Biological Physics: Energy, Information, Life

14 Chapter 1. What the ancients knew[[Student version, December 8, 2002]]

buildingmodel

mathematical

analysis

experimentally

testable

quantitative

predictions

observed

facts

physical model

Figure 1.4:(Diagram.) What biological physicists try to do.

In the best case, the results of step (c) give the sense of getting something for nothing: The
model generates more structure than was present in its bare statement (the middle part of Fig-
ure 1.4), a structure moreover usually buried in the mass of raw phenomena we began with (left
end of Figure 1.4). In addition, we may in the process find that the most satisfactory physical
model involves some threads, or postulated physical entities (middle part of the figure), whose
very existence wasn’t obvious from the observed phenomena(left part), but can be substantiated
bymaking and testing quantitative predictions (right part). One famous example of this process is
Max Delbr ̈uck’s deduction of the existence of a hereditary molecule, to be discussed in Chapter 3.
We’ll see another example in Chapters 11–12, namely the discovery of ion pumps and channels in
cells.
Physics students are heavily trained on the right end of the figure, the techniques for working
through the consequences of a mathematical model. But this is not enough. Uncritically accepting
someone’s model can easily lead to a large body of both theory and experiment culminating in
irrelevant results. Similarly, biology students are heavily trained in the left side, the amassing of
many details of a system. For them the risk is that of becoming an archivist who misses the big
picture. To avoid both of these fates, one must usually know all the details of a biological system,
then transcend them with anappropriatesimple model.
Is the physicist’s insistence on simplicity, concreteness, and quantitative tests on model systems
just an immature craving for certainty in an uncertain world? Certainly, at times. But other times
this approach lets us perceive connections not visible “on the ground” by viewing the world “from
above.” When we find such universality we get a sense of havingexplainedsomething. We can also
get more pragmatic benefits: