A 2016 experiment by Austrian scientists, led by Julia Rubino, investigated cause-and-effect relationships in quantum mechanics. It revealed that in quantum theory, the order of events cannot be definitively defined initially. In essence, the experiment presents a scenario where Alice sends a gift to Bob, or vice versa, but in reality, they simultaneously exchange the same gift. This perplexing phenomenon challenges our understanding of causality in quantum mechanics.

The Experiment
The experiment involves sending a photon through two sets of optical devices labeled “Alice” and “Bob.” These devices manipulate the quantum state of the photon differently. Depending on whether the photon passes through Alice before Bob or vice versa, the outcome varies. To simplify, think of the photon as a gift. Alice prefers gift-wrapping, while Bob opts for ribbons. If Alice handles the gift first, she wraps it and then passes it to Bob for ribbon tying. If Bob goes first, Alice’s wrapping conceals Bob’s ribbon, resulting in a different outcome.
Quantum Uncertainty
Things become more intricate with the photon, as “Alice” and “Bob” as optical devices can perform various actions with probabilities, offering more than two possible outcomes.

Who Goes First?
In the experiment, a quantum switch controls the photon’s path, determining the sequence of actions by Alice and Bob. To blur cause-and-effect relationships, scientists place the switch in superposition, effectively implying that both Alice and Bob act first. This doesn’t precisely reflect reality, but it underscores the enigmatic nature of the quantum world beneath our familiar reality.
Uncertainty Principle:
At scales as minuscule as the Planck length (10^-33 cm), vacuum fluctuations cause space to behave like “quantum foam.” While space seems smooth at larger scales (10^-12 cm), it exhibits roughness at 10^-30 cm, embodying the uncertainty principle in quantum mechanics.
Conclusion
Mateusz Araujo, a team member, suggests that “time itself can be uncertain in these situations.” The perplexities in quantum mechanics arise from our ignorance, as they don’t align with our macroscopic classical experiences.
Mati Hoban from the University of Oxford notes that this experiment pushes the boundaries of quantum mysteries and entanglements to their limits. It hints at potential applications in quantum communications and computations, possibly reducing resource requirements. However, its practical utility remains uncertain, as computational problems often diverge from real-world applications.
Source: This article is adapted from http://nauka.offnews.bg/news/Novini_1/Nov-kvantov-fokus-Redat-na-prichina-i-sledstvie-e-neopredelen_55146.html. The original research was published on the arXiv.org portal of Cornell University Library under the title “Experimental Verification of an Indefinite Causal Order” and can be found at https://arxiv.org/abs/1608.01683.