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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 ...
Singer Asvj's user avatar
1 vote
0 answers
59 views

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 ...
RC_23's user avatar
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2 answers
75 views

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 ...
San26's user avatar
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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? $8.314 \frac{J}{mol×K}=8.314 \frac{J}{mol×°C}$
Malcolm's user avatar
1 vote
1 answer
52 views

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?
Manish's user avatar
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2 answers
160 views

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 ...
Gabriel Tellez's user avatar
16 votes
4 answers
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 ...
cms's user avatar
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1 answer
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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/...
russell.price's user avatar
4 votes
1 answer
217 views

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 ...
niobium's user avatar
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0 answers
47 views

Spin-squeezing scaling with the number of particles in one-axis twisting hamiltonian

I am exploring the one axis twisting (OAT) hamiltonian $\hat{H}=\chi S_z^2$ with $S_z=\sum_{i=1}^N\frac{\sigma_z^i}{2}$ and considering the initial state to be $\left|\psi(0) \right>=\left|+x\right&...
Camilo160's user avatar
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1 answer
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Abbreviations for energy and torque

What are correct abbreviations for torque and energy in US Customary and S.I. systems? Both include distance and force. Do we write N-m and lbf-ft for both torque and energy? I always heard (years ago)...
Bill's user avatar
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1 vote
0 answers
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Techniques to measure/picture very low (cryo) temperatures [closed]

I am looking for an experimental technique to measure/create a temperature map of a small object (few mm diameter), like in terms of IR camera. However, all IR-cameras I encountered measured ...
Agnieszka's user avatar
  • 195
2 votes
1 answer
79 views

Fisher information of parametric channel

Suppose $\Phi_\theta$ is a quantum channel whose action can be written for any state $\rho\in \mathcal S(\mathcal H_S)$ in the Stinespring representation as $\Phi_\theta(\rho)= \text{Tr}_E(U_\theta (\...
Quantastic's user avatar
4 votes
2 answers
91 views

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 ...
SpectraXCD's user avatar
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1 answer
64 views

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 ...
Shinnaaan's user avatar
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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?
NotAScientist's user avatar
3 votes
2 answers
467 views

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,...
Yif's user avatar
  • 33
3 votes
8 answers
1k views

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 ...
Dinesh Katoch's user avatar
-1 votes
1 answer
79 views

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 ...
elechris's user avatar
1 vote
2 answers
76 views

Definition of proportionality constant in Coulomb's law

My textbook, which was written before 2019, says: $k$ [Coulomb's constant] is defined in terms of $c$ to be precisely $k=(10^{-7}\ {\rm N\cdot s^2/C^2)}c^2$ $\implies\displaystyle\frac{1}{4\pi\...
Shoes's user avatar
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1 vote
0 answers
66 views

Melting point of water [duplicate]

Wikipedia says about melting point that: The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K); this is also known as the ice point. But why it is very close and ...
Harjot Dhillon's user avatar
1 vote
2 answers
80 views

Is Coulomb written/found on any object/product as its unit?

Today I had physics class and the unit Coulomb made me wonder as to why it was even invented when I don't see it being used anywhere. When I asked where it may be shown just like bulbs have x Watts ...
Moon_Hawk77's user avatar
-1 votes
1 answer
60 views

I am confused about the apparent fact that the numerical values of the Planck units are unchanged by exchanging $\hbar$ for $h$ [closed]

Can you confirm that each of Max Planck's original equations for Planck units included $h/2π$. If yes, does this explain how $ħ$ can be used In Planck's formulas without changing the numerical results ...
Frank Burdge's user avatar
0 votes
2 answers
167 views

Why isn't the charge of an electron taken to be "$-1.6 \times 10^{-19}$ coulombs" [closed]

Even though we refer to electrons being negatively charged, why is it that we don't write the charge as "-1.6 × 10^-19 coulombs"?
Darth Nandan's user avatar
1 vote
0 answers
35 views

Connection between quantum non demolition measurement and the Heisenberg limit in metrology

Quantum non demolition measurements are those that do not feed back action noise into the measured observable. For discrete systems, this leads to the somewhat trivial case of where the observable (...
j.foobles's user avatar
-2 votes
2 answers
103 views

Why can the Ampere not be defined as the flow of $n$ Coulomb in $n$ seconds?

1 Ampere is defined as the flow of 1 Coulomb of charge in one second. However, I do not understand why it cannot be defined as the flow of n Coulomb of charge in n seconds. This definition is ...
Smarika Singh's user avatar
22 votes
6 answers
4k views

How do you know mercury changes monotonically with temperature if mercury itself is used to make the thermometer?

In the book I am reading recently "Concept of physics" volume 2 by professor H.C. Verma it says that (I am just summarizing the main points chronologically) Energy is transferred from hot ...
Rahul Einstien's user avatar
-3 votes
3 answers
115 views

What exactly is kg? [closed]

Kilogram literally makes no sense. We say that we are 10 kilogram by using the weighing scale but according to its definition, it is the amount of matter contained in an object. So how much matter is ...
Hatadi V's user avatar
0 votes
1 answer
60 views

Correct way to reference to a percentage value's category/label

When a percentage is written as, for example: 20% of sold products were of product A and 80% product B. What is the correct concept / word to use to describe "product A"? Is it units? As ...
AJP's user avatar
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0 votes
2 answers
314 views

Notation: units with negative exponents

I'm not sure if this belongs on Meta or here but: In many scientific journals, books, and posts on this site and others, I see the negative exponent convention used for units, e.g. $\mathrm{N} \,\...
RC_23's user avatar
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0 votes
0 answers
27 views

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 ...
Tooba's user avatar
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1 vote
0 answers
28 views

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 ...
Michael.Andy's user avatar
-1 votes
5 answers
148 views

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 ...
Maxim Lazovsky's user avatar
6 votes
1 answer
251 views

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 ...
KF Gauss's user avatar
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4 votes
3 answers
1k views

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 ...
SpectraXCD's user avatar
0 votes
2 answers
89 views

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 ...
Steve's user avatar
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1 vote
3 answers
381 views

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 ...
MajorDave's user avatar
1 vote
1 answer
95 views

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 ...
moonblink's user avatar
  • 947
1 vote
0 answers
160 views

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 ...
alisaleh's user avatar
-6 votes
2 answers
127 views

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?
Wanna be a scientist's user avatar
2 votes
1 answer
38 views

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 ...
Koh Yi Min Jason's user avatar
1 vote
1 answer
88 views

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 ...
Jagerber48's user avatar
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0 votes
0 answers
50 views

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 ...
qmstatmech's user avatar
10 votes
4 answers
3k views

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 ...
j riv's user avatar
  • 409
0 votes
0 answers
80 views

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, ...
ramcdougal's user avatar
4 votes
1 answer
136 views

How to reinvent measurement units?

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.) ...
Ri Di's user avatar
  • 151
2 votes
2 answers
210 views

Why use units of $\rm 1/Hz$ instead of $\rm s$?

The Wikipedia page for Planck's constant frequently includes the constant in units $\text{J/Hz}$ or $\text{J} \times \text{Hz}^{-1}$. Is there a reason these units are used instead of $\text{J} \times ...
weathergirl7's user avatar
13 votes
4 answers
5k views

How it was found that 12 g of carbon-12 has Avogadro's number of atoms? [duplicate]

I just want to know the history of finding Avogadro's number of atoms in 12 gm of C-12 and why C-12 only? Like I was asking how scientists came to conclusion that there is exactly 6.023*10^23 atoms in ...
S.Rishabh's user avatar
  • 147
1 vote
0 answers
54 views

How much does the International Prototype Kilogramme weigh?

From [some date in] 1889 to 20 May 2019 the International Prototype Kilogramme in Paris (called the ur-kilogram by German-influenced writers) weighed $1.000\,000\,00kg$. It seems very hard to find out ...
Martin Kochanski's user avatar
0 votes
2 answers
207 views

Why the specific factor of $2 * 10^{-7}$ is used while defining 1 ampere?

while defining an ampere through force between two current-carrying wires, I noticed that they considered the force between two wires as $2 \times 10^{-7}$ forming a definition as follows:- "One ...
Shaishav Sharma's user avatar

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