The term "physicalism" may be interpreted in at least two different ways. First, it may be taken to mean the claim that the stuff that the laws of physics describe is all there is in the universe. There is no mysterious other stuff, no magic spray applied to reality above and beyond the photons and electrons, etc., all of which behave strictly in accordance with physical laws. This sounds like a simple enough claim, at least to the extent that one ought to be able to say whether or not one agrees with it, but (bear with me) I can not say for sure whether or not I agree with it.
I am not a physicalist, as the term is usually understood, because of the intractability of the Hard Problem within the bounds of physicalist explanations. People who take the Hard Problem seriously are sometimes called dualists (usually pejoratively) because of the presumption that a rejection of physicalism implies a belief that there are two fundamental kinds of stuff in the universe: mind and matter.
Indeed, dualism suffers from some serious problems. One of the most critical is that of the interaction of the two kinds of stuff, mind and matter. If they are fundamentally different, how can they influence each other? How can our minds be aware of our physical bodies and senses, and how can our minds control our bodies? Descartes (Western philosophy's quintessential dualist) thought that the pineal gland formed some kind of special junction that allowed this to happen, a hand-wave on his part, really. If, however, consciousness is built into physical reality in a way analogous to the way in which mass (for example) is, then there is still only one kind of stuff in the universe, and we can preserve a monist view of reality.
The second interpretation of the term "physicalism" is the somewhat stronger claim that not only is the stuff that physics describes all there is, but that the laws of physics are a complete description of that stuff (or will be, as soon as we complete our laws of physics). I would argue that this second, stronger type of physicalism is definitely false, whether or not you buy any of the Hard-Problem-of-consciousness arguments.
A good physicist (which is to say a philosophically humble physicist) will tell you that physics provides a way of predicting the outcomes of certain experiments, and that is all. Strictly speaking, the famous Copenhagen interpretation of quantum mechanics applies across the board - "shut up and calculate." Physics is not metaphysics - it does not pretend to describe the ultimate nature of reality. As a matter of fact, it can not, even in principle, describe reality "all the way down".
Each hard science rests, in a sense, on the science below it (biology rests on chemistry, chemistry rests on physics). This is to say that, for example, once all the facts about the physics of the universe are fixed (all the physical laws and all the positions and momenta of all physical particles), it is automatically true that the chemistry of the universe must be the way it is, and it could not be any other way. The physical laws and facts necessarily entail all the chemical laws and facts. Another way of saying this is that the facts about the chemistry of the universe are a logical consequence of the facts about the physics of the universe. Once the physics is locked in place, so is the chemistry. There is simply no way you could have two universes that were physically identical, but chemically different. In the same way, the chemical facts, in turn, logically entail the biological facts, and so on up through the layers of science. As far as the hard sciences are concerned, once God invented physics in all its detail, He was done - He had no more work to do to invent chemistry or biology. The fancy philosophical word for this is supervenience. We say that chemistry supervenes on physics, because chemistry constituitively depends on physics.
Each layer in this pile of science consists of a) extrinsic functional properties (which, taken together, support or implement the layer above), and b) intrinsic properties (which are supported, or implemented, by the extrinsic functional properties of the layer below). The field of biology studies biological entities which behave the way they do ultimately because of their chemistry. Chemistry studies compounds which behave the way they do ultimately because of physics, which these days means quantum mechanics. Quantum mechanics behaves the way it does because…?
At the lowest layer of physics we can, in principle, only know the extrinsic functional properties, those which give rise to the macroscopic physical world we see around us. All we have to describe the world at that level are mathematical equations (called Schrodinger equations, as it turns out, in the case of quantum mechanics). We do not know, and we can not know, the intrinsic nature of matter and energy described (with nearly 100% accuracy) functionally by these equations. That is, we can say quite accurately how matter and energy behave at the lowest levels, in terms of how they impinge on other matter and energy, but we can't say anything beyond that about what it is that is doing the behaving. Something's functional characteristics are perfectly described by these equations, but we will never be able to know what that something is. Some people (including most practicing physicists) say that there is no "something else" besides a perfect functional description, and that once you have specified how something behaves at the lowest level of physical reality, there is nothing left to talk about. At the very least, it makes no sense to speculate about such things.
To use an analogy from computer science, it is as if each layer of natural science could be thought of as a program module. Each module is implemented a certain way, and each presents an API (application programming interface) to the level above. Each module makes use of, or calls down into, the API presented by the level below. Each module does not, and should not, know or care how the level below is implemented, as long as the lower level module faithfully presents the correct API. But suppose that out of curiosity, although we operate at a certain level, we wonder how the API we use at that level is implemented. So we read the source code of the module below and find that it, in turn, relies on an API presented to it by a module still further down. It seems a bit absurd to me to suppose that at some low level we get to the magic API that just is - that is, the API that exists only as an API, but which is not implemented at all!
Gregg Rosenberg once used an analogy with the game of life (Rosenberg 1998). The game of life consists of a (possibly infinite) two dimensional grid of bits. That is, each square of the grid is either on or off, 1 or 0. There is also a clock of sorts, in that we speak of the state of the grid at time t, where t is an integer. We begin the game with some configuration of on and off squares on the grid, at time 0. For each subsequent tick of the clock, the state of each square on the grid depends on the state of its eight surrounding neighbors at the previous tick according to the following formula: a square that is on in tick t will stay on in tick t+1 if at tick t it had two or three on neighbors; a square that was on at tick t but had any other number of on neighbors will be turned off in tick t+1; any square that was off at tick t but had exactly three on neighbors will be turned on in tick t+1.
Much has been written about the fascinating complexity that arises out of these simple rules. Rosenberg asks us to imagine the game of life as toy physics, and consider a universe in which the rules listed above were the only laws of physics. He then asks, not whether or not consciousness could exist in such a universe (it has been shown that one can implement a universal Turing machine - a computer - in the game of life) but if such a universe could exist at all. What does it mean to have a pure game of life universe? What does it mean for a square to be on or off? These are properties whose only specification within the game is that they be distinguishable from each other: what is on? It's not off. What is off? It's not on. The properties of on and off are circularly defined, and the rules, then, are defined in terms of these circularly defined properties. Whenever we implement the game of life, we represent on and off with checkers on a board, or more often, electronics in a computer. For us, these properties must be instantiated by some substrate.
You couldn't have a "pure" game of life universe. There is no such thing as a "bare" property, characterized entirely in terms of its contrast to other properties, but this is exactly what a pure game of life universe asks us to imagine. Rosenberg used the game of life as a toy physics to make the point, but as he says, our own real physics is in no better shape. It is more complicated, so the circle is a bit larger, but "pure" physics in our world makes no more sense than it does in the game of life world.
Physics is a castle in the sky, an elaborate structure built on a foundation of nothing. Or rather, built on circularly defined terms, much like a mathematical system. Each of the lowest-level things that physics deals with (the fundamental particles and forces) is defined mathematically in terms of the other particles, forces, or some constants. Everything in physics, then, is defined relationally, in terms of the other things in physics. Physics gives us a schema, a description of causal dynamics, but it is inherently silent about the stuff doing the causing, except insofar as it defines a behavioral role for the stuff in terms of its influence on other stuff. Physics, then, is sort of like a playwright who writes the dialog but leaves the casting to someone else.
Any system whose parts obeyed the same relations among themselves, or whose parts interacted according to the same patterns of interaction that our physics do would automatically have identical physics to our universe, no matter what its parts "really" were. We could, in principle, transpose our physics to another universe made of entirely different stuff, as long as the causal dynamics of that stuff matched perfectly the causal dynamics of the stuff that instantiates our physics.
The bottom line is that, given a complete and perfect set of physical laws and physical facts, even though all the other hard sciences would be locked in place, God would have still more work to do before he had a complete recipe for a universe. He could create any number of different universes, made of different stuff, but which were physically identical (and thus chemically identical, and biologically identical, etc.), as long as the structures of the causal dynamics among whatever He chose to make each universe out of were identical. It would be impossible for an inhabitant of any of those universes, from within the science of physics, to get underneath the physics and see the intrinsic nature of the matter out of which his or her particular universe was made. This is true completely without regard to any questions of consciousness.
There is only one kind of stuff in the universe; it's just that physics is inherently incapable of completely describing that stuff. This problem of the unknowability of the intrinsic properties of the lowest level of reality is going to be a problem for (or at least an aspect of) any physics (as the science of physics is currently construed), and is not particular to quantum mechanics. There is always going to be, in principle, a hole at the lowest level of our descriptions of the natural sciences.
Some people, when confronted by the fact that physics all comes down to circularly defined equations and/or algorithms, draw exactly the wrong conclusion: that our universe is mathematical or algorithmic at its core. Since no matter how advanced our particle accelerators, no matter how true our theories, all of physics must rest on abstract equations, then abstract equations must lie at the bottom of the physical world. Electrons, by this way of thinking, are made of information, quarks are algorithms. This idea is sometimes called "It from bit".
But the map is not the territory. Just because all of our ways of talking about physics must, in principle, bottom out in a cluster of equations, it does not follow that the stuff we are talking about is made of equations. There is still something down there doing the equating, so to speak. We just can't know what it is, or anything about it other than its outwardly efficacious participation in the causal mesh, which is described by the equations. As our technology becomes more and more refined, we can represent more and more information with less and less physical stuff (vacuum tubes to transistors, to integrated ciruits, with ever more diodes crammed on a chip). To imagine, however, that the universe itself has perfected its "technology" to the point where it can leave base, coarse phsyical matter behind entirely, and instantiate "pure" information, information in itself, is nutty. It is an old, familiar kind of nuttiness, however. It is the same late medieval Platonism that lead thinkers to hypothesize concentric crystaline spheres of ever more rarity and fineness around the earth, with angelic ether filling the void between them, and producing music we are too base to hear.
So on one hand we have a hole at the lowest level of our best descriptions of reality, and on the other hand we have an inconvenient extra ingredient, consciousness, that doesn't seem to fit anywhere in our descriptions, but probably lives at a pretty low level. The idea that the extra ingredient might fit in the hole has been explored by Whitehead, Russell, and Rosenberg. It is essentially the idea behind panpsychism.
"All right," one might reasonably argue, "Maybe we can't know what a quark really is, we can only know exactly how it behaves. So what? At that minute level, what difference could it possibly make? My world and my understanding of it, including the laws of physics, remain exactly the same, no matter what the intrinsic nature of a quark really is." To base a theory of consciousness on this unknowability within science of the lowest levels of reality, we have to say not only that this hole at the bottom of physics is filled by some form of proto-consciousness, but that there is some way this stuff scales up to the level of human minds. This is known as panpsychism's aggregation problem. A quark could be experiencing (or proto-experiencing) all it wants, but as long as its behavior ends up conforming to the behavior dictated by the physical laws pertaining to quarks, that proto-experience stays at the quark level. The larger world could still be built out of the structural dynamics whose foundation is this proto-stuff. For large, complicated things like ourselves to be conscious in some special way that outruns anything we might expect to emerge from the structural dynamics, we have to explain how this stuff scales up from the level of a quark to the level of a mind, and we also have to say how it could be meaningfully efficacious: what could it possibly buy us in terms of its effect on the world beyond simply instantiating the lawful low-level regularities that science has already mapped out so accurately? How could it do so in a way that added anything to what we already get out of our physical laws and facts, but that did not also violate those laws and facts?