The Double Slit — Knowledge Base

The simplest possible setup

A source, a barrier with two narrow slits, and a detection screen. That is all. We will fire three different things at this barrier and watch where they land on the screen.

The double-slit apparatus. A source fires particles (or waves) toward a barrier with two slits. Beyond the barrier, a detection screen records where each one arrives. The answer to "what pattern appears?" is the central mystery of quantum mechanics.

Three cases, three patterns

Bullets (classical particles)
Two piles, one behind each slit. No surprises. Each bullet goes through one slit or the other, and lands in a clump directly behind it. The pattern is the sum of two single-slit patterns.

Water waves
An interference pattern: a series of bright and dark bands. Each wave passes through both slits, splits, and the two outgoing waves overlap. Where crest meets crest, you get a big wave (bright band). Where crest meets trough, they cancel (dark band).

Electrons (quantum objects)
An interference pattern — exactly like the waves — built from individual dots. Each electron arrives at a single point, but the overall distribution of many electrons shows bright and dark bands. The pattern is NOT the sum of two single-slit patterns, and that is everything.

This is not weird. This is what a wave does. The weird thing is that each electron arrives as a single dot (it is localised, like a particle) but the distribution of many dots shows interference (it spreads, like a wave). An electron behaves as if it goes through both slits at once — and arrives at a single point.

Bullets: two piles. Waves: continuous interference fringes. Electrons: individual dots that build an interference pattern — single-particle impacts with a wave-like distribution. The electron pattern is NOT the sum of the two single-slit patterns.

Particles — localised, never interfere
Waves — spread, interfere, build continuous patterns
Quantum objects — localised arrival, wave-like distribution

The heart: which path?

Now the crucial variation. Put a detector at each slit that tells you which slit each electron goes through. Watch the screen.

The interference pattern disappears. You get two piles — exactly what bullets produced. The act of knowing which path the electron took destroys the interference.

Without which-path information, electrons build an interference pattern. With a detector at the slits (determining which path each electron takes), the interference vanishes — replaced by two classical piles.

Interference — both paths available, not known
Two piles — which path known, interference destroyed
Detector — the act of measuring collapses the pattern

This is not about disturbance. It is not that the detector "bumps" the electron. You can design a detector that interacts so gently it cannot possibly knock the electron off course — and you still lose the interference. The information alone, even if never looked at by a human, is enough.

The core insight of quantum mechanics in one sentence: A quantum object can explore multiple paths simultaneously — but only as long as you do not know which path it takes. The moment which-path information exists (even in principle, even if no one reads it), the paths separate and the interference vanishes.

What it means

The double slit contains every concept you need for QM:

Observation	Concept it installs	Why it matters later
Electrons arrive as dots	Particle-like detection	QM objects always interact in localised, countable units
Dots form an interference pattern	Probability amplitude	The pattern is the square of a wave-like amplitude — the core of the QM maths
Two slits open ≠ sum of one slit	Superposition of paths	An electron takes both slits at once — superposition is the basis of QM states
Detector destroys interference	Measurement collapses superposition	Which-path information selects a single outcome — the measurement postulate
Gently detecting still destroys it	Information, not disturbance	The collapse is driven by the existence of recordable information, not mechanical force

Every concept above is present in this one experiment. The rest of QM is working out the mathematical rules for exactly how superposition, amplitude, and measurement behave in more complex systems.

Where does it stop?

Large objects — baseballs, planets, people — do not show double-slit interference. Why? The boundary is not a size. It is information leakage. A baseball can never pass through two slits with unknown path because its environment inevitably records which path it took — air molecules bounce off it, photons scatter from it. The which-path information exists even if no detector is placed. The baseball behaves classically because its quantum coherence has been destroyed by its own environment.

This is called decoherence, and it is why classical physics emerges from QM. An isolated electron can stay coherent long enough to reach the screen. A baseball decoheres in about 10⁻³¹ seconds — it is classical before it has moved a fraction of a proton diameter.

The boundary is not a law — it is a statistics problem. Large objects have too many parts interacting with too many other parts. Information leaks faster than the object can do anything quantum. The classical world is what quantum mechanics looks like when you cannot stop information from leaking.

Grok check

Prediction, not recall.

An electron is fired at a double slit and arrives at the screen as a single dot. Does this mean it went through one slit? How would you test this without destroying the interference?
You set up a detector that records which slit the electron passes but you never read it. Does the interference pattern survive? Why or why not?
A molecule of 100 atoms is fired through a double slit. It shows interference. A grain of sand of 10²⁰ atoms is fired through — it does not. What changed? Is it just the number of atoms?
The double-slit pattern is not the sum of two single-slit patterns. What does this tell you about the mathematical rule that governs quantum probability?

Question 2 is the one people argue about. The answer — information alone, without a conscious observer — is the key to understanding that QM is not about consciousness. It is about what nature allows to stay coherent.

Next: Superposition & the State — the formal engine: how a quantum state encodes possibilities, and what happens when you ask a question.