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. It could say that the probability of observing it in a position X is 0.9, the probability of observing it in position Y is 0.05 and so on. The sum of all should be 1 as with all probabilistic predictions.

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.

The famous Schrödinger’s cat thought experiment is an attempt to explain this weirdness. In this hypothetical scenario, a cat is placed in a sealed box alongside a mechanism that releases poison if a radioactive atom decays. When the atom decays it is subject to quantum theories and is probabilistic. According to quantum theory’s Copenhagen interpretation, the atom exists in a superposition of both decayed and not decayed until it is observed. So, the cat must simultaneously be both alive and dead until the box is opened. This is because before it is observed, the system’s state is a quantum function that has the probability for both to happen. Erwin Schrödinger created this paradox to show the absurdity of quantum physics.

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.

The Higgs-Boson particle is what they called the God Particle as it is this field that gives mass to the other particles.

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