An interactive explainer, inspired by Adam Becker's What Is Real?
Quantum mechanics predicts experiments perfectly, yet no one agrees on what happens when you look: does observation change reality, split it, or merely reveal it?
Fire particles one at a time at a barrier with two slits. With no one watching the slits, single particles paint an interference pattern, as if each went through both slits as a wave. Turn on a which-path detector and the stripes vanish into two plain bands. This page draws the physics directly on a 2D canvas because the experiment itself is flat: source, slits, screen.
Idle: the screen is blank. Fire a particle to begin.
Reduced motion detected: particles land instantly without flight animation.
The math is settled; its meaning is not. Drag each slider to step through one measurement event, frame by frame.
What it claims: before measurement there is only a wave of possibilities; the act of measuring collapses it to one definite outcome.
What it costs: "measurement" is never defined. Where does the quantum world end and the classical one begin?
What it claims: nothing collapses. Every outcome happens, each in its own branch of a splitting universe. You are one branch of you.
What it costs: an unimaginable number of parallel worlds, and hard puzzles about why outcomes have the probabilities they do.
What it claims: particles are real and always have positions; an invisible guiding wave steers them. Measurement just reveals where the particle already was.
What it costs: the guiding wave acts instantly across any distance, sitting uneasily with relativity's speed limit.
A single qubit sits in superposition: 70% chance of reading 0, 30% chance of reading 1. Its state is drawn as a vector tilted between the poles of a sphere. Press Measure and watch each interpretation tell a different story about the same click.
The vector leans toward 0. Copenhagen: it will snap to a pole when measured. Many-Worlds: both poles will occur, in branches weighted 70/30. Pilot-wave: the answer is already fixed; we just do not know it yet.
Five quick questions. Instant feedback, no scoreboard shame — 0/5 correct so far.
What Is Real? by Adam Becker tells the human story behind everything on this page: how Bohr's Copenhagen view became orthodoxy, how Bohm and Everett were sidelined for asking what the math means, and why the question is roaring back. If this page made you curious, the book will make you obsessed. It earns its spot on any list of books to fall in love with science.