SI Units Redefinition

guilty as charged. it gets worse, though: meters are originally based on the size of the earth, but humboldt's expedition (?) fucked up the measurement and now we're stuck with a meter that's significantly too short, nominally defined as the distance light travels in 9192631770/299792458 cycles of the hyperfine transition radiation frequency of cesium-133. you probably think this is a goddamned joke but it's noti ask you, what the fuck kind of number is 9192631770?

It is not a unitless constant like pi though. Like the speed of light, Planck's constant has units which is problematic (how can you define the meter in terms of the speed of light if the speed of light depends on the definition of the meter?) Thus, you simply assign a value to the constant to break the unit dependency.

This will still have some arbitrary constant, much like the meter and the second.

I thought they were moving towards basing all units of measure on the speed of light?

You can change the units if you want, but it's still a physical constant, and it could be stated in purely physical terms. For example, according to Wikipedia, the second is defined thus:"Since 1967, the second has been defined as exactly "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" (at a temperature of 0 K)."Therefore, the meter may be defined

Here's the physics world articleSI units to be defined by universal constants..https://physicsworld.com/a/si-gets-a-makeover/

Because in order to measure something in terms of a constant, you cancel out all the other units except the one you're measuring.I.e., if you know how to measure a second, you could define the meter based on the speed of light by creating light in a vacuum and timing it.Meanwhile, you can't define a meter or a second in terms of pi, because it has no units. It's just a ratio of the surface area of certain classes of object or shape to thheir dimensions.

Interesting question, in that kilogram & second are only really handy ways for humans to be able to compare things to other things reliably. Using constants is presumably a more reliable way for the higher level comparisons to be reproducible. It's turtles all the way down...

I have always had one question about defining measurements in terms of universal constants. Perhaps it is a silly one.What if universal constants are, in the long run, variable? What if over the course of 500 years the speed of light, or the strength of gravity, changes?If the measurements used to define these constants are defined in terms of the constants themselves, how would we see change? Perhaps this is a silly question in practice. If so, I would be interested in the 'theoretic

Four of the seven physical constants used to define the seven base SI units will change, so a bunch of physical constants that currently have exactly defined values will start being subject to measurement uncertainty. So μ_0 will no longer be exactly 4π × 10^-7 H/m, k_C = 1/4πε_0 will no longer be exactly 8,987,551,787.3681764 N m^2/C^2, and 1 mol of carbon-12 will no longer have a mass of exactly 12 g.