In 1935, Erwin Schrödinger devised an insidious thought experiment. He imagined a box with a cat inside, which could be killed at any moment by a deadly mixture of radiation and poison. Or it might not be killed at all. Both outcomes were equally probable.
But the consequence of thinking through this situation was much more shocking than the initial setup. According to quantum theory, there wasn’t just one cat inside the box, dead or alive. There were actually two cats: one dead, one alive—both locked into a state of so-called superposition, that is, co-present and materially entangled with one another. This peculiar state lasted as long as the box remained closed.
Macrophysical reality is defined by either/or situations. Someone is either dead or alive. But Schrödinger’s thought experiment boldly replaced mutual exclusivity with an impossible coexistence—a so-called state of indeterminacy.
But that’s not all. The experiment becomes even more disorienting when the box is opened and the entanglement (Verschränkung) of the dead and the live cat abruptly ends. At this point, either a dead or a live cat decisively emerges, not because the cat then actually dies or comes to life, but because we look at it. The act of observation breaks the state of indeterminacy. In quantum physics, observation is an active procedure. By taking measure and identifying, it interferes and engages with its object. By looking at the cat, we fix it in one of two possible but mutually exclusive states. We end its existence as an indeterminate interlocking waveform and freeze it as an individual chunk of matter.
To acknowledge the role of the observer in actively shaping reality is one of the main achievements of quantum theory.