Complex sentence: The speed of light in a vacuum provides a convenient universal relationship between distance and time, so in physics (particularly in quantum physics) and often in chemistry, a jiffy is defined as the time taken for light to travel some specified distance.
Split into simpler sentences: The speed of light in a vacuum provides a convenient universal relationship between distance and time. Accordingly, in physics (particularly in quantum physics and often in chemistry), a jiffy is defined as the time taken for light to travel some specified distance.