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Quantum Gravity Could Reverse Cause and Effect

A well-known scientific notion is Schrödinger’s cat, the unlucky feline in a box that is simultaneously alive and dead until the box is opened to reveal its true state. Well, now let’s consider Schrödinger’s time, a situation in which one event can be the cause and effect of another event at the same time. This scenario may be inevitable in any theory of quantum gravity. This field of physics seeks to combine Albert Einstein’s theory of general relativity with the working principles of quantum mechanics. In a recent paper, scientists have created a mashup of the two concepts by imagining starships near an enormous planet whose mass slows time. They hypothesize that the starships could find themselves in a state where causation is reversed i.e. one event could end up causing another event that actually happened before it.

Study co-author Igor Pikovski, a physicist at the Center for Quantum Science and Engineering at Stevens Institute of Technology in New Jersey, held, “One can devise this kind of scenario where temporal order or cause and effect are in superposition of being reversed or not reversed. This is something we expect should take place once we have a full theory of quantum gravity.”

Quantum time

The famous Schrödinger’s cat thought experiment expects a viewer to imagine a box holding a cat and a radioactive particle, which, after decaying, will kill the unfortunate feline. By the principle of quantum superposition, the stuck cat’s survival or death is equally likely until actually measured. So till the box is opened, the cat is simultaneously assumed to be alive and dead. In quantum mechanics, just like Schrödinger’s cat, superposition means that a particle can simultaneously exist in multiple states.

The new thought experiment published on Aug. 21 in the journal Nature Communications, associates the principle of quantum superposition with Einstein’s theory of general relativity. General relativity states that the mass of a giant object can slow downtime. Pikovski said that this concept is well established as true and measurable because an astronaut orbiting Earth will experience time just a pinch faster than his or her twin back on the planet. This is the reason why falling into a black hole would be a very slow experience.

Consequently, if a futuristic spacecraft was positioned near a massive planet, its crew would experience time just a trifle bit slower than that experienced by people in a fellow spacecraft stationed farther away. Now, let’s apply quantum mechanics in this scenario and imagine that the planet is superpositioned simultaneously near to and far away from the two spacecraft. In this superpositioned state of two ships experiencing time on different timelines, cause and effect could get unsteady. For instance, if the ships are asked to conduct a training exercise in which they fire at each other and dodge each other’s fire with full knowledge of the time the missiles will launch and intercept their positions. If there’s no massive planet nearby interfering with time’s flow, this will be a straightforward exercise. But due to the presence of the massive planet, the ship’s captain might not account for the slowing of time, the crew might dodge too late and get destroyed. With the planet in superposition i.e. present simultaneously near and far, it would be virtually impossible to tell whether the ships would dodge too late and destroy each other or whether they would move aside in time and survive.

Pikovski added, “ What’s more, cause and effect could be reversed, Imagine two events, A and B, that are causally related. A and B can influence each other, but in one case A is before B, while in the other case B is before A” in a superposition state. That means that both A and B are simultaneously the cause and effect of each other”.

Fortuitously for the likely-confused crews of these hypothetical spacecraft, Pikovski supposed, they would possibly have a mathematical way to analyze each other’s transmissions to confirm that they were in a superpositioned state.

In real life, obviously, planets don’t move around the galaxy in a haphazard manner. But according to Pikovski, this thought experiment could have practical implications for quantum computing, even without working out an entire theory of quantum gravity. By applying superpositions in computations, a quantum-computing system could simultaneously evaluate a process as a cause and as an effect.

“Quantum computers may be able to use this for more efficient computation,” he commented.

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