Joe Bugeja writes

When Einstein raised relativity, it was not all immediately testable, that
came later.

1) It 'explained' quite a few puzzling experimental results.

2) There was no counter experiment that contradicted it.

At which point it was really a good hypothesis.

3) Various tests were devised that it explained very much better than
competing hypotheses.

When it slowly became a theory.

--
Oz
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