The atom, which is the building block of matter is mostly empty. Our body is made of atoms, 99.99% of our body is made up of emptiness of atoms. Why then it appears solid? एक अणु या एटम जोकि किसी भी भौतिक वास्तु का आधार है, वह मुख्यतः शून्य स्थान है, एकदम खाली। परन्तु हमें यह शून्यता महसूस नहीं होती, क्या सचमुच एक अणु खाली है, या भरा हुआ है? यदि कोई आपसे पूछे की आपके अपने शरीर में कितना शून्य स्थान है, तो सही जवाब होगा की आपके शरीर में ९९ % से ज्यादा शून्य ही है, फिर वो आपको महसूस क्यों नहीं होता?
The densest city of our world is Manila, the capital of Philipines. There are 41515 people in 1sq km of Manila. Mumbai is the second densest city of the world with 28505 people per sq km. No matter how high the population density in your city is, the universe is mostly empty space. Or is it so? If someone asks what part of your body is empty space, some sane one may answer that around 99.99% of your body is actually empty space. How true is that?
The densest material on earth is the metal osmium. The nucleus of the Osmium atom contains 76 protons and 114 neutrons. The nucleus of any metal, any atom is the densest part of our universe. In fact, the densest matter of our universe is Neutronium which is a hypothetical highly hot plasma of neutrons that may be found at the core of neutron stars. Or maybe there is some more dense matter such as the Quark Matter.
Anyways, the size of an atom is around 1e-10, and the size of the nucleus of most dense heaviest metal with the biggest nucleus is around 1e-15 and this is despite the fact that most of the matter of an atom resides within the nucleus. So yes, most of an atom can be said as empty. And since our whole body, every organelle, every cell, every tissue, every organ is made of atoms, we can say that most of us is empty, IF the atom is empty.
The idea of emptiness of an atom began from the famous Rutherford experiment in which he bombarded a thin gold foil with the radioactive matter. He found that most of the alpha particles travelled freely while crossing the atoms and only a few reflected or deviated. Which meant that most of the atom was free, or Empty. So thus began the idea of emptiness within the solid dense atoms. Of dense elements such as gold. Still, when you push your finger in the soft pillow, it doesn’t seem empty. So why it doesn't feel empty despite the atoms being empty?
For that, the quantum nature of electrons and other fermions is responsible, along with another famous idea of Pauli’s exclusion principle which says that no two fermions with the same spin can have the same spot at any time. It is NOT a law but a principle because there are some particles that do not follow Pauli’s exclusion principles and they are known as Bosons. Also, being too small, electrons follow quantum mechanics. So they do not lose energy consistently, rather they lose or gain energy in Qantas, and hence, they do not fall back in the nucleus to neutralize things despite the strong electromagnetic force between nucleus and electrons. In addition, electrons are continuously moving particles and in fact, it is difficult to predict their position at any given time.
All these forces and facts at the quantum level make it sure that the saying “empty space is never really empty” comes to be true.
If electrons are point-like objects, then it is true that atom is mostly empty. But it doesn’t seem so, rather the continuously moving, dancing electrons in their cloud appear to be as some material field. And this field fills the whole of the atom. So maybe there is no empty space within the atom.
Why then most of the alpha particles passed through the atom without any deviation when Rutherford tried the experiment? Oh, that answer is simple. Light passes through the glass, in general, you cannot see a transparent glass. That doesn't mean the glass is empty. So maybe the electron cloud or electron field doesn't react with the alpha particles so easily and lets them pass through.
Let me remind you, that almost all leptons pass through a nucleus as freely as alpha particles pass through an atom. But that doesn't mean that even nucleus have huge empty space. There are neutrinos that pass through most of the things without having any detectable effect on them. That doesn't mean that everything is empty. Rather, it only means that despite the whole thing is filled up to the brim to us, for neutrinos, almost the whole universe is empty. For Dark Matter or dark energy, it appears to be more empty. But it is not empty. That is, emptiness is a subject of relativity. For light, glass doesn't exist, for alpha particles, electrons aren't enough to restrict them. For leptons, the nucleus is almost empty. But, none is empty. The nucleus is full of matter field made of neutrons and protons, and outer atom is full of matter field made of electrons.
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