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.
358
questions
3
votes
2
answers
413
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,...
- 33
3
votes
5
answers
897
views
Relationship between bel and decibel
Bel is a unit of $log_{10}$ of ratio of two quantities.
1 Bel = $\log_{10}\frac{P_1}{P_2}$
On Wikipedia it says: 1 decibel = $\frac{1}{10}$ bel
According to this definition then,
1dB = $\frac{1}{10}$ $...
- 321
-1
votes
1
answer
65
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 ...
- 1
1
vote
2
answers
55
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\...
- 75
1
vote
0
answers
65
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 ...
1
vote
2
answers
70
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 ...
- 21
-1
votes
1
answer
54
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 ...
0
votes
2
answers
110
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"?
- 17
1
vote
0
answers
23
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 (...
- 19
-2
votes
2
answers
53
views
Why can the Ampere not be defined as the flow of $n$ Coulomb in $n$ seconds? [closed]
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 ...
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 ...
- 355
-3
votes
3
answers
98
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 ...
- 7
0
votes
1
answer
53
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 ...
- 277
0
votes
2
answers
139
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} \,\...
- 8,458
0
votes
0
answers
23
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 ...
- 771
1
vote
0
answers
23
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 ...
- 157
-1
votes
5
answers
140
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 ...
5
votes
1
answer
167
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 ...
- 7,384
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 ...
- 151
0
votes
2
answers
70
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 ...
- 15
1
vote
3
answers
218
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 ...
- 21
1
vote
1
answer
86
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$), which does not take into account the ...
- 927
1
vote
0
answers
85
views
Why are the seconds/minutes/hours units are universally used, while other measures have several units? [closed]
There are several units to measure length like meter, foot, and rod. The same is true for weight. While time seem to have a universal unit which is the second. Was there other measures of time that ...
- 11
-6
votes
2
answers
113
views
What are light years? like you know how people measure some things in light years? what is light years [closed]
so basically you know how scientists measure how far things are but in space and they measure in light years well what are light years and how long are light years?
2
votes
1
answer
36
views
Confusion on metrological terminologies in physics
I had came across many physicists or engineers use words like Measurable, Detectable, Observable and Quantifiable.
Are those words synonymous in physics in general, in the field of metrology or in ...
1
vote
1
answer
63
views
Relativistic Time-Keeping and Sharing
Suppose there are 2 ships which keep time using atomic clocks. The atomic clocks are the same build so it is know that the two keeps keep proper time at the same rate. Suppose that the two ships ...
- 12.9k
0
votes
0
answers
45
views
How is the SI unit for mass defined (as of 2019)? [duplicate]
So, the SI unit of mass is defined by taking an exact value for Planck's constant, which is units of J$\cdot$s = kg m$^2$ s$^{-1}$, and meter and second already defined in ways I understand. I don't ...
- 33
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 ...
- 409
0
votes
0
answers
69
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, ...
- 101
0
votes
0
answers
15
views
Fastest most accurate orientation measurement of an object while intermediary objects are involved to pass relative information
I am looking for the fastest most accurate way to have the orientation of an object measured optically while intermediary objects are involved between the source and the target. An example scenario ...
- 101
4
votes
1
answer
125
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.) ...
- 151
2
votes
2
answers
150
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 ...
- 21
13
votes
4
answers
4k
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 ...
- 147
1
vote
0
answers
53
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 ...
- 1,970
0
votes
2
answers
139
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 ...
0
votes
0
answers
35
views
Base units in CGS and technic systems
What are the base units in the CGS and the technical system of units? Is there not seven? How could it be that what is fundamental in any system is not in other?
By the way, how many minimal "...
- 6,379
7
votes
1
answer
368
views
Illogical choice for the new definition of the charge of the electron by the 28th General Conference on Weights and Measures
The present definition of the Coulomb, since May 20th 2019, is that one electron has a charge of exactly
$1.602176634 \times 10^{-19} $C
Previously, the Coulomb was defined (through the Lorentz force ...
- 4,153
3
votes
2
answers
133
views
What limits mechanical watch precision? [closed]
It seems like good mechanical watches gain or lose +/- a few seconds per day. 5 s/day corresponds to a fractional frequency stability of $5\times 10^{-5}$. What limits this stability?
Obvious ...
- 12.9k
5
votes
2
answers
210
views
What determines the existence of a physical dimension, as in dimensional analysis?
Dimensional analysis in physics is based on the idea that only commensurable quantities can be added, subtracted, equated or otherwise compared. The five dimensions which appear in dimensional ...
3
votes
2
answers
137
views
Metrology: How are the most accurate measurements made? [closed]
How is a most accurate measurement made when there is no other equipment to verify it? Consider you base your apparatus on a set of theory and assumptions, and the result does not match prediction. ...
- 35
6
votes
4
answers
4k
views
"Earth spinning faster will break GPS and atomic clocks".. Umm, how?
Since it's just come out that Earth is apparently spinning about a milliscecond and a half faster.. I see a bunch of "articles" claiming this will.. somehow.. break atomic clocks and make ...
- 93
2
votes
3
answers
166
views
Calling angles dimensionless and simultaneously distinguishing between numbers and angles is inconsistent, isn't it?
The SI Brochure says that each angle is the ratio of two lengths - i.e. a number - and therefore a derived and dimensionless quantity. On the other hand, the papers [1] and [2] suggest that angles are ...
- 1,781
3
votes
1
answer
106
views
In what sense is this a feedback control?
I am reading a couple of papers on control enhanced parameter estimation. One is titled "Optimal Feedback Scheme and Universal Time Scaling for Hamiltonian Parameter Estimation" (arXiv ...
- 439
18
votes
5
answers
6k
views
Is the definition of a meter tautological?
The speed of light is defined as $c=299{,}792{,}458\,\mathrm{m/s}$, and a meter is defined as the distance that light travels in a $1/299{,}792{,}458=1/c$ of a second, but then we would have defined a ...
- 307
3
votes
1
answer
64
views
What is a "cycle time" in Allan deviation formula for atomic clock instability? And why does having more independent atoms reduce $\sigma_y(\tau)$?
The usual formula for clock instability is given as
$\sigma_y(\tau)\approx\frac{\Delta f}{f_0\sqrt{N}}\sqrt{\frac{T_c}{\tau}}$
First off what do each of these symbols really mean?
What is $T_c$? The ...
-5
votes
1
answer
115
views
Why there is no unit of energy? [duplicate]
In the SI System (https://en.wikipedia.org/wiki/International_System_of_Units) there are 7 units, but no unit of energy, though this is surely the fundamental unit of physics.
Yet the Joule is usually ...
- 671
1
vote
1
answer
81
views
The definition of Time and second
From the book "University of Physics 15th edition", in chapter 1, they talk about the fundamental units. They stated that the definition of unit of time is based on an atomic clock, where 1s ...
- 15
0
votes
1
answer
70
views
How is the Heisenberg Uncertainty calculated value added to a measurement error bar?
It is not clear to me if the calculated HU for a specific experiment is added to the measured value Gaussian curve together with the statistical errors (i.e. added to the total error bar) of this ...
- 3,768
3
votes
2
answers
105
views
Can the SI kg be realized without superconducting josephson junctions?
This question is motivated by me trying to finish my answer to this question.
In the 2019 SI redefinition the kg was redefined in terms of Planck's constant (and the second and meter):
$$
1 \text{ kg}=...
- 12.9k
29
votes
3
answers
3k
views
What technology is needed for an individual to reproduce the current SI meter and kilogram from scratch?
With the 2019 redefinition of the SI base units, I wonder what kind of technology is needed to reproduce the meter and kilogram in practice from scratch with a tolerance of ±0.1 mm and ±0.1 g? ...
- 415