8.2 Quantum Physics

If you think that the theory of relativity was ridiculous, you have something worse coming with Quantum Physics. This is one of the most difficult concepts to grasp as it goes against all our intuitions. I will try my best to explain this.

Just like there was a lingering issue about the speed of light, solving which led us to the theory of relativity, there were a couple of lingering issues around smaller particles - Some small particles were behaving like waves and some waves were behaving like particles. Trying to find an explanation for these phenomena led us to Quantum Physics.

I said that in classical physics, if you know the position of all matter at one time, you can predict their state in the future (and even the past). Imagine you see a ball that is flying in empty space. You see it go in front of you at 20kmph. Theoretically, you can predict where exactly it will be after five minutes, unless there is something else that could impact its motion. In practice, there are limitations in knowing its exact position because our ability to measure its position and speed is never 100% precise. But in theory, we know where to find it after five minutes.

Quantum physics puts theoretical limitations to our ability to do this. The classical physics idea of being able to predict the future position of the object works in our day-to-day life because the objects we deal with in our day-to-day life are big compared to the size of particles which quantum physics deals with. When we try to do the same thing with smaller particles like electrons and protons, it becomes hard to predict its future state from its current state. All we can say is its probability of finding it in a particular state and the equations of quantum physics help us do it.

This is roughly how it works:

Any particle’s state is written in the form of what is called a wave function. The wave function tells us the probability of different states the particle could be in, when we observe it. Think of it like a weather map. For something like a coin toss, you can say ‘50% heads, 50% tails’ and they add to 100%. But position isn’t like a coin toss - there are endless places the particle could be. So instead of a simple percentage for each spot, the wave function gives you a kind of ‘likelihood map.’ Where the map is darker, you’re more likely to find the particle there. Add up the whole map, and you get 100% - the particle has to be somewhere.
The particle’s state (wave function) evolves with time according to Schrödinger’s wave equation. Yes, he did more than talking about cats. Maybe after a few seconds, the probability of observing that particle in X is 0.05 while that of observing it in Y is 0.9.
When we observe the particle and try to measure its position, the wave function “collapses” into a precise state. This is the measurement we take. The value we measure depends on the probability that is set by the wave function. So unlike classical physics, we cannot be 100% sure where the particle will be in the future. We could theoretically find the particle in a position that has a very low probability too.
Then the wave function evolves again according to Schrödinger’s wave equation.
There is another complication that fits within the above system that is described by Heisenberg’s uncertainty principle. If you measure the particle’s position accurately, you won’t have precise momentum of the particle and vice versa.

Why don’t we see this magic in our day-to-day life? As objects get bigger and bigger, the wave function tends to have a very high probability for the object to be present in one state.

If the particle’s position is known only when we make measurements, where was the particle when we before we made the measurement? This is still an open debate which is dealt with by the interpretations of quantum physics.

Erwin Schrödinger came up with a famous thought experiment - a cat in a sealed box with a tiny bit of radioactive material. If the atom decays, the cat dies; if it doesn’t, the cat lives. According to one popular way of reading quantum theory, until you open the box and look, the theory doesn’t tell you the cat is alive or dead - it simply refuses to give a definite answer.

Pop-science often phrases this as “the cat is both alive and dead at the same time.” That’s catchy, but it’s not quite what the theory says. Schrödinger himself wasn’t claiming the cat was somehow two things at once, and he wasn’t trying to prove quantum physics was nonsense - he helped build the theory. He invented the cat as a joke, to show what happens when you take that particular reading of the theory too literally and stretch it to everyday objects. A tiny particle refusing to pick a definite state is strange enough; a cat doing it is absurd - and that was exactly his point.

Shocked cat meme

As expected, this turned the fundamentals of physics upside down. There are other magical phenomena like entanglement which come out of it which we won’t discuss here. Quantum Field Theory (QFT) is the culmination of years of work in the field of Quantum Physics, which is considered the best explanation of reality we have today. This theory works on the idea that all that we consider particles (like electrons, protons, neutrons and many more) are fundamentally just fields which pervade throughout the universe. We are not talking about one field per each electron. There is a single field that permeates throughout the universe for electrons. All the electrons we observe are just excitations of this one field in different places. Similarly, there is a field associated with every other particle that covers both matter and forces we observe - Muon field for Muons, Electromagnetic field for photons, Higgs field for Higgs-Boson particles, and so on. The field equations describe how these fields evolve.

You may have heard the Higgs boson called the “God particle” - as if it’s the one thing that gives everything its mass. The real story is more interesting and less dramatic.

In our best model, there’s an invisible field spread across the whole universe - the Higgs field. When tiny elementary particles like quarks and electrons move through it, they get slowed down, and that resistance is what we call their mass. Think of it like walking through water: the water drags on you, and that drag is a bit like mass.

But here’s the twist most pop-science skips: most of your mass doesn’t come from the Higgs at all. The protons and neutrons inside your atoms get almost all their mass from the energy holding their quarks together - not from the Higgs field. The Higgs matters for the fundamental building blocks, but your bathroom scale is mostly measuring binding energy.

And the nickname? It was a publisher’s joke. Physicist Leon Lederman originally wanted to call it the “goddamn particle” because it was so hard to find. His publisher cleaned it up to “God particle,” and the name stuck.

We also built the standard particle model based on this, which lists all the particles which we believe are out there. There are a total of 17 particles which cover all the matter we see around us and also the following forces:

Electromagnetic force
Strong nuclear force
Weak nuclear force

What it hasn’t explained though is the Gravitational Force. For Gravity, we still rely on Einstein’s theory of relativity. This is one of the biggest unsolved problems for physicists - Reconciling Quantum theories with Einstein’s theory of relativity to build a single theory of everything.

8.2 Quantum Physics

Further Reading