The quintessential illustration of quantum mechanics remains that of Schrödinger's (long-suffering) cat, both dead and alive - simultaneously - inside of a box. According to the example, it is only once we open the box that the cat 'chooses' its state. This picture is meant to emphasize a key feature of quantum mechanics: the superposition principle. According to this principle, objects are not in well-defined states of existence. Rather, multiple states of existence are inhabited at the same time, each with a certain assigned probability. In the example of the cat, there is a 50% chance it is dead and a 50% chance it is alive - and both eventualities are happening contemporaneously...until we open the box and force the cat to decide.
Much-loved as this thought experiment is, it's time to let the cat out of the bag: if we had a real cat, in a real box, whose life depended on the quantum state of a subatomic particle, it would be EITHER alive or dead, not both. The superposition principle as we know it does NOT work at macroscopic scales. This is why you can say for certainty that you are seated in your chair right now instead of distributed across a superposition of positions. So...what's going on here? Why doesn't quantum mechanics work any more for large objects?
The short answer is: we don't really know. It's possible that the superposition principle itself - the sacred pillar of quantum mechanics - breaks down for objects larger than a certain scale. If that's the case - which can only be determined through future cutting edge experimental research (some proposed highly complex and challenging such experiments are shown in the image below) - the long-term prognosis for our feline friend may be grim.
Will the quantum superposition principle stand the test of time? We have emphasized that this question is neither crazy nor heretical. Objective modifications of quantum mechanics can be set up that agree with all observations and experiments so far, while describing a tangible breakdown of quantum theory at the macroscale. Whether quantum mechanics is universally valid is thus not an issue of conviction or metaphysical reasoning, but an empirical question, to be answered only by future experiments.