Black holes: a little history (and II)

1993/03/01 Arregi Bengoa, Jesus Iturria: Elhuyar aldizkaria

• Astrofisika

The works that suggested the existence of black holes were not taken into consideration, since no star was known to present similar characteristics to those of them. And. Hagihara, for example, presented a work in 1931, calculating all the geodesic results of the theory of general relativity discovered by Schwarzschill. When drawing conclusions, Hagihara himself pointed out that the existence of black holes was very difficult, since considering the volume of a black hole that the Sun could have another mass, the density should be ¹⁰¹⁷ times greater than that of water. The highest density star known then (the white nano, friend of Sirius), ^however, was 6.104 times higher than that of water.

However, as often happens, not all astrophysicists thought the same thing. Or. Lodge, in 1923, greatly relativizes the density problem by presenting the following example: If in order for the Sun to become a black hole its mass would have to be inserted into a sphere of radius 3 km, or if in the case of the Earth we had to put all its mass in a sphere of radius 1 cm, considering a group of stars the problem is not so serious. For example, a group of stars of the mass ₁₀₁₆ ^{Mo (Mo,} _mass of the Sun) could be incorporated into a volume of 1,000 light years, with a density of 10-15 g/cm3. The presence of this type of materi does not seem impossible and circulates through the limit of the black hole. However, with the theory of relativity it began to develop through quantum mechanics, roads were soon opened with prediction or, at least, with the possibility of small bodies of very high density.

1931 S. Chandrasekhar and L. D. Landau showed that there is a higher limit for the mass of white dwarfs. If the mass of the white nano is 1.4 times greater than that of the Sun, the pressure of gravity is greater than the degenerated pressure of the electrons holding the star, it causes a contraction. In these conditions protons and electrons are melted giving neutrons. The degenerate pressure of neutrons prevents collapse, as for the first time Landau predicted that a neutron star would form. R in the coming years. Oppenheimer developed the whole neutron star theory. H. 1939 Snyder and G. With Volkhoff he published an article about it. It also showed that the degenerate pressure of neutrons had a higher limit (about two or three Solar Masses). Neutron stars above this limit contract and there are no more seasons until the so-called black hole occurs.

After these works a long parenthesis was opened in this field of astrophysics caused by the Second World War. The invisibility of the black holes and the inability to detect them at all through the instruments of then extended the parenthesis further. But when the number of observations increased and, above all, the quality improved, astrophysicists resumed this field and the sixties were very rich. J. 1967 A. Wheeler first used the name “black hole”. In the same year W. Israel showed that black holes without rotations are absolutely spherical.

The radius of the sphere, that is, the radius to the limit of the events of the black hole, only depends on the mass, so two black holes of the same mass would be exactly the same. By then R. If the turning speed is constant, the dimensions and shape of the black hole depend on their mass and turning speed, according to his 1963 study by Ker on black holes with turning motion. As for its shape, we can say that the black holes that are spinning, like the Sun or Earth, expand in the equator, being the smaller polar diameter. Of course, turning speed values cannot be large indefinite. Just as the spin too fast would undo the star, it would also prevent it from being a black hole. As an example, the spinning speed of a three-mass black hole like the Sun is not estimated to exceed 5,000 turns/s.

So far we have not mentioned at all the electromagnetic characteristics that black holes can have, but certainly they are particularities to consider. In short, stars, and in general stars that can form black holes, have a clear electromagnetic activity. Aware of this, the electric charge was very early in the development of the study of black holes. 1916 H. Reissner and, independently, G in 1918. Nordstrom released the equations of the theory of general relativity for the case of loaded mass.

If we add these results to those of Schwarzschild they give us a description of the loaded black hole. In this case we must also define the roof as to the load that a black hole can have. Otherwise, the repulsion force between loads of the same sign would prevent the black hole. Specifically, the charge is proportional to the mass of the black hole, and it is estimated that when it is ten times greater than the mass of the ^Sun it can be around 1020 C.

However, it is not considered that there may be black holes that are not neutral. Since the electric force is much more violent than the gravitational force, the matter charged against the black hole would attract with much force, just as it would move away from the same load. Therefore, the balance would soon be achieved. However, in 1965 there was also a study of the black holes of the rotating Kerrs.

Summarizing, then, we can say that the space-time geometry of stable black holes, and therefore all properties, only require three descriptive parameters: mass, angular momentum and load. As a result, only four types of black holes are distinguished: no turning or loading, no turning but loaded, rotating without load and with load and rotation movement. Remembering what was said about the charge and considering that all the stars seen in the Universe have rotation movement, we can say that the only natural result of gravity contraction is the third.

Many interesting things can still be said about the behavior of black holes, and we will try it in the next issue. At the moment, to finish, there is only another note. The fact of being able to summarize all the particularities of the hole to the three parameters mentioned, tells us that the black hole has no type of “memory”, that is, analyzing the black hole we can not know the peculiarities of the body or whatever it has produced, except for the mass, the movement of rotation and perhaps the approximate load. About the nature of the creator we can hardly say anything.

EPHEMERIS SUN: March 20, 14 h 40 min (UT) enters Aries. Spring begins. LUNA Growing room Full moon Recent searches New moon (UT) 1 and 3115 h 46 min 4 h 10 min 89 h 46 min 154h 16min 237h 14min PLANETS MERCURY: it will not be easy to see during the month of March. However, in the second half we could try it at dusk. VENUS: Next April will go through the lower conjunction position. So throughout the month of March the height is losing in the sky and at the end of the month we will lose it. MARTITZ: We can see it almost all night. As soon as it darkens the view and at night it will travel through the sky to disappear. JUPITER: On March 30 Jupiter is in opposition. So throughout the month we can see it in good condition throughout the night. SATURN: leaves the conjunction. At dusk it begins to appear every time before. At the end of March we will have it in the sky as soon as we darken.