Black Hole Information Paradox

English physicist Stephen Hawking is known for his work on black holes , ranging from singularities to quantum theory of gravity. One of the most infamous paradoxes of physics is the Black Hole Information Paradox, which is a consequence of two contradictory theories of physics .

In one of the previous articles, we have understood a black hole, its properties and the way its formed. But, what happens to the matter and energy that travels inside it? Does it get lost or does it manifest itself in some other form in some another corner of the universe.

At this point, it is important to understand, that despite its enormous gravitational pull, black holes themselves release exact black body radiation what is known as “Hawking Radiation” (named after physicist Stephen Hawking) beyond their event horizon. With time, this radiation makes black holes lose mass and evaporate completely after a certain point.


Hawking Radiation

Similar to the first law of thermodynamics or law of conservation of energy, information also remains conserved throughout the universe. Here, information is used to collectively refer to matter and energy and also its specific states, as unique as a fingerprint viz. spin, mass.Take the example of a paper burning – we know that the products of combustion are smoke, ash, carbon dioxide, and water vapor which are chemically different from the reactants. But, on an atomic level, they are intact and are conserved throughout the universe. Thus, information can neither be created nor destroyed.

The radius beyond which light cannot escape the clutches of a black hole is known as the event horizon. The energy emitted from this horizon is consistent,and has a temperature inversely proportional to the mass of the black hole. It is independent of the information entering it and has no trace of the objects that fell into it. It is this radiation that makes black holes lose mass with time, till it disappears completely.


The information entering the black hole is preserved through a special phenomenon known as time dilation. Special relativity suggests that time, itself depends on the observer and slows down for objects traveling at speeds comparable to that of light( relativistic speeds). Near the event horizon, objects gradually reach relativistic speeds due to a high kinetic energy, till it attains the speed of light, at which point, time freezes. No discernible time passes through the object and thus no change is observed. Thus, all information that has entered in the past is preserved in the event horizon as static entities.

The above two theories are in contrast to one another. On one hand, we see that information is preserved through time dilation, and on the other, the black hole itself disappears with time leaving no trace of any existing information. This begs the question, whether the information entering the black hole, is indeed lost, thereby violating the conservation principle.


In July 2004 Stephen Hawking,proposed a solution to this infamous paradox of physics which is incomplete at best.

In his paper, he stated that the information preserved on the event horizon indeed influences the outgoing Hawking radiation through quantum perturbations, thereby preserving the net information of the universe. That being said, these mysterious spots in dark corners of our universe are far from being fully understood, which makes studying them all the more interesting.


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