We cannot measure time. We have no idea why time is unidirectional.
We claim to measure time periods and the passage of time, though we have no idea what it is that is passing.
We impute time periods to the observation of changes. We assume that the changes being observed are stable and regular. We used to assume that the earth orbited the sun in a stable and regular manner with every completed orbit taking what we called a year. We now know that the orbit is neither stable nor regular and is no longer accurate enough for use as the standard measure of a time period. We used to assume that the earth’s rotation around its own axis was stable and regular but now know that this rotation is slowing and days are getting longer by about 2.3 milliseconds per century. Of course, to be able to say that, we need a “second” defined independently of a “day” defined by the rotation of the earth. The modern definition of a “second” is now based on the vibration of a caesium atom.
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom in its ground state at a temperature of 0 K.
This assumes that this radiation is stable and regular. We assume that the 9,192,631,770 periods taken to constitute a second are each identical to the other. (Why it should be so is of course magic). For all practical purposes and relative to the duration of the lifespan of the human species it may well be so. But over the long, long term it cannot be so.
The earth-moon-sun system, the solar system and even our galaxy are all losing energy. Even all vibrating atoms must be losing energy for any radiation to occur or for any vibration to take place. For the “radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom in its ground state at a temperature of 0 K” to remain stable for ever requires an energy input which does not exist. Why any radiation should be regular is still a matter of magic.
What a “second” was at the Big Bang and before is unknown. But since then, it follows that “seconds” then were shorter than “seconds” are now. Every “second” now must be shorter than every “second” to come.
Of course what is even more magical is our fundamental assumption that the passage of time itself is stable and regular. We have no clue as to what laws of the universe require such stability or regularity and the why of any such laws is still in the realm of magic.
The magical speed of an inconstant time
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