Fundamental constants are also set by a consortium of national metrology institutions and the resolution of the defined constants are usually on par with existing measurement ability and eventually become the bottleneck of measurement uncertainty as technology advances. This is the reason they are periodically redefined.
For example, if there is a constant called Bump’s constant and it is set at 1.23, it is not that the actual constant is 1.230000000000000, it’s that it is only known to two decimal places. It could actually be 1.230… or 1.239… or anything in between. So if you have a unit that you measure that is defined by frequency * bump’s constant, you are really limited in how accurate that measurement is by the resolution of the constant regardless if you can measure frequency to a part in 10^13.
As discussed in this video, the official kilogram is losing weight, simply from erosion (and possibly decay of some isotopes? I didn’t re-watch it). Earlier systems for measuring temperature relied on some specific physical thing like “the ambient temperature in the deepest chamber of King Tut’s tomb”, and it’s pretty obvious why that’s a problem and we ended up switching to other systems.
Treating one property of a specific physical object as a fundamental constant presents some major problems (even if it has copies, which are not perfectly consistent, despite our best efforts). Having to visit a certain location to calibrate your instruments is not least among them.
It’s real pretty, but breaking THE kilogram into a million pieces would force science to improve a little bit faster.
Isn’t the kilogram defined as the weight of 1000 grams, were a gram is the weight of a cube of water, one centimeter per side? A 10x10x10 cm cube of water should be usable as a reference kilogram.
Water with what proportion of deuterium vs. regular hydrogen, and O-18 vs. O-16? At what temperature? Under what pressure?
Yes, that was the basis for why the standard was chosen as it was, and it’s good enough for many purposes, but it’s not good enough to be the standard for all measurement.
Um…OP cites the thread which this one is a spinoff of–and it’s not a prepositional grammar thread. It’s about the status, use, fact of, whatdoyoucallthis and whatdoyoucallthat of weight and mass.
Yes, coppers! Come get me! I’m junior modding the fuck out of this thread! Top of the world, Mom!
There will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre and candela are already defined by physical constants and it is only necessary to edit their present definitions. The new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements.
Wasn’t it in the plans to replace it with a silicon sphere sized to a mind numbing degree of precision?
The idea being to change the standard kilogram to an exact number of silicon atoms represented to the best technical ability in said sphere.
Wait, they’re re-defining the mole as well as the kilogram? Does that mean that they’re abandoning the relationship between the mole and the gram, or that they’re redefining the atomic mass unit?
Read a fascinating book some time back about the histories of various measures - from the king’s foot, to each town having its marketplace standard, to the physical representations and beyond.
As I recall, solids simply decay - or erode as an earlier poster said. Not significant unless you are attempting to measure with this extreme degree of accuracy. And such things as dust, or oil from one’s hands, could throw it off. When the meter was in metal, it needed to be at a constant temp or it would expand/contract…
Not sure if it was that or another book on the subject, that said the meter was originally calculated incorrectly, due to one of the teams of French surveyors fudging their data.