A real physics experiment where a choice made now seems to affect what already happened
In 1999, physicists Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan Scully ran a real experiment, later published in the peer-reviewed journal Physical Review Letters, testing a proposal first made in 1982. The result has been widely, and mostly incorrectly, summarized online as proof that the future can change the past. The real physics is stranger and more precise than that: a photon's recorded behavior, wave-like or particle-like, appeared to depend on a choice made about its entangled partner after the first photon had already been detected. This page covers what the actual, published experiment shows, and where the popular retrocausality version of the story goes wrong.
// Field Guide
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Ten rules and daily protocols for treating reality as a system you can read, matched against a real, peer-reviewed experiment where measurement itself is the strange part.
Get the guide: $8 instant download →The setup used entangled photon pairs, particles linked so that measuring one instantly correlates with the state of the other, regardless of distance. One photon of each pair traveled to a detector recording position, building up a pattern. Its entangled partner traveled a longer path to a second set of detectors that could either preserve or erase which-path information, effectively deciding after the fact whether the first photon's path could, in principle, still be known. When the researchers later sorted the first detector's data according to what the second detector recorded, they found that the sorted subsets showed wave-like interference or particle-like behavior depending on that later choice.
The popular retelling claims a choice in the present rewrites what a particle did in the past. The real physics, as understood by the field, is narrower and does not involve sending any information backward in time or violating causality. No signal travels faster than light, and no observer can use this effect to send a message to the past; the correlation only becomes visible after both measurements are compared, which requires ordinary, forward-in-time communication between the two detector sites. What the experiment genuinely demonstrates is that quantum entanglement lets a later measurement determine which statistical pattern is present in already-recorded data, a real and profound feature of quantum mechanics, just not the "changing the past" story it gets turned into online.
This establishes that a real, peer-reviewed experiment, published in a leading physics journal and independently replicated in multiple forms since 2000, demonstrated a genuine and counterintuitive feature of quantum entanglement involving measurement choices and correlated particle behavior. This is confirmed by the original 2000 Physical Review Letters publication and its subsequent replications.
What this does not establish is retrocausality, backward-in-time signaling, or any violation of standard causality. Physicists are broadly in agreement that no information travels backward in time in this experiment. The genuinely strange part is real. The popular explanation of why it's strange is usually wrong.
Paper: Kim, Y.H., Yu, R., Kulik, S.P., Shih, Y.H., Scully, M.O. "Delayed 'Choice' Quantum Eraser." Physical Review Letters 84, 1. Published January 3, 2000.
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