Complex sentence: Paul Dirac in 1937 speculated that physical constants such as the gravitational constant or the fine-structure constant might be subject to change over time in proportion of the Age of the Universe, and experiments conducted since then have put upper bounds on their time-independence.
Split into simpler sentences: Paul Dirac in 1937 speculated that physical constants such as the gravitational constant or the fine-structure constant might be subject to change over time in proportion of the Age of the Universe. Experiments conducted since then have put upper bounds on their time-independence.