It occurred to me while discussing a different question on this exchange that in order to understand space and time we must first agree on how it's defined in terms of measurement. My current understanding is that time is defined as how long it takes light to go a known distance. Length is defined as how far light gets in a certain time. The problem of course is the definitions are paradoxical. To know time you must first know length and to know length you must first know time.
Is there an agreed gold standard for defining time that doesn't rely on knowledge of space or vice-versa? I would be most interested to know.
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Qmechanic
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Derek Seabrooke
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2Does this answer your question? What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole? – John Rennie Nov 16 '21 at 13:30
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3Search for definition second for lots more info. – John Rennie Nov 16 '21 at 13:31
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I not asking for the definition of the second, much less a kilogram. My question is fundament to how length and time itself is defined. Is radioactive decay the answer for time? If so it would imply that if cesium didn't exist time would cease to exist? – Derek Seabrooke Nov 16 '21 at 13:36
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Please. If cesium didn't exist, people would pick some other decay. If no radioisotopes existed, people would pick the resonant frequency of some material. If resonance didn't exist, people would keep using the orbital period of the Earth like the ancients. – Connor Behan Nov 16 '21 at 14:37
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Yes, that was a bit rhetorical, but here's a more philosophical question: If light didn't exist would space exist? – Derek Seabrooke Nov 16 '21 at 15:03
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Of course. In a pitch black room you can walk three steps forward. You have then measured out a length equal to three steps. – Marius Ladegård Meyer Nov 16 '21 at 15:22