In everyday life, causes always precede effects. But new experiments suggests that no such restriction applies in the quantum world
15 January 2020
BREAKFAST in my house is a causal affair. The kettle boils because I have switched it on. The toast acquires its golden crust because I put it in the toaster. The butter makes its way to the table because I removed it from the fridge. For all the weirdness that the universe throws at us, these are simple truths that we can take for granted. The past is the past. The present precedes the future. Cause comes before effect. Except when it doesn’t.
Physicists have started to realise that causality might not be as straightforward as we thought. Instead of cause always preceding effect, effects can sometimes precipitate their causes. And, even more mindbogglingly, both can be true at once. In this version of events, you would be opening the fridge because the butter was already on the table, and your toast would be perfectly golden both before and after you put it in the toaster. You wouldn’t just be making breakfast your breakfast would also be making you.
Playing fast and loose with causality does more than make for confusing mornings. It could shake physics to its very foundations. No longer having a definite order of events goes against the picture of the universe painted by general relativity, and even hints at a reality beyond quantum mechanics, the best model we have of the subatomic world. Allowing causality to operate in both directions could allow us to combine these two theories into a single framework of quantum gravity, a goal that has eluded us for over a century. The end of causality as we know it …