Quantum Gravity Could Reverse Cause and Effect

Quantum Gravity Could Reverse Cause and Effect

Stephanie Pappas

Thus, if a futuristic spacecraft were near a massive planet, its crew would experience time as a little bit slower than would people in a fellow spacecraft stationed farther away. Now, throw in a little quantum mechanics, and you can imagine a situation in which that planet is superpositioned simultaneously near to and far away from the two spacecraft.

Time gets weird

In this superpositioned scenario of two ships experiencing time on different timelines, cause and effect could get wonky. For example, say the ships are asked to conduct a training mission in which they fire at each other and dodge each other's fire, knowing full well the time the missiles will launch and intercept their positions. If there's no massive planet nearby messing with time's flow, this is a simple exercise. On the other hand, if that massive planet were present and the ship's captain didn't take the slowing of time into account, the crew might dodge too late and be destroyed.

With the planet in superposition, simultaneously near and far, it would be impossible to know whether the ships would dodge too late and destroy each other or whether they would move aside and survive. What's more, cause and effect could be reversed, Pikovski said. 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, Pikovski said. That means that both A and B are simultaneously the cause and effect of each other. Fortunately for the likely-confused crews of these imaginary spacecraft, Pikovski said, they would have a mathematical way to analyze each other's transmissions to confirm that they were in a superpositioned state.

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. By using 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 said.