Questions without answer
The advantage of holidays: one does not need to be serious all the time… and this is probably the only time when the really serious questions can come to the surface. I have jotted down here three of them, each one followed by an explanation of the context – I have got a few more questions, and more specific ones, but these will becomes research projects for my students and I am not going to reveal them here.
Question 1: is quantum physics imposing on us by its sheer weight?
The inadequacy of classical physics to describe nature is an established fact. Post factum, even the source of the problem has been clearly identified: the “classical prejudice” consists in believing that a definite value (true or false) can be assigned to any physical property at any time. But physics is not in a state of despair: positively, quantum physics has scored an impressive list of successes and is presently unchallenged. But what is quantum physics?
There is no characterization of quantum physics in terms of a small set of physical principles or phenomena. Relativity arises from the constancy of the speed of light, while the only compact definition of quantum physics is the description of its mathematical recipes. The legitimacy of those recipes is justified a posteriori: the number of correct predictions that are obtained is so large, that one must be very careful before even daring challenging a theory with such achievements. This is very reasonable, but should we just be content with it? I would really like to be able to say what quantum physics is in a few sentences, instead of burying the question under the sheer weight of the number of its achievements.
This question has no answer yet, but the answer may come one day through works like http://arxiv.org/abs/1112.1142. The next two questions are more undefined.
Question 2: how necessary is quantum physics?
Here it gets very speculative – but don’t forget, I was on holidays. It is well known that our universe is extremely well tuned, and this suggests prima facie that it has been tuned by an intelligent being. For several reasons, it seems desirable to have at least an alternative to such a conclusion. The currently fashionable alternative goes along the following lines: our universe would be extraordinary if it were unique; however, if all kind of universes are being “tried” in parallel, there is nothing astonishing in us living in the “right” one – by definition, if the universe is the wrong one, we cannot be there.
At first, this solution is meant to convey the idea of universes similar to ours, but in which the values of some physical constant differ. One may also admit that universes with more (or fewer) dimensions co-exist with ours. However, to be rigorous, one must admit the possibility of universes with laws that are absolutely different from ours: not only in the value of constant or in the number of dimensions, but in the very meaning of what “matter” is. Or maybe not? Maybe some features of our universe, duly extrapolated, are a super-universal necessity? Maybe reality is more constrained than speculative logic? As you see, we are not going to find the ultimate answer to this question – but it is good at times to sit at the verge of nonsense and feel its vertiginous call.
Question 3: emergence, seriously?
When Nobel Prize winners feel the need to contribute seriously to humanity, they write books and give talks about their vision of the world in prophetic terms, hoping that future will vindicate part of their vision. In this exercise, particle physicists then tend to adopt a bottom-up approach: everything is made of elementary particles and, in principle, everything could be explained at that level (though admittedly we are happy not to have to, when it comes to putting a satellite in orbit). Condensed matter physicists like to convey a different view, one in which the mess… sorry, the complexity they deal with every day is presented as irreducible: they like then to say that there is new physics emerging at larger scales than that of elementary particles. Their favorite example is the quantum Hall effect: it cannot arise without disorder in the atomic arrangement, and yet most of its features are described only in terms of elementary constants (think it calmly: each atom around itself sees some disorder, but somehow all the atoms together manage to forget the details of the mess and act in a clean, universal way). Sounds nice… but somehow, emergentists have never managed to look consistent in my eyes.
On the one hand, as noted above, operational emergentism is a necessity: it is a practical impossibility to use many-body quantum physics to describe the motion of a satellite, no human-built computer will ever be able to do that. But if this is the meaning of emergence, it is trivial. The deep question is whether emergence is real. Let us ask it this way: is nature doing physics from bottom-up, performing the computation that we can’t dream of simulating? Or, on the contrary, does it really have layers of complexity? When hard pressed, it seems to me that even the emergentists are scared of what their idea may ultimately mean, namely that order may appear in some cases from nothing below: a very suspicious conclusion, especially for the evolutionist cosmogony of our time…
You want my opinion? Well, when I was a teenager and all my friends started smoking, by that very fact I started finding smoking silly. In the same vein, since the multiverse is so popular, I believe in one single universe, ours; since everyone believes that everything is quantum, I believe that there may a real boundary where the quantum behavior stops; and I am sympathetic with the emergentists. On holidays, one can afford to be wrong.