Quantum Mechanics is simply a group of theories that offer math models (scientific formulas) explaining how physical phenomena works at microscopic size or atomic and subatomic levels. About a dozen different proposals by various scientists set up different ideas how the clockwork of quantum mechanics actually runs.
THESE THREE ARE AMONG THE MOST POPULAR:
The most widely accepted version of quantum mechanics is from Niels Bohr and Werner Heisenberg: The measurable properties of an atom—a system of submolecular particles—is its quantum state: represented either by a matrix, (like a spreadsheet); or a formula called the wave function—repre¬senting a map of possibilities.
When a submolecular system such as an atom comes into contact with the real world, the phenomena can be ‘measured’ using the Born rule, a recipe for approximating probabilities for a given quantum state. During a measurement, an observer causes an instantaneous change in the nature of the submolecular system and it is this change that is measured.
THE GUIDING FIELD
Many physicists, including Albert Einstein for a while, favored rewriting the way quantum mechanics was understood to include a real physical field of force that controls the motion of a particle.
But this idea becomes difficult to prove and justify because several particles, are involved in measurements of phenomenon. Not all subatomic particles move in our familiar three dimensional space but some behave in an abstract space with more than 3 dimensions. Another point of contention is that the guiding field (a force yet to be explained) has actions from a distance that transmit instant physical effects to other subatomic particles over large distances.
The many-worlds theorem offers a single quantum state of the world. When something measures or checks which of two shells an electron traversed, for example, the quantum state does not collapse onto one shell, but instead the world actually splits into two shells.
The observers in the real world, reside on one ‘reality’ and are unaware of the other. Thus, the universe really branches out like a tree into a vast multiverse in which every possible outcome actually occurs in infinite and distinct, real universes. The theory makes extraordinary demands on our imagination. The failure to quantify the “measurements” that lead to alternate worlds and its difficulty in justifying the Born rule make this theory the most difficult to prove with scientific processes.