Delayed-Choice Quantum Eraser
The past wasn't changed. You just chose which slice of the data to look at.
Companion to Quantum Mechanics Without the Math
Quantum Eraser: Postselection Demo
Same data, different subsets → different patterns
Key insight: The data never changes. You're choosing which correlated subset to highlight.
The Setup
Entangled photon pairs. Signal hits the screen first. Idler measured later. Four detectors: D1/D2 erase which-path info, D3/D4 preserve it.
The "Paradox"
Sorting by D1 shows fringes. Sorting by D3 shows blobs. Did the later measurement change what the earlier photon did?
The Resolution
No. The raw data never changes. You're postselecting—choosing which correlated subset to examine. The patterns were always in the joint state.
Notice: D1+D2 together shows no pattern (fringes and anti-fringes cancel). This is why you can't send information backward in time—the unconditional signal data contains no trace of the idler measurement choice.