Polchinski's paradox, also known as the information paradox, is a thought experiment and puzzle related to the behavior of information in black holes within the context of quantum mechanics. It is named after Joseph Polchinski, the physicist who proposed it.
According to the principles of quantum mechanics, information is not meant to be lost or destroyed. Instead, it is believed to be preserved and can, in principle, be retrieved or reconstructed. However, the classical understanding of black holes suggests that anything that falls into a black hole is lost forever due to the presence of a singularity.
The paradox arises when we consider the Hawking radiation, a theoretical prediction by physicist Stephen Hawking. Hawking showed that black holes are not completely black but emit radiation due to quantum effects near the event horizon. This radiation carries away energy from the black hole, causing it to gradually lose mass and eventually evaporate completely.
If a black hole evaporates, the information about the matter that formed the black hole seems to disappear with it. This poses a conflict with the principles of quantum mechanics, which state that information cannot be lost. According to quantum mechanics, if a black hole were to completely evaporate, the information of the matter that fell into it should somehow be encoded in the outgoing Hawking radiation.
Polchinski's paradox suggests that the evaporation process of a black hole, governed by quantum mechanics, is inconsistent with the classical notion of a black hole destroying information. It implies that either the information is somehow encoded in the Hawking radiation and can be recovered, or there must be a modification to our understanding of black holes and the behavior of information within them.
The paradox has led to numerous proposed solutions and sparked intense research in the field. Some proposals involve the idea of "firewalls," which are hypothetical intense barriers of energy near the event horizon that would destroy any matter falling into a black hole, thus preserving the information. Other ideas involve the notion of "black hole remnants" or modifications to the laws of quantum mechanics at the black hole scale.
The resolution of Polchinski's paradox remains an active area of research and a significant challenge in the quest to understand the nature of black holes and the behavior of information in the presence of extreme gravitational forces.