# Questions tagged [metrology]

The study of measurements

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### What does the symbol $μ$ represent?

I know that μ represents the prefix micro. However, I read in few books (reliable, at least in my country) and other resources, that the symbol μ represents micron(micrometre) too. Now, if the ...
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### Zeptosecond smallest unit of time ever measured? How?

There is this newly discovered zeptosecond discovered by scientists. I have seen some articles calling it smallest unit of time ever measured. How can it be possible if Yoctosecond already exists ...
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### Why is the Planck constant an exact number with defined value?

I found here that the Planck constant is defined as an exact number: $6.626 070 15\times10^{−34}\ \mathrm{J/Hz}$. How could this be done? Shouldn't it be a quantity with uncertainty measured by ...
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### Does speed of light being constant make time a derived unit?

This is a paradox I'm trying to understand. I'm not tackling relativity yet. I'm still working through Walter Lewin's lectures on electro magnetism. However, I understand base and derived units pretty ...
43 views

### What is the difference between current and the ampere?

I have always thought that current has the same definition as the ampere. When this problem comes up: So I instinctively choose answer A. But according to the my book, B is the answer?!?! But why? ...
2k views

### What's exactly the new definition of kilogram, second and meter? [duplicate]

Could one explain this? Technically a kilogram (kg) is now defined: […] by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which ...
67 views

### What do we exactly mean by charge and why the elementary charge of electron is $1.602176634×10^{−19} C$? [closed]

A coulomb is 6.241 x 10^18 elementary charges. But what exactly is a coulomb? And what do we mean exactly by a charge? how was it measured?
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### Quantum Fisher information for Gaussian states without eigenproblem?

Given a Gaussian-preserving interaction (including a unitary operation and losses) for a Gaussian input, I want to know if there is an "easy" way to compute the Quantum Fisher information without ...
123 views

### New S.I. and the second definition standard

The new SI defines the second as the hyperfine splitting transition time associated to the frequency of that transition of caesium-133, 9192631770 Hz, that is 12 decimals precision, but...I wonder two ...