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The fundamental laws of Nature that govern the physics of atoms, elementary particles, light quanta, and everything else that is small and light, have been found to fit in one grandiose framework called Quantum Mechanics. The internal logic of this framework appears to be counter intuitive and mysterious, while extremely coherent and effective. In this general introduction the author explains why he feels the need for expressing his views on how to understand these facts, even if many known scientists have already given extensive reviews of numerous ideas and approaches.

In the most basic deterministic models that one can imagine, a transition from a down-to-earth, deterministic formulation towards a fundamentally quantum mechanical notation, is not mysterious at all, but instead points towards a general picture of what quantum mechanics might really mean.

The world of atoms and molecules is described by exactly the same expressions as the determinisric world described in the previous chapter, yet here, a world consisting of ‘real things’, seems to be missing. As soon as one makes some very basic assumptions about locality and causality, it seems to be possible to prove that such a world cannot exist at all. Have other assumptions been made? Can deterministic theories exist that generate strong correlations in space-like directions? What does mean?

An outline is given of the interpretation of quantum mechanical expressions if one assumes that what is called Quantum Mechanics today, refers to an underlying reality as in Chap. , even if many authors claim to have evidence to the contrary, as is usually brought forward. If there exists a sub-microscopic classical world of real things, is that the same classical world as the world of large things such as planets, rocks and people? How does the notion of enter into quantum mechanics?

Even if we still face problems in formulating concise theories that explain the resolution of various apparent paradoxes, one may simply such a theory to exist, and that it will one day be found. This leads to the CA (Cellular Automaton) Interpretation. We now formulate the rules.

The universe has a wave function, which, in terms of the classical degrees of freedom, collapses continuously, yet fully in agreement with Schrödinger’s equation. We formulate how to look at various notoriously ‘quantum mechanical’ phenomena, such as superposition, exponential decay, and the double slit experiment. What can we say about the ‘quantum computer’?

There are a few things the reader must know about the gravitational force, and the problems theoreticians have in reconciling this force with quantum mechanics.

The fact that is easy to add as a feature in deterministic models, while it seems inevitably to lead to loss of unitarity in quantum mechanics, requires our attention. One could demand that our deterministic models should all preserve information, but it is more interesting to ask what happens if we do start with deterministic models with information loss. Here it is shown that such models do allow for a quantum interpretation as well, but they lead to internal local symmetries, as well as an . We do have local gauge symmetries in elementary particles, and we do have an arrow of time in thermodynamics.

It would have been highly instructive if we could come forward with models that describe a universe a little bit like ours: a deterministic sub-microscopic world whose laws lead to the existence of things such as atoms and molecules that are fundamentally quantum mechanical. This has not, or not yet, been achieved. Among the obstacles found that actually stand in the way for many sophisticated theories of our universe, one stands out: the hierarchy problem.

A number of fundamental questions of theoretical physics are directly related to the question of interpreting quantum mechanics.

The conclusion of Part I, the non-technical (or somewhat less technical) part of this book, is that a completely convincing model of our universe is lacking. This should be a model that shows how quantum mechanical behaviour, with all its mysteries and paradoxes, can emerge from a local, deterministic theory. Yet we also find that one can assume its existence to arrive at a picture of our quantum mechanical universe that makes perfect sense.