# 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 ...
23 views

### 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?
1 vote
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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?
1 vote
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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 ...
33 views

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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 ...
40 views

### 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?
1 vote
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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 ...
1k views

### 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 ...
49 views

### 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 ...
1 vote
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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 ...
114 views

### 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 ...
111 views

### 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 ...
1 vote
113 views

### "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 ...
588 views

### 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 ...
59 views

### 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 ...
1 vote
163 views

### 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 ... 49 views

### 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 ...
1 vote
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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. ...
1 vote
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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 ...
38 views

### 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 ...
84 views

### 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 ...
114 views

### 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 ...