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The mystery of the delayed choice experiment: How does a photon retroactively decide whether it is a wave or a particle?
The delayed choice experiment in quantum physics was proposed by John Archibald Wheeler to challenge our conventional understanding of the behavior of photons. These thought experiments focus on a key proposition: Photons seem to be able to "choose" whether to behave as waves or particles as they travel from one place to another. How does this phenomenon that cannot be explained by classical physics show the peculiar characteristics of quantum?
Delayed selection experiments show how quantum behavior depends on the collision environment rather than on the properties of the photon itself.
The core intention of these experiments is to close a "loophole" in the traditional double-slit experiment, that is, photons seem to be able to sense the experimental device and adjust its behavior according to the arrangement of the device. If we let the photon make choices during the experiment, will it affect its particle and wave behavior? This question is at the root of these experiments.
Wheeler's delayed choice experiment
There are two main versions of Wheeler's delayed choice experiment: one is the traditional double-slit experiment, and the other is the "cosmic interferometer" experiment. In a double slit, any given photon travels through two paths and is determined before reaching the detector. If you decide whether to build an interferometer while the photon is traveling, the display of the results will also change drastically.
In different combinations of experimental designs, photons end up exhibiting particle or wave properties, depending entirely on when the observer makes the measurement.
In tests of the cosmic interferometer, Wheeler explored what would happen if photons from a distant galaxy passed through a massive galaxy (acting as a gravitational lens). This further challenges the laws of causality, as these photons appear to have made their choices long ago.
Inspiration from the double-slit experiment
In the double-slit experiment, we observe that photons can pass through two slits at the same time and interfere with each other, which means that its behavior depends on whether we make changes to the experimental setup. If we randomly insert interferometers involved in measuring photons before they enter the screen, interference patterns will appear. This phenomenon led researchers to question: Did the photon choose the identity of the particle or the wave?
If the screen is removed, the interference phenomenon disappears, which means that the nature of quantum is an uncertain process.
Current experiments and challenges
With the advancement of science and technology, new techniques allow delayed choice experiments to be performed more accurately in the laboratory. Not only were these experiments scientifically challenging, they also led to a stream of new observations and discoveries that further deepened our understanding of wave-particle duality.
Today, researchers are using sophisticated quantum devices to manipulate the behavior of photons, and these operations may ultimately produce interesting results and even reflect potential consequences in the future given our choices. Will these delayed-choice quantum experiments challenge our ideas of cause and effect in the future?
