# Questions tagged [metrology]

For questions about the science of measurement, such as how units are defined or how practical measurements are connected to those definitions.

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### Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
1 vote
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### Mass of the Prototype $\rm kg$

With the new SI kg based upon Planck's constant $h$, is there any plan to eventually measure the former IPK using a Watt Balance, and determine how far its mass deviates from the new definition of 1 ...
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### What is the key difference between SI and MKS units?

In MKS as well SI system, the length, mass and time are measured in the units meter, kilogram and second, respectively. Then how the SI units are different from MKS units? What are the key ...
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### What is the value of the ideal gas constant in J/(mol*°C)? [duplicate]

I've seen in some sites that it is the same as in J/(mol*K), is that true? $8.314 \frac{J}{mol×K}=8.314 \frac{J}{mol×°C}$
1 vote
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### Fundamental and derived physical quantities

I read that fundamental physical quantities are independent of each other but, if we write length = velocity x time, then length depends on the time interval so how come it is independent?
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### Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
4k views

### How long is a second on the moon?

The Office of Science and Technology Policy, part of the government of the United States of America, issued this statement outlining the need for a Lunar Standard Time (LST) standard. They state: For ...
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### Clarification regarding the meaning of Universal Time UT1

I've been reading the book "From Sundials to Atomic Clocks: Understanding Time and Frequency" by James Jespersen and Jane Fitz-Randolph which is available at https://www.nist.gov/system/...
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### Has there been a big change in 1983 when the definition of the metre changed?

The metre was defined at the end of the $18^{th}$ century as the ten-millionth part of the quarter of the meridian (from the north pole to equator). Then, from $1983$ the definition changed for the ...
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### What was defined first and how? The ampere or the vacuum permeability?

I've been looking up the history and evolution of the seven base units and am currently checking out the ampere. What I've found is that 1A is defined as the current in a wire which would experience a ...
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### Kilogram is SI unit but not gram at the same time meter is SI unit and not kilometer. What decides whether a unit should be considered as SI or not?

centi, milli,kilo,micro etc all are prefixes used in SI system of measurement. So in centimeter both the prefix-centi and meter belong to SI system but centimeter is not SI unit. If it is not SI unit ...
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1 vote
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### Is there something special about the 7 fundamental quantities or are they just convention? [closed]

I mean could we do as good with any fewer (or more) fundamental quantities, or is seven really a magical number when it comes to them?
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### The relationship betwen Equivalent Megatons and Yield

This article defines a concept called Equivalent Megatons (EMT) and gives a formula of calculating it in terms of Yield (measured in megatons). In evaluating the destructive power of a weapons system,...
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### Relationship between bel and decibel

Bel is a unit of $\log_{10}$ of ratio of two quantities. $$1\,\text{bel} = \log_{10}\frac{P_1}{P_2}$$ On Wikipedia it says: $1\,\text{decibel} = \frac{1}{10}\,\text{bel}$. According to this definition ...
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### What was the exact triple point of water in the pre-1954 definition of Celsius and in the post-2019 definition of Celsius?

In 1954 the definition of Celsius was changed in terms of absolute zero and the triple point of water. And the triple point was chosen to be exactly 0.01C. Since there is no physical reason for the ...
1 vote
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### Do entangled coherent states violates Heisenberg limit?

In the ideal quantum information version of metrology, a maximally entangled state is viewed as the best resource for quantum metrology. The optimal phase uncertainty of the NOON state reaches to the ...
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1 vote
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### What are the similarity and difference between quantum fidelity estimation and parameter estimation problem?

Quantum fidelity estimation is to estimate the similarity between two quantum states or process. Could quantum fidelity be viewd as a parameter? And what are the similarity and difference between ...
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### Isn't defining the unit of length in terms of the speed of light kind of faulty? [closed]

well I was reading Halliday and it defined the standard of length as follows "The standard of length, which corresponds to exactly 1.0 meters, is the distance travelled by light in a vacuum ...
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### What is stopping optical clocks from redefining the second?

Optical clocks, based on optical transitions either in cold atomic lattices or trapped ions, have been shown to up to one million times better accuracy/precision compared to the cesium microwave ...
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### Defining the second by an alien civilization [closed]

(The above question could be phrased better, so feel free to suggest.) Like many, I was imprecisely told that the second is 9192631770 oscillations of caesium valence electron (or smth along those ...
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### Why do we measure plane angle in radians and solid angle in radians and steradians respectively rather than degrees? [duplicate]

Recently, I learnt about physical quantities. When i got to know about plane angle and solid angle, i had a doubt that even though they are just angles, why do we measure it in radians or steradians ...
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1 vote
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### Is it possible to have non-linear relationship between temperature scales?

Is it possible to have a hypothetical temperature scale that does not follow a linear relationship with another temperature scale? For example, the graph formed between the readings taken by a ...
1 vote
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### Describing the strength of a magnetic field

There is one thing I can never get right. If I want to describe the strength of a gravitational field I can use the standard gravitational parameter ($\mu$), which does not take into account the ...
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1 vote
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### Why are the seconds/minutes/hours units are universally used, while other measures have several units? [closed]

There are several units to measure length like meter, foot, and rod. The same is true for weight. While time seem to have a universal unit which is the second. Was there other measures of time that ...
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### What are light years? like you know how people measure some things in light years? what is light years [closed]

so basically you know how scientists measure how far things are but in space and they measure in light years well what are light years and how long are light years?
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### Confusion on metrological terminologies in physics

I had came across many physicists or engineers use words like Measurable, Detectable, Observable and Quantifiable. Are those words synonymous in physics in general, in the field of metrology or in ...
1 vote
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### Relativistic Time-Keeping and Sharing

Suppose there are 2 ships which keep time using atomic clocks. The atomic clocks are the same build so it is know that the two keeps keep proper time at the same rate. Suppose that the two ships ...
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### How is the SI unit for mass defined (as of 2019)? [duplicate]

So, the SI unit of mass is defined by taking an exact value for Planck's constant, which is units of J$\cdot$s = kg m$^2$ s$^{-1}$, and meter and second already defined in ways I understand. I don't ...
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### If we had just invented the first clock, and we only had a calendar system, how would we set the time of day for the first time?

I've noticed there are extensive answers on this website about the accuracy of atomic clocks and how they reference the time between each other with the average of time between each other, but I ...
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### How accurately do we know how long a meter is?

A meter is, by definition, a specific fraction (1/299792458) of the distance light travels in a vacuum in one second. How accurately do we know how long this is? In principle, it's exactly defined, ...
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Imagine you're in deserted island. You will eventually need to know how much there is of something or how long is some thing. Is there a way to get all main measurement units (kg, m, °C, m$^3$, etc.) ...