Questions tagged [metrology]

The study of measurements

Filter by
Sorted by
Tagged with
0
votes
2answers
91 views

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 ...
-1
votes
2answers
58 views

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 ...
16
votes
3answers
4k views

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 ...
8
votes
5answers
2k views

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 ...
0
votes
2answers
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? ...
11
votes
3answers
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 ...
1
vote
1answer
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?
0
votes
1answer
77 views

Kg or kg: notation

Often in many elementary and secondary school books frequented by students aged 6 to 13 years old still write $\mathrm{Kg}$ instead of $\mathrm{kg}$. Many teachers give to the students this notation $\...
1
vote
2answers
69 views

How robust is the international atomic time standard against malicious actors?

This answer explains the international atomic time standard (TAI). The fact that the Russian GLONASS and American GPS systems are referenced to their national clock systems, which are then ultimately ...
69
votes
3answers
6k views

What reference clock is an atomic clock measured against?

I looked at a few of the other posts regarding the accuracy of atomic clocks, but I was not able to derive the answer to my question myself. I've seen it stated that atomic clocks are accurate on the ...
0
votes
1answer
36 views

What is the effect of a flat mirror's surface flatness as a function of beam wavelength

Mirror surface flatness is specified as a multiple of $\lambda$, where $\lambda$ is the wavelength at which the surface flatness was tested, and is typically $\sim 632\ nm$. A $4\lambda$ surface ...
4
votes
1answer
44 views

Why Fizeau interferometers are much more popular than point diffraction interferometers for optical metrology?

If somebody wants to measure surface error of for example concave spherical surface - in most of the optical companies around the world Fizeau interferometer will be used, with relatively large, high-...
0
votes
0answers
26 views

How to handle uncertainty introduced by low resolution measurements

When trying to convert a continuous variable (running time of a piece of code) into a quantised one (s on a clock), I've been trying to work out the effect of instrument resolution on the uncertainty ...
0
votes
3answers
89 views

How do we introduce an hour as a time unit?

Historically, people measured time with the help of naturally recurring phenomena (say, a day). Then a day was split into smaller chunks (an hour, a minute, a second). Suppose we define an hour as 1/...
1
vote
2answers
196 views

Why does $\epsilon_0=1$ in CGS units?

This Wikipedia article gives a table of certain constants in different systems of units. I noticed that in Gaussian and electrostatic CGS the value of $\epsilon_0$ equals the dimensionless $1$. I ...
1
vote
2answers
55 views

What are the accepted abbreviations for Million Years and Billion Years?

I'm putting together some charts and the time axis in one shows billions of years, the time axis in another shows millions of years. I've seen in various publications $my$ and $myr$ (also $ma$) for ...
1
vote
1answer
53 views

Phase unwrapping in interferometry

I observed that in "interferometry" is used a process called phase unwrapped. This process can be done by using complicated algorithms but I was not able to found an example more simplistic. If somone ...
5
votes
1answer
223 views

Is there a need for the unit mole? [duplicate]

If 1 mole of X is approximately $6.02 \cdot 10^{23}$ X, why is it a unit? How does it have the characteristics of a unit? Is it just a different way of writing the Avogadro's constant? Does amount ...
0
votes
0answers
31 views

Is there a use of the SI units gray, sievert and katal in pure physics

Since the gray and the sievert are associated with relative biological effectiveness of the radiation and the katal is associated with catalytic activity, do the gray (Gy), sievert (Sv) and katal (kat)...
0
votes
0answers
39 views

Why isn't length a derived quantity? [duplicate]

I was going through the first few lessons in my physics course and came across a section which defined some of the fundamental units in classical mechanics ( length, mass and time ) and found that the ...
0
votes
2answers
116 views

Why is current measured in coulombs per second and not in electron per second?

If electricity is produced by flow of electron, that is current, then current should be measured in electron per second, why is it measured in coulombs per second?
-2
votes
2answers
129 views

Definition of 1 second [closed]

"One second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom." Can we alternatively define it as, Frequency ...
0
votes
5answers
160 views

Why is Coulomb's constant not unity? [duplicate]

Just as in Newton's 2nd law the value of constant was completely arbitrary (as said in ncert textbook class 12 physics) and was chose to be 1. Why not Coulomb's constant is chosen as unity?
17
votes
3answers
4k views

Why is ampere still a base unit? [duplicate]

The ampere is still a base unit, according to the SI brochure. However, in my perception the recent redefinition of units effectively defines the Coulomb as e/(1.602 176 634 × 10^−19), and the ampere ...
0
votes
2answers
42 views

Source for updated values of physical constants

Looking for the value of the Molar Gas constant. I am unsure because NIST has 8.314 462 618 J/(mol*K) while Wikipedia says ...
2
votes
0answers
48 views

Why is there a Moore's Law for atomic clocks?

From the NIST website: If you add the more recent data points, like this one in 2019, which achieved $~10^{-17}$ fractional instability, the trendline continues. Why should there be a Moore's law ...
13
votes
3answers
601 views

Why was the Planck constant $h$ fixed to be exactly $6.62607015\times10^{−34}\text{Js}$ and not some other value?

So apparently in May, all of the SI base units were redefined to be relative to the Planck Constant $h$, instead of relying on physical objects like the Kilogram Prototype in a Paris Basement. Planck'...
4
votes
5answers
442 views

Why is there no constant factor in the equation $E=mc^2$? [duplicate]

As I understand, the metric system starts with an arbitrary weight, and calls it $1\, \mathrm{kg}$, then the volume of this weight in water gives the meter. And then the energy is defined based on the ...
-5
votes
2answers
219 views

Why is the ångström not a metric unit? And why is the ångström spelled with the Swedish/Finnish letters “å” and “ö”? [closed]

The website here http://unitsofmeasure.org/ucum.html tells us whether every unit is metric or not. Metric units can be multipled by a power of 10 and can be combined with a prefix. 1 ångström is ...
0
votes
1answer
33 views

Electrostatics, measurement of charge

How did physicists come up with a standard for measuring charge? How an electrical measurement of charge was made?
0
votes
1answer
265 views

Why $1 \ \mathrm{fm}$ is often used in modern physics?

In the International System, we all know that a one femto (or one Fermi) is equal to $$1 \ \mathrm{fm}=10^{-15} \ \mathrm{m}$$ What is the historical reason why this unit of measure was adopted as ...
0
votes
1answer
401 views

Why is the triple point of water defined as 0.01 degrees Celsius and not 0?

This was stated in my thermo dynamics lecture today and I tried to ask my lecturer why it was not just defined as 0 since 0.01 seems weirdly specific. She was mentioning something about the order in ...
1
vote
6answers
157 views

Standard Definition of speed of light and metre

The speed of light is the speed at which lightwaves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per ...
-3
votes
1answer
60 views

Changes to Planck constant historically [closed]

I looked all over to get the various accepted value of the Planck constant since 1900. But there is never any record about the history of this mysterious number.
2
votes
1answer
127 views

Why is a Coulomb the charge of $6.24 \times 10^{18}$ electrons? [duplicate]

Where does the $6.24 \times 10^{18}$ number come from? How was it historically derived? I know that $1$ C $=$ $1$ A s but that just pushes the question down another step, and another and another, at ...
3
votes
0answers
137 views

What is the difference between quantum sensing and quantum metrology?

The title is mostly self-explanatory. Both terms get thrown around a lot. I used to think quantum sensing uses harmonic oscillators / bosons and quantum metrology spins, but this doesn't seem to ...
1
vote
1answer
82 views

How are energies determined in gamma ray calibration standards?

In operating scintillation gamma ray detectors, certain gamma ray standards are used for calibration: Energy (KeV) Na-22, 511 Mn-54, 835 Co-57, 122 How are ...
5
votes
3answers
159 views

Can we determine the date just from sky observations?

Assuming everyone "freezes" for several thousand years. Then everyone unfreezes at the same time. Could we tell how much time passed based on sky observations without fancy telescopes? (Just started ...
-3
votes
2answers
103 views

Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...
-2
votes
1answer
381 views

How to increase Sensitivity and precision of a measuring sensor?

Considering the definition that Sensitivity: Relation between the maximum of range, the sensor can $\frac{i}{p}$ to the corresponding $\frac{o}{p}$ given by the sensor due to it. Ex: A pressure ...
1
vote
1answer
104 views

Does Fisher Information quantify only the precision of the instrument?

Looking at perspective from estimating the actual value from a set of data measured by the instrument. Does Fisher information just quantify the precision of the measurement? What does it say about ...
0
votes
0answers
43 views

Uncertainty in length measurement

I am trying to measure the length and the uncertainty in length of a fiber optic cable for measuring the speed of light. So far, I have measured it the standard way: Using a standard meter scale. I ...
2
votes
1answer
83 views

Why does one Coulomb equal that strange number? [duplicate]

Why is one Coulomb equal to 6.24 *10^18 ? This number is weird . why wasn't it a nice number like 10^20. I have some guesses about this.
5
votes
1answer
88 views

What is the advantage of a cesium atomic clock over a hydrogen maser as a frequency standard?

Even though both of them are frequency standard, and both are used simultaneously in almost every apex meteorology institute to keep the time, the cesium atomic clocks are more used in this field what ...
5
votes
0answers
127 views

How do measurements of $G$ account for variations in the local gravitational field?

One of the notable discrepancies of modern physics is the current disagreement of measurements of the gravitational constant $G$, well beyond reported uncertainties (and agreeing only to about 1 part ...
4
votes
3answers
350 views

How can I explain what a kilogram is using Planck's constant? [duplicate]

I want to understand what $1\ \mathrm{kg}$ represents. For example: I know that $1$ second is equal to $9\,192\,631\,770$ transitions from the microwave radiation that a cesium-133 atom (at $0\ \...
2
votes
0answers
25 views

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 ...
1
vote
1answer
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 ...
1
vote
2answers
77 views

Is there a maximum quantum advantage in sensing?

This is a rather conceptual question. Quantum sensing takes advantage of entanglement (and other quantum properties such as squeezing) to get variances that scale much better than the ones one can ...
3
votes
3answers
419 views

Why are there $1 / 1.602176634 \times 10^{-19}$ electrons in a coulomb?

Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ...

1
2 3 4 5 6