At the moment, the unit of measurement of time is the second. In the International System of Units (SI), a second is defined as “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 caesium 133 atom.”
It has not always been so, as you might have guessed. Prior to 1956, the second was defined as the mean solar second, which is 1/86,400 of the time the earth takes to spin completely around on its own axis. From what I’ve read, the mean solar second is not a reliable measurement because of inconsistencies in the speed of the spin, altered by the gravity of the moon and the resulting tides, which slow the rotation.
From 1950 to 1956, physicists (and possibly others) responsible for defining international measurement units agreed to redefine the second as the ephemeris second, based on the speed of the earth’s orbit around the sun in 1900. That speed was determined using predictions from the “Tables of the Sun,” published by astronomer Simon Newcomb in 1895, based on observations of the sun’s position between 1750 and 1892.
There have been other adjustments. The present definition, as shown in the first paragraph, was adopted in 1967. For more fascinating information, plus a snippet about the leap second, go to this link.
You might ask, “Why is he posting this?” The answer is not straightforward, nor it is necessarily logical. My exploration of the official measurement of time began with an attempt to learn whether time has any physical properties. Yes, I know, that sounds absurd, but I wasn’t sure. And, as it stands, I’m still not. Anyway, I wondered if time possessed physical properties that might be changed in some fashion under the right conditions. You know, the way gravity can “bend” light. (Yes, if what little I understand from physics [and that is smaller than small] is correct, light is energy and E=MC2, so light can be convertible to mass, and mass is susceptible to gravitational pulls. Thus, light travelling in a straight line through space can be bent by gravity. Or so I’m told.)
Well, it seems to me time as we understand it is limited to beings in our own solar system, inasmuch as we are the only ones we know of who can measure “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 caesium 133 atom” and, moreover, the only ones who care about or even experience time in the way we do.
This got me wondering about light-years and whether light-years would have the same meaning to a being on a planet in another solar system. I think not. As I was wondering whether time would be relevant in other places, I started wondering whether our understanding of the elements might be extraordinarily limited, as well, though we keep hearing that our space probes and über-telescopes detect elements with which we are familiar in other galaxies.
There is so much to know. There is so much we cannot and will not ever know. But if we cannot know and will never know, doesn’t that mean the first sentence in this paragraph is wrong? If we cannot and will not know it, it is unknowable. I will leave it there.