Imagine two intrepid astronauts, Jack and Andres, determined to investigate black holes. His goal is to explore what goes on inside. What will happen to them?

For their investigation Jack and Andres choose a black hole whose mass is ten times that of the Sun and radius 30 kilometers, 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. Jack, adventurous and daring, will be the one who travels inside. Andres, pragmatic and realistic, will stay in orbit observing the trip.

Once their watches are synchronized, our beloved astronaut heads towards him. As he approaches, Jack feels the black hole pull 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 a meter and a half is enough to appreciate it. And not only that. Jack feels as if he is being squeezed laterally in a straitjacket, as every point of his body is directed toward the heart of the hole. This combination of stretch and compression is increased in such a way that it shreds our unfortunate friend into literally one long noodle.

In a short time, the remains of the astronaut approach the border that separates the interior of the black hole from the exterior: it is the event horizon or limit of non-return, where the escape velocity coincides with that of light. Jack knows that once pierced he will never be able to get out of there. From Andres’ point of view things are very different. For him, comfortably installed in the tranquility of his orbit, his friend’s trip is absolutely abnormal. All of Jack’s movements become progressively slower and he sees that time inside the ship runs slower and slower until it stops completely at the event horizon. However for Jack the trip lasted a few seconds.

The election has been completely suicidal. A black hole of ten solar masses causes an acceleration fifteen million times greater than the terrestrial acceleration on the event horizon. The human body can only withstand about ten times the acceleration of gravity at the Earth’s surface, so Jack 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 Jack 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, their inevitable destiny is to plunge into 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 the journey is flawed because it does not take into account quantum processes occurring at 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 churrusca everything it touches. This solution has a but… and a big one. The big problem is that it requires that at the event horizon you 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 that quantum theory requires- 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 tell if we are on the surface of a planet or are being carried through space at constant acceleration. Polchinski and Marolf say that this is not true 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 as it was thought, but then what stops working is quantum mechanics. The commotion they created was outrageous. Either solution was unacceptable as it implied that one of the two most important theories of 20th century physics does not work on black holes: if you save one you have to bury the other. And for now nobody knows where to go.