Constraint Counting - Einstein Strength

Einstein Strength

Everyone knows that Albert Einstein said that a physical theory should be as simple as possible, but no simpler. But not everyone knows that he had a quantitative idea in mind.

Consider a second order partial differential equation in three variables, such as the two-dimensional wave equation

It is often profitable to think of such an equation as a rewrite rule allowing us to rewrite arbitrary partial derivatives of the function using fewer partials than would be needed for an arbitrary function. For example, if satisfies the wave equation, we can rewrite

where in the first equality, we appealed to the fact that partial derivatives commute.

Einstein asked: how much redundancy can we eliminate in this fashion, for a given partial differential equation?

Read more about this topic:  Constraint Counting

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    If my theory of relativity is proven correct, Germany will claim me as a German and France will declare that I am a citizen of the world. Should my theory prove untrue, France will say that I am a German and Germany will declare that I am a Jew.
    —Albert Einstein (1879–1955)

    To ask strength not to express itself as strength, not to be a will to dominate, a will to subjugate, a will to become master, a thirst for enemies and obstacles and triumphant celebrations, is just as absurd as to ask weakness to express itself as strength.
    Friedrich Nietzsche (1844–1900)