In part 1 of this mini-series we looked into the meaning of the word quantum. We saw that it relates to the *package-like* behavior of energy transmissions in an atom, and this behavior is one of the most important characteristics of quantum physics.

Another very important feature of quantum physics is the seemingly *random behavior* of particles. In this post we will dive into what that means, and why this strange behavior does not translate into your everyday life. We are going to describe a bunch of radio-active atoms and see why it’s impossible to determine *which* atoms are going to decay, yet it’s possible to say *exactly* how long it takes for half of the bunch to decay. If you have 100 radio-active atoms, how can you know exactly when 50 are left without knowing which are going to decay? We will explore that here.

Let us start the journey by exploring what happens at a concert, before we continue to jump headfirst into the details of quantum physics and how this strange behavior on microscopic level translates into normal behavior in our macroscopic everyday life.

## The Audience At A Concert Behave Like Particles

Picture yourself at an open-air concert. Maybe you have been to outdoor music festivals where thousands of people attend a concert at the canopy stage. In my mind right now, I picture the Orange stage at the Danish Roskilde Festival where I over time have seen big artists like Björk, Green Day, Foo Fighters, Metallica and many more. This was in the before-times.

Anyway, you are at this concert now, surrounded by thousands of people. Some are close to the stage, some are close to the bar, some are in crowded spaces and others are in less dense crowds. But they are all in this confined space that belongs to the stage.

Let’s say there are 100,000 people in the audience. Being in the audience it can look crowded and chaotic, but if you see the concert from above it looks a lot more orderly.

When the concert has started you know the audience area will be full, and when the concert is over you know it will be empty. By observing enough concerts you can even determine exactly how long it takes for the area to be half empty. Calculations will show that precisely 15 minutes after the end of the concert, half of the audience will have gone away.

Despite this very precise calculation that allows you to determine when exactly 50,000 people will be left, it can never let you predict the movement of the individual person.

If you zoom in to any of the people there, e.g. Robert on the 5th row, you have zero chance of knowing when he will leave the concert. He might leave after 5 minutes or he might be the last man standing. This applies to anyone in the crowd.

Radioactive particles behave the same way. You have zero chance of knowing which atom will decay next, but you can very precisely determine when half of the atoms will have decayed. This time-frame is called an atom’s half-life. It’s a measure of the time it takes for half a quantity of atoms to decay, regardless of what this quantity is to begin with. The half-life for 10 million Radium nuclei is the same as the half-life of 100 Radium nuclei (given it is the same isotope).

## Quantum mechanics is unpredictable – yet it provides predictions

The key to understand this is not unlike that of understanding how we humans behave. You can not predict what goes on in one person’s mind, but you can predict how a group of people will behave. This is essentially what traffic is: the predictable behavior of people in groups.

Much the same way, quantum mechanics tells us that it is not possible to determine how one atomic nucleus is going to behave, but it allows us to calculate very precise probabilities of how a group of nuclei will behave. The more you think about this, the weirder it probably seems.

Luckily for us, it means that as long as we use many atoms at the same time, we can be very sure of what will happen. This allows us to use radioactive materials for diagnostics, photography and energy production in an extremely controlled and predictable manner. If the unpredictable nature of quantum mechanics applied to large scales, these applications would be impossible as we would have no control over how long it takes for a certain radioactive material to leave the body or how high an impact it would have.

Quantum mechanics is at its core a description of probabilities more so than a description of behavior. So, if you are going to dive further into the amazing world of quantum mechanics, remember that concepts are more about knowing the probability than knowing the action.