“Quantum mechanics is magic”, said the physicist Daniel Greenberger. The theory contains so many peculiar features, each of which appears to be in contradiction with our current understanding of the world.

The Heisenberg uncertainty principle, stating that not all pairs of physical observables can be known simultaneously with arbitrary high precision, is linked to the idea that any observation influences the system being observed. In fact, the more precisely we want to measure a physical quantity, the more we need to disturb the system, in turn affecting other physical quantities.

Quantum tunneling tells us that a particle confined inside potential barriers of finite height have always a chance to escape, no matter how height they are or how low is the energy of the particle.

Contrary to a classical particle, a quantum particle occupies an extended region of space when it’s not observed. This spread is described by a probability distribution, and it’s rooted in the wave-like behavior of quantum objects. The latter is especially manifested by the interference fringes occurring when the object moves through obstacles.

While properties of classical particles (e.g. energy) appear able to assume any possible value with continuity, properties of quantum particle can take only discrete values, in steps called quanta.

When a quantum system is observed, it assumes a definite state rather than a probability distribution of different states. On the other hand, while the system is not observed, its state is free to evolve in time according to the Schödinger equation. Therefore, if an observer keeps looking at the system, its time evolution will never occur as if its time was frozen: a quantum Zeno paradox.

The fact that quantum systems can be in superpositions of different states is one of the most striking features: to put it in Schrödinger’s words, it would be as we were able to see cats simultaneously dead and alive!

Dig deeper into quantum mechanics with these videos:
What is the Heisenberg Uncertainty Principle?
Schrodinger's cat: A thought experiment in quantum mechanics
What can Schrodinger's cat teach us about quantum mechanics?