QuantumPhysics.dvi

1.4 Reductionism

The success of modern science, including physics, is due to ever advancing experimentation,

as well as to the success of the reductionist approach to understanding the Universe. The

basic assumption is that the behavior of larger objects may be determined, at least in prin-

ciple, in terms of the dynamics and interactions of its constituents.Thus, the structure of a

molecule can be understood in terms of the dynamics of the atoms out of which it is built.

The structure of the atoms and the strength of their mutual interactions can, in principle,

be calculated from the quantum electro-dynamics of electrons andnuclei. The structure of

the nuclei is governed by the strong and electro-magnetic forcesbetween their proton and

neutron constituents, whose own structure may be understoodin terms of quarks and gluons.

Does this mean that we need to control the dynamics of quarks andgluons in order

to understand semi-conductors, for example? Remarkably, the answer is no, at least in

most circumstances. The reason is that physics at two widely different length scales (or

equivalently energy scales) tends to decouple from one another. For most of chemistry, you

do not need to know the detailed nuclear structure of the atoms in the molecules, and it is

enough to know their mass, charge and spin. Occasionally though, such as with radioactive

nuclei, the nuclear structure and its modifications under radioactivity do have chemical

consequences.

In summary, we shall be able to go a long way in describing Nature if we can understand

well various dramatically simplified approximations to a problem. Such simplified cases give

modelsfor physical behavior. A good model will capture the essence of the physical phe-

nomenon while relaying to the background less important and/or more complicated aspects.

In this course, we shall use such simplified models throughout.