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 __

r^{2}

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.v^{2 }

Now equating the kinetic energy with the potential energy we get

v^{2} =__G. m__

r

Now imagine a mass with gigantic (and i really mean it) mass and really tiny radius

such that __G.M __=c^{2}

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=mc^{2}) 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.

Oct 02

# Black holes-Not exactly Black!!

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