I wanted to add the tag philosophy of science firstĺy but that's a tag not suited I guess, even if it is about a philosophical problem in physics. Though I' m not sure even about that.

I think it's more or less a question about the interpretation of quantum mechanics. Yes, again.

At first sight the question is simple. The answer to it, that is. Something can happen without me knowing. But in principle I could have been witness of the happening. So the answer would be "They can't".

But if something happens in quantum mechanics, then in principle, I could not have witnessed it. The very act of witnessing would had changed the happening. But why can a happening then not go on without me having knowledge of the happening? Can't I just make up what happens objectibely? Say that I know the wavefunction completely. I can find out how it develops in time. The usual view is that the wavefunction gives mere probabilities of all kinds of happenings when the wavefunction develops freely. And only upon an interaction, a measurement, the happening becomes realized. The many happenings happening simultaneously are collapsed to one of them. Which one is purely arbitrarily. Pure chance or random. Each happening has a weight to occur.

Now why can't it be that there actually was one happening all the time?. Hidden variables were exactly made up for this to occur. Off course (?) the hidden variables cannot be known. They are hidden. Which means that we cannot know which actual path was taken. We measure a single path but this was not the actual path. It is just the final piece of a random but determined total path. So the only advantage the hidden variables have is to satisfy our longing for a determined path.

But why can't we place measurement devices along the path to measure the path at every piece of it? Is this impossible in principle because every time we make a measureme t the multitude of paths collapses? Would the path be a different one as the unknown path when not measuring it (only at the end one time)? What about an electron track in a bubbleb chamber?