January 19, 2025
Quantum Gravity Could Reverse Cause and Effect
Obviously, in real life, planets don't move around the galaxy willy-nilly. But the thought experiment could have practical implications for quantum computing, even without working out an entire theory of quantum gravity, Pikovski said.
Obviously, in real life, planets don’t move around the galaxy willy-nilly. But the thought experiment could have practical implications for quantum computing, even without working out an entire theory of quantum gravity, Pikovski said.

You’ve probably heard of Schrödinger’s cat, the unfortunate feline in a box that is simultaneously alive and dead until the box is opened to reveal its actual state. Well, now wrap your mind around Schrödinger’s time, a situation in which one event can simultaneously be the cause and effect of another event. 

Such a scenario may be inevitable in any theory of quantum gravity, a still-murky area of physics that seeks to combine Albert Einstein’s theory of general relativity with the workings of quantum mechanics. In a new paper, scientists create a mashup of the two by imagining starships near an enormous planet whose mass slows time. They conclude that the starships could find themselves in a state where causation is reversed: One event could end up causing another event that happened before it. 

“One can devise this kind of scenario where temporal order or cause and effect are in superposition of being reversed or not reversed,” said study co-author Igor Pikovski, a physicist at the Center for Quantum Science and Engineering at Stevens Institute of Technology in New Jersey. “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 asks a viewer to imagine a box holding a cat and a radioactive particle, which, once decayed, will kill the unfortunate feline. By the principle of quantum superposition, the cat’s survival or death is equally likely until measured — so until the box is opened, the cat is simultaneously alive and dead. In quantum mechanics, superposition means that a particle can exist in multiple states at the same time, just like Schrödinger’s cat. 

The new thought experiment, published Aug. 21 in the journal , combines the principle of quantum superposition with Einstein’s theory of general relativity.

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