Quantum mechanics describes the strange behavior of photons, electrons and the other particles that make up the universe. Among its many outstanding mysteries, is the quandary of causality—whether events happen in a particular order. Generally, we experience things that seem to be triggered by earlier events. Astrophysicist Brian Koberlein writes, in an article for Forbes, that our lives follow a series of causes and effects—but, could an effect ever trigger a cause? In physics, this thought experiment is known as retrocausality. It is a concept of cause and effect where the effect precedes its cause in time.
In complex systems, order can be observed through entropy—basically the trajectory from ordered to disordered provides a clue about the direction of the event (e.g., a cup falling and shattering into a dozen pieces). However, there are quantum experiments where physicists try to mix up the order of cause and effect. As Koberlein explains it, quantum objects can sometimes behave similar to particles, and sometimes seem to behave like waves. These properties reveal themselves in different kinds of experiments. In the double slit experiment, when a beam of photons shines against a barrier with two slit openings, left unmeasured, the photons take on a wave behavior. However, if a detector is placed by each slit to measure which one each photon goes through, then the photons do behave like particles.
Things become fuzzier still in the delayed choice experiment, where you measure which slit each photon passes through, and measure where each photon strikes the distant screen, but before looking at the results, you destroy the data on which slit each photon passes through. In 1999, the experiment was conducted, and physicists discovered that delayed choice does determine the outcome. “[T]hat would mean destroying the data about the photons going through the slit would give the ‘wave’ interference pattern, even though the ‘particle’ data was collected at the time,” writes Koberlein. In other words, an effect can trigger a cause, and it is possible for the present to cause an outcome in the past.