For their research, Javier and Andres choose a black hole whose mass is ten times that of the Sun and a radius of 30 kilometres, located in one of the spiral arms of the Milky Way. This black hole appeared 6 billion years ago after the death of a star as a supernova. Javier, adventurous and daring, will be the one who travels inside her. Andrés, pragmatic and realistic, will stay in orbit observing the trip.
Once their watches are synchronized, our dear astronaut sets course towards him. As he approaches, Javier feels how the black hole pulls harder on his feet than on his head because gravity is stronger the closer we are to the center of the massive object. As the Earth’s gravitational field is weak, our bodies do not notice this effect, but in the intense field created by the hole, a difference of one and a half meters is enough to appreciate it . And not only that. Javier feels as if they were compressing him laterally with a straitjacket, since all the points of his body are directed towards the heart of the hole. This combination of stretch and compression is increased in such a way that it crushes our unfortunate friend into, literally, a long noodle.
In a short time, the remains of the astronaut approach the border that separates the inside of the black hole from the outside: it is the event horizon or limit of no return, where the escape velocity coincides with that of light. Javier knows that once transferred he will never be able to get out of there.
From Andrés’ point of view, things are very different. For him, comfortably ensconced in the tranquility of his orbit, his friend’s journey is absolutely abnormal. All of Javier’s movements become progressively slower and he sees that the time inside the ship passes more and more slowly until it stops completely at the event horizon. However, for Javier the trip has lasted a few seconds.
The election has been completely suicidal. A black hole of ten solar masses causes an acceleration on the event horizon fifteen million times greater than the Earth’s . The human body can only withstand about ten times the acceleration of gravity on the Earth’s surface, so Javier would have ceased to exist 3,000 kilometers from the hole. To cross the event horizon with some comfort, they should have chosen a hole with a mass greater than 100,000 solar masses. These are only found in the center of galaxies. In this case, when crossing the horizon, Javier would not notice anything extraordinary. No jolt or radical change in the fabric of space would herald its arrival at the event horizon. Once inside, its inevitable destiny is to rush to the heart of the black hole. Unfortunately, no astronaut can reach the center alive because gravity takes care of pulverizing everything.
However, the journey may not end like this. In 2012, Joseph Polchinski of the California-based Kavli Institute for Theoretical Physics and Don Marolf, a theoretical physicist at the University of California, said that this view of travel is flawed because it does not take into account quantum processes occurring on the horizon. of events. What will really happen to Javier is that when he reaches the event horizon… he will burst into flames! The poor thing will have collided with a kind of cosmic firewall that prevents anything from entering the black hole and scorches everything it touches. This solution has a but… and a very big one. The big problem is that it requires that in the event horizon we have to get rid of a fundamental aspect of general relativity, the equivalence principle. In other words, accepting the idea of the firewall and thus preserving the integrity of the information that falls into the black hole -something required by quantum theory- requires sacrificing Einstein’s equivalence principle, the pillar of general relativity and which says that if we are locked in a closet there is no way to distinguish whether we are on the surface of a planet or being carried through space at constant acceleration. Polchinski and Marolf say that this does not hold at the event horizon. Aware of what this meant, in their article they also presented another solution to the problem: the firewall does not appear to be everything as it was believed, but then what stops working is quantum mechanics.
The commotion that they formed was órdago. Either solution was unacceptable because it implied that one of the two most important theories of 20th century physics doesn’t work on black holes: if you save one you have to bury the other. And for now nobody knows where to go.
