Fundamental Interactions

Fundamental Interactions

When we want to ask questions about the fundamental nature of our reality, we can sometimes arrive there by questioning assumptions that seem too trivial to be meaningful.

Let’s start with “Why is a brick wall solid?”

If we recorded someone walking into a wall in slow motion with a scanning tunnelling microscope, this is what we’d see:

  • A person’s nose atoms collide with the atoms in a brick,
  • Nose atoms are less firmly stuck together than brick atoms, so as force is applied their shape gets distorted and pushed aside,
  • Distortion of nose shape ruptures blood vessels and tissue.
  • Pain receptors send signals to the brain. Ouch.

But why do the atoms collide in the first place? Why is matter solid where something else, like light, is not?

The answer comes from something called the electromagnetic force. It works by giving electrons (in the outer layer of an atom) a negative charge that resists the negative charge of other electrons. When two atoms get too close, their electron clouds repel each other like two opposing magnets.

Magnets actually work on the same principle. A magnet is a piece of metal that has been exposed to a powerful magnet during its smelting process. It forces all of the electrons in the metal to spin in the same direction, and their charge compounds and can be felt beyond the atomic level.

When you look at the world this way, it’s like every surface is covered in a thin coat of outward-facing magnets, and that is why we can’t walk through walls.

It is also what gives everything from a brick to your body its structure. If that resistance wasn’t there, then every physical thing in the universe would collapse into a puddle.

So far, we know of only one way that you can walk through a solid wall. The wall must be a few atoms thin, and you must be the size of an atomic nucleus. You’ll be able to stroll back and forth through the wall, through the gaps between atoms.

This electromagnetic force is one of four fundamental interactions of the universe, and together they describe how all particles in the Standard Model of Quantum Mechanics interact with all other particles. In real person talk, they are the four types of glue that hold the universe together.

Here they are:

  • Electromagnetic force creates the charge of particles including electrons, as well as electromagnetic radiation including light.
  • Gravity is the curvature of spacetime around an object that has mass. It keeps our feet on the ground, planets and moons in orbit, and rotates the galaxies.
  • The strong nuclear force holds atoms together by attracting quarks together, and protons and neutrons together.
  • The weak nuclear force is a more esoteric interaction that causes some forms of radiation.

How they work is pretty well understood when you dig down to the level of Quantum Mechanics (All except one. Gravity.)

The forces each come from absolutely tiny, unique types of particles, and they wildly differ in their characteristics. For instance the strong and weak nuclear forces can be felt only within the atom, whereas electromagnetism and gravity seem to have infinite range and can be detected across galaxies.

They also have different relative strengths. While the strong nuclear force is localised to within the atom, it is 1038 times stronger gravity (an incredible 10 with 38 zeroes after it). We don’t really have any analogy tha communicates how huge this difference is, other than perhaps the giant rocket used in the Apollo 11 launch, the Saturn V, is about 1011 times stronger than a housefly.

This incredible strength that holds atoms together is the source of the energy of the atomic bomb.

The relative strengths of the fundamental interactions.
The relative strengths of the fundamental interactions.

What is interesting about this is that if any one of these ratios of strength or distance were different by even the slightest degree, then the universe would be unrecognisable.

If gravity were a fraction of a percent weaker, then interstellar dust would never have coalesced into stars, planets, or galaxies. If it were slightly stronger, the universe would be engulfed in ever expanding black holes. Neither would have resulted in the conditions for chemistry and life.

These ratios are exquisitely positioned like four tightrope walkers balancing on a rope the width of an atom. This improbability had led many scientists to speculate that perhaps there are billions and billions of other universes, each with fundamental interactions with slightly different relative strengths.

Our universe may be one of a few ‘lucky’ universes, where everything worked out to create the conditions that make life possible. If that is the case, then we inhabit a very special place in a expansive multiverse of endless possibilities.