Causality: Why It Can't Be Measured ✎ express
The quantum world plays a strange game with causality. If in everyday life salt is always added to soup before serving, here the ingredients end up in the bowl at the same time, and the chef doesn't remember the order. Physicists asked: can we measure this 'recipe'? The answer is no. Researchers developed clear criteria for measurability: a device must distinguish states unerringly. But causal order, like a shuffled deck of cards, doesn't retain memory of which suit was on top. This loss of information is quantum entropy: chaos erases the past. The most striking thing is that in the lab, they create loops where A causes B while B simultaneously causes A. Such a 'switch' turns conventional logic on its head. In contrast, relativity strictly forbids signals from outrunning the speed of light, preserving causality. Understanding the rules of the quantum looking-glass not only changes philosophy but is also essential for developing quantum computers and unraveling the mysteries of black holes.
In the quantum world, you can create a situation where event A causes event B, and at the same time B causes A. This is called a quantum switch.