The momentum of an object is calculated by multiplying its mass with its speed. A driving truck that weighs two tonnes has a higher momentum than a car of half a ton driving at the same speed. It will take more time or energy for the truck to reach that speed, but also more time or energy to come for it to a complete stop. It is perfectly possible to calculate where the truck or the car will be, given a certain momentum and time.

However, this rule does not hold for anything smaller than an atom, as Heisenberg showed in his 1927 paper. Heisenberg explained that the more accurately you know a particle’s momentum, the more difficult it becomes to know where it actually is, and vice versa. He coined this the uncertainty principle, and it was based on Schrödinger’s idea that the electron acts as a wave instead of a particle.

Before the electron is observed, it is a wave. But Heisenberg’s uncertainty principle states that once the wave function interacts with its measuring system (e.g. light), it collapses and becomes a particle with a definite place. This then makes it impossible to know the particle’s momentum.

This only holds true for subatomic particles or for objects moving near the speed of light. But let’s imagine it didn’t. Enter Schrödinger’s Cat, a well-known thought experiment published by Schrödinger in 1935. Here’s how it goes: A cat sits in a closed box with a bottle of poison. If the cat touches the bottle, the bottle breaks and releases its poison. The cat dies instantly. If the cat doesn’t touch the bottle, it survives.

In this scenario, the cat has two possible states of being: it is either dead or alive.

Here’s where it gets weird. According to quantum theory, because the box is closed and no one knows what’s happening inside, the cat is in both states. It is both dead and alive. It is only when you open the box and observe the cat that a certain state would be chosen and the cat would be either dead or alive. In this sense, the observer decides about the life of the cat.

It might sound ridiculous, but this is exactly how things work on a subatomic level. Particles exist in all their possible states until they are observed and decide on one particular state. Humans are uncomfortable with this ambiguity, as evolution has taught us to perceive life as either/or. Something either is or isn’t. But on a subatomic level, things can both be and not be.