Soft-haired black holes

At present, we have two separate theoretical formulations of physics explaining how the Universe works at the cosmic and microscopic scales. 

For the cosmic scale, we have Einstein’s theory of general relativity, which beautifully accounts for gravity and all of the things associated with it – orbiting planets, colliding galaxies, gravity waves, bending of light, black holes and dynamics of the expanding Universe as a whole. For the microscopic scale, we have quantum mechanics, the theoretical basis of modern physics that explains the atomic and subatomic world with the highest precision. A wacky theory that says a particle can exist simultaneously at two different places or we can be dead and alive at the same time, quantum mechanics incorporates the concepts of quantisation of energy, wave-particle duality, time reversibility, the uncertainty principle and the correspondence principle to describe the motion and interactions of particles. 
One consequence of the principle of reversibility is that information_ mass, position, speed, acceleration, temperature, charge, spin _ of a particle is never truly lost. This means by reversing all the parameters, we can run the whole Universe forwards and backwards. In other words, we can always construct the future configurations of a system from the initial state and vice versa. This is known as the conservation of information. If this rule is broken, energy may be created or destroyed, threatening one of the most essential underpinnings of physics – conservation of energy.
General relativity states that the ultimate fate of sufficiently massive stars is black holes. Because of their nature, we can only see the outside of black holes; we are completely shut out from their inside. Moreover, black holes are perfectly featureless. The only properties they exhibit to the wider cosmos are their mass, charge and spin. This fact led the American physicist John Wheeler to coin the phrase ‘black holes are hairless’. Whatever is going on in the interior cannot be seen because no ‘hair’ sticks out of the event horizon – the invisible boundary that is the point of no return. Simply stated, we can’t penetrate the ‘bald’ event horizon.
The hairless black holes present a challenging conundrum: We don’t know whether a black hole actually deletes its autobiography, ‘forgetting’ its past and its progenitor’s composition, or preserves it somehow in a way we don’t know yet. However, since according to general relativity, black holes exist forever, we might argue that the information is not really lost. It’s just that the information is inaccessible. 
In 1974, British astrophysicist Stephen Hawking dropped a bombshell when he hypothesised that black holes have a temperature. They can, therefore, slowly evaporate and disappear. This is known as Hawking Radiation. 
Indeed, if black holes do evaporate, what happens to the information that describes an object swallowed by these enigmatic entities? With Hawking Radiation, all information about the distinct identity and properties of objects going into a black hole would be completely erased from the Universe. This incompatibility, known as the information paradox, pits a central tenet of quantum mechanics against the cornerstone of general relativity. 
Several hypotheses have been proposed to resolve the paradox. One of them involves yet to be discovered objects called ‘white holes’– the opposite of black holes, in which the flow of time is reversed and nothing can fall in, only out, information included. Then there is the chance that black holes never quite evaporate completely. They only shrink down to incredibly small sizes, thereby, preserving all the information that fell into the black hole. Or perhaps the information is somehow copied from inside a black hole to outside, so that when the black hole is destroyed the outside copy remains. 
Another possibility that has been suggested is maybe black holes leave behind a tiny ember that contains an enormously compressed version of all the information that fell into the black hole. And finally, there are proposals in which information is believed to be encoded on a black hole’s event horizon in ways hitherto unknown to us.
Earlier this year, in a publication in the journal Physical Review Letters, Hawking and his two colleagues claimed that they may have found a way around the paradox, at least in part. They are predicting the existence of information-preserving massless particles known as ‘softhair’ that form a halo around black holes. According to them, these are low-energy quantum excitations that contain the information for the things that were consumed by black holes. A drawback of the theory is it cannot explain how information could be exchanged between a black hole and the soft hair.
Although the theory of soft hair is more a qualitative account than a rigorous mathematical description, it nevertheless represents a promising resolution to the information paradox. 
Finally, with more esoteric theories, hypotheses and conjectures expected to be proposed in the future, we may be able to get an even better view of black holes, drawing ever closer to their event horizon. But for now, the Universe still hides from us the mystery of what lies inside them. 

The writer is Professor of Physics at Fordham University, New York

Soft-haired black holes

At present, we have two separate theoretical formulations of physics explaining how the Universe works at the cosmic and microscopic scales. For the cosmic scale, we have Einsteins theory of general relativity, which beautifully accounts for gravity and all of the things associated with it orbiting planets, colliding galaxies, gravity waves, bending of light, black holes and dynamics of the expanding Universe as a whole. For the microscopic scale, we have quantum mechanics, the theoretical basis of modern physics that explains the atomic and subatomic world with the highest precision. A wacky theory that says a particle can exist simultaneously at two different places or we can be dead and alive at the same time, quantum mechanics incorporates the concepts of quantisation of energy, wave-particle duality, time reversibility, the uncertainty principle and the correspondence principle to describe the motion and interactions of particles. One consequence of the principle of reversibility is that information_ mass, position, speed, acceleration, temperature, charge, spin _ of a particle is never truly lost. This means by reversing all the parameters, we can run the whole Universe forwards and backwards. In other words, we can always construct the future configurations of a system from the initial state and vice versa. This is known as the conservation of information. If this rule is broken, energy may be created or destroyed, threatening one of the most essential underpinnings of physics conservation of energy. General relativity states that the ultimate fate of sufficiently massive stars is black holes. Because of their nature, we can only see the outside of black holes; we are completely shut out from their inside. Moreover, black holes are perfectly featureless. The only properties they exhibit to the wider cosmos are their mass, charge and spin. This fact led the American physicist John Wheeler to coin the phrase black holes are hairless. Whatever is going on in the interior cannot be seen because no hair sticks out of the event horizon the invisible boundary that is the point of no return. Simply stated, we cant penetrate the bald event horizon. The hairless black holes present a challenging conundrum: We dont know whether a black hole actually deletes its autobiography, forgetting its past and its progenitors composition, or preserves it somehow in a way we dont know yet. However, since according to general relativity, black holes exist forever, we might argue that the information is not really lost. Its just that the information is inaccessible. In 1974, British astrophysicist Stephen Hawking dropped a bombshell when he hypothesised that black holes have a temperature. They can, therefore, slowly evaporate and disappear. This is known as Hawking Radiation. Indeed, if black holes do evaporate, what happens to the information that describes an object swallowed by these enigmatic entities? With Hawking Radiation, all information about the distinct identity and properties of objects going into a black hole would be completely erased from the Universe. This incompatibility, known as the information paradox, pits a central tenet of quantum mechanics against the cornerstone of general relativity. Several hypotheses have been proposed to resolve the paradox. One of them involves yet to be discovered objects called white holes the opposite of black holes, in which the flow of time is reversed and nothing can fall in, only out, information included. Then there is the chance that black holes never quite evaporate completely. They only shrink down to incredibly small sizes, thereby, preserving all the information that fell into the black hole. Or perhaps the information is somehow copied from inside a black hole to outside, so that when the black hole is destroyed the outside copy remains. Another possibility that has been suggested is maybe black holes leave behind a tiny ember that contains an enormously compressed version of all the information that fell into the black hole. And finally, there are proposals in which information is believed to be encoded on a black holes event horizon in ways hitherto unknown to us. Earlier this year, in a publication in the journal Physical Review Letters, Hawking and his two colleagues claimed that they may have found a way around the paradox, at least in part. They are predicting the existence of information-preserving massless particles known as softhair that form a halo around black holes. According to them, these are low-energy quantum excitations that contain the information for the things that were consumed by black holes. A drawback of the theory is it cannot explain how information could be exchanged between a black hole and the soft hair. Although the theory of soft hair is more a qualitative account than a rigorous mathematical description, it nevertheless represents a promising resolution to the information paradox. Finally, with more esoteric theories, hypotheses and conjectures expected to be proposed in the future, we may be able to get an even better view of black holes, drawing ever closer to their event horizon. But for now, the Universe still hides from us the mystery of what lies inside them. The writer is Professor of Physics at Fordham University, New York