Black holes are certainly among the strangest objects in the cosmic catalog. Predicted by Albert Einstein's general relativity equations, their existence is no longer debated today, the collaboration of the Event Horizon Telescope having released the first image of a black hole's structural appearance in 2019. But despite all the observational clues we have about these objects, we still don't know precisely what's inside. Several theories, such as superstring theory and loop quantum gravity, have provided theoretical answers, but the question still remains unresolved today.
The radius of a black hole is proportional to its mass, this means that the density of a black hole is inversely proportional to its mass. In other words, the smaller a black hole, the denser it will be, and the stronger its gravitational pull will be near its event horizon. To try to enter a stellar black hole is to take the risk of being immediately spaghettified , i.e. being stretched and then destroyed by the extreme tidal forces of the black hole.
Supermassive black holes are much less dense, so it is possible to approach them, and even pass their event horizon without running the slightest risk (putting aside of course the ultra-energetic radiation coming from the accretion disk that would roast any astronaut on the spot). However, Einstein was clear on this point:whether or not one manages to enter a black hole, its central singularity would be the end point of the journey. But is this really the case?
Over the years, scientists have investigated the possibility that black holes could be wormholes leading to other galaxies. They might even be, as some have suggested, a path to another universe. Such an idea has been circulating for some time:Einstein teamed up with Nathan Rosen to theorize bridges connecting two different points in spacetime in 1935.
But the theory gained traction in the 1980s when physicist Kip Thorne — one of the leading experts on the astrophysical implications of Einstein's theory of general relativity — sparked discussion about whether objects could physically cross them. However, it seems unlikely that wormholes exist.
Indeed, Thorne, who lent his expert advice to the production team of the film Interstellar, wrote:"We see no object in our universe that could become a wormhole as we age> in his book The Science of Interstellar. Thorne explains that travel through these theoretical tunnels will most likely remain science fiction, and there is certainly no solid evidence that a black hole could allow such passage.
If black holes lead to other galaxies or other universes, there could be its exact opposite at the end of the tunnel. For example, a white hole, a theory put forward by Russian cosmologist Igor Novikov in 1964. Novikov proposed that a black hole is related to a white hole that existed in the past. Unlike a black hole, a white hole allows light and matter to exit, but matter cannot enter.
Physicists continued to explore the potential connection between black holes and white holes. In their 2014 study published in the journal Physical Review D , physicists Carlo Rovelli and Hal M. Haggard asserted that "there exists a classical metric satisfying Einstein's equations outside of a finite region of spacetime where matter collapses into a black hole then emerges from a white hole ". In other words, matter absorbed by black holes can be shed, and black holes can themselves become white holes when they die.
Far from destroying the information it absorbs, the collapse of a black hole would be stopped. Instead, it would experience a quantum bounce, allowing information to leak out. If so, it would support a proposal by former Cambridge University cosmologist and theoretical physicist Stephen Hawking, who in the 1970s explored the possibility that black holes emitted particles and radiation at the sequence of quantum fluctuations near their horizon.
Hawking calculated that the radiation would cause a black hole to lose energy, shrink, and disappear, as described in his 1976 paper published in the journal Physical Review D . According to his assertions that the radiation emitted would be random and would not contain any information about what was absorbed, the black hole, during its evaporation, would erase all the information contained.
This meant that Hawking's idea was at odds with quantum mechanics claiming that information cannot be destroyed. Hawking's idea led to the "black hole information paradox" and has long intrigued scientists. Some have said Hawking was simply wrong, and the man himself even said he made a mistake at a scientific conference in Dublin in 2004.
In their 2013 study published in Physical Review Letters , Jorge Pullin of Louisiana State University and Rodolfo Gambini of the University of the Republic of Montevideo, Uruguay, applied loop quantum gravity to a black hole and found gravitational dynamics consistent with the existence of an outflow of the material. The results lent further credence to the idea of black holes serving as bridges in spacetime. In this study, the singularity does not exist, so it does not form an impenetrable barrier that ends up crushing everything it encounters. It also means that the information does not disappear.
Yet physicists Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully thought Hawking was right. They worked on a theory known as the AMPS firewall, or black hole firewall hypothesis. According to their calculations, quantum mechanics could turn the event horizon into a huge wall of fire and anything that comes in contact would burn in an instant. In this sense, black holes lead nowhere, because nothing will ever be able to penetrate them.
This, however, violates Einstein's theory of general relativity. A person crossing the event horizon shouldn't feel any particular effects, because they would be in free fall and, according to the principle of equivalence, this person would not feel the extreme effects of gravity. And even if this did not violate general relativity, this assumption would conflict with quantum field theory.
Hawking went so far as to say that black holes might not even exist. “Black holes should be redefined as metastable bound states of the gravitational field “, he wrote. There would be no singularity, and while the apparent field would move inward due to gravity, it would never reach the center and therefore would never form a singularity.
