People often portray Black-holes as giant vacuum cleaners that destroy all that wearies too close. Though that statement itself is only an exaggeration, you certainly won’t be surprised to know the truth is even more unshakable. Here we address the first fear of the wandering mind which has just come to know these very real mathematical monsters. On their first introduction to the subject, most beginners learn that –

“NOTHING ESCAPES THE BLACKHOLE!”

, we don’t and won’t say that is a wrong statement! But it is undoubtedly an incomplete one. After discovering these mathematical phenomena (which we call black hole) in the equations of Einstein’s general theory of relativity, most did believe the clause mentioned above, but having a skeptic’s eye is a property of physics wizzes. As many of us have in us, a hard-boiled notion that some things in our universe are indestructible, e.g.: – Energy, Entropy, Charge, or more generally- “the quantum information” any well-versed student in science should be pretty-much convinced that these quantities never change- a bunch of numbers that you add up before and after something happens (explosions, change of states, zombie apocalypses), remains unchanged! A primary student may summarize this phenomenon as – The conservation laws. But a more advanced student will go on to say that mass-energy, charge, angular momentum, baryon, and lepton numbers are conserved. There is no reason why they should be! But they do.

Our universe follows this fundamental law of – “MAKING SENSE,” i.e., things don’t appear out of no-where, and things don’t happen without a cause. This notion of having specific preserved quantities is known as- “conservation of quantum information.”But blackholes with their enormous mass give rise to horizons; these horizons are a boundary of no return. Anything that goes into the black hole forever vanishes from the view of an outside observer. Thus, an outside observer can only measure three parameters – its mass, charge, and spin. This phenomenon is also the preposition of the famous ‘no hair theorem,’ which states that blackholes have no hair. One can put whatever in a blackhole (cats, dogs, astronauts, and even talkative wives) and all it will remember is their mass, charge and angular momentum. Does this imply that blackholes destroy the information of what they swallow! – NO, the information is still there, inside the blackhole, forever out of our reach but still intact (at quantum scales). If only there had been some way to get stuff out of the blackhole.

Physicist Steven Hawking proved just that. He found- “blackholes aren’t so black!!”, they emit particles. There is some very spooky quantum physics behind this idea. Still, the impossible was achieved. It came to be known that blackholes emit photons of energy inversely proportional to their mass. This is a ridiculously slow process, a blackhole with the mass of our sun will evaporate in 2 * 1066 years. That is a ridiculously long-timescale, orders of magnitude larger than the current age of the universe.

But here is the catch! Blackholes emit photons, but these emitted photons are random, resembling the emission of a hot object (black-body spectrum). This is where the – ‘conservation of quantum information’ goes out of the window; basically, hawking’s idea implies that the radiation only depends on the blackhole’s mass and not on the baryon number, charge, or even if the dumped particle was matter. Everything – neutrons, electrons, photons, unfortunate cats, etc. come out as random photons, and information is destroyed. This is what the paradox is, but as always, like any other paradox, this represents a lack of understanding.

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