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The secret choice of a photon: can its behavior really be determined only after the experimental setup?
The depths of quantum physics often challenge our basic understanding of reality, as exemplified by John Archibald Wheeler’s delayed-choice experiments in 1978 and 1984. . These thought experiments highlight a core idea of quantum theory, suggesting that photons should not be given any concrete substance during their flight from their entry point to their final moments.
These experiments closed a loophole in the traditional double-slit experiment, showing that quantum behavior depends on the experimental setup.
The concept of delayed choice disrupts our understanding of causality, particularly when it comes to how a photon of light "decides" to travel as a particle or a wave. Early researchers claimed that photons seemed to "sense" the experimental apparatus they passed through and adjust their behavior accordingly.
Wheeler's concept of delayed choice experiment
Wheeler's delayed-choice experiment demonstrated that the propagation model could not consistently explain quantum theory. In this experiment, the photons have two equal paths from the source to the detector. The key to this experiment is that the choice between the two paths is delayed until just before the test.
The goal is to ensure that any particle or wave traveling through a quantum system has crossed two distinct paths before choosing a way to proceed in an experiment.
Or from the perspective of a "cosmic interferometer," Wheeler imagined a quasar billions of light years away bending and interfering light through a massive galaxy. As photons travel in the direction of Earth, they have to "decide" to go around this massive galaxy as particles, or as waves, taking both paths at once.
Universe version of the experiment
When such photons reach Earth, astronomical observations will see the image of two quasars separated by the gravitational lens effect. Some researchers point out that if a photon is emitted as a particle, it should be detectable in only one telescope. The choice of photons seems to have been set hundreds of millions of years ago. However, researchers have begun challenging this setup by routing the output of two telescopes into a beam splitter.
The results of observations of this system were surprising: one output showed strong interference, while the other was almost zero, indicating that the photons experienced self-cancelling quantum effects in the process.
Wheeler took his thinking even further, and began to question whether all this meant that the behavior of photons was traceable and self-selected. In other words, when astronomers place or remove a beam splitter at one moment in time, those photons from millions of years ago could retroactively make different choices, which has captured people's imagination.
Discussion and reflection on the double-slit experiment
The idea of delayed choice can also be found in the classic double-slit experiment. When set up based on the behavior of photons, how would the light pass through the double slits differently depending on the detection equipment? Does this question confuse many scientists? The double-slit experiment reveals the wave-particle duality and prompts people to think: How does a photon become a wave or a particle at the moment of "decision"?
The experiment's predictable results were not affected regardless of when the detection screen was inserted or removed, challenging our fundamental understanding of quantum phenomena.
In this challenge, scientists also began to explore Bohm's interpretation, which states that photons or electrons follow the laws of classical physics and are influenced by quantum potential on this basis. This theory provides a different perspective on the connection between quantum behavior and history, but the view also faces the challenge of being inconsistent with relativity.
Future experiments and thoughts
Researchers moving toward designing delayed-choice experiments based on quantum effects have finally revealed the deep structure of wave-particle duality. These experimental explorations not only expand our understanding of the quantum world, but also prompt a rethinking of the role of observers. As the latest delayed-choice experiments, their boundaries with classical physics are becoming increasingly blurred, and they present novel wave-particle coexistence phenomena.
As experiments progress, the scientific community is approaching a key question: Is the behavior of photons really determined by our choices? Perhaps, on a deeper level, this is all just part of the complex workings of nature?
