Questions tagged [metrology]

The study of measurements

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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 ...
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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 ...
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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 ...
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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}=...
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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? ...
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CGS of resistance

What's the CGS unit of resistance? Is it the same as that in SI system, i.e. ohm (Ω)? I googled but found no explicit answer.
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What is a joule? I find the definition confusing

This is the definition on Wikipedia: It is equal to the amount of work done when a force of 1 newton displaces a body through a distance of 1 metre in the direction of the force applied. I take that ...
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Notation used to denote the unit $\rm mm^3$

For example, to denote the volume of a cube with the sides $1mm$ we write $1mm^3$. But I think it should be written as $1 (mm)^3$ because the prefix $m$ denote the factor $10^{-3}$ hence $$[1mm^3=10^{-...
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Thermometers and the Celsius scale

Why is the Celsius scale not useful for making extremely accurate temperature measurements? We know that $\delta L/L=\alpha \delta T$. By setting $\delta T$ to $1°$C we can find how much mercury or ...
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Why should a clock be "accurate"?

Having read that atomic clocks are more accurate than mechanical clocks as they lose a second only in millions of years, I wonder why it is necessary for a reference clock to worry about this, if the ...
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Conversion factors derivation

Unit conversion chart Queries regarding MKS to CGS system for the following formulas : Force $=$ Newton (MKS), Dynes (CGS) $$\mathrm {1\ N = 10^5 \ dynes}$$ Work $=$ Joule (MKS), Ergs (CGS) $$\...
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What is the simplest reason why an "entropometer" cannot exist?

Entropy is a measure of statistical nature. Thus, knowing the entropy value of a system requires knowing something of its internal structure, the hierarchy of microstates, and the values of their ...
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Why is 1 newton defined as 1 $\rm kg · m/s^2$?

From my limited understanding, one newton is defined as the amount of force that gives a mass of 1 kilogram an acceleration of 1 meter per second squared. What I don't understand is why it corresponds ...
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h-bar limits measurement precision. What limit arises for h-bar itself?

The question is simple. h-bar, or $\hbar$, limits the precision of every measurement, books tell us. For example, length measurements are limited by the Compton wavelength. What limit for the ...
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Resources for studying quantum metrology

I want to learn about quantum metrology topics like von-Neumann hamiltonian, ABL rule etc., typical case studies of interpretation of measurement, quantum contextuality etc., discussion on what is ...
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Why does the SI unit use $\rm kg$ as it's unit of mass? [closed]

Since the kilogram has a prefix in the the list of SI units would it be wise to redefine the mass of the platinum iridium cylinder at the international bureau of weights and measures as 1g rather than ...
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Why aren't liters a SI Unit? And why isn't there a unit for volume?

In the International System of Units, there is the second for time, the metre for length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for density, and ...
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Why are Amperes and moles both base units?

Current and "amount of substance" are base units The SI system treats both electric current and "amount of substance" as a quantity that is measured in "fundamental" or &...
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Why are there just 3 main units ($L$,$T$,$M$) in physics?

Most physics books define physical units in terms of length, time and mass. Some books add temperature. And yes, the SI unit system has 7 base units, but some are clearly redundant. Why are exactly ...
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Has anyone charged an object with 1 coulomb? Why was such a ridiculously large charge chosen as the unit of charge?

The fact that two balls charged with 1 coulomb each would repel/attract each other from a distance of 1 metre with a force sufficient to lift the Seawise Giant would suggest me otherwise, but has ...
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What happened to the Planck charge on Wikipedia? [closed]

Wikipedia used to have a nice article about the Planck units, with contributions made directly by Don Page. I recently visited the current article and the Planck charge kind of disappeared. I also saw ...
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How do we actually build the new kilogram?

Once upon a time the kilogram was a piece of metal preserved in a vault located in a basement. If I needed to build a high precision balance all I needed to do was tuning the the value of my kilograms ...
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Good book/resource/chapters on the foundations and science of accuracy, precision, and measurement?

I guess measurement and accuracy is a science, and thus, I assume there's foundations and universal theory that can be learned from scratch, independently of international conventions used, just like ...
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Is an ideal Cesium Standard atomic clock more accurate than Einstein's thought experiment light-clock?

This question comes to mind to clarify points in an earlier question. Basically my question is this: According to current international standards the second is defined by the Cesium Standard which is ...
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How is time and space defined? [duplicate]

It occurred to me while discussing a different question on this exchange that in order to understand space and time we must first agree on how it's defined in terms of measurement. My current ...
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What is the concise form of physical constants? (What is the number in the parentheses?)

According to nist : Proton mass $(m_p)$ : Numerical value $: \mathrm{1.672 \ 621 \ 923 \ 69 \ × \ 10^{-27} \ kg}$ Concise form $\ \ \ \ \ \ \ \ \ \ : \mathrm{1.672 \ 621 \ 923 \ 69(51) \ × \ 10^{-...
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What is the name for 100 litres? [closed]

1 litre = 1 litre 10 litres = decalitre 100 litres = ? 1000 litres = kilolitre Is there a scale for the naming like there is for data?
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How much is the inherent quantum-mechanical uncertainty in the definition of the second?

Inspired by this other question. The second is defined such that the electromagnetic radiation whose energy equals the hyperfine splitting of the ground-state of the Cs-133 atom has a frequency of ...
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How long is a second?

According to the latest definition of the SI units in 2019, a second is defined as "$9,192,631,770$ periods of the transition between the two hyperfine levels of the caesium-133 atom $\Delta\nu_{{...
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Is there a nice formula to represent POVMs $\Pi$ in a Bloch-vector-like form?

A way to write a quantum state is to use the Bloch vector representation, i.e., $$ \varrho = \frac{1}{2}\left(\mathbb{I}_2 + \boldsymbol{r} \cdot \boldsymbol{\sigma}\right) $$ In general a POVM for a ...
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Fundamental quantities in physics

I observed that the fundamental units like meter, kilogram, ampere, kelvin and candela are all indirectly dependent on one single fundamental value 'second' and each other. For example: Meter:- 1 ...
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Scientific Notation for Different Units of Charge

Sorry in advance for the basic question, I’m pretty new to physics. I’m doing some electromagnetism homework and so far in class we’ve used only nano and micro coulombs in our force and electric field ...
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Are inverses of fundamental SI units also physical units?

I know that Hertz (1/s) is a physical unit, but are there physical units for 1/m or 1/kg too? For example, in the formula for resolvance of a diffraction grating, $\frac{\lambda}{\Delta \lambda} = mN$,...
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Limits of electromagnetic source detection? [closed]

What distance from the observer a lightbulb must have so you can't say the light was from this lightbulb? Is it possible in astrophysics to detect a farest star or galaxy only measuring separate ...
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How is atomic time defined in Hartree units?

As in Planck units, I believe Hartree units have a fundament unit of time, Atomic Time, something like $2.4188843265857(47)×10^{−17}$ s. My question is what exactly is it that happens in this time? ...
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How to account for nominal resolution in calibration measurements?

I have a real world question but I think it'll be clearer if I phrase it like a "homework" question so here goes*: A physicist is trying to use a laser measuring device to determine the ...
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Is the speed of light exactly 299792458 m/s?

I want to know if there is any uncertainty in the measurement of the speed of light. If yes, please mention the uncertainty.
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2 answers
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Why does the metric system use the Celsius scale rather than the Fahrenheit scale?

Part of why I ask this is because the Fahrenheit scale has a higher degree of precision than the Celsius scale. And the Kelvin scale could just as easily been adapted to be an extension of the ...
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Why are centimetres still in use? [closed]

When SI units were adopted in the UK old, imperial, units were dropped but with a few exceptions for very specific purposes eg pints for beer, pounds and ounces for babies birth weight and feet and ...
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How are the seven base units related to each other?

How are the seven S.I. fundamental or base units of measurement of physical quantities interrelated or dependent on other base units: Universal constants define these seven SI basic quantities. For ...
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Subatomic natural units

In High Energy Physics it seems to be common use to measure everything in terms of eV powers, by assuming $\hbar = c = 1$ (dimensionless). Often times this system of units is referred as Planck units, ...
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12 answers
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Definition of a joule

I'm not getting the definition of a joule. From the definitions I've read if I apply one newton of force to any object, now matter how heavy/ how much mass it has, over one metre in a single direction ...
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Is a second now defined using 9,192,631,770 Hz or 9,192,631,770.2 Hz of a caesium atom?

This is obviously splitting hairs, to an extent, but I am genuinely curious... And it does make a difference... In several places, I have seen that .2 added on to the figure... Did the length of a ...
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When and why are units measured in degree?

I was always under the impression that a unit is measured in degree if it is a relative unit. While this is true for temperatures and angles, I cannot find any reference to this definition or any ...
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Symbol of quintal

Depending on the source, I have come across the symbol of quintal as 'q', 'qt'and 'qtl'. Is there a standard symbol for quintal. If yes what is it?
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Why the need for more sophisticated atomic clocks?

Why is there such a huge effort in building more sophisticated atomic clocks when the current ones achieve $1$ second of error every few hundred million years? Some achieve frequency stability close ...
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9 answers
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How are physics theorems so accurate, relative to fixed measurement systems? [duplicate]

I admit this sounds like a dumb question. I know very little about physics, but I've always wondered that given the fact that: A unit of measurement is a very specific constant value. Then how can a ...
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Expansion of universe effects on mass

It has been noted that the metal alloy model previously used to characterize the value of 1 kilogram has been steadily increasing in mass due to unknown causes. Could this be a local effect of ...
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168 votes
21 answers
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How do you make more precise instruments while only using less precise instruments?

I'm not sure where this question should go, but I think this site is as good as any. When humankind started out, all we had was sticks and stones. Today we have electron microscopes, gigapixel cameras ...
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Why did the scientific community stop using hydrogen as the main measure for the atomic mass unit?

The only thing I could find is that the community switched form one hydrogen atom to 1/16 of Oxygen-16 (which they thought to be the only isotope that exists at time because they had no concept of ...
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