Ever wondered what your life might look like if you’d made a different choice at a pivotal moment? Maybe you took one job and turned down another, left a relationship, or moved to a new city. What if, each time you made a decision, other versions of you continued living out the other choice—each one living a completely different life based on the alternate choices? It sounds like something out of science fiction. And at the same time, this sci-fi scenario might more closely reflect reality than you think.

At the most fundamental level, everything can be described by quantum mechanics. These “physical systems”—atoms, light, all matter—begin in a smooth, wave-like way. [1] An atom can be both here and there. Before that atom is “observed,” its state is not just one thing. It’s a coherent blend of all the things it could possibly be at the same time. Note that this is not just a mathematical abstraction. It’s how the universe actually behaves at the scale of the very, very tiny.

But any physical system doesn’t exist in isolation. Every quantum system—atoms, light, and all matter—is surrounded by its environment. In this case, that includes light, air molecules, thermal vibrations, everything in the natural world. These are external factors that are constantly interacting with the quantum systems. When that happens, each interaction leaves a tiny trace of the system’s state in

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