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Def Time/Feynman: it's how long we wait. It can probably not be defined any further. It is important how we measure it.
Time Measurement/Measurement: one possibility: use something that changes periodically.
History: to be able to use the day as a measure, they had to make understand that days are not always of the same length; a different instrument was needed for that, e.g. an hour glass.
But with that they had not yet proven that they are periodic! >Covariance.
We can simply say that our definition of time is based on the repetition of an obviously periodic event.
Vom Wesen physikalischer Gesetze München 1993
Vorlesungen über Physik I München 2001