One of the basic unsettled questions of quantum physics is why we don't see quantum superposition in everyday objects. At the quantum level – and before being “measured” – mass and energy exist simultaneously as both wave and particle. The classic examples are light and electrons. Photons exist as both wave and particle and manifest as either depending on how it is observed. Similarly, electrons do not exist, in reality, as tiny “planets” circling the nucleus in neat orbits but in clouds of probabilities that may be “found” as a particle in a particular “place” only when measured. Everything that exists at the quantum level – the realm of the very tiny – shares this dual nature as wave and particle. It can be more accurately described as a wave function.
If everything were to remain in quantum superposition in the macro-world we inhabit, Schrödinger's cat – and everyone else's – would be both alive and dead at the same time. We don't see in that way because superposition seems to breakdown when things get large. The wave function has collapsed and we see either waves or particles, i.e., individual, unconnected, single state things. Why?
The easiest answer might be that we don't see quantum superposition at the macro level because when we look at the world, we as conscious observers collapse the wave function. Light, sound, touch, smell, taste all enter our perceptual mechanisms and, interacting with brain and mind, are perceived. The world is there when we observe it because the act of observation collapses the wave functions around us even if nothing else did. But does this mean that if a tree falls in a forest with no one there to hear it, it doesn't make a sound?
One answer might be yes, the unobserved falling tree makes no sound. The basic reality of the universe may be thought of as one all-inclusive wave function in which everything is entangled. The universe is one big cloud of probabilities. Nothing exists per se until observed. But that verges on solipsism. So, science has considered a variety of other mechanisms for decoherence of quantum superposition – collapsing the wave function of anything tiny before it can get very big. It may happen simply because as things get bigger, they get more complex. They interfere with each other, fall out of phase, or vibrate at different frequencies. The latest theory posits that as mass slows down – dilates – time, even the gravity of earth would be enough to pull entangled particles into different time streams.
But at least some aspects of the macro-world do work through quantum effects. The efficiency of photosynthesis arises from quantum mechanical effects. Quantum mechanics may explain how birds use magnetic fields to navigate and our sense of smell. It may be that the cosmos is an entangled universal wave function that decoheres only at the boundary of individual acts of “observation.” But the observers would not simply be conscious human beings but any living thing interacting with its environment? Might the definition of life be that which breaks wave functions?