Einstein's Notion of a Principle Theory

Einstein scholar Galina Weinstein

Einstein's distinction between principle theories and constructive theories is methodological rather than metaphysical. Principle theories such as thermodynamics and relativity articulate empirically distilled constraints that delimit admissible microphysical models, while constructive theories remain provisional and revisable....

In late 1919, following the British eclipse expeditions that confirmed the light-bending prediction of general relativity, Albert Einstein agreed to write an explanatory article for The Times of London. Written in German as “Was ist Relativitätstheorie?” and published in English as “Time, Space, and Gravitation” [7, 8], the article was intended not merely as popularization, but as a methodological clarification of the kind of theory relativity is.

In the essay, Einstein contrasts constructive theories (konstruktive Theorien) with principle theories (Prinziptheorien) [7]. This distinction is not merely classificatory but methodological and epistemological in character [22].

Here is that 1919 Einstein paper:

There are several kinds of theory in physics. Most of them are constructive. These attempt to build a picture of complex phenomena out of some relatively simple proposition. The kinetic theory of gases, for instance, attempts to refer to molecular movement the mechanical thermal, and diffusional properties of gases. When we say that we understand a group of natural phenomena, we mean that we have found a constructive theory which embraces them.

But in addition to this most weighty group of theories, there is another group consisting of what I call theories of principle. These employ the analytic, not the synthetic method. ...

The special relativity theory is therefore the application of the following proposition to any natural process: "Every law of nature which holds good with respect to a coordinate system K must also hold good for any other system K' provided that K and K' are in uniform movement of translation."

The second principle on which the special relativity theory rests is that of the constancy of the velocity of light in a vacuum.

No, relativity was not developed as a principle theory. FitzGerald proposed the relativity length contraction in this 1889 paper:

I have read with much interest Messrs. Michelson and Morley's wonderfully delicate experiment attempting to decide the important question as to how far the ether is carried along by the earth. Their result seems opposed to other experiments showing that the ether in the air can be carried along only to an inappreciable extent. I would suggest that almost the only hypothesis that can reconcile this opposition is that the length of material bodies changes, according as they are moving through the ether or across it, by an amount depending on the square of the ratio of their velocity to that of light. We know that electric forces are affected by the motion of the electrified bodies relative to the ether, and it seems a not improbable supposition that the molecular forces are affected by the motion, and that the size of a body alters consequently.

This appears partly inspired by this 1988 Heaviside paper. That is, solid objects are held together by electromagnetic forces, and those fields were known to be warped by motion. Relativity was the constructive consequence.

The relativity principle that laws holding in K must also hold for K' was essentially what Lorentz proved in 1895.

Einstein seemed to disavow all of this when quantum mechanics was discovered. He was happy to avoid the question of how the FitzGerald contraction works on the molecular level, but refused to accept a quantum theory that did not explain the Heisenberg uncertainty on an atomic level.

It is interesting that in 1919 Einstein was still using 1895 terminology, and not saying that the laws of nature must be in covariant equations, or that the laws must be well-defined on a non-Euclidean manifold.

Weinstein posted some other goofy papers recently, including this on Einstein EPR entanglement, and this comparing Heisenberberg-Schroedinger to the P=NP problem. The Heisenberg and Schroeding theories were mathematically equivalent, but she has a whole paper analogizes them to things that are completely different.