Questions tagged [terminology]

Use this for questions relating to the proper use of physics terminology or nomenclature.

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Unable to make sense of - Heat, a form of energy [duplicate]

I have referred to some textbooks on Thermodynamics and they more or less explain heat in the following manner: Consider a system which has some energy content. In order to increase the energy content ...
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Terminology question: in-phase or out-of-phase?

Suppose that in a chain of many coupled oscillators, the displacements of two consecutive particles, in a normal mode of oscillation with frequency $\omega$, are given by $$x_p(t)=A_pe^{i\omega t}$$ ...
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What is the difference between photoelectric effect and photo-ionization?

Photoionization, ionization by a photon, and the photoelectric effect aren't they identical? If not then what is the phenomenological difference between them?
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Why is this the exact shape of expectation values in the path integral formalism?

This question is about expressions of the form $$ \langle x_f, t_i | \hat{x}(t) | x_i, t_i \rangle = \frac{1}{N} \int_{x(t_i) = x_i}^{x(t_f) = x_f} \mathcal{D} x~x(t)e^{i S[x]}. $$ In the following ...
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What is $B(N)$ crystal structure? What does this nomenclature stand for

Is it basic cubic with 20 atoms? I can't find the explanation for this nomenclature online. Maybe I could find it in a textbook, but if someone answers it here, other people can just google it.
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Can you explain me the definition of wave number as defined in theoretical physics?

Wavenumber, as used in spectroscopy and most chemistry fields, is defined as the number of wavelengths per unit distance. The corresponding formula is $$k=\frac{1}{\lambda}.$$ However, in theoretical ...
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What is an eigensystem? Could you provide a simple example? [closed]

Also, what is the difference between an eigensystem and the eigenspace?
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Ordinarily continuous function of the wave function

I just started studying quantum mechanics using the textbook Introduction to Quantum Mechanics by Griffith. Under the section of solving the Shrodinger equation for a Dirac delta potential, he ...
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Terminology: does this situation correspond to an anisotropic but linear dielectric?

Let us assume that for a dielectric the relation $${\vec D}=\epsilon\vec E$$ holds where $\epsilon$ is independent of $\vec E$. However, let $\epsilon$ is not a scalar number but a tensor (or a $3\...
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6 votes
3 answers
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Difference between point force and force

In high school, we used to draw a free diagram, and we are asked what are the forces acting on the object. When we represent these forces, we represent them using vectors going out from a point called ...
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Why kinetic energy is called kinetic energy and not potential even though it has a potential to do work?

I lift a block of mass $m$ by a height $h$ in gravitational field. I have done a work of $mgh$ on the block. This energy is stored in the block when it is at height $h$. At height $h$, the block has ...
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NLO and NLL difference [duplicate]

The next-to-leading order (NLO) Feynman diagram is the next leading process having more vertices than the tree-level diagram, but what is next-to-leading-logarithm (NLL)?
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How are the Bloch equations non-linear?

This question is similar to the following, but I have expanded the question moderately: Nonlinearities arising from linear equations The Bloch equations are described by the following vector equation (...
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Aren't all objects luminous in a sense?

In my physics class, I learned about "nonluminous objects" - these are objects which don't produce their own light. But, don't all objects emit light by black body radiation? So are all ...
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Why is the state of a quantum system called "Density $\textbf{Operator}$"?

In quantum mechanics, a $d$-dimensional pure state is represented by a vector belonging to a $d$-dimensional Hilbert space $\mathcal{H}^d$. A mixed state is represented by a density matrix $\rho \in \...
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What do you call $ \frac{d^2 r}{dt^2}$ in polar coordinates? [duplicate]

In polar coordinates, one finds centripetal acceleration as: $$ a_c = \frac{d^2 r}{dt^2}- \frac{v^2}{r}$$ Where $|r|$ is distance from center to particle, $v$ is tangential velocity. My question is ...
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When or why to use the $\equiv$ symbol in place of the $=$ symbol?

In literature, I read the following: A typical relationship*, often appearing in the literature, is: $$|-\nabla(\bar p+\rho g z)|\equiv \rho g J=q(\mu w+\rho Bq^m)$$ The nomenclature does not define ...
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What is the name of the instrument that separates substances using an electromagnetic field, similar to spectrography

As far as I remember, the elements of the substance are passed across an electromagnetic field in such a way that they seperate as the electromagnetic field alters their trajectory. I also seem to ...
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Are seismic waves and shock waves same?

Are shock waves (those produced by high speed jets) and seismic waves (produced during earthquake) the same? In many places I have seen seismic waves referred to as shock waves. So are these two same?
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Terminology of $SU(3)_F$

From a terminological point of view, what is the relatioship between the flavor symmetry group $SU(3)_F$ of strong interaction and the group $SU(3)$ (without subscript $F$) of 3x3 unitary matrices ...
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What is the difference between Bohr-Sommerfeld and Wilson-Sommerfeld quantisation rule?

Is Bohr-Sommerfeld and Wilson-Sommerfeld quantisation rule the same thing? If not, what is the difference?
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What is passive gamma ray emission?

I was trying to find the meaning of passive gamma-ray emission through the internet. I haven't found any helpful article except some research paper just denoting the word passive ray emission. They ...
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What does "hard tail" mean in the context of astronomical spectra?

I am currently reading about neutron stars and came across this sentence: "Often the radius is underestimated because only a hot spot emits or the spectrum contains a hard tail." I can find ...
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1 answer
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Is there a name for this type of wavevector?

When we say wavevector we often mean a vector $\mathbf k$ that is related to the direction and wavelength of a plane wave, given by $e^{i\,\mathbf k\cdot\mathbf x}$. I have to write something about ...
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In simple English what is meant by "tagging", "triggering", "selection criteria" and "reconstructed events/particles/masses" in particle physics? [closed]

I am currently reading the original Higgs boson discovery paper, "Observation of a new boson at a mass of $125$ GeV with the CMS experiment at the LHC", HIG-12-028, which can be found ...
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de Sitter space vs de Sitter universe

I have heard of the term de Sitter space. From this post user G. Smith writes, De Sitter spacetime is curved; specifically, it has the same positive scalar curvature at every point. Likewise, when I ...
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4 votes
1 answer
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Why does "chiral symmetry" in the Altland-Zirnbauer classification mean something different to other contexts?

The Altland-Zirnbauer classification of random matrices is based on three symmetries: time-reversal, charge conjugation, and a third which is sometimes referred to as "chiral" or "...
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Why do we use different differential notation for heat and work?

Just recently started studying Thermodynamics, and I am confused by something we were told, I understand we use the inexact differential notation because work and heat are not state functions, but we ...
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1 vote
1 answer
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"Classical field configuration" - QFT

I often encounter the term "classical field configuration" in the scope of QFT, but I have a hard time interpreting what it really means. If I understood it correctly, then a general field ...
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5 votes
2 answers
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Why are non-Newtonian fluids called non-Newtonian when they follow Newton’s third law?

To my understanding, Newton’s third law states that for every action there is an equal and opposite reaction. Therefor if I punch the non-Newtonian fluid harder, there will be a harder reaction force ...
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2 votes
3 answers
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What is the difference between direction and angle?

For example, "angle the man must steer his boat" vs. "direction the man must steer his boat". Are direction and angle the same in this case? Or does it mean I have to find the ...
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What is the correct name for the 4-space of special relativity

I refer to the 4-space commonly used to describe event-points $(x_0,...,x_3)$. A massive particle traces out a time-like path in such a space, since it cannot travel with a speed greater than or ...
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2 votes
1 answer
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Why do we use "vector-like" to mean "non-chiral", in particle physics?

I've been reading some stuff about searches for vector-like quarks and vector-like leptons, and I'm a little confused about the terminology. Also, I'm a little new to this, so bear with me. As far as ...
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On the language use of quantum mechanics: "state $\rho$" or "density matrix $\rho$ of the mixed state"?

For pure states one usually uses the bra-ket Notation and then uses language e.g. "the state $|\psi>$..." Is it also common to say similarly for mixed states, which are usually written as ...
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Source spectral index

Whilst studying the propagation of cosmic rays (CRs) through our galaxy, I was comparing simulated data to data measured by the Alpha Magnetic Spectrometer 02. I am studying the proton flux within CRs ...
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Vector dimensions and spatial dimensions

Are there any differences between vector dimensions and spatial dimensions? I have seen people talking about it a lot. And while talking about extra dimensions, they always bring these two terms up. ...
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1 vote
1 answer
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What is the difference between variational principle, principle of stationary action and Hamilton's principle?

In advanced mechanics, we learn about the variational principle, the principle of stationary action, and the Hamilton's principle. I feel that the difference between them is not very clearly organized ...
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What is Spin Asymmetry?

In CMS paper of single top quark production in association with a $Z$ boson in proton-proton collisions, that i'm reading, says that, the spin asymmetry of the top quark is related to its's ...
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What do freeze out and freeze in mean in the context of cosmology?

I have read that when the interaction rate is much greater than the Hubble expansion rate a species is in thermal equilibrium the hot plasma, when the interaction rate fall below the Hubble expansion ...
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1 answer
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Why are people using 'lightyear' as a measurement of time? [closed]

I came across 2 people in an open forum, who are college students, who that agree using the term 'lightyear' as a measurement of time is correct. Their context was "This country's technology is ...
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2 votes
3 answers
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What does mean to say that "the problem is reduced to quadratures" and why is it useful?

In classical mechanics, what does it mean to say that "the problem is reduced to quadratures"? And why is that useful? In the answer, bobbyphysics remarked that reduction to quadratures ...
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The "Proper Time Experiments" of C. O. Alley et al.: Did the airborne clocks run longer than the groundbased clocks (rather than running faster)?

Description and results of the so-called "proper time experiments" which were carried out 1975-76 based at the Patuxent Naval Air Test Center, a.k.a. "the Maryland experiment", are ...
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5 votes
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In the Langevin dynamics: neglecting inertia. A mathematician trying to understand physics terminology

If we write the Langevin equation: for a particle with mass $m$, position $x$ and velocity $v$, with some damping coefficient $\gamma$, $$ m dV(t)=-\gamma V(t)dt+dW(t) ,~~~~~~~dX(t)=V(t)dt.$$ Then as $...
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What do we mean when we say "an atom is at energy level 1"?

I know that when we say an atom is excited, we actually mean that one of its electrons either moves to another energy level or to another orbit. But when we say that an atom is in a certain energy ...
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Well-behaved metric

What is a well-behaved metric in general relativity (GR)? Should every metric be well-behaved in GR? And what is the mathematical description for this kind of metric and what does it mean physically ...
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1 answer
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What is "antipodenpunkte" (this is German)? [closed]

What does "antipodenpunkte" mean? If you can't find a word about Einstein on the Internet, who should know it? "In addition, the question arises: can we see stars very close to our ...
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11 votes
2 answers
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If a measurement has 5% error, can we say it has 95% accuracy?

Most often when, in a numerical problem, it is demanded that we calculate the accuracy of the final result, we write the final result in terms of the error. So I want to know if, in a measurement, ...
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Why do excitations not exist in non-quantum fields? [duplicate]

In my understanding, where the value of a quantum field [i.e. a field with discrete values] is 0 it is said to be in the ground state, and where it is not 0 it is said to be an excitation. Why do we ...
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What is the difference between 'Equation of motion' and 'Transport equation'?

I think this is a simple question with a not so easily explained answer. What is the difference between the Equation of motion and Transport eq? Is Navier stokes equation an 'Equation of motion' or a '...
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Can two waves be considered in phase if the phase angle is a multiple of 2$\pi$?

Question is essentially what the title states. Wavefront is defined as the locus of points that are in phase. So I wanted to know if the locus would be the points of only a single circle or multiple ...
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