A solution to the black hole information paradox

It is a paradox that has long troubled scientists. A black hole is a region of spacetime where the effects of gravity are so powerful that supposedly nothing, including light and other forms of radiation, can escape from it. They are hard for a human brain to comprehend, but mathematically they make sense. In fact, our own Milky Way galaxy is believed to be centred around a supermassive black hole.

A major challenge in physics involves resolving different theories with one another. Quantum physics works beautifully when describing minute particles, and special relativity applies when things are moving very quickly. The reason why these theories endure so well is that they both reduce down to familiar Newtonian physics, so the theories complement rather than contradict one another.

The theory surrounding black holes is not so well behaved. A fundamental principle known as determinism is that if all the information about a system is known, then one can predict exactly what is going to happen, and work out what has already happened. In practise, most real life systems are much too complicated to gather all the information, but by using simplified models excellent predictions can be made about past and future events. This assumption that information does not just disappear means that scientists can make informed theories about the history of the universe.

However, in the mid-1970s, Stephen Hawking suggested just that – that black holes will sometimes evaporate or explode, and that they will take all the information contained within them with them. In other words, by observing the present, we cannot predict future events or visualise the past, because unknown amounts of information simply no longer exists.

On a practical level this might not seem so disastrous. But the consequences to physics would be enormous, as many of the most fundamental laws of physics are built on the assumption that the universe is deterministic. Once one crack is found in determinism, many other theories start to unravel.

A new paper authored by Malcolm J Perry, Andrew Strominger, and Stephen Hawking claims to make solid steps towards resolving this paradox of the lost information. It may be that determinism still holds, but some other assumptions require rethinking. First, the vacuum state has previously been taken to be a uniform state, but Strominger now suggests that there are infinitely many different vacuum states. Second, the cutely named ‘soft hairs’ may resolve the paradox. ‘Soft’ in this context just means very low or zero energy. ‘Hairs’ are photons (a particle representing a unit of light) and gravitons (a hypothetical particle that mediates the force of gravity). The depositing of soft hairs on black holes may mean the information is recorded, not lost forever.

The paper itself does not claim to draw any absolute conclusions. It also contains some pretty brutal mathematics, but if your interest has been piqued then Strominger has provided a lengthy ‘translation’ of the paper for an interview with Scientific American. If this article is enough of theoretical physics to satisfy you for a while, then what has been learned by this new research? Even celebrated great physicists such as Hawking will sometimes go back and adjust or contradict their earlier work. Such is the evolving nature of scientific understanding. Also, that without most of the public knowing it; since the 70s one of the most fundamental postulates on which we have understood our universe has been decidedly shaky, and only now may be reinforced.

 

Image: NASA/JPL-Caltech

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