I think the Double-Slit experiment suggests that - just as there are many possible futures that something can have - there are also many different versions of the past. Things in the quantum world can have any history that you want them to have; or all such histories at once. We are looking at alternate timelines here. Whether they also represent alternate universes - as some believe - or simply vanish into the realms of zero possibility after being observed - is not something that has so far been demonstrated through experimental observation.
What the Double-Slit experiment proves - is that the past can be just as indeterminate as the future.
The "observer" is nothing mystical.
The way that I understand things, the observer can be anything at all that is capable of being affected by the quantum characteristics of an object. In that sense, an "observer" does not require any intelligence at all - and can even be a single molecule of air. Or it can be an electron. Or it can be a quark. It doesn't MATTER what the observer is, as long as it is something that can be affected by the quantum state of SOMETHING ELSE.
The Double-Slit experiment is an example of a MACRO "OBSERVER".
Most "observers" are microscopic, and are not witnessed by any human intelligence at all. That is why the macro world - of people and planets and stars, ad infinitum, does NOT appear to have quantum characteristics. Any indeterminate characteristics it might have had, has already been collapsed by the group behavior of trillions upon trillions upon trillions of atoms and molecules.
Once again, we note that the "observer" can be anything which is affected by a quantum object.
A single molecule of air would certainly be affected by the quantum characteristics of a free electron. The wave function of the electron would then be seen to collapse; at which point, it might be described as a particle - as opposed to a wave.
That same molecule of air might not be affected by a photon, however, since it might be virtually transparent to it. NOR would fiber optic cable be affected by the quantum state of a photon, since this is ALSO transparent. So the photon in either case, could penetrate quite a few molecules without disturbing anything - and losing it's quantum coherency.
The idea of alternate timelines, in my opinion, ALSO explains quantum entanglement.
Two or more quantum entangled objects can be separated by any distance; they can be half-way across the room, half-way across town, or half-way across the galaxy. Distance is irrelevant. As soon as the quantum state of ONE is collapsed, the quantum state of the OTHER is collapsed. There are some people who view this as a faster than light transmission. Others, see the quantum objects as having communicated information into the past. In my opinion, NEITHER of these things are taking place.
In my view, both quantum entangled objects had indeterminate histories. Or to put it another way, they had indeterminate timelines. BOTH histories - or timelines - were equally valid; until an "observer" of some kind collapsed the quantum wave function of at least ONE of these objects. When the history of ONE object was determined, the history of the OTHER object was determined as well - even though they might have been separated by many light years of space.
No faster than light transmission occurred.
Both histories were EQUALLY REAL - and ALWAYS EXISTED - until ONE HISTORY was made determinate.
Of course, even though no faster than light transmission of quantum characteristics occurs, this does not rule out the faster than light transmission of INFORMATION - since selecting the history of an object in one part of the universe - will instantly determine the history of it's quantum entangled partner in another part of the universe. So if you like - you can have faster than light communication - without anything actually moving faster than the speed of light.
And that is quantum physics - mysticism removed.