2.5.3 The end of physics?
Suppose for the moment that quantum field theory, or string theory or M-theory, or some other theory no one has yet heard of, does turn out to be the much sought-after superunified theory. Suppose it is unique and is so wonderfully compact that it can be printed on the front of a T-shirt. What would such a theory really tell us about the world?
Looking on the positive side, the theory should indicate the fundamental entities of which the world is composed, whether they are particles, strings, quantum fields, or whatever. The theory should also indicate the truly fundamental constants of Nature (it may be that Planck's constant, the speed of light and so on are not really as fundamental as we think), and it should certainly indicate all the fundamental processes that can occur - the elementary processes from which all other processes are composed. This would be the ultimate realisation of reductionism, the view that every phenomenon can be reduced to some more elementary set of phenomena, all the way back to a set of truly fundamental entities and interactions.
Being rather more negative, a fundamental theory of everything might not really tell us very much at all. It is hard to believe, for example, that even a supremely intelligent scientist equipped with as much computing power as he or she could desire could set to work from the theory of everything and predict the existence of the Earth, let alone something like my choice of breakfast today. They might show that the existence of an Earth-like planet, or an egg-like breakfast was consistent with the theory of everything, but that's a long way from predicting particular cases. There are several reasons why a theory of everything will probably not really be a theory of all that much. Here are some:
The problem of initial conditions. In a fully deterministic theory, such as Newtonian mechanics, the present is determined by the past. To predict particular eventualities in the present Universe we would therefore need to know the initial state of our Universe. It may not be impossible to determine these cosmic initial conditions, but it's not clear, and it is hard to believe we will ever know for sure.
The problem of indeterminacy. We have seen that when it comes to predicting particular events quantum physics is limited to making probabilistic predictions. It seems certain that quantum physics will be an underlying principle of any conceivable theory of everything, so the predictions may always be limited to possibilities rather than particular eventualities. Some might hope, as Einstein did, that quantum physics will eventually be shown to be incomplete and that a full theory will replace probability by certainty, but all current indications are that this is not going to be the case and that the one thing that's certain is that uncertainty is here to stay.
The problem of emergence. Reductionism was originally a biological doctrine which aimed to reduce biology to more fundamental sciences such as chemistry and physics. It was opposed by the doctrine of emergence which claimed that even if all physical and chemical phenomena were known it would not be possible to predict biological phenomena because new properties emerged at the level of biology that were not contained in any of its parts. These doctrines are now used generally in discussions of science, including physics. To give a physical example: water is wet and it is made of molecules, yet no molecule is wet: the wetness is a property of the water that emerges when large numbers of molecules come together. Most physicists would expect a satisfactory explanation of the wetness of water to make contact with fundamental principles (somehow, the wetness of water must be implicit in the electrical interactions of its molecules) and, in this sense, they are reductionists. But it often happens that complex phenomena require explanations on many different levels, and it would be wrong to dismiss the higher levels as being unimportant, or uninteresting to the physicist. The interactions of atoms and molecules are now understood - at least in terms of the fundamental laws that operate. Yet a wealth of unexpected phenomena continues to emerge in the physics of atoms, molecules, solids and liquids, showing that there is much to explore in physics above the most fundamental level. The challenges are as much to do with understanding the consequences of known laws as with discovering new ones. Perhaps the ultimate challenge will be to provide a chain of understanding that links fundamental principles to truly complex phenomena, such as how a brain works.
For all of these reasons, and others you can discover for yourself, it seems safe to conclude that physics has a healthy future that might well include a theory of everything, but which is very unlikely to be ended by such a theory.