Classical → Quantum

Everything a classical physicist assumes about the world, and what quantum mechanics replaces each assumption with — the conceptual rupture that makes QM necessary.

Requires
Nothing — this is the physics ground. Every other quantum page stands on these ideas.
Installs
Determinism → Probability · Definite Properties → Superposition · Measurement as Passive → Measurement as Active

The classical picture

Before quantum mechanics, physics was built on three assumptions so deep that nobody bothered to state them. They were simply what physics was.

Determinism
If you know the position and momentum of every particle now, you can predict every future state exactly. The universe is a clockwork.
Definite Properties
Every object has a position, a momentum, an energy — regardless of whether anyone looks. Properties exist whether measured or not.
Passive Measurement
Looking at something does not change it — or if it does, you can account for the disturbance and recover the true value.

These three assumptions work beautifully for baseballs, planets, and billiard balls. They are why you can aim a rocket at Mars and hit it. They are also all wrong for atoms, electrons, and photons.

Where they break

The trouble began in the late 19th century, when measurements became precise enough to see the cracks. Three experiments, three failures:

Blackbody radiation
The classical prediction said an oven should emit infinite energy at high frequencies. It does not. Planck found that energy comes in discrete packets — quanta — not continuous amounts.
Photoelectric effect
Light below a certain frequency ejects zero electrons, regardless of intensity. Classical waves cannot explain this. Einstein showed light is absorbed as discrete particles (photons), not as continuous waves.
Atomic stability
An orbiting electron should radiate energy and spiral into the nucleus. Atoms do not collapse. Electrons occupy stable orbitals — standing waves, not orbits. (This is already installed by the Chemistry Orbitals page.)
The pattern: Every failure traces back to one root: nature is discrete where classical physics expects continuity. Energy is quantised. Light is quantised. Electron states are quantised. QM is the theory that takes discreteness as its starting point.

The quantum replacement

QM does not deny that classical physics worked so well for so long. It replaces each assumption with a more general one that contains classical behaviour as a special case — one that applies when objects are large.

Determinism → Probability Classical exact path predictable Quantum many possible paths, weighted Definite Properties → Superposition Classical position: x momentum: p both defined Quantum ↑ → ↓ ← many possible values at once until measured Passive Measurement → Active Measurement Classical observer sees what is there Quantum probe probe changes system
Each panel: Classical assumption on the left, quantum replacement on the right. The quantum picture is not a rejection of classical physics — it is a more general framework that explains why classical physics works for baseballs and fails for electrons.

The pattern so far

Three assumptions broken. Three replacements. The structure repeats at every level of QM:

ClassicalQuantumWhy it matters
Objects have positions Objects have wavefunctions A wavefunction spreads out, interferes, and only collapses to a position when measured
Measurement reveals reality Measurement creates reality Asking "which slit?" forces the particle to pick a slit — and destroys the interference pattern
Nature is continuous Nature is discrete Energy levels, spin, charge — all come in fixed-size packets
You can know everything Some pairs of things cannot both be known Position and momentum are not independent — the more you know one, the less you know the other

The right column is not a philosophy. It is a set of predictions, tested billions of times, that always come out the same way. The next page — the double slit — is the one experiment that shows you all of it at once.

The one sentence that captures everything: When no one is looking, a quantum object explores all possibilities at once. When someone looks, it picks one — and the answer depends on how you looked. The rest of QM is working out the rules that govern this behaviour.

Grok check

Prediction, not recall. If the three replacements are solid, you can answer these without looking anything up.

  1. The photoelectric effect showed that light below a certain frequency ejects zero electrons, no matter how intense the beam. Which classical assumption does this violate?
  2. If objects do NOT have definite properties until measured, what does "the electron is here" actually mean before you look?
  3. Why did classical physics work so well for 200 years if its core assumptions are wrong? What determines whether the classical approximation is valid?
  4. A quantum state is described as "exploring all possibilities at once." If the measurement picks exactly one, what determines which one it picks? (This is the hardest question in QM — but you already have the beginning of an answer.)

Question 3 is the one that stops people from dismissing QM as "just philosophy." Classical physics is not wrong — it is a limit. The limit is large objects. The boundaries of that limit are what the next pages explore.

Next: The Double Slit — the one experiment that contains the entire engine of quantum mechanics.