Let me also say what I don't mean by final, underlying, laws of physics. I don't mean that other branches of physics are in danger of being replaced by some ultimate version of elementary particle physics. I think the example of thermodynamics is helpful here. We know an awful lot about water molecules today. Suppose that at some time in the future we came to know everything there is to know about water molecules, and that we had become so good at computing that we had computers that could follow the trajectory of every molecule in a glass of water. (Neither will probably ever happen, but suppose they had.) Even though we could predict how every molecule in a glass of water would behave, nowhere in the mountain of computer printout would we find the properties of water that really interest us, properties like temperature and entropy. These properties have to be dealt with in their own terms, and for this we have the science of thermodynamics, which deals with heat without at every step reducing it to the properties of molecules or elementary particles. There is no doubt today that, ultimately, thermodynamics is what it is because of the properties of matter in the very small. (Of course, that was controversial at the beginning of the century, as you'll know if you've read a biography of Boltzmann, for example.) But we don't doubt today that thermodynamics is derived in some sense from deeper underlying principles of physics. Yet it continues, and will continue to go on forever, as a science in its own right. The same is true of other sciences that are more lively today and in a greater state of excitement than thermodynamics, sciences like condensed matter physics and the study of chaos. And of course it's even more true for sciences outside the area of physics, especially for sciences like astronomy and biology, for which also an element of history enters.

- Steven Weinberg, "Towards the Final Laws of Physics," in Elementary Particles and the Laws of Physics, p. 64-65 (1987)

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