The Experiment
Detection Patterns
No Which-Path Info
✓ Interference!
Which-Path Recorded
✗ No interference
After "Erasing"
✓ Hidden pattern!
The Apparent Paradox
The signal photon hits the detector BEFORE we measure the idler photon. Yet our later choice of measurement seems to retroactively determine whether interference occurred!
Does the future affect the past?
The Resolution: No Retrocausality!
Looking at ALL signal photons together, you see NO interference. The pattern only emerges when you sort the data by the idler photon's measurement outcome.
Two shifted interference patterns are hiding in the noise—but they cancel out when combined. "Erasing" just means measuring in a way that lets us sort and reveal them.
How It Works
- Create entangled photon pairs: A special crystal splits each incoming photon into two entangled photons—a "signal" and an "idler."
- Signal goes to double-slit: The signal photon passes through the double-slit apparatus toward a detection screen.
- Idler carries which-path info: The idler photon is routed so that its path correlates with which slit the signal used. This "marks" the path.
- Interference vanishes: Because the idler "knows" which slit, the signal photon's interference pattern disappears. Entanglement = distinguishability.
- "Erase" the information: Instead of measuring which path the idler took, we measure it in a different basis that mixes both paths. This "erases" the which-path info.
- Sort and reveal: By correlating signal detections with idler measurements, two shifted interference patterns emerge! They were there all along—just hidden.
The Deep Truth
Why It Seems Like Retrocausality
The signal photon is detected before we choose how to measure the idler. If our choice determines whether the signal "went through both slits" (wave) or "one slit" (particle), it seems like we're changing the past!
The Quantum Mechanical Reality
The signal photon doesn't "decide" to be a wave or particle. It's always in a superposition of both paths. Entanglement correlates outcomes, not causes.
When we look at ALL signal photon hits together, we see a blob—no interference. But hidden in that blob are two interference patterns, shifted relative to each other, that perfectly cancel out.
When we "erase" by measuring the idler in a special basis, we get two possible outcomes. If we then sort the signal photon data by which idler outcome occurred, each subset shows interference!
The Critical Point
You cannot see interference without the idler measurement data. The "eraser" doesn't send information backward—it just gives you a key to unlock patterns that were always there in the correlations.
This is why faster-than-light communication is impossible: you can't know which interference pattern you're in until you receive (at light speed or slower) the idler measurement result.