A quantum can hold "both." Why is it that we only ever see one? ── the close of the story of ℏ
So far we have seen the wave function \(\psi\) hold several possibilities in superposition. An electron passes through both slits "at once," and an atom holds several states together. The famous Schrödinger's cat is a thought experiment that pushes this to the extreme ── a cat in a superposition of "alive" and "dead." Yet what we actually see is always one or the other. Either a living cat or a dead one. Why does superposition vanish at every observation and collapse into a single result (the measurement problem)? And why, in the first place, do everyday things look classical from the start? The star of this finale is decoherence ── when a quantum entangles with its surrounding environment, interference (the evidence of superposition) quietly disappears, and it comes to look like a classical "one or the other." Here, we bring the story of \(\hbar\) to a close.
As we saw in Episode 2, quantum states can be added ── \(\psi=a\,\psi_{\text{alive}}+b\,\psi_{\text{dead}}\). This is not "we don't know whether it's alive or dead (a lack of knowledge)" but a state in which both really are superposed. The evidence is interference (Episode 2's \(2|\psi_A||\psi_B|\cos\Delta\varphi\)). It is precisely because an electron holds both paths in superposition in the double slit that interference fringes appear. Superposition is an everyday affair in the microscopic world, confirmed again and again in experiments.
Yet when you detect an electron on the screen, it is found as a particle at "some single point" of the fringes (Episode 2). When you look at the cat, it's either alive or dead, one or the other. The superposition "transforms" into a single result at every observation ── naively this is called the collapse of the wave function. But what, physically, is "collapse"? Why, when, and which one gets selected? ── This is the greatest puzzle of quantum theory (the measurement problem). The following idea sheds modern light on much of it.
For an isolated quantum system, superposition is kept cleanly and interferes. But real objects cannot be perfectly isolated from their surrounding environment (countless air molecules, photons, heat…). When a system entangles with its environment, the phase relationship between the two branches of the superposition leaks out into the environment and gets scrambled. Then ──
The interference term that produced interference in Episode 2 is averaged to zero by the scatter of phase leaked into the environment. What remains is a simple sum of probabilities \(|\psi_A|^2+|\psi_B|^2\) ── this is the classical "one or the other" of "either A or B" itself. The evidence of superposition (interference) disappears, and the system looks like a collection of classical alternatives. This is decoherence.
What matters is that once information about "which one it went through" leaks into the environment, interference vanishes. This is why, if you peek at "which slit it went through" in the double slit, the fringes disappear. Peeking = entangling with the environment (the measuring device). Superposition is not destroyed but diluted into a vast entanglement of system and environment, so that looking at the system alone it appears classical.
Below is the pattern on the screen of a double slit. The slider is the entanglement with the environment (the strength of decoherence). At zero (isolated), crisp interference fringes ── superposition is alive. As you raise it, information about "which path" leaks into the environment, and the fringes gradually fade, ending in a smooth single hump (= the classical distribution obtained by simply summing "either A or B").
Even though the superposition itself is not destroyed, looking at the system alone the fringes vanish and it appears as a classical "one or the other" ── this is the mechanism by which "a quantum looks classical."
Now we can answer "why is everyday life classical." The more macroscopic the object, the more overwhelmingly it entangles with its environment. Even a single speck of dust is struck by a staggering number of air molecules and photons every second ── decoherence is almost instantaneous (for a cat-sized object, there are estimates that superposition vanishes faster than \(10^{-20}\) seconds). So the superposition of a cat or a ball has already fallen into a classical "one or the other" before we can even look, and we never lay eyes on a macroscopic superposition. On top of this, since the \(S/\hbar\) of Episodes 1 and 3 is enormous, the phase was turning furiously to begin with and interference is delicate ── all the more classical. "Quantum or classical" was not a difference of nature but a matter of degree of how fiercely something entangles with its environment (= whether \(\hbar\) effectively matters).
That superposition and interference are real, that entanglement with the environment loses the interference term (coherence) and makes the system look like a classical probabilistic mixture, and that the decoherence of macroscopic systems is extremely fast ── these are established physics, confirmed in experiments too (interference of ever-larger molecules; the main source of error in quantum computers).
But honestly ── decoherence explains "why interference vanishes and it looks like classical alternatives," but it does not by itself explain "why this one particular is realized and the others vanish (the unique result and its probability \(|\psi|^2\))." This is still an unsolved problem where interpretations diverge ── Copenhagen (posits collapse as a rule), many-worlds (there is no collapse, all branches are real, and we are on one branch), and others. This piece states, as established physics, everything up to "decoherence produces the classical appearance," and does not venture into the "interpretation of reality" beyond that. Schrödinger's cat is a thought experiment for sharpening this very question.
A quantum holds several possibilities in superposition, and the evidence is interference (Episode 2). But upon observation only one is seen (the measurement problem). The key is decoherence ── when a system entangles with its environment, the interference term leaks into the environment and is averaged to \(0\), so looking at the system alone it appears as a classical "one or the other." Macroscopic objects entangle overwhelmingly with the environment, so this is almost instantaneous; and since \(S/\hbar\) is enormous, everyday life looks classical from the start. Quantum versus classical is not a difference of nature but a matter of degree. (Though "why a unique result, and the probability \(|\psi|^2\)" is an unsolved problem where interpretations diverge.)
And the backbone of all six episodes of "Quantum That Clicks" is just one thing ── \(S/\hbar\). Particle and wave are the exchange linked by \(\hbar\) (Episode 1); the phase of \(\psi\) is \(S/\hbar\) (Episode 2); the sum of phases over all paths drops the classical limit as \(S/\hbar\to\infty\) (Episode 3); confinement makes the wave discrete (Episode 4); \(\hbar\) spans the smallest area of the position–momentum seesaw (Episode 5); and entanglement with the environment plus an enormous \(S/\hbar\) gives rise to the classical (Episode 6). Classical mechanics is the limit \(\hbar\to0\). This is the same structure as "Relativity That Clicks"'s \(\beta\to0\) and "Fields That Click"'s \(\varepsilon\to0\) ── Newton's world is always the limit of "some dimensionless ratio." Quantities with units are stage props; only the ratio matters. In the story of \(\hbar\) too, we have carried through what you said at the very start: "start from there and it never gets complicated."
Print / make PDF: ⌘+P (Ctrl+P on Windows). On screen, raise "entanglement with the environment" and the interference fringes vanish into a classical single hump. Click "See the answer" to open each solution.