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The term Black Hole has only recently been coined. It was first used in 1969 by the physicist John Wheeler and described effectively a two-century old idea.
The studies began in 1783, when John Mitchell, one of the great forgotten scientists of the XVIII century published an essay in The Philosophical transactions of the Royal Society of London where he stated that a star with a large mass and density would present such a gravity as to prevent light from getting out. A beam of light emitted from the surface of this star would be drawn back by the star gravitational attraction. Mitchell understood that a great lot of stars with such characteristics could exist. His great intuition was to imagine that the light leaves a star as we consider it a rocket leaving the surface of the planet. To completely escape Earth’s gravitational attraction and travel through space, a rocket needs a 11/Km/sec velocity n upwards, that is to say, more than the terrestrial gravity attracts it downwards. Mitchell knew nothing about rockets on the moon but he did know that, theoretically a largest star could exert a gravitational attraction such as to swallow the light rays that travel at the speed of 300,000 Km/s. John Mitchell calculated that in a celestial a body with a big mass the gravity would be such as to prevent light to escape from its surface, and theorized that an object with the bigger mass than the universe could be invisible. In 1795, the great French mathematician Pierre Simon de Laplace calculated that light could not have got out of quite massive bodies, the dark bodies as he called them. However, it was only in 1939 that scientists found out that Black Holes could really exist, and in the atomic era it finally became known how a black hole is formed. In 1939 J. Robert Oppenheimer and a student of his, Hartland Snyder, showed that a cold, big mass star is bound to collapse indefinitely, thus becoming a Black Hole. Oppenheimer and Snyder’s work, which came out almost contemporarily to Oppenheimer-Volkoff’s about neutron stars, drew the same conclusions: black holes could exist. They could be real objects, not only mathematic games of people sharing an interest in Einstein’s theory. In the Sixties, when Einstein’s theory of general relativity came back in fashion, black holes were thoroughly studied and their features clarified in detail. Furthermore, in the mid-sixties, scientists calculated that there can’t be stable dead stars bigger than three solar masses and as we commonly observe stars (not yet collapsed) which have much bigger masses, astrophysicists have taken into serious consideration the idea that black holes are scattered about in the cosmic space. To completely understand how a black hole is generated, men have had to wait and live the atomic era, when scientists began to comprehend what happens inside a star. A star is composed of three main parts: the visible surface, called photosphere, a gas mass containing most of the star mass, and a small central nucleus. The nucleus has to counterbalance the mass gravitational push and carries out this task exerting a pressure. A star can realize such pressure through the nucleus’contorsion: the gas is compressed, heats up and generates enough pressure to sustain itself. This contraction, however, would provide a star with energy for only 15 million years, whereas we know that the Sun is 4.57 billion years old. Therefore, there must be another source of pressure: this source is the thermonuclear fusion. In a star like the Sun, thermonuclear fusion reaction occurs between two atoms of hydrogen that generate one of helium. When hydrogen is over, a star begins to contract. If, during the contraction temperatures of 108 K are reached, the reaction of fusion occurs between the Helium atoms. As helium fuses, it produces Carbon and Oxygen, Carbon fuses into Neon and Magnesium; Oxygen into Silicium and Sulfur and Neon, Magnesium, Sulfur and the rest fuse into a series of reactions (so far only partly understood) to generate Iron. From iron no other reaction takes place, and so the nucleus starts to contract. If the star is less than 1.5 solar masses big, (one and a half the sun mass) the matter density itself generates enough pressure to sustain the star (degenerating pressure). A white dwarf is born, a super dense star, not bigger than our earth. One of the first dwarfs to be discovered was the one which orbits around Sirius, the brightest star in the sky, a winter sky colossus called Sirius B. This star concentrates a mass close to that of the Sun in a volume nearly equal to the earth’s. It is then extremely dense. One has to imagine that a box of matches full of solar matter would weigh 15 grams, while filled with Sirius B matter would have a weight of 10 tons if it were on the Earth. Instead, if the star features more than 1.5 the solar mass, the degenerating pressure is no more sufficient. The neutrons collapse onto the nucleus and the star becomes a super dense star with a mass equal to the sun enclosed in a sphere with a 20 Km diameter, about the size of New York. There, the matter collapses and becomes so dense that the quantity of matter equal to 1/100th of a pin-head would weigh as much as 24 elephants. A neutron star is born. Yet, if the star features more than 3 solar masses, the collapse is inevitable. The mass of the star gets concentrated in an infinitely small as well as infinitely dense point. Gravity is so high it doesn’t even let the light out; that’s why it looks black: only a black hole is visible in space. However, how a black hole may show up all its power is a matter which Professor William Hawking is closely concerned with. Born exactly 300 years after Galileo Galilei’s death, Hawking has the same professorship as Isaac Newton at Cambridge University. Hawking’s mind moves freely not in Newton’s universe, but in Einsteins’s one. We are used to thinking about gravity – Hawking says – as a force which attracts objects to the earth and the earth to the Sun, but Einstein had the great idea of considering gravity as an effect of the space and time curvature in presence of very big bodies. Einstein understood that nothing can exist in a certain space without existing in a certain time simultaneously. Space and time are linked together to form the flexible frame dimensional structure of the universe: the so-called space-time. Space-time is almost impossible to imagine because our sensory universe is limited to our everyday three-dimension experience.
The best way for us to get into Einstein’s universe is to imagine that space and time are like an elastic plan. If space-time were empty, the plan would have absolutely no reliefs, but big bodies like the earth and the sun bend the elastic surface of space-time producing a curve. This curvature represents Einstein’s concept of gravity. The bigger is the mass of a star or a planet, the deeper is the curvature of space-time around it and consequently the bigger is its gravity. Imagine to launch onto a plan something extremely heavy like a star collapsing on itself and you will find a universe full of holes. While a giant star gets cold as long as it implodes, it bends the space-time around itself more and more. When it reaches a particular critical mass, it will literally create a black hole in the space-time. Objects can precipitate into it but can never get out of it. One of the most brilliant experts of black holes, Phil Charles, looks for them. Phil has found strong signals that show the presence of a black hole in a not far area of our galaxy. As he points out, looking for these objects is an extraordinary way of get ting closer to the borders of modern physics. By day Phil Charles holds lessons of theoretical astrophysics at Oxford university and by night he passes from theory to practice looking for black holes with the biggest telescopes on the Earth: Las Palmas and Hawaii in the north hemisphere, in South Africa, Chile and Australia in the southern one. The searchers of black holes exploit the best instruments to peruse the deep space looking for these mysterious objects: from the x-ray satellites and the orbited telescope Hubble, to the best optical or radio-wave telescopes on earth. Black holes cannot be seen by definition since light can’t get out of them. Official science accepted the idea that black holes could exist only in the 90s. Theory tells us that inside black holes all that man knows about the universe and its laws is no longer worth.
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