Quantum mechanics is one of the most ambitious and successful theories ever created by science.
It unifies thousands of separate theories under one roof to explain the basic building blocks of everything in the universe, with the exception of gravity, black holes, the big bang, dark energy, and dark matter; each one a major mystery in its own right.
It’s also incredibly accurate. Physicist Richard Feynman has compared the accuracy of its experimental predictions as equivalent to estimating the width of North America to within one hair’s breadth of accuracy.
For such a monumental theory, it seems like hardly anyone understands it. Let’s borrow a bit of its ambition and try to fix that.
This is a basic rundown on how quantum mechanics works.
If you could look at the universe with a special microscope that’s so fine that it could see not just inside of atoms, but inside of the protons and neutrons that make up atoms, the first thing that you would notice is that everything is made up of unusual kinds of waves.
Just like waves made out of water come out of the ocean, these waves come out of several energy fields that underlie the universe. Describing how exactly this happens is the job of quantum field theory.
When we look closely at these waves, we can discover that they can be sorted and categorised, and this arrangement is called the ‘Standard Model’. It’s the core of quantum mechanics, like it’s equivalent of the periodic table.
This is roughly what it looks like, and we’ll be going through the major players and how they interact.
The waves are unusual because when they come in contact with each other, they bounce off each other like particles do, but the rest of the time they seem to act like waves. Because of this, it’s standard practice for scientists to simplify these waves by just calling them particles, which makes understanding some aspects of quantum mechanics a bit easier.
The waves have a few unusual properties like this, but that is to be expected. Our brains evolved to understand our world, where boulders are solid, the sky is blue, and water is wet. But what we find intuitive is not the true nature of reality.
What we intuitively understand is so far away from the tiny world of quantum mechanics that it may as well obey the laws of a different universe. All of the things above that we’re used to are properties that have emerged from the more fundamental behaviour of the universe that comes from quantum mechanics.
Ok. So there are two types of waves that make up our understanding of matter. Those that look like this, which are called Bosons:
And those that look like this, which are called Fermions:
If you’ve read the article ‘What is Quantum Field Theory‘, this shape is their ‘wave function’.
The waves come in different sizes, which are always multiples of a certain fundamental unit of energy called ‘Planck’s constant’. Very loosely, it’s a bit like the quantum mechanics equivalent of the kilogram or litre.
Bosons come in a certain range of sizes that are WHOLE multiples of Planck’s constant, like 1, 2, or 3 litres. Just like milk cartons in the supermarket, you can’t get anything in between.
Fermions come in a certain range of sizes that are HALF multiples of Planck’s constant, like 0.5 or 1.5 litres.
Bosons and fermions only being able to exist in these exact amounts, or ‘quantities’, is the origin of the name ‘quantum mechanics‘.
Because scientists often refer to bosons and fermions as particles, they usually call the size and shape of the waves their ‘spin’ or their ‘angular momentum’. Spin makes a huge difference in how they behave.
Fermions can never be in the same place at the same time, or they bump into each other. Like apples.
Bosons can merge and overlap with each other. Like waves in a pond.
Because fermions are the main ingredient in atoms, atoms also bump into each other. We might not intuitively understand that something like a boulder is made up of fermions but by feeling how solid it is, we do intuitively understand the effects of their spin.
Electromagnetic radiation, which includes light and radio waves, is made of a type of boson called a ‘photon’. Photons are waves in the electromagnetic field, which is one of the energy fields mentioned above. Their ability as bosons to share the same space is why radio stations can broadcast different signals across a city at the same time without them interfering with each other.
Bosons and fermions often interact with each other, and the rest of the standard model is determined by these interactions. This happens because the waves that cause bosons and fermions often interfere with other’s fields, like high voltage power lines interfering with a radio signal.
Phew. Okay. This article is pretty dense. Let’s take a rest here for a moment before continuing.
Alright, let’s go.
The first interaction between bosons and fermions to know is between a boson called the ‘gluon’ and a type of fermion called the ‘quark’. A gluon is a wave in the gluon field, but quarks can stretch and distort this field too, making the two attracted to each other a bit like the north and south poles of a magnet.
There are a few different pairs of quarks. Up and down, charm and strange, and top and bottom. But the only ones that really matter for us are the ‘up’ and ‘down’ quarks. The others can only be made inside of particle accelerators, where they exist for billionths of a second before breaking apart.
Up and down quarks stick together with the gluons which ‘glue’ them together. Two up and one down quarks create the proton. Two down and one up quarks create the neutron.
If you’ve read the article ‘What an Atom Looks Like‘, you might recognise protons and neutrons as components of the atom.
Other configurations like four or more quarks, or with different types of quarks, aren’t stable and they break down in less than a nanosecond.
You might also remember that protons have a positive charge, and neutrons have a negative charge. This happens because quarks also stretch and distort the electromagnetic field, which attracts positive and negative ‘charges’ towards each other.
There’s another type of fermion to know, called a ‘lepton’.
The main difference between leptons and quarks is that leptons don’t distort the gluon field, so they don’t ‘glue’ anything together. They only interact with other particles through electromagnetic charge.
Just like quarks, leptons have three types. They are called electrons and electron neutrinos, muons and muon neutrinos, and taus and tau neutrinos. Again like quarks, only the first pair is stable.
Electrons have a negative charge, and are attracted to the positive charge of the quarks within a proton. This is the second interaction to know. It’s one of the major forces that holds atoms together.
Electron neutrinos have a neutral charge, making them unusual, ghostly particles that interact with almost nothing else, and are detectable only in some forms of radiation that are mediated by the obscure W and Z bosons.
The final particle to know is the Higgs boson, which is a wave in the higgs field. Quarks hit the higgs field like a tennis ball skimming a pool, slowing them down and giving them some mass. The Higgs boson has been a white whale in The Standard Model for decades until it was discovered in 2013.
That was a wild ride but that is about it. The Standard Model does a fantastic job of bringing together subatomic particles and most of the forces under one complicated but cohesive framework.
As yet, it still fails to account for gravity, dark matter, or dark energy. It’s predicted that gravity comes from another type of boson that has not yet been discovered, called the ‘graviton’. Finding this tiny particle is the goal of a theory of everything, while dark matter and dark energy remain a mystery.