delayed-choice quantum eraser · kim et al. 1999 · real, verified, misexplained everywhere

The experiment that looks like time travel — but isn't.

Here is the most notorious "that can't be legal" result in physics. A photon is split into an entangled pair: the signal flies through a double slit to a screen; its idler twin flies off to a detector where you decide — possibly after the signal already hit the screen — whether to record which slit it went through or to erase that knowledge. When you erase, an interference pattern appears. Choosing now seems to reach back in time and change what the photon did. Run it and watch — then watch the catch that dissolves the paradox.

The raw screen (D0) — everything

Every signal photon, unsorted. This is all a person at the screen ever sees.

Sorted by the idler (coincidence)

The same photons, split by which idler detector fired.
Fire some pairs to begin.
Why there's no time travel — the honest resolution. The trick everyone leaves out: the raw screen never changes. Whether you mark or erase, whether you choose early or late, the total pattern at D0 is always a smooth featureless blob — no fringes appear or vanish in real time, so nothing you do to the idler sends any signal back to the screen. The interference is only recoverable after the fact, when you take your screen data and sort it by which idler detector fired. In the erase case the "fringe" detector (D3) and the "antifringe" detector (D4) each pull out a beautiful interference pattern — but they are exactly complementary: cos²(φ) and sin²(φ), which sum to 1. Where D3 has a bright fringe, D4 has a dark one, so added back together they wash into the same flat blob you already saw. The interference was always latent in the correlations; erasing doesn't create it, and sorting doesn't change the past — it just reveals which photons were secretly in step. No information travels backward, relativity is safe, and the "retrocausal" thrill survives only as long as nobody mentions that you needed the idler results — from the future — to draw the fringes in the first place. Spooky, real, and completely law-abiding.