massmetrologyphysical constantssi-units
Scientists are going to redefine the kilogram by using the value of Planck's constant. What is the physics behind this would-be definition of a kilogram? How is Planck's constant related to kilogram?
EDIT:
I read some articles about it and watched this video https://youtu.be/Oo0jm1PPRuo
But I couldn't understand it, since I don't know much about Quantum Theory. It would help me a lot if I get a simpler explanation
Following on from Jon's comment above, this is from the NIST Website (Dated June 21, 2016), discussing a more accurate measure of the kilogram, and the involvement of Planck's constant.
A high-tech version of an old-fashioned balance scale at the National Institute of Standards and Technology (NIST) has just brought scientists a critical step closer toward a new and improved definition of the kilogram. The scale, called the NIST-4 watt balance, has conducted its first measurement of a fundamental physical quantity called Planck’s constant to within 34 parts per billion – demonstrating the scale is accurate enough to assist the international community with the redefinition of the kilogram.
Planck’s constant lies at the heart of quantum mechanics, the theory that is used to describe physics at the scale of the atom and smaller. Quantum mechanics began in 1900 when Max Planck described how objects radiate energy in tiny packets known as “quanta.” The amount of energy is proportional to a very small quantity called h, known as Planck’s constant, which subsequently shows up in almost all equations in quantum mechanics. The value of h – according to NIST’s new measurement – is 6.62606983x10-34 kg∙m2/s, with an uncertainty of plus or minus 22 in the last two digits.
The NIST-4 watt balance has measured Planck’s constant to within 34 parts per billion, demonstrating that the high-tech scale is accurate enough to assist with 2018’s planned redefinition of the kilogram.
I have found the answer to my own question: The Kibble balance. For the technical details, see this article.
Best Answer
So a second used to be defined as one 86,400th of one day: but now we have these cool things called atomic clocks which measure time very, very precisely: we have therefore decided to define the second as a certain number of ticks of a certain type of atomic clock, so that anybody who needs really high precision can have it. With these new clocks we can see that actually the length of a day is very complicated and, in fact, we can see that our days are slowly lengthening due to tidal forces. Our definition used to line up with the old definition of the second extremely well, but that has become less so as the Earth slows its rotation; right now there are "leap seconds" injected into our time system to try to keep 12:00AM at midnight, so some days are 86,401 seconds long.
Similarly a meter used to be defined as one 40,000,000th of the circumference of the Earth, but when light-interferometry became easier and we had learned that actually, light moves at the same speed $c$ in all directions for all inertial reference frames, we redefined meters in terms of the distance that light travels in one second. As before, we tried very hard to get the new and old definitions to match up for all practical purposes. But as a result, our value for the speed of light has infinite precision; if light were any faster than it happens to be, then the meter would by definition be longer to make light travel at exactly $299,792,458\text{ m/s}$. So choosing a natural constant to have a fixed value in our units, or choosing a unit to be a certain size, are the same general procedure.
Similarly in the early 1900s we found out that light comes in lumps of energy; Planck's constant relates the frequencies of these photons to their energies: so for example a photon of frequency $f$ has energy $E = h f.$
We can do the same trick that we did with $c$, but now with $h$: now that we have been able to measure $h$ to very high precision in our conventional units, we can define $h$ to infinite precision by redefining how we measure energy.
Right now we measure energy in units of $\text{kg}~\text{m}^2/\text{s}^2.$ Remember that we defined $\text m$ in terms of the distance that light travels in one second, so really these units of $\text m/\text s$ correspond to some fixed number times the already-numerically-fixed speed of light $c$. So if we don't change how we measure $\text{m}$, then this redefinition of energies so that $h$ is now an infinite-precision constant, demands that we now measure $\text{kg}$ by this new standard. Essentially, one could say that the photon corresponds to a mass $E = m c^2$ and therefore any measurement of energy is a measurement of mass.
This last point isn't 100% off-base: if you consider an electron which annihilates with its antimatter relative the positron, the mass of both of those must be converted entirely into photons, and this is usually two photons travelling in opposite directions in the center-of-mass frame, and you could measure the energy of either of these photons to determine the mass of the electron. So you use your super-amazing atomic clocks to measure the frequency of the photons, we have fixed $h$ so that tells you $E$, and now you have measured the mass of the electron as $m = E/c^2.$ That's the basic idea.