THE BIRTH OF A BLACK HOLE: HOW DOES IT FORM? The universe is a strange and mysterious place,
that hides hundreds of secrets and incredible phenomena. Probably, this is the reason why
some people are always scared when talking about space and astronauts. And what can be
more frightening and attractive than a black hole? Hundreds of articles have been published
on these singular objects, which happen to be the most exciting topic to study in astronomy.
Last year, when the first image of a black hole was taken by the Event Horizon Telescope, showing
us the fantastic hole located in the center of the M87 galaxy, the entire scientific community and
even the world population was totally captivated. Imagine them like incredibly large amounts of mass
gathered in small volumes, with such a strong gravitational force that doesn’t leave anything
escape from the object, not even light. As a matter of fact, their name furnishes a good
symbolic explanation: like holes, everything continues falling in them, with no way out.
"Abandon all hope, ye who enter here," reads the inscription Dante placed at the gates of
Hell in his Inferno. Indeed, this would be a good description of this phenomenon,
but don’t be too scared, you won’t die swallowed by a black hole. Instead, have you ever thought
about the origin of these objects? How can they form? Are they created only in a single way or
are there different types of formation? How long does the “birth” of a black hole last? How can we
discover the origins of this phenomenon? These and other questions will be answered in this video,
so, if you are interested, stick with me and I’ll tell you everything in a moment.
We are going to examine one of the most important objects of modern physics, so,
in order to understand the topic better, we’ll proceed step by step starting from the beginning.
Who firstly came out with the idea of the existence of black holes? The idea of a
body so massive that even light could not escape was briefly proposed by astronomical
pioneer and English clergyman John Michell in a letter published in November 1784. However,
when physicists discovered the wavy nature of light in the 19th century, Mitchell’s theory
was abandoned, as it wasn’t so clear how and if gravity could interact with light radiation.
To sum up, let’s directly move to 1915 when maybe the most fascinating theory in physics
was published by a Swiss man. Do you know its name? Albert Einstein. He firstly developed
Special Relativity in 1904, its famous “annus mirabilis” in which he published even another
article about the photoelectric effect which was worth a future Nobel Prize in 1921.
Around ten years later, in 1915 he published General Relativity, which happens to be its
most important work, and one of the most crucial theory in physics. But what do General Relativity
states? How we can intuitively understand such an advanced model? Shortly, Einstein tried to
apply his Special Relativity to gravitational phenomena such as planets orbiting around a
star or stars colliding at high velocities. He came out with an important result: space and
time are “deformed” by the mass of an object. What does it mean? Imagine the space-time
continuum as a big cloth sheet. When a sphere with a mass (the Sun) is put on the centre, the sheet
curves. If another lighter object (the Earth) tries to go on a straight line through the sheet,
it starts to move circularly around the first one as a result of the curvature of the sheet.
How time is deformed by the mass of a star? Time is more dilated and slowed down when it’s
near the object. Moreover, the heavier the body, the slower the flow of time will be.
What’s an important consequence of this model? Light is influenced by gravity, because it must
follow the curvature of spacetime. As an example, if a big black hole is positioned between us and
a bright star, light would deviate according to the deformation created by the mass of the hole.
However, not only Einstein gave a strong contribution in “discovering” the theoretical
existence of these objects. As a matter of fact, just a few months after the publication in 1915,
the physicist Karl Schwarzschild found the solution of General Relativity equations
for a spherical mass and for a point mass. Just some time after Schwarzschild, Johannes Droste,
a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote
more extensively about its properties. This solution had a peculiar behavior at what is
now called the Schwarzschild radius, where it became singular, meaning that some of
the terms in the Einstein equations became infinite. What were the two main strange
characteristics of these “singularities”? Firstly, the force of gravitation would be
so great that light would be unable to escape from it, the rays falling back to
the star like a stone to the earth. Secondly, the mass would produce so much curvature of the
space-time metric that space would close up around the star, leaving us outside.
Therefore, the main discoveries were made in the so-called “golden-age”,
starting around the 60’s when black holes became the most predominant and mainstream subject of
research. In 1958 David Finkelstein gave the famous name of “event horizon” to the surface
delimited by the Schwarzschild radius. What does this definition mean? Roughly speaking, the event
horizon is the boundary within which the black hole's escape velocity is greater than the speed
of light. To be more precise, it is the limit in which all the possible paths taken by light start
to fall in the black hole, with no way out. Does it seem difficult? Don’t worry, here’s a simple
analogy. Within the event horizon, all light is directed in the hole similarly to how we move in
the positive direction of time, it’s an immutable order of the phenomenon that can’t be altered.
And do you want to know a scary thing about these strange objects? When an object falls
into a black hole, any information about the shape of the object or distribution of charge
on it is evenly distributed along the horizon of the black hole, and is lost to outside observers.
What does it mean? Practically, we can consider the object like a dissipative system, similarly
to a membrane with friction or electrical resistance. Whatever disappears in the hole
is totally inaccessible to external observers. This behavior is so unusual that it’s known
as the “black hole information loss paradox”. But let’s move towards the main topic of this
video. What is the origin of black hole? To answer this question, we need to talk
about stars. Stars? Why? Because in Astronomy almost everything is linked
to these breath-taking shiny celestial bodies that enlighten our dark night sky.
Scientists have understood that stars have a different evolution according to their
mass. They are mainly characterized by the nuclear reactions that happen in their core:
nuclear fusion is pivotal for life creation since the reactions generate enough radiation to warm
the planets orbiting around and even balances the gravitational attraction that would make
the celestial body collapse on itself. How is that possible? Nuclear reactions dissipate a
very small part of the mass involved in the process, generating a huge energy according
to the famous Einstein’s equation E=mc2. These enormous quantities of energy are
then emitted through electromagnetic radiation at different scale (visible light, UV ray, gamma
rays or microwaves). These emissions are able to heat up matter, but even to exert a tenuous
pressure on particles. As a matter of fact, the radiation emitted by stars acts like a wind that
balances gravitational contraction. Although the pressure of radiation is irrelevant on earth,
in space it can be observed really well. As a consequence of that, Japanese researchers in
2010 were the first to launch a probe, IKARO, that exploits stellar wind to move in space.
Returning to the evolution of stars, heavier ones usually have a shorter life, because they consume
hydrogen and other important elements (such as helium) for nuclear reactions at a faster rate.
Throughout its life, the star spends different amount of time in various stages, according to
the main element which is being burnt in the core. In the case of massive objects, 8 or more times
heavier than our Sun, the process of nuclear reactions in the core stops with the production
of iron. Why? Because the fusion of its nucleus would require too much energy to happen. What
is the result of this? The internal pressure of the star, which was initially given by the
pressure of radiation, dramatically falls and fails to oppose to gravitational contraction,
generating what we call a “gravitational collapse”. Practically speaking, the star
collapses under its weight as there’s no more pressure balancing the natural contraction.
Now, there’s one question we need to ask: is there an inferior limit for the collapse of the star?
Well yes. And it is linked with the extraordinary theory of Quantum Mechanics. The most important
principle of this model is the Heisenberg uncertainty principle, which states that we
can’t know both the position and the momentum of a particle. More precisely, it states that the
product of the uncertainty in the position and in the angular momentum of an object must be higher
than a constant value. So, if we know well the coordinates of a particle, we’ll have a high error
on its momentum and viceversa. Together with this, it’s important to explain another concept which
is Pauli exclusion principle. Shortly, it states that two different fermions (which are a class of
particles including the electron) cannot be in the same quantum state. What does it mean? It means
that if two particles have the same position, they must have different momentum. What is the
application of these principles to a collapsing star? Well, in some cases the contraction is
stopped by the so-called degeneration pressure. The more densely particles get packed together and
are forced to share the same space, the more they must differ from each other in terms of
their momentum. In turn, the greater the range in momentum, the greater the fraction of particles
with high momentum, and these exert pressure on their surroundings. So this process allows the
condensation of matter into an exotic denser state. The result is one of the various types
of compact star. Which type forms depends on the mass of the remnant of the original star left if
the outer layers have been blown away. The mass of the remnant, the collapsed object that survives
the explosion, can be substantially less than that of the original star. Remnants exceeding 5
solar masses are produced by stars that were over 20 solar masses before the collapse.
For smaller remnants, the collapse gets to the formation of a neutron star, that
is a very small object composed of ultra-dense degenerated neutrons exerting a strong pressure
that balances its gravitational contraction. If the mass of the remnant exceeds about 3–4
solar masses, either because the original star was very heavy or because the remnant collected
additional mass through accretion of matter, even the degeneracy pressure of neutrons is
insufficient to stop the collapse. No known mechanism is powerful enough to stop the implosion
and the object will inevitably collapse under its weight: this is the birth of a black hole.
Do you want to know something extraordinary about this event? Due to time dilation, which is very
significative in these massive regions of space, an external observer won’t never see the end of
this process. What does it mean? Practically, a person observing the collapse would see the
infalling material slow and halt just above the event horizon. Light from the collapsing material
takes longer and longer to reach the observer as time is strongly deformed by the high amount of
mass. Thus, the external observer never sees the formation of the event horizon; instead,
the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away.
In conclusion, black holes are usually formed by the remnants of a collapsing massive star more
than 8 times heavier than our Sun. Therefore, a lot of problems must still be solved,
mostly regarding the possible death of a black body. Many things are going to be
studied to give us a proper perspective of these incredible objects that continuously
“swallow” everything within the event horizon. "This video ends here! Thanks for watching
everyone! Did you find this topic exciting? Are you amazed by the formation of a black hole? Do
you think that is possible that humanity will be able to get closer to one of these objects?