IDEAS THAT CHANGE THE WORLD
When we lean on a wooden table, press our feet against the floor or stub our little toe on the corner of the bed, our senses give us one certainty: matter is solid, compact and full.
Yet modern physics shows us that this certainty is an illusion of our minds. At the atomic level, everything we touch, including our own bodies, consists almost entirely of nothing.
If that is the case, why can we not simply walk through a wall or pass our hand through a table? The answer lies in the invisible forces governing the quantum world. Let us explore what matter is made of.
The composition of matter: how big is an atom?
To understand how “empty” matter is, we need to change scale and examine the structure of the atom. In the simplified model we learn at school, an atom consists of a central nucleus (protons and neutrons) surrounded by an orbiting cloud of electrons.
To visualise the actual proportions, let us use a thought experiment. Imagine an atom as a football stadium with 80,000 seats. The nucleus would be a fly sitting in the middle of the pitch, and the electrons tiny specks of dust whizzing around the stands. And all the rest of the space? Completely empty.
This means that more than 99.9999999% of the volume of the matter around us is empty. If we could remove all the empty space in the atoms making up the entire human race, all 8 billion people on Earth would fit comfortably into the volume of a sugar cube.
If it is all empty, why does matter feel solid?
If atoms are empty shells, what stops the atoms in our hand from slipping between the empty atoms of a brick wall?
The first reason is electromagnetic. Electrons carry a negative electric charge. When you bring your hand close to a wall, the electron cloud of your atoms approaches that of the wall’s atoms. Since like charges repel each other with a force inversely proportional to the square of the distance, this generates enormous electrostatic repulsion.
In fact, you have never touched anything in your life. When you think you are “touching” an object, you are simply sensing the repulsive force between your electrons and those of the object. You are literally floating on a cushion of electromagnetic force at an infinitesimal distance.
Pauli’s “exclusion principle”
But electric repulsion is only half the story. The real reason you cannot pass through walls is a fundamental principle of quantum physics discovered by physicist Wolfgang Pauli: the “exclusion principle”.
The fundamental building blocks of matter (such as electrons) belong to a family of particles called fermions. One of the universe’s inviolable laws states that two identical fermions cannot occupy the same quantum state at the same time in the same place. When you push your hand against a wall, you are trying to force the electrons in your body to occupy the same energy spaces already occupied by the wall’s electrons.
Quantum mechanics offers insurmountable resistance to this attempt. The “solidity” we perceive is not contact between two rigid surfaces, but the consequence of a mathematical law of the universe that prohibits particles from overlapping. Without the exclusion principle, gravity would cause all the atoms in your body to collapse into an infinitely dense point, and you would sink to the centre of the Earth.
