Posted:

**Jan 05, 2019 9:05 pm**romansh wrote:I thought that was a neat essay.

A few years back I noticed that (in our meso world) pressure had the same units as energy density. (I understand Dyson is not referring to our meso world here). Strange thing is I don't recall ever having being taught that pressure can be seen as energy density, though it seems intuitively obvious.

The essay is excellent. One of the highlights for me is how the mode of discussion was seen to change at least twice in the development of Maxwell’s EM theory and later quantum mechanics.

The ideas of measurable quantities like pressure and mechanical motions were replaced with ideas about field intensities that were manifestly mathematical abstractions - the unit of electrical field intensity is the square root of a joule per cubic metre and only the square of such an abstraction yields a measurable energy density.

Similarly, the two ‘layers’ of quantum mechanics are the abstract wavefunction, equivalent matrix elements or state vectors that are either squared or are the product of two abstractions to give the probabilities of measurement outcomes.

Maxwell’s EM is seen best as a theory of fields rather than mechanical levers and pulleys and the language of quantum mechanics is that of geometry rather than words alone. Is what I took away from the read.