In 1978, John Archibald Wheeler proposes a thought experiment. A photon is sent toward a Mach-Zehnder interferometer. At the entrance, a beam splitter divides the beam into two possible paths. At the exit, a second beam splitter recombines the paths. If the second beam splitter is in place, the two paths interfere. The photon behaved as a wave, traveling both paths simultaneously. If the second beam splitter is removed, a detector clicks on one path or the other. The photon behaved as a particle, traveling a single path.
Wheeler proposes removing or inserting the second beam splitter after the photon has already passed through the first. The experimenter's choice, made after the photon has entered the interferometer, determines what the photon did before.
Jacques et al. (2007) carry out the experiment at the École Normale Supérieure. A quantum random number generator decides, after the photon has passed through the interferometer, whether the measurement will be wave-like or particle-like. The results confirm the prediction of quantum mechanics. When the second beam splitter is inserted late, interference appears. When it is removed late, interference disappears. The photon conforms to the future choice.
Kim et al. (2000) push further with the delayed-choice quantum eraser. Two entangled photons are produced. One is detected immediately. The other is sent to a device that can, after the detection of the first, erase or preserve the which-path information. When the information is erased, the correlations between the two photons show an interference pattern. When it is preserved, the pattern disappears. The future erasure of information retroactively restores past interference.
Standard quantum mechanics accounts for these results without invoking any signal traveling back in time. The wave function does not collapse in the past. What changes is the basis in which the correlations are analyzed. The interference pattern appears only in the post-selected subset, never in the total raw data. No information travels to the past. No signal can be sent.
And yet. The photon that traverses the interferometer at $t_1$ produces statistics at $t_1$ that depend on a choice made at $t_2 > t_1$. The formalism explains it.
Doctrine
The photon's past is not fixed until the measurement is made. What took place depends on what will take place. The formalism requires no retrocausality to describe it. The result has every appearance of it.
Vecteur ouvert
Quantum mechanics forbids sending signals to the past. It does not forbid the past from depending on the future. The distinction between the two is the finest boundary in physics.
