In almost any imaginable theory an experiment involves the preparation of a system in what we call a “state” and the subsequent observation of various quantities which we call “observables”.

The measurement process has several stages, and a lot of confusion about what is meant by an “observer” in QM arises from not keeping them separate.

Two of these stages are often identified with the notion of “collapse”.

One is where a small part of the world appearing to be in a pure state (which is a linear superposition of the eigenstates of any particular observable) interacts with some part of the external world which is not in a pure state in such a way that the small part ends up appearing to be in a classical statistical mixture of eigenstates. After this interaction is complete the observer still may not know which eigenvalue applies (ie what the observed value of the measurement will be) but the situation is no more (nor less!) mysterious than that of a coin toss which has not yet been observed. For this kind of collapse (which has been understood in principle since von Neumann but for which in the last few decades specific more detailed examples have come to be touted as “decoherence”) the part of the world “causing” the collapse could be anything from surrounding thermal radiation, to an actual measurement instrument, or a cat, or another human observer (cf “Wigner’s friend”) who learns the truth before you do.

But if you are the observer we are interested in, then the system remains in a mixed state until you become aware of the result, and collapse of the classical probability distribution happens only in your mind.

So tl;dr there is no collapse of the wave. What there is are first *decoherence of the wave *(which can be caused by interaction with almost anything that is even slightly complicated), and then later *collapse of the resulting probability distribution* which is where you and your “consciousness” come in.