Question: What are some of the great questions in physics today?
David Albert: Sure. There's a glass of water on the table beside me. Someone asks, how do I know there's a glass of water on the table beside me? And the answer, the kind of answer that the whole structure of Western scientific knowledge is very deeply committed to, is something like this: there's light in the room, some of the light bounces off the glass, some of the light that bounces off the glass enters my retina; that causes certain electrical excitations in my retina; that in turn causes certain electrical excitations in my optic nerve; that causes various chemical and electrical changes deeper in my brain, and after some finite number of some steps, my brain is in the state that corresponds to having the impression that there's a glass of water sitting on the table. And needless to say, it's crucial to this story that every one of the steps I just described occurs in full accord with whatever the fundamental laws of physics happen to be. Good.
It was noticed about 80 years ago that if one supposes that the fundamental laws of the world are quantum mechanics, if one supposes that the fundamental physical laws of the world are the ones that we get in quantum mechanics textbooks, this story that I just told about how I know there's a glass of water on the table radically falls apart. And it can't fall apart; we can't imagine how else to begin to tell the story of how I know there's a glass of water on the table. So there's a problem; there's some kind of problem at the foundations of quantum mechanics. This problem has come to be called, for reasons that are obvious, given what I've just said, the measurement problem in quantum mechanics, or the observation problem. And there has been a project under way ever since then to imagine how to fiddle around with the fundamental laws of quantum mechanics in such a way as to hang on to all of the very good predictions that the theory makes, but so as to make it possible to tell the story of how I know that there's this glass on the table at the same time.
Now, this is, at the end of the day, a scientific project. The fact that this story collapses indicates that something must be amiss with the fundamental laws of the theory, and what needs to be done in order to correct this is an essentially scientific job of writing down new laws that are going to be compatible with all of the good predictions that quantum mechanics makes and with the possibility of telling the story that I just described. On the other hand, the problems that we run into are peculiarly philosophical sorts of problems. What is going to count as a solution to this problem? What’s the minimum that we need in order to be able to tell a story like this? These are questions, questions of epistemology, questions of the relationship of our beliefs to the external world and so on and so forth that philosophers have been dealing with for a long time.
So philosophers can be, or people with philosophical training or philosophical sensitivity can be, helpful here in trying to frame very precisely what the problem is, what would count as an adequate solution to the problem, so on and so forth. That's a good example, and an example which has been very, very active over the past 25 years or so, of a problem that comes up at what used to be thought of as the boundary between physical investigation and philosophical investigation.