Because this passage in an article on how cause and effect might be subject to superposition doesn’t make sense to me:
In 2019, [Caslav Brukner at the University of Vienna, Austria] published a paper that took this idea a step further. He wanted to build a picture of causality that reflected the full complexity of the world, merging the notions of temporal superposition from quantum mechanics with general relativity’s prediction that time seems to pass more slowly in stronger gravitational fields. His thought experiment imagines a scenario in which two spaceships – operated by sworn enemies we shall call Alice and Bob – synchronise clocks before readying their photon cannons to fire. Then, at precisely 1200, each of them fires a photon at the other’s ship. But there is a plot twist: Bob’s spacecraft is docked near a dense planet. According to general relativity, objects such as this with strong gravitational fields would cause nearby clocks to slow. So, time should run slower for Bob, and he would get Alice’s photon before his clock shows 1200.
So far, so classical. But, Brukner asks, what if you could put that massive planet into a quantum superposition state, so that it is close to both Alice and Bob, and affects both of their clocks? In that scenario, the impossible seems to happen: a superposition state is created where Alice’s photon arrives at Bob’s spaceship before he sends his, but Bob’s photon also reaches Alice before she sends hers. [“In the quantum realm, cause doesn’t necessarily come before effect,” Kelly Oakes, NewScientist (18 January 2020, paywall)]
Or would they? Wouldn’t both photons slow down? They’re traveling the same path, albeit in opposite directions.
Down at the quantum level, things are so bizarre – allegedly – that they feel like a hack, a kludge, not something natural.
