Why is de Broglie’s hypothesis still called a hypothesis and not a “theory”?

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Considering that de Broglie’s hypothesis on the wave-particle duality of matter has already had an experimental grounding (the Davisson-Germer experiment), why is it still labelled a “hypothesis”? Are there still reasons why it is still not considered a theory?

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In this case, think of the “*hypo*” in hypothesis to mean “narrow in scope” rather than “unsupported by experiments”. This is a bit different from the typical usage of the word.

In Physics, “theories” tend to have broader scope. E.g., theory of relativity, theory of gravity, etc. These are big things that can explain or predict many different kinds of phenomena.

Comparatively, de Broglie’s hypothesis is limited to one aspect of the wave-particle duality, and may be considered *a part* of the larger theory of quantum mechanics.

> why is it still labelled a “hypothesis”? Are there still reasons why it is still not considered a theory?

There’s not necessarily a deep reason – the name just stuck like that. There are some comparable examples that spring to mind from maths: the Poincaré conjecture is still called that even though it has a proof, while Fermat’s last theorem was called that long before a proof was published. Naming schemes tend to have a lot of random inconsistencies.