An Event Requires an Observer.

Imagine we are students rushing between classes and see a big banner on the bulletin board. The design catches our attention, and we stop to read. It promotes a new course the school just opened. The banner says it’s about the study of what the world is doing when we are not looking.

We reread it many times since it seems more like a joke than a class description. But it turns out it’s valid, and the class is about quantum mechanics, a.k.a. quantum physics.

Quantum mechanics might seem like a field where physics meets philosophy. However, it’s a well-observed phenomenon, and its basic principles are extremely well-tested. No mobile phone or computer would work without quantum mechanics. Semiconductors depend on this field of science and are the essence of every chip inside any piece of technology.

The fundamental principle of quantum mechanics states that every measurement of an object must also consider the one doing the measuring, which we’ll call the observer.

Think about traveling to a new destination. We decide we are going to visit Argentina, a beautiful country in South America. We travel to Buenos Aires and spend a couple of days in the capital city. We visit all the historical buildings, watch a tango show, and walk by the river. We then travel back home and meet a friend who was also in Argentina. But she went to Mendoza, a region close to the Andes. She spent most of her time visiting wineries and hiking the largest mountains in the entire continent. Did we experience the same country?

We returned home with a particular description of Argentina, based on the city we chose to visit. The country is the same, but the experiences were different. They are both valid measures of Argentina. If someone were to ask us or our friend which country we visited recently, we’d both answer Argentina. And if we like crossing countries off our list of desired destinations, we’d both cross it off. However, just because we went to Buenos Aires doesn’t mean we know everything about Argentina. Our choice determined what we learned from the country.

Similarly, the result of any measurement in quantum mechanics depends not only on what we are measuring but also on the observer, i.e., the one doing the measuring.

When we visit a new country, the choice of city or cities determines how we measure it. Different visitors will have distinct impressions of Argentina, even though it’s one country. Just as two people can have unique experiences of a place, the outcome of measuring things in quantum physics depends on the observer.