For centuries, physics aimed to describe a predictable universe. If you knew the position and speed of an object, you could calculate its future precisely. A thrown ball follows a path; planets move in orderly orbits. Generally these things do what we would intuitively expect. If we throw a ball then we can knock a can off a wall at some distance. Nature appeared to operate like a clockwork machine.
As the 2 slit experiment shows, electrons do not behave like that and a new, and far stranger and unintuitive description was needed. At the smallest scales, particles do not behave like tiny billiard balls with definite properties. Instead, physics describes a world governed by probabilities, uncertainty, and phenomena so strange that even Einstein resisted accepting them.
We find that it is not possible to precisely describe things like the position and speed of an electron in the way we would expect of something larger, such as a billiard ball. What we can do instead is specify the probability of an electron being in a particular region of space and the probability that it has a velocity in a certain range.
Probability Replaces Certainty
In ordinary life, probability usually reflects incomplete knowledge. A coin toss is unpredictable because we cannot measure every detail of the motion. In principle, however, the result is determined.
Quantum mechanics introduced a more radical idea: some events appear fundamentally probabilistic. A radioactive atom may decay now or a thousand years from now, and physics can predict only the likelihood of each outcome, not the exact moment.
This was deeply unsettling to those who liked the idea of a deterministic Universe. Nature itself no longer seemed fully determined.
Observation Changes the Situation
The famous double-slit experiment revealed behaviour unlike anything in ordinary experience. Electrons fired one at a time create an interference pattern associated with waves, as though each electron somehow travels through multiple paths simultaneously.
Yet when scientists attempt to observe which path the electron takes, the interference disappears. The act of measurement changes the result.
In classical physics, observation simply reveals what is already there. In quantum mechanics, observation appears inseparable from the behaviour being observed.
Entanglement
Even stranger is the phenomenon of entanglement. When two particles interact, quantum mechanics can link their properties together so completely that they behave as parts of a single system, even when separated by enormous distances.
Measuring one particle instantly determines the corresponding property of the other. The connection remains regardless of the distance between them.
Einstein famously dismissed this as “spooky action at a distance,” because it seemed to violate the ordinary idea that objects are influenced only by their immediate surroundings.
Experiments repeatedly confirmed that these correlations are real and cannot be explained by hidden pre-existing instructions carried by the particles.
Quantum mechanics is the most successful scientific theory ever created in terms of predictive accuracy. Modern electronics, lasers, and quantum computing all depend on it.
Yet despite its success, the theory resists ordinary intuition. It suggests a universe in which certainty gives way to probability, observation cannot be cleanly separated from reality, and distant particles remain mysteriously connected.
Quantum mechanics works extraordinarily well. Understanding what it ultimately says about reality remains another question entirely.