So, I will try to explain this as best as possible. I am literally on the first chapter of a classical mechanics book. The chapter is discussing all the basic SI units...the meter, the second, etc. It's these units that I am having such a hard time understanding. They seem so easy but my mind keeps wondering and it's making it harder to understand them. I will list my difficulties below: I understand that the meter was created by using the earth as a basis of its measurement. This was eventually inscribed in a metal stick with two sharp scratches. So, the meter was set...more or less (because the scratches gave a +/- error--ie: do you measure at the valley of the scratch, the beginning of the slope, or the top end of the slope) and there were several other variances that made it not a reliable measurement. The second was created based upon our solar day--1/86,400 of a solar day is a second. And that leads me into the question of the sidereal day and the solar day and how they actually even measured a measurement like 1/86,400 of a day hundreds of years ago and knew exactly when the earth turned a full solar day. That's a whole other question I don't understand. But anyways...from my understanding...since the meter was set with those scratches, we eventually had the accommodate that measurement to be the length light travels in 1/299,792,458 of a second. We had to fit that fraction of a second to fit our meter that was already made. But how do they know it's exactly 1/299,792,458 seconds of "light length". My guess is that the difference between 1/299,792,458 and 1/299,792,459 second of "light length" would be the difference of measuring the scratch mark in the valley vs the top edge of the valley. I hope you understand what I'm getting at there. And I know I skipped a standard there with the krypton, but it's still the same question. We let's say I understand that it's 1/299,792,458 seconds of "light length". That is set. Well that comes from the second. Which is based off a Cs atom. Well, that definition has it at 0K. Which I thought was impossible, so I don't even know how they got that. And then on top of that! I was reading that the meter actually has an absolute error of something! I can't find that article again but I believe it was bigger than something that we actually know...like the radius of a proton. So, if the meter potentially has an error bigger than that, then how do we know the radius of a proton!! Ahhhh!! Lol So, what I'm trying to say is that I think the meter and the second are, in my opinion, the two most important SI units (yeah, kg I guess too). My understanding is that we have come up with standards that don't differ from one universe to another and we have closely matched those standards to the previously known standards. My confusion is that I think they all depend on each other...so if there is error with one, it only is magnified in the next SI unit that depends on it. Also, I understand that this doesn't have big implications for the day-to-day world....like it's not going to matter that a bolt on a car is a picometer off, but at the atomic level, I do feel like it makes a difference. I hope my questions are obvious enough..and if not, I hope you understand where I am going with this, because sometimes I don't even understand what I am trying to ask. Lol PS: could anyone tell me what 1/300,000,000 of "light length" is. I'm just curious. I know it's obviously smaller than a meter since it s a shorter amount of time, but what is it? 9/10 of a meter? My brain is fried right now and I can't even figure that out lol Thanks you.