I'm Emily Conover, physics writer for Science News. Scientists have redefined the kilogram, basing it on fundamental constants of nature. Why? How? What's that mean? AMA!

...how is chemistry class gonna be now?

It actually shouldn’t be much different at all. None of the formulas you use will change, and none of the mass measurements will differ. Something that weighed a kilogram yesterday will still weigh a kilogram today. So for the general public, scientists have tried really hard to keep things from changing.

Scientists, on the other hand, will have an easier time making measurements. For example, if you want to measure a tiny mass of micrograms or smaller, you still have to make a comparison to a full kilogram. That means that there's more uncertainty in your measurement. Now that the kilogram will be based on a fundamental constant, you can use that constant to measure out a mass of any size you like. Plus, we don't have to worry about whether the size of the kilogram changes over time.

Can you please elaborate more on your second point? I'm not sure I got it

Sure! So, say you want to measure an object that has a mass of around a microgram. That's a billionth of a kilogram. Right now, if you want to know how much a microgram is, you have to take a kilogram (the mass of the lump of metal in France) and split that up into a billion tiny pieces. It would be much easier if you could just say "here's how much a microgram is" and use that to measure your object's mass. The new definition, based on Planck's constant, allows you to do that. You can use Planck's constant to determine the mass of an object of any size. You aren't required to start with a mass the size of a kilogram and split it up.

Will "Le Grand K" still be stored in the same place under the same conditions just as a historical artefact? Or since it no longer really matters as much will they no longer bother keeping it in such a controlled location?

They are planning to keep Le Grand K under the same conditions for some period of time and to keep studying it. They want to understand if its mass has changed over time, and if so, how much. Now that there is a new definition of the kilogram, Le Grand K might change in mass relative to the new definition. I imagine eventually people will lose interest in it, but I don't really know how long that will take.

How is the definition of a mole changing?

Previously, the mole was defined as the amount of substance that has as many atoms (or molecules, electrons, what have you) as there are in 12 grams of carbon-12. Avogadro's constant was a number that scientists measured, that told you how many atoms that was. Now, Avogadro's constant will be a fixed number with no error on it, that is no longer possible to measure. It just is what it is. A mole is then the amount of substance containing exactly that many atoms. And as a result, a mole of carbon 12 will no longer be exactly 12 grams. There will be some measurement error on that number.

With so many opinions for the basis of the new standards, how difficult was it to agree on the new standards? And how were they decided (by a committee, voting, specific studies,ect)?

The decision today was based on a vote of 60 delegates of member nations of the General Conference on Weights and Measures. That’s the body that decides about changes on the International System of Units (the SI). That decision was unanimous in favor of changing to the new SI.

But there was a long process leading up to this. In 2005, a group of scientists started to push for a new definition of the kilogram. http://iopscience.iop.org/article/10.1088/0026-1394/42/2/001/meta They laid out a plan for how to replace it with a definition based on the Planck constant. People quickly agreed that having a physical artifact define the kilogram was not a great idea and that this was a good plan.

Then in 2011, the General Conference agreed on some requirements for how well Planck’s constant would need to be measured for the change to take place. Scientists met those goals, and that’s what led to today’s vote. I believe there was some arguing behind the scenes (as there always is in science) about whether the measurements of Planck’s constant were in close enough agreement to make the change, but it was eventually decided that there was.

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It’s for historical reasons. Before the French revolution, there was the idea to make a standard called the grave, essentially a kilogram. After the revolution, that was changed. Since people often wanted to measure things out in smaller amounts in their daily affairs, it was decided to use the gram for practical purposes. However, it was hard to make a standard for such a small mass, so the standard that was stored in the archives became a lump of metal, the kilogram.

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Yes, the kilogram is base unit for mass in the SI, which means those are the standard units that are used all over the place, including in the definition of constants. I haven't heard any talk of going from kg to g. Presumably that's because it's mostly a notation thing, not a practical thing. It doesn't make it more difficult to make measurements or develop new technologies, it just confuses people sometimes. :-)

What is the basic problem being solved by changing the basis to the Planck's constant? In reality how often are scientific instruments being calibrated to the physical Kilo under lock and key? Isn't this calibration already more or less a virtual ideal that's shared as a standard for worldwide calibration on the majority of physical instruments?

Ok to answer your first question, there are a few problems that this change solves:

1. The ideal is that the units should be accessible to anyone, anywhere. The kilogram isn’t that way, only certain people can access it, very rarely, and everyone else has to work with imperfect copies.

2. The kilogram is also not completely constant/stable. What if someone dropped it and a chunk broke off? We would all suddenly weigh more! More realistically, gunk collecting on the surface can easily change its size. You don’t want a kilogram that changes in size, because then your measurements will change as it changes.

3. The kilogram isn't based on a fundamental constant, which limits how useful it is. If you want to measure masses much smaller than a kilogram, you start getting bigger and bigger errors on your measurements. That's because you have to start with a large mass, and slice it up into smaller pieces. Basing the kilogram on Planck's constant means you can pick a mass of any size, and measure it directly, rather than starting from a much larger mass.

The official kilogram prototype, called Le Grand K, is only taken out of the vault and used a few times a century. I believe it's been used four times since 1885. In 2014 it was used to help in this effort to redefine the kilogram. It requires three people, each with their own separate key, to open the vault. It's kind of crazy that scientists ascribe this much importance to one physical object!

Did you consider the impact that this might have on the world's drug dealers?

I haven't asked any scientists specifically about that topic. So, no, I did not consider that. But, I can tell you that the size of the kilogram doesn't actually change with this shift, so the amount of your preferred drug that you would get when purchased by weight would also not change.

Given that these units are now going to be expressed in terms of fundamental units; do you think that education at the primary level is going to shift in a way that will focus on using natural units?

I don't think so. We are so used to working with the standard, everyday units of the metric system/SI. They just make so much more sense on a human level. Usually, we want to measure mass in terms of something we can hold in our hand, like a kilogram. In natural units, physicists measure mass in terms of eV (masses of particles, for example) But that doesn't make sense on our human level. We'd be working with huge numbers all the time if we did that. And this change to the SI doesn't affect that fact. I think the SI is here to stay!

How easy/hard is it to measure the Planck Constant?

If you want to measure it extreme precision, it's hard! To redefine the kilgoram, scientists measured the constant to within 10 parts per billion. To do that, they used an instrument called a Kibble balance. It compares electromagnetic forces to the force of gravity, to determine mass. Current running through a wire in a magnetic field generates a magnetic force that counterbalances the mass. Precise quantum mechanical methods of producing voltages mean that the mass can be connected to Planck’s constant.

The scientists that did this work have spent years perfecting it. They really dedicated their lives to it, to the extent that several of them got tattoos of the Planck constant in celebration! You can see a picture at the bottom of my article https://www.sciencenews.org/article/official-redefining-kilogram-units-measurement?tgt=nr

What does this mean for that national weights and measure lab and all the state ones?

People will still keep using the labs to calibrate their scales as they have in the past. But within the labs, they won't be referring back to the official kilogram prototype anymore. Instead they will use a device called a Kibble balance, which uses the value of Planck's constant to determine the mass of an object. At some point, certain industries might start making/using their own Kibble balances, and they won't have to rely on the measurement institutes anymore to calibrate their scales. That's one of the big goals of making this change.

Will fancy scales used to weigh things commercially all need to be recalibrated?

So the way scales are normally calibrated is that they use weights that are sent to measurements institutes (like NIST in the US). Those are calibrated against copies (or copies of copies) of the kilogram. This process shouldn't change much. The change will happen at the measurement institutes, where, rather than using the kilogram to determine the mass of their weights, they'll use what's called a Kibble balance, which measures mass using Planck'c constant. At some point, certain industries might start making/using their own Kibble balances, and they won't have to rely on the measurement institutes anymore to calibrate their scales. That's one of the big goals of making this change.

Could you please name some of the important things which are going to be affected a lot by the redefining of the unit?

It's a little hard to know. Basically this sets us up for the future, to develop new technologies. What those are is not clear. It may help in the pharmaceutical or nanotechnology industries, in which measuring very small masses is important. Historically, having well defined units based on fundamental constants has been important for advancing science and tech. Just for one example, this change is similar to one that was made in 1983, when scientists redefined the second to base it on the speed of light. That redefinition allowed scientists to make technologies that rely on precise timing info, like GPS.

Also, the change of the ampere will have an effect on electrical measurements. The International System of Units dealt with electric current/voltage so poorly that in 1990 scientists came up with sort of a stopgap measure, to allow them to make measurements that were as precise as they needed to be. They used certain quantum effects to set the size of a volt. But that stopgap took them out of the normal system of units. Now, these units are being reincorporated into the standard system, but that results in a change in the units. The volt will change by about 0.1 parts per million, so a tiny tiny amount. That matters for people making quantum measurements.

Wouldn't it also resolve any confusion about unit of weight on other planets?

Weight is a force, not a mass. Your weight depends on gravity, so that will be different on different planets. But for masses, it will make it easy to compare with anyone anywhere in the universe! One scientist once told me that if we ever met aliens, and we told them we used a little metal cylinder to measure masses, “we’d be the laughingstock of the galaxy.” Now, we can easily compare mass measurements with aliens if we were ever to meet them :-) Aliens also have access to Planck's constant, so we'd just have to figure out how to convert our units of mass into whatever units they use.

Does this in turn affect the derivation of the Liter? I know Liters and kilograms used to be linked through water, but the relationship isn't as used anymore, correct?

Yes, the relationship between the kg and water isn't used anymore. This change shouldn't affect the liter, I don't think, because the liter is based on the meter (as it's a unit of volume) and the meter isn't changing in this SI revision. In fact, the meter was already based on a fundamental constant, the speed of light! It's been that way since 1983. Now the rest of the units are catching up.

I thought it was all just based around water?

1 cubic centimetre of water = 1 milli litre water

1 litre of water =1 kilogram

Edit: Oh, we started with this measurement then based every other measurement off of water?

We don't use the liter of water standard, although that was how the kilogram was initially designed. It's true that a kilogram is approximately the mass of a liter of water at a certain temperature, but it's not exact. Currently, the kilogram is based off the mass of a metal cylinder that is kept in a vault in France. So now, we're doing away with that idea. Now the mass of the kilogram will be based off a fundamental constant, Planck's constant, which is an important parameter in quantum mechanics. We're switching over to a system of units that is appropriate for our new, quantum mechanical understanding of the world. So you can think about it in the following way: We started out using some amount of water to define the kilogram, and now we're using quantum mechanics. That's quite an improvement in our scientific prowess!