> Physicist's definition of time: "it is what is measured by a clock" I hate this quote, because it makes people think there is something unique about time. Th

> Physicist's definition of time: "it is what is measured by a clock" I hate this quote, because it makes people think there is something unique about time. There is, but not because of this. We could similarly say "a meter is what a ruler measures" (defined by how far light moves in 1/299792458th of a second; a second is defined as 9192631770 Hz, or the transition of 2 hyperfine levels of a CS-133 atoms). We can do this with any SI unit! What is an Ampere? You gotta define charge and frequency (time). What is a kilogram? You gotta define Planck's constant, the speed of light, and... frequency. Kelvin? Planck's constant, frequency, and kilo. Candela? Also depends on frequency. Mol? Well... actually this one is unique. Truth is that these are not as "well defined" as one might think. They're kinda made up and arbitrary, and that's perfectly okay. In particle physics you often set hbar = 2pi = c = 1 (or if you're Mills, you include i). If we meet aliens, they would very likely have a different set of units, but their physics would look identical (they'd be isomorphic, i.e. we could translate between them). So what is time? Distance? Mass? Energy? These are still very open questions at a base fundamental level. We treat them like axioms but even math still questions axioms and would love to remove them (and just like math, axioms are typically developed in hindsight). Someone might give you some definition for these things and you might be satisfied, but that doesn't mean there is a question one level deeper that's unknown. Beware this trick, as you'll fool yourself into thinking you know more than you do (it's quite common, and we all do it. I'm not trying to stand on a pedestal, every human does this and in abundance. Sweep things under the rug, but don't forget that you've done this). (also, this quote is usually attributed to Einstein, not Feynman) For info on SI units: https://www.bipm.org/en/si-base-units/mole https://www.bipm.org/en/si-base-units/mole