Wave-particle duality and quantum measurement interactive
Looking at an electron changes it
The double-slit experiment reveals that measurement isn't passive observation—it's an interaction that fundamentally alters quantum behaviour. When you detect which slit an electron passes through, the wave interference pattern vanishes and particles appear. Turn off the detector and the pattern returns, showing each electron somehow passes through both slits. Drag the slider to control detection strength.
Double-slit detector
Wave behaviour intact
Detection strength determines which quantum behaviour emerges
Both slits — interference visible
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When no detector is active, each electron exists in a superposition state—it genuinely passes through both slits simultaneously as a probability wave. The two paths interfere with each other, creating the characteristic bands of light and dark on the screen. This isn't about ignorance of which slit it "really" went through; the electron doesn't have a definite path until measurement forces it to choose one. Quantum mechanics predicts that even if you send electrons one at a time, hours apart, the pattern still builds up—each individual electron interferes with itself.
Know This
The act of measurement fundamentally changes quantum behaviour—not because of clumsy equipment, but because observing particle properties like position destroys wave properties like interference.