The incomplete veil: from macroscopic to the microscopic

 It is natural to assume that scientists need to probe a system at the microscopic scale to learn about what is happening at that scale. If we take this view we will necessarily be pessimistic about the "bottom-up" research strategy for quantum gravity advocated by Bei Lok Hu. It goes from macro- to micro-, the opposite to the more popular approaches of string theory and loop quantum gravity. However, the history of science shows that we can learn a lot about microscopics from probing systems at much greater length scales. Here are some examples.

Following Perrin's experiments and Einstein's theory of Brownian motion, almost all scientists believed that atoms were not just a mathematical convenience but did exist and were the basic constituents of liquids and solids. All this was before X-ray diffraction allowed the more direct study of crystals at the atomic scale.

Crystallography was pretty much settled as a field before there was any direct evidence of the atomic constituents and their spatial arrangement. Cleavage of crystals, facets observed in minerals, and group theory provided a complete classification of all possible crystal structures. Observations of crystal facets and different modes of sound can be sufficient to determine (or at least constrain options for) the crystal class. 

Figure from Traité de minéralogie (1801) by Rene Hauy See also this.

In 1935, Linus Pauling proposed the crystal structure of common ice without any information from X-ray crystallography. He only used the measured value of the residual entropy, simple models of hydrogen bonding, and the Bernal-Fowler ice rules.

In 1961, the biochemist Peter Mitchell deduced the mechanism of the synthesis of ATP, the molecule responsible for energy transport in cells, without knowing any details of the molecular structure of cell membranes. He reasoned from thermodynamics and the fact that there was an electric potential across the cell membranes. His work led to the discovery of the enzyme ATP synthase, a molecular motor. The underlying physics is beautifully described by Phil Nelson in his text, Biological Physics.

I see two important and related lessons for today from these historical examples.  

1. We have access to amazing computational power and microscopic probes. However, before rushing off to use them, ponder what constraints on the microscopic might be deduced from macroscopic observations.

2. Given that a quantum theory of gravity seems so elusive more resources might be invested in the macro- to micro- strategy.

Aside: Overall, I think this post is going against the strong claims that Bob Laughlin makes in "The Dark Side of Protection", chapter 12 in A Different Universe.