The double-slit experiment and why the observer may be the most important part of physics.
Classification: SIMULATION THEORY | Confidence: EXPERIMENTALLY DEMONSTRATED ANOMALY
Thomas Young fired light through two slits in 1801. Expected two bright lines. Instead: an interference pattern. Light behaved as a wave. But then physicists fired electrons one at a time through the slits. Still got the interference pattern. Each electron went through both slits simultaneously — as a wave. Until you watched. The moment you observed which slit it passed through — the pattern vanished. Solid particles. Two lines.
This isn't philosophy. It's measured. In 1978, John Wheeler proposed the "delayed choice" experiment. Photons emitted before the decision is made — yet the choice of how to measure them (wave or particle) retroactively determines what the photon "was" at emission. The future measurement affects the past event. Causality bends.
Quantum mechanics says particles exist in superposition — every possible state simultaneously — until observed. The act of measurement collapses the wave function from probability into definite reality. Before observation: a spread of possibilities. After: a single outcome.
The Copenhagen interpretation: the observer causes collapse. Many-worlds: every outcome branches. Pilot wave theory: particles always have definite positions, guided by a hidden wave.
But none of these explain WHY observation collapses the wave function. That's the puzzle.
If reality is computational — like a simulation — the observer effect makes perfect sense. The universe only needs to render what is being observed. Unobserved quantum states are computational optimizations: don't calculate what no one is looking at. When you look, the simulation computes the exact result.
This is exactly how video games work. Minecraft only renders chunks around the player. Deep space is low-detail until you turn the camera toward it. The quantum unobserved state is the simulation's LOD (level of detail optimization).
The real crisis: where does the classical world end and quantum begin? Schrödinger's cat is alive AND dead until observed. A grain of sand is not in superposition despite being made of quantum particles. Where is the threshold?
Decoherence theory explains why large objects don't show quantum effects — interaction with the environment collapses superposition. But decoherence doesn't fully explain the definite outcomes we observe. The math predicts probabilities, not actual results.
Physics has known for 200 years that observation affects physical reality at the quantum scale. This is not interpretation. It is experimental fact, reproduced countless times. Why observation collapses wave functions remains the deepest open question in physics.
For the simulation hypothesis: the observer effect is exactly what you'd expect if reality renders on demand.