8 Billion People Compressed into a 1.5 cm Sphere

Assuming an average weight of 60 kg per person, the total mass of humanity would be 4.8 * 10^11 kg. He explained that if this immense mass were compressed into super-dense nuclear matter, like a neutron star, its radius could shrink to approximately 1.5 cm.

Professor Ji Woong-bae explained from a quantum mechanical perspective that about 99.999% of an atom is empty. The nucleus and electrons that make up an atom occupy a very small volume, with the space between them appearing empty. However, complex principles are at play where these empty spaces combine to form the solid surfaces we perceive as matter.

He stated that if a hydrogen atom were enlarged to the size of a 10-won coin, that same coin, expanded proportionally, would be comparable to the size of the entire Earth. This analogy vividly illustrates how much empty space exists between the nucleus and electrons inside an atom.

While the total diameter of an atom is on the order of 10^-10m, the nucleus is only about 1/100,000th of the atom's total size, on the order of 10^-15m. He explained this is like a tiny grain of sand, 1 to several millimeters in size, placed in a 100m stadium, emphasizing how extremely small the volume occupied by the nucleus is within the atom.

One of the fundamental principles by which objects remain solid is the 'Pauli exclusion principle.' Professor Ji Woong-bae explained that there is a strict rule that particles like electrons cannot occupy the same quantum state. This is similar to theater seats, where each electron has its 'assigned seat' that no other electron can occupy. These unique states of electrons prevent matter from being compressed further, and the resulting repulsive energy generates the solidity of matter.

Professor Ji Woong-bae emphasized that even if the space around the nucleus and between electrons is described as 'empty space,' it is strictly different from the truly empty vacuum state of outer space. Inside an atom, phenomena like quantum fluctuations occur, and complex quantum mechanical interactions, such as the superposition of wave functions of the nucleus and electrons, take place. Therefore, the space inside an atom is not simply nothingness, but rather a state where particles exist in quantum mechanical superposition.

The main reason atomic structures are difficult to forcibly compress is Heisenberg's Uncertainty Principle. Professor Ji Woong-bae explained that if electrons are forcibly confined to a very narrow range around the nucleus, the subtle laws of quantum mechanics, specifically the uncertainty principle, come into play. This principle states that an particle's position and momentum cannot be simultaneously measured with perfect accuracy; if the precision of one increases, the uncertainty of the other increases. Therefore, confining electrons to a small range drastically increases the uncertainty in their momentum, leading to an enormous increase in their kinetic energy. This makes the atomic system extremely unstable, generating a strong repulsive force that resists further compression.

Neutron stars are born when massive stars undergo gravitational collapse in the final stage of their evolution. Professor Ji Woong-bae explained that during this process, immense gravity causes protons and electrons to combine, transforming into neutrons and neutrinos. That is, a reaction where 'one proton and one electron combine to yield one neutron and one neutrino' occurs, leading to the formation of an extremely dense star composed solely of neutrons. The interior of a neutron star is thus filled with these newly formed neutrons, exhibiting the properties of nuclear matter.

Neutron stars are among the densest objects in the universe. To explain the immense density of neutron star material, Professor Ji Woong-bae used an analogy. He stated that if a piece of neutron star material the size of a sugar cube were taken, this small piece could weigh approximately 1 billion tons. This mass is far heavier than the entire human population compressed, illustrating an astonishing example of how densely packed matter in a neutron star is.

Removing the empty space within atoms and compressing the entire human population yields unimaginable results. Professor Ji Woong-bae explained that if all the empty space within the atoms of each of the 8 billion individuals were removed, the entire human population could fit into something the size of a single grape. This starkly demonstrates the quantum mechanical fact that most of the matter we perceive is actually empty space.

Professor Ji Woong-bae explained that inside a neutron star, the 'strong nuclear force' acts to bind particles tightly together, drawing them as close as possible. Simultaneously, there is also 'electrical repulsion' pushing particles apart. The balance between these two forces allows the neutron star's extreme density and stable structure to be maintained.

Due to extreme pressure and density, the matter inside a neutron star forms very unique structures, referred to as 'nuclear pasta.' Professor Ji Woong-bae explained that various forms appear depending on how particles are arranged. Structures where particles clump together like spherical droplets are compared to Italian dumplings, 'gnocchi'; structures gathered like elongated filament strands are called 'spaghetti'. Additionally, structures stacked in flat plate-like layers are referred to as 'lasagna,' indicating that various pasta dish names have been assigned to these nuclear structures.

Professor Ji Woong-bae explained that the Planck unit system was created to neatly normalize fundamental physical constants used in physics, such as the speed of light, Planck's constant, and the gravitational constant, to 1. In other words, it is a tool for efficiently expressing constants that operate within various physical laws, including quantum mechanics, the law of gravity, and thermodynamic laws. Therefore, he pointed out that the perception of Planck units as representing the inherent smallest size or time unit of the universe is somewhat exaggerated.

What happens if the universe is compressed endlessly? Professor Ji Woong-bae explains that shrinking the universe makes it almost identical to its state immediately after the Big Bang. This works similarly to the principle of adiabatic compression, where volume reduction leads to a temperature increase. In a system where energy is conserved, if the universe's volume decreases, the kinetic energy of particles rapidly increases, causing it to return to an extremely hot and dense state. This aligns with the Big Bang theory, which posits that the early universe was very hot and dense.

When the universe is compressed and its temperature becomes excessively high, the principles of matter binding also change. Professor Ji Woong-bae explained that if the universe becomes too hot, protons and electrons cannot even remain tightly bound by their forces. In an extremely high-temperature environment, atoms and particles that constitute matter disintegrate, becoming a 'soup of particles' where more fundamental particles like quarks scatter. This phenomenon provides an idea of how high-energy the early universe was.

The four fundamental forces governing the universe are gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. Professor Ji Woong-bae explained that there is a hypothesis that in the early universe, these four forces were not separated from each other but were unified into a single force called 'Superforce.' Each force has its unique mediating particles, such as photons and gluons, but it is inferred that in the extreme environment of the early universe, they existed as a single, undifferentiated entity.

Modern physics is actively researching to unify the four fundamental forces of the universe into a single theory. Professor Ji Woong-bae stated that various studies and experimental evidence have so far confirmed how the electromagnetic force and the weak nuclear force were connected as a single force. However, a complete theory that integrates all forces, including the strong nuclear force and gravity, remains an unknown territory and is one of the greatest challenges in physics.

Directly recreating the super-hot, super-dense environment of the early universe on Earth is impossible. Therefore, scientists use particle accelerators to indirectly simulate such conditions. Professor Ji Woong-bae explained that since the early universe was in an extremely high-energy state, immense energy is required on Earth to split and scatter particles. Particle accelerators accelerate particles to high speeds and collide them, creating a high-energy state similar to that immediately after the Big Bang, thereby contributing to the exploration of the universe's fundamental secrets.