[sanxiyn]

> Scientists don't know whether the BIPM prototype is losing mass, perhaps because of loss of impurities in the metals, or if the witnesses are gaining mass by accumulating contaminants.

Can we stop this nonsense? It would be a big problem if it were true, but it isn't. It's the later (contamination weight gain) and we have fairly good understanding of what's going on. For example, see https://phys.org/news/2013-01-kilogram-weight.html

[DonbunEf7]

As Veritasium explains [0], all of the reference kilograms have drifted, and some appear to weigh less than they used to. So even if we know the precise mechanism of action of the drift, it doesn't help with the fact that our measurements are less reliable than they used to be.

Of course, what's probably happened is that our measuring tools have gotten more accurate!

[0] https://www.youtube.com/watch?v=ZMByI4s-D-Y

[cmurf]

The kilogram is a measure of mass, not weight which is a measure of force. In order to really figure out mass, I'd think the localized gravity has to be accounted for. e.g. GRACE maps gravity variation around the earth by measuring the acceleration difference between a pair of satellites.

[mattashii]

The measurements were taken at the same place at around the same time, so fluctuations in local gravity should have been negligible compared to actual mass fluctuations.

[astrodust]

Do they account for potential variations in gravity where they've been doing the measurements? Given how ridiculously sensitive the equipment is the position of the moon probably has an impact on the readings they're getting.

[tankenmate]

Balance scales work independently of gravity, both sides of the scale are equally affected.

[stickfigure]

Only if the moon is directly overhead.

[tankenmate]

Down to eight significant figures it is an issue (effect of the moon's gravity on the surface of the earth peaks at 1.1 × 10−7 g), but you can always point the normal of the axial plane of the balance in the earth relative right ascension direction of the moon to avoid this effect (regardless of the moon's relative declension, δ).