Tech UPTechnologyHunting for white holes

Hunting for white holes

The idea of pulling something out of nothing is a classic illusionist effect. They show their empty hat, reach in and extract a rabbit, a dove or a bouquet of flowers from there. No matter how skilled the magician is, no matter how incredible the effect, we all know there is a trick, but it is not always like that. In the universe it is possible that there are some colossal magic hats from which, very from time to time, it seems as if matter and energy are literally emerging from nowhere: we are talking about white holes. “If a black hole can swallow a Mercedes, the corresponding white could certainly eject an identical car,” explains Paul Halpern, a physicist at the University of Sciences in Philadelphia (USA) and author of the book The Cyclical Serpent. Prospects for an Ever-Repeating Universe (1995). Although, unfortunately, what really devours the former is light and interstellar dust, so that is what we should expect its luminous reverse to regurgitate.

The origin of this strange phenomenon is in the theory of formation of a black hole formulated by the German scientist Karl Schwarzschild in 1916, while fighting on the Russian front during the First World War. Because paradoxical as it may seem, the first to find a solution to the equations of general relativity – the modern theory that describes gravity – was not its creator, Albert Einstein, but this astronomer six years older, director of the Leibniz Institute for Astrophysics from Potsdam, who died of bullous pemphigus – a rare skin disease – shortly after developing the new concept. His solution showed that a mass contained in a point has no exterior, since it causes such a distortion that space closes around it and isolates it from the rest of the universe. And this cleavage occurs at a distance from the central point that only depends on the mass concentrated there, the so-called Schwarzschild radius or event horizon . Every particle that passes through it never returns.

Said like this, it is quite clear, but physicists had a hard time understanding the physical meaning of this limit: is it a tangible, real barrier? Seen from the outside, if we throw an object into the black hole we will never see it cross the event horizon, because time slows down as it gets closer to it and it would take forever to reach it. However, from the point of view of the object nothing extraordinary happens, since in a matter of minutes it crosses this border without problems; You will only realize that you have done it because you cannot get out.

Moreover, when crossing the horizon, time and space exchange their roles: if outside we can move anywhere, but we are always dragged forward in time at a speed of sixty seconds per minute, inside we move –Within certain limits– by time, although we will go inexorably towards the central singularity.

Now, if we look carefully at Schwarzschild’s solution, we discover that … it is not one, but two! The equations that describe the ultimate collapse of a celestial body into a black hole can be read backwards, as an outward expansion of an object from a singularity. Or, what is the same, a white hole .

We had to wait until the mid-1950s for a way to visualize and understand this gibberish to develop. It was Martin Kruskal, a plasma physicist at Princeton University (USA) – at that time very interested in general relativity – who came up with a coordinate system to describe the structure of black holes by means of a only a model of equations, which united the flat space-time on the outside –and away from the hole– with the extremely curved inside. Most strikingly, there was no hint of singularity on the Schwarzschild horizon.

Kruskal had the brilliant idea of describing the phenomena from the perspective of a beam of light thrown into a black hole , although he never bothered to publish it. Only John Archibald Wheeler, the American theoretical physicist who named black holes such, realized the importance of this work. Wheeler wrote an article on the calculations, put Kruskal’s name on it, and published it in 1960 in the journal Physical Review . Later, the Englishman Roger Penrose perfected the representation of Kruskal and turned it into a diagram. In this way, everyone is happy: for mathematicians, the key to understanding these objects is the Kruskal metric; and for physicists the essential idea is provided by the graphical version known as the Penrose diagram .

What do we deduce from all this? That white holes are the mirror images of black holes . If one does one thing, the other does just the opposite and invested in time. Thus, while the event horizon of a black hole is a place that cannot be exited, the anti-horizon of a white hole cannot be entered. If the former swallows everything, his hypothetical counterpart expels him.

You don’t have to be very perceptive to realize that the big bang looks very much like a white hole : all matter and energy that exists today was created in that sudden mega-explosion . “It’s amazing how much the movie of the big bang would look like going backwards to the instantaneous gravitational collapse of a fireball,” says Halpern. Or conversely, if we rewind the movie that shows the destruction of energy falling into the central singularity of a black hole, we would seem to be witnessing the moment when the universe was born.

In 1965, the Soviet Igor Novikov and the Israeli Yuval Ne’eman independently developed the first fairly detailed theory of the origin of white holes, baptized by Novikov as lagging nuclei. According to both physicists, the vast majority of the universe arose from the big bang, but with the passage of time, fragments of considerable size have continued to appear from the lagging regions of the original explosion .

With this idea in hand, Alon Retter and Shlomo Heller suggested in 2012 in New Astronomy magazine that the cosmos was actually born from a white hole, which they called small bang, and that it is a spontaneous phenomenon : all matter is expels in one go. Therefore, unlike blacks, they can only be detected around the event itself. Would Gamma Ray Bursts (GRBs) be associated with extremely energetic bursts in distant galaxies, that is, white holes? The idea is not far-fetched, since GRBs are listed as the most luminous explosive events in the cosmos.

The truth is that they have been looking for them for many years. When the first indirect proof of the existence of black holes was obtained in the 1970s, efforts to find their antagonists were redoubled. And in this context , quasars appeared, objects very far away –and therefore located at the time when the universe was young– that continuously emit large amounts of energy. Quite a few thought they finally had conclusive proof, but a jug of cold water dampened their illusions.

Indeed, Douglas Eardley of the California Institute of Technology (Caltech) detected in 1974 that the Novikov and Ne’eman solutions were very unstable and would disintegrate almost immediately. The cause is very simple: the white hole would die buried by the layers of matter and energy accumulated around it.

Imagine a Schwarzschild white hole, with a central singularity from which matter and energy sprout, which in turn is surrounded by an anti-horizon. But this energy emission does not escape into space, but rather accumulates in the outer fringe of the anti-horizon. Thus, layer after layer, we will have a white hole surrounded by a dense protective screen of energy, which Eardly called the blue sheet .

Following the rules of general relativity, the ultra-energetic wall causes that region of space to deform abruptly and the event horizon of a true black hole emerges. According to Nick Herbert of Stanford University (USA), “universes like to have their own lethal doses of light and matter contained to form blue sheets, which suffocate newborn white holes in their cradle”. As the calculations show, this asphyxia process depends on the mass: for one equivalent to ten suns, the conversion would take place in less than a thousandth of a second; for another with a mass of a million suns, in little more than a minute.

So there are no white holes in the entire universe? The Italian theoretical physicist Carlo Rovelli – one of the founders of loop quantum gravity, the rival idea to string theory and currently a professor at the University of Aix-Marseille (France) – believes that all is not lost. Applying the rules of quantum mechanics to the world of white holes – as Stephen Hawking did with his dark replicas – Rovelli argues that black holes bleach after undergoing a quantum transition. And matter, falling on them, bounces.

Then there must be a time when the event horizon changes to anti-horizon. And this is where quantum theory comes to lend a hand, thanks to a well-known and no less mysterious phenomenon: the tunnel effect . Without it, it is impossible to understand radioactive decay, when a particle trapped in the nucleus of an unstable atom manages to overcome the barrier that prevents it from going outside. The laws of classical physics forbid it, as it does not have enough energy to overcome the bonds to which it is subjected.

In the case of black holes, the fact that they experience the so-called Hawking evaporation – according to which, due to quantum effects, they slowly evaporate until they disappear – is what allows a peculiar tunnel effect to take place. For Rovelli, just when the black hole has shrunk to a point where space-time can no longer contract, it becomes white.

If so, where would we find them? Well, they could be behind the mysterious dark matter of the universe, only detected to date by its indirect gravitational effects. The Italian physicist has calculated that only one tiny white hole is needed for every 10,000 cubic kilometers, much smaller than a proton and with a mass of approximately one millionth of a gram – “equivalent to the mass of a twelve-centimeter human hair” – to account for all the dark matter found in the Sun’s galactic environment. These white nanoholes would not emit radiation, and because they are infinitely small, they would be invisible, like dark matter. If a proton were to hit one, it would just bounce back. “They can’t swallow anything,” Rovelli sums up.

And if the idea of the existence of these ultramicroscopic entities wasn’t outlandish enough, Rovelli suggests – hold on to your chair – that some white holes could predate the big bang. Not only that, but these objects from a previous universe could help explain why time flows forward in ours.

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