Do black holes have hair?

Black holes are strange objects. They are accumulations of extremely compact matter, which exerts such huge gravity that at less than a certain distance (the event horizon) nothing can escape their attraction, not even light. Hence their name.

The existence of black holes had been predicted in the 18th century by the English geologist John Michell and the French astronomer Laplace. At that time nobody paid attention, but from 1915, when Einstein formulated the theory of General Relativity, the interest in these mysterious objects grew. It was soon concluded that when a massive star exhausted its ability to produce nuclear fusion reactions, no force of nature would be able to overcome the gravitational pull of the remaining matter, resulting in a black hole. But for a long time there were doubts about their real existence, for the theory seemed to predict that the matter located inside a black hole would occupy a zero volume and therefore would have an infinite density. As physicists usually suspect that infinity is a mathematical concept that cannot happen in real life, there were two possibilities: either black holes do not exist, or Einstein's theory would have to be modified so that they wouldn’t have an infinite density.

The end of the universe

Will the cosmos expand indefinitely, or will its expansion stop one day? What could stop it? It is clear only gravity could do it. The expansion of the universe, which makes galaxies separate, goes against the gravitational attraction, which tries to hold all bodies together.

If we look at Einstein's cosmic equation of general relativity, the question of whether gravity will succeed in stopping the expansion of the universe depends on the relative values and signs of the three terms in the equation. Depending on them, three things can happen:

Mistakes in popular science in the media: Stephen Hawking didn’t discover everything

Stephen Hawking

Stephen Hawking has been in the last decades a scientific icon for the media. His painful personal situation turned him into a celebrity who inevitably attracts attention. Therefore, the media have a tendency to exaggerate his scientific work, attributing to him achievements that weren’t his, which he would be the first to repudiate, if he were still among us.

For example, on the occasion of his death, the following headlines appeared in several media:

•         AlDiaNews.com: Stephen Hawking, author of the Big Bang theory dies at 76

•         ElTiempoHoy: Creador de la teoría del Big Bang y los agujeros negros: fallece Stephen Hawking a los 76 años. (Creator of Big Bang’s theory and black hole theory: Stephen Hawking dies at 76).

•         Gizmodo Univisión: Del Big Bang a la Teoría del Todo: el increíble legado científico de Stephen Hawking (From the Big Bang to the Theory of Everything: Stephen Hawking’s incredible legacy).

Virtual particles

Werner Heisenberg

Heisenberg's uncertainty principle, one of the consequences of quantum mechanics, makes possible the birth of virtual particles in the void, apparently transgressing the principle of energy conservation, the most holy in physics. The reason is that Heisenberg’s principle can be expressed in several ways, one of which relates the uncertainty about the energy to the uncertainty about time:

DE.Dt≥ħ/2

This expression can be interpreted in the sense that a pair of objects, each with energy E, can appear spontaneously in the vacuum, provided that they lasts at most a time Dt<ħ/(2E). These pairs of objects are called virtual particles. One of these particles is always matter, the other antimatter, and their duration, according to this principle, is ridiculously small. A virtual electron, for instance, would last 1.3×10^-21 seconds (just above one sextillionth of a second). The higher the mass (energy) of the virtual particle, the less time it will last. After that time, the two particles will annihilate each other and disappear. Due to their short duration, the existence of virtual particles has not been experimentally verified.

Is it possible for these virtual particles to become real under certain circumstances? Yes it is, and it is believed that there are at least two situations (somewhat drastic, it is true) where this could happen.