light. That’s right! Black holes actually bend gravitational space around it so much that even light cannot escape it’s pulling power. Scientists have named this as the infinite space time curvature!! At this point the state of the system is called singularity. In the state of singularity the some of Einstein’s equations along with some other big equations give weird solutions the details of which i will not bore you with. But what i will say is that any mass near this point of singularity  will have time running down slower for it!(Time travel theories anyone?!!)
Why are scientists so obsessed with it?
Well for one thing they hold vital clues as to the origin of the universe. It’s true. The requirement of the formation of black holes is gigantic mass and small radius. That was the condition of the universe just a few moments before and after the big bang. So when that small point exploded in the event of the big bang many black holes were formed as the universe was expanding. So the black holes we study including the one at the centre of our galaxy is a by product of the big bang. So we can actually figure out where it all began!
So how are they formed?
Well like i told you the big mass and small radius is a good condition for the formation of black holes. So lots of black holes were formed after the big bang. Also lots of black holes are formed when massive stars collapse due to their own gravitational pull.  But of course once formed it doesn’t stay together forever. It slowly decays by means of a radiation called Hawking’s radiation.
So where do these masses disappear into?
Well that is something no one is sure of! Yes that’s true. Nobody really knows what happens to all that sucked up mass. While it is true that a lot of the mass is emitted as energy from the accretion disks.  In fact accretion disks are some of the brightest stuff in the universe. Not so black any more right?
But scientists have found that this energy emitted as energy does not equal the energy obtained from the mass energy equivalence (any one remember Einstein’s epoch changing equation? e=mc2) energy obtained from the mass swallowed. (Though it is more efficient than fusion or fission as they use of 0.4% of the mass to be converted to energy while accretion disk formation uses 40% of the relative rest mass!)In fact information is lost inside the black hole. It would have been ok if black holes existed forever but the fact is they don’t. They decay through hawking’s radiation and hence it seems that the information is truly lost. In quantum mechanics this is the same as violation of conservation of energy!
So next time your physics teacher tells you that nothing violates the conservation of energy you might wanna raise your hand and ask him/her about black holes. If he is genuinely interested in learning he will certainly try to help you explain better.

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