New measurement will help redefine international unit of mass | Hacker News Reader

New measurement will help redefine international unit of mass

phys.org

41 pointsdnetesn8 comments

This is really interesting - in a geeky way.

Until now all fundamental units were defined in terms that depended on the constants of nature.

For instance:

- the metre as the length of the path travelled by light in a vacuum in 1/299 792 458 seconds.

- the second as: 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

And so on EXCEPT for the kg.

The kg was the only odd one out. There was this object stored somewhere and when people wanted to measure very well the kg they had to go there and try to measure the mass of that object as accurately as possible... which is a rather odd thing to do when aiming for accuracy.

Now we finally have the means to express the kg with a physical experience in universal terms.

It is called "le grand k" and is kept in France. Even with these new measurements, they are still just a representation of the original grand k.

Indeed, but the hope is that you could (in principle) lose or damage the grand k, and be able to get on with life. Just like with the second and the meter, the kg will be defined using an agreed upon number that provides the best possible agreement with the old artifact. Then the artifact will have been replaced by a number whose value is largely of historical interest.

Interesting background in http://www.npr.org/templates/story/story.php?storyId=1120033... The standard kilogram has been losing mass over time (when compared to a collection of copies). Apparently, we have been getting progressively less banana... (about 50 parts per billion between these comparison exercises)

I thought I was getting a new unit to buy bananas in other than lbs... turns out they just measured Planck's constant very accurately in an impressive manner.

Slightly disappointed.

Since when was lbs international unit?

Sad truth is that you buy bananas in lbf and in most of the world we use Kgf.

This world is lost I tell you, can't even pay for something according to it's mass.

Amusingly, all of the "English" units used in the US are defined in terms of metric units. For instance there's no standard inch. Rather, the inch is exactly 2.54 cm.

Reply on news.ycombinator.com