At the heart of quantum mechanics lies a paradox. All experimentation has shown that a particle has a dual nature - the so-called quantum duality. It is, at the same time, a particle or 'quanta' of energy, and a wave. It has been shown that a single photon, fired at two slits in a screen, passes through both slits simultaneously and creates an interference pattern with itself on a photosensitive detector behind the slits. And it's not just two slits. The same effect will be seen with three, four, five slits or as many slits as you like.
Furthermore, it has also been shown that, when we try to observe a particle on its journey through the two slits, by placing a detector behind one of them, the interference pattern disappears and the particle behaves like a perfectly respectable single entity and passes dutifully through the slit with the detector and not through the other one.
Observation has (apparently) caused the waveform to collapse into a particle. Seemingly, our observation of it has changed the nature of reality with respect to the particle being observed and it now only has one future.
The conclusion is inescapable, even if a little disturbing. All particles have many futures, possibly infinitely many futures, and they all take all of them.
The experimental data showing the collapse of the wave into a single future by our observation of it has also lead some people to conclude that somehow we create reality by detecting it, even leading some to speculate that there was no reality until mankind was there to observe it.
This conclusion is the 'Copenhagen Interpretation' of quantum duality and has lead to the multiverse hypothesis, where all possible universes, representing all possible futures potentially co-exist but we determine which one by our observation.
Cue, Schrödinger's Cat.
In an attempt to repudiate this view, Erwin Schrödinger devised a thought experiment in which a live cat is placed in a sealed box, with air, food and water, and a phial of cyanide linked to a device which breaks it, so killing the cat, on detecting a particle emitted by a single atom of a radioactive isotope.
In this system, the cat's future is inevitably linked to a single quantum event - a radioactive decay - which is purely random and independent of any other event. The future of the isotope is either decayed or not and the future of the cat is either alive or dead. However, since the emitted particle will exist simultaneously in both possible futures, and the isotope will thus be both decayed and not decayed at the same time, so the cat will be simultaneously both alive and dead.
However, if the Copenhagen Interpretation is correct, this paradox will only be resolved when we open the box and observe the state of the isotope. Only then will the cat's future be determined; until that point, according Copenhagen, the cat will be both dead and alive.
This, of course, is a highly anthropocentric view of reality and assumes that observation is a uniquely human ability. In fact, it's naive. 'Observation' is carried out by detecting the effect(s) of a particle interacting with one or more other particles. Observation is witnessing the effects of quantum entanglement, when the future of one particle becomes entangled with that of another, and this has been happening since the beginning of time regardless of whether humans were present to witness it or not.
Schrödinger had intended his thought experiment to show the illogicality of the Copenhagen Interpretation but it failed to do that. What it lead people to conclude is that we discover which future we are in when we observe reality. When we open the box we discover whether we are in a future in which the cat is alive, or one in which it is dead. The multiverse hypothesis is not scratched by Schrödinger's cat.
The late, great Richard Feynman, working at Caltech, went some way towards resolving this problem. He showed that all possible histories with respect to a single particle can be expressed as a probability distribution expressing the 'sum over histories' and that this distribution is the wave we see when we observe the wave nature of a quantum event.
He has also shown that for complex systems, these waves 'decohere' to produce what may be a single future. This apparently refutes the multiverse hypothesis, but it may not do. It is still possible to view the future histories of small objects like atoms and molecules as having multiple possible futures because we know they, like particles, take all possible paths through spacetime. It could be that decoherence occurs only above a certain level of complexity.
The largest objects which have been shown to pass simultaneously through both slits in a two slit experiment are molecules of buckminsterfullerene (C60) consisting of sixty carbon atoms arranged in a geodesic - the dome-shaped structures designed by the architect Buckminster Fuller. Sixty atoms is large for inorganic molecules but still quite small for organic molecules, and many orders of magnitude smaller than an organism such as a cat, dead or alive. And we know that if we throw a dead cat at a couple of slots in a wall, it won't go through both, don't we? In fact, unless our aim is good, it'll most likely go through neither and we'll see the dead cat bounce.
So what do we make of this? Small objects have many possible futures, yet larger objects have only one - and we don't yet know where the dividing line is...
It could also be that what we see as 'now' is an advancing front of decoherence as we move into an array of futures. That NOW is only the interface between our macro-reality and micro-futures operating at the quantum level.