marlon said:
I am sorry but i have no idea what this means. "the scale of a human body"...C'mon man...what is that about ?
Well, the first question is: do you think that the principles of quantum theory apply universally, or not ?
I mean:
when you have a system of, say 5 electrons and 5 protons, does quantum theory apply to it or not ?
When you have a system of 200 electrons and 200 nucleae, does quantum theory apply to it or not ?
When 20000 particles ? When 10^6 particles ? When 10^10 particles ?
When 10^25 particles ? When 10^80 particles ?
The "scale of the human body" is simply the number of particles that are concerned in your average human body. Does quantum theory apply to that system or not ?
By the above phrase "does QM apply", I mean: can we construct the Hilbert space of the system with all its considered degrees of freedom.
I would guess that most people would say "yes", right ? Only, most people would think that quantum theory somehow reduces to Newtonian physics in a very good approximation ; just like special relativity does. This is unfortunately not true. Quantum theory does NEVER reduce to classical physics. The only thing that MWI tries to do, is to explain why *an observer* gets *the impression* of a classical world. I know that in elementary QM courses, one goes quickly over the issue with Ehrenfest's theorem. But Ehrenfest's theorem does NOT show that QM reduces to classical physics ; because it ASSUMES already a transition to classical physics by using expectation values.
Of course you can also be of the opinion that quantum theory as we know it today, is a limiting case for "small" systems, and that Newtonian physics is another limiting case for large systems. I do consider that possibility ; but one thing is sure: we haven't gotten a clue then, what that theory is. And it would, in any case, have serious consequences.
No most researchers stick to the "original" interpretation of QM, ie the fact that measurement breaks superposition and all other info is GONE. It did not get entangled or... what ever...
Let's be clear: I also stick to that "view" to do practical calculations of course. But it should be clear that it is a totally undefined procedure which violates locality and which has no grounds in any physical process, every physical process we know off being described by a unitary time evolution operator which cannot implement it. I think most people who have given it any thought realize that there is something highly fishy in this view.
This is also the vision i have on this.
The only thing that bothers me is where do we make the distinction between QM and Newtonian physics ? At what distance scale ?
Well, as you know, the distance scale is at least 50 km if we accept the Vienna experiments. An entangled pair of photons 50 km apart still shows entanglement, so this is a system of 50 km diameter that needs to be described by quantum theory.
But ofcourse, the fact that this border is not "clear" or known, does NOT imply that there is something wrong with the underlying formalism...
It is entirely possible that QM and classical physics are two limiting cases of a yet unknown theory and that collapse occurs "for real". But the implications this would have are quite drastic, in that it would certainly imply non-local actions (with all the consequences for relativity).
EDIT : you know, Vanesch, i have really read some of the links to papers you provided on the measurement problem. We have had some small discussions on this before. But i still really do not understand what all the fuzz is about. Even this Bell-thing is not clear to me. I understand what it is about but i do not get the content of this theorem. Even in college, it seemed very superfluous to me. Perhaps it is me; but i feel like i am missing out on something here.
Yes, I think you're missing something then. If you're not struck by Bell's theorem, I think you're missing something. I would say that it is one of the most shocking results you could ever think of.
I use QM almost every day for my ab initio simulations. I look at QM from a very pragmatic point of view. The QM formalism works because i can calculate physical quantities with it, that are ofcourse correct. So, i am happy...That is all for me.
That's an attitude that many people have. But to me, it is a very strange view. It looks a bit like chemistry when people still thought that you needed a "living force" to work with organic compounds. They considered their chemistry not "universal": certain things obeyed the laws of chemistry as they knew it, and other things, well, were the result of "living force". With the same atoms. But that didn't mean they could not do their anorganic chemistry well in the lab.
To me it sounds strange that you have a certain theory that you apply to well-choosen things, and in the middle of the game, you change the *fundamental* rules: you switch to classical physics. And not in the sense that you can now allow yourself to do so because it is a good approximation, but it is a *totally different* theory with incompatible axioms.
This is not the same as, say, switching from GR to Newtonian gravity, because we know that Newtonian gravity is an approximation to GR for weak fields. So when you do Newtonian gravity you can think of it as a numerical appoximation while you're in fact still doing GR.
Another example is geography: you know that the Earth is round, but when looking at the map of your city, you don't mind using a flat map. That's simply because it is a good approximation. But switching to Newtonian physics (not even relativistic physics because that's impossible) from QM is NOT an approximation. At no point Newtonian physics is a limiting case of QM.
The reason why you cannot use classical relativistic physics when switching from QM to classical, is that you need to project your state *at a certain time* (the time of the measurement). When your quantum system has a certain spatial spread (in other words, if it contains more than one particle), then it is not clear *what reference frame to use* to do the projection in.
Now, the only way to avoid that difficulty is to avoid projection all the way. But you see that if you're going to invent a theory in which there *IS* such a collapse, you're going to have to negociate extremely well with relativity!
To put it differently, let's go back to the end of the 19th century. Imagine the "UV catastrophe": when you apply the well-founded statistical idea of equidistribution of energies to modes of the classical EM field, you get of course the UV-divergent spectrum for black body radiation. Now, some people might consider that a fundamental problem and others might say: well, in the case of the EM field, simply apply the well-working Planck curve for black body radiation, what's the problem ?
The problem is of course that you switch rules halfway the game! Suddenly, statistical physics doesn't apply "in the same way" to the EM field as it does to gas molecules.
