In the real world, the past uniquely determines what the future will be like. For example, if a physicist knew exactly how the Universe was born, he could calculate his future for any time and for any point in space.
However, a mathematician at the University of Berkeley has discovered that certain types of black holes can tear that rule apart. And if someone ventured into one of those relatively "benign" black holes, he would survive, although he would come out of the experience with all his past erased. And with an infinite number of possible futures ahead. The study has just been published in Physical Review Letters.
It is not the first time that statements like this have been made and in the past physicists have invoked a sort of "cosmic censorship" to avoid them. That is to say, Nature itself prevents with something catastrophic (usually a horrible death) that an observer actually enters a region of spacetime (a black hole) in which his future is not uniquely determined.
This principle, proposed for the first time 40 years ago by the physicist Roger Penrose, tries to protect a very concrete idea, determinism, which is key to any physical theory. In other words, given the past and the present, the physical laws of the Universe in which we live allow no more than a possible future. But what if there were exceptions?
According to the physicist of the University of Berkeley Peter Hintz, in fact, the calculations show that in some specific types of black holes of a Universe like ours, which expands at an accelerated rate, it is theoretically possible to survive, and thus pass through from a deterministic reality to another that is not.
How life could be in a space in which the future can not be predicted is something that is not clear. And Hintz himself clarifies that his finding does not mean that the equations of Einstein's General Relativity are wrong when describing the evolution of the Universe.
"No physicist," he explains, "is going to travel to a black hole to measure it, it is a mathematical question, and from that point of view, Einstein's equations become more interesting." In other words, the question of how that life could be with infinite futures can only be addressed with mathematics, although "it has physical, almost philosophical implications, what makes it really interesting".
Stretched as a chain of atoms
Black holes are strange objects that take their name from the fact that nothing, not even light, can escape its enormous gravity. So if anyone got too close and crossed the point of no return, known as "event horizon," he could never get back out of the black hole.
Also, no one could survive by getting too close to a black hole, even if it is small. In fact, the tidal forces near the event horizon would be intense enough to turn anything, or person, into a "spaghetti", stretching it to a simple chain of atoms.
But the thing would be different if we crossed the horizon of events of a big black hole, one with hundreds or billions of times the mass of a star, like those that are in the center of many galaxies. In those cases, the step "to the other side" would occur, more or less, without incidents.
And since it is theoretically possible to survive the transition from our world to that of one of those huge black holes, physicists have wondered what that world could be like. To find out, they have resorted to Einstein's equations, which work well until the moment when the observer reaches the center of the black hole or singularity, where the
curvature of space-time becomes infinite in theoretical calculations.
Even before reaching the center, however, a hypothetical explorer of black holes (who could never communicate what he sees to the outside world) might encounter some strange and deadly things
To carry out his calculations Hintz has studied a specific type of black hole. One that does not turn, that has electric charge and that according to the theories has another additional barrier, the so-called Cauchy horizon, within the event horizon.
It is there, on the Cauchy horizon, where determinism is broken, where the past no longer determines the future. Numerous physicists, including Penrose himself, argue that even if an observer had survived there, he could not cross Cauchy's horizon, since he would be annihilated.
But let's continue with our argument. As an observer approaches the horizon, time slows down, as the clocks pass more slowly within a strong gravitational field, and since the light, gravitational waves or anything else that is in the black hole will fall inevitably towards the horizon of Cauchy, the observer who is also falling towards the same place will see how all that energy (which would be all the energy of the Universe) enters at the same time as him. And, despite having come so far, it would be irremediably destroyed by that energy.
Hintz, however, realized that this might not apply to a universe that expands faster and faster, as it does with ours. And as spacetime stretches and separates more and more, a large part of the distant Universe will not affect the black hole at all, since that energy can not travel faster than light.
In fact, the only energy available to fall into the black hole would be the one that contains the observable horizon, that is, the volume of the Universe that the black hole can expect to see throughout its existence. For us, for example, the observable horizon is greater than the 13,700 million light years that we can see into the past, because it also includes everything we will see forever in the future. The accelerated expansion of the Universe will prevent us, in effect, from seeing beyond a horizon of approximately 46,500 million light years.
In this scenario, the expansion of the Universe counteracts the amplification caused by the dilation of time within the black hole. And in certain situations, it cancels even completely. In smooth, non-rotating, electrically charged black holes (the so-called Reissner-Nordström-de Sitter black holes), an observer could, according to Hintz, survive by passing through Cauchy's horizon and, therefore, enter a non-deterministic world.
"There are some solutions to Einstein's equations," says Hintz, "that are perfectly smooth, without bends, without tidal forces that tend to infinity, and in which everything behaves perfectly well to the horizon of Cauchy and beyond." For the physicist, in those cases, going through the horizon would be a painful but brief experience: "after that," he continues, "there are no more bets, in some cases, like in the black hole of Reissner-Nordström-de Sitter, one I could avoid the central singularity and live forever in an unknown Universe. "
The truth, according to the researcher, is that it is very unlikely that there are electrically charged black holes, since they all attract matter with opposite charges that cancel out until they become neutral. However, the mathematical solutions for loaded black holes are used as approximations of what would happen inside rotating black holes, those that we see around us and that probably constitute the norm.
But for Hintz, the mere possibility of an exception is disturbing. The idea that an electrically charged star collapses in a black hole; that after someone travels inside that black hole; that also manages to cross the horizon of Cauchy and manage to reach a region of the Universe in which, despite knowing the complete initial state of the star, it is not possible to predict what will happen ... "That region - says Hintz - it would no longer be determined by the full knowledge of the initial conditions, and that is something very problematic. "
Hintz's article has already begun to elicit reactions and new scientific articles, one of which aims to show that most "well-behaved" black holes will not, after all, violate determinism. Something with which, of course, the researcher disagrees.