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

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71k views

What does the magnitude of the acceleration mean?

I am a little confused as to what the magnitude of acceleration is and what it means.
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2answers
202 views

What will happen if we use a speed greater than light speed and find a body'motion and energy relative to it?

In Einstein's papers, he used light speed as a reference speed. What if we use a greater finite speed and do the same calculations. Won't this greater speed then be the limit.
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2answers
1k views

What is the difference between classical thermodynamics and statistical mechanics? [duplicate]

What is the difference between classical thermodynamics and statistical mechanics? To me, they are greatly different but are different approaches for explaining same thing. But I do prefer ...
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1answer
127 views

How does Dirac form this conjugate imaginary equation?

On page 30 of Dirac's book $$\xi|P\rangle = a|P\rangle\tag{12}$$ He then says Suppose we have a solution of (12) and we form the conjugate imaginary equation, which will read $$\langle ...
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3answers
84 views

In Orbital Mechanics what is the quantity described below called?

I seem to recall that $r^2 \dot{\theta}$ is a conserved quantity in orbital mechanics, which I just proved using the Euler-Lagrange equations. Namely via: $ \mathcal{L} = \frac{m}{2} (\dot{r}^2+r^2 ...
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2answers
86 views

What is the difference between toy models and normal models?

Here is the short description of scientific model: an imperfect or idealized representation of a physical system And the definition of toy model: a simplified set of objects and equations ...
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1answer
794 views

What is an effective potential in classical mechanics?

What is an effective potential in classical mechanics? I have read the wikipedia article and David Tong's lectures notes, but I didn't understand how an effective potential simplifies a situation or ...
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3answers
119 views

“Complete” confusion

The word "complete" seems to be used in several distinct ways. Perhaps my confusion is as much linguistic as mathematical? A basis, by definition, spans the space; some books call this "complete" -- ...
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2answers
125 views

Why can't we precisely define physics? [closed]

While reading a textbook, I came across this statement: "A precise definition of physics is neither possible nor necessary." I was curious why it is not possible but the textbook never ...
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205 views

What does excitation in an atom mean?

This is not regarding the definition of excitation itself. What I'm having trouble with is the difference between "electron excitation" and "atom excitation". When I electron inside the atom gets ...
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3answers
319 views

What is actually a conservation law?

Though in his lectures, Feynman didn't define conservation law, he did use it while explaining divergence theorem: [...] heat is conserved. That is, no heat is generated inside the material and ...
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3answers
304 views

Ideal, isotropic fluid and stress tensor

An ideal fluid is the one which cannot support any shearing stress. It also doesn't have viscosity. My question is what does it mean by a fluid to be isotropic? Is an ideal fluid necessarily isotropic ...
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3answers
267 views

Generic term comprising everything that can be represented with a number and a unit?

I am looking for the generic term comprising all of the following: $23.42\,\text{m}$ $200\,\text{K}$ $123\,\text{MeV}$ $ħ$ with other words, everything that can be reasonably represented ...
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4answers
393 views

Can physics (ever) explain intrinsic properties of nature? [closed]

I may be totally off with this quite abstract (?) question(s). But still, here are some closely related sub-questions: Is there a list of currently "known" intrinsic properties of nature? How ...
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1answer
72 views

Looking for the name of a particular device [closed]

Please move this if it's not in the right location. I'm looking for the name of a device that I frequently see in many scenarios, specifically that of an office/library which can be described as ...
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3answers
61 views

What does “fidelity” mean?

In particular I am interested in whether it is more closely related to "precision" or "accuracy". So a somewhat mathematical description might be appropriate. For example the word "fidelity" occurs ...
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2answers
106 views

What does it mean by saying the generators of translations transform as vectors under the Lorentz Group?

The commutator of generators of Lorentz transformation and translation is as follow: $$[M^{\mu\nu},P^\sigma]=i(P^\mu\eta^{\nu\sigma}-P^\nu\eta^{\mu\sigma} ).$$ Then from this we usually say that the ...
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3answers
271 views

Does lithium-6 “decay” when hit by a neutron?

I am talking about the nuclear reaction $$ ^6\text{Li} + n \rightarrow\ ^4\text{He} +\ ^3\text{H} + 4.78\text{MeV} $$ A neutron hits a lithium-6 nucleus and together they form an alpha and triton ...
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2answers
530 views

How to tell the order of a Feynman diagram?

How can we know the order of a Feynman diagram just from the pictorial representation? Is it the number of vertices divided by 2? For example, I know that electnro-positron annihilaiton is first ...
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1answer
92 views

Usage of the terms Raman, Stokes and anti-Stokes scattering

I am confused over the correct usage of the terminology for "Raman scattering", "Stokes scattering", "anti-Stokes scattering", or even "Stokes-Raman scattering" and "anti-Stokes-Raman scattering". Is ...
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2answers
153 views

What distinguishes the particles we chose as matter from their antimatter equivalent? [duplicate]

Back before we knew about antimatter we just called everything matter. Ignoring CP-violation for a moment, there is nothing special about matter versus antimatter. Once we knew about antimatter it ...
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1answer
104 views

What is it called when two particles are associated so that what happens to one happens to the other?

There was some experiment that I read about some time back in which two particles (or the same particle, but split into two) were sent in opposite directions, but when something happened to one, it ...
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1answer
105 views

What is *uplift* in respect to extra dimensions and their stability?

What is uplift in respect to extra dimensions and their stability? It's notoriously hard to find something on this, as all possible keyword combinations pull up plethora of unrelated Google hits.
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927 views

A basic confusion about what is an atom

Wikipedia defines atom as The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. and defines electron as: The ...
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600 views

When to use $f$ and when $\nu$ signifying frequency?

When to use $f$ and when $\nu$ signifying frequency? I guess that when you mean frequency of electromagnetic wave, you use $\nu$, and $f$ otherwise?
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1answer
64 views

What is the different between a dark state and a ground state?

In a atomic quantum system, typically discussing in quantum optics, there is something called dark state. A dark state is a state of a quantum system that does not emit any photon. A ground state also ...
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1answer
185 views

Meaning of the term 'bulk'

I have recently started reading literature on 2 dimensional systems in Condensed matter. While reading, I frequently came across the word 'bulk'. Sometimes it referred to 2-D and sometimes to 3-D. I ...
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1answer
53 views

What is “forward peaking”?

In "Research and Development for a Gadolinium Doped Water Cherenkov Detector" the phrase "forward peaking" is used to describe a signal. This comes up in lots of other contexts too, but I still can't ...
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2answers
75 views

What is the difference between the potential $V$ and the effective potential $V_{eff}$?

What is the difference between the potential $V$ and the effective potential $V_{eff}$? Some times when solving problems, an effective potential $V_{eff}$ is defined and its usually equal to the ...
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1answer
45 views

What process happens in an IT nuclear decay?

I've been researching medical isotopes and alot of them decay by an IT path. Does anyone know what IT stands for? And what physical process is happening? Example: ...
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300 views

Eternal Black Holes

What is the definition of an eternal black hole? Studying white holes and the term appears in relation to this field of research.
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1answer
137 views

The meaning of “heralded photon”

I am not a native English speaker, and I have just started to study physics in English. However, I came across the term heralded photon while I was reading a review article about optical quantum ...
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1answer
155 views

Theorem or Conjecture? [closed]

I understand the definitions "theorem" and "conjecture" in mathematics, but I wasn't sure for physics. I mean, if it's proved mathematically, it's a theorem, otherwise it's a conjecture. But for ...
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2answers
4k views

Magnitude refers to number or number with units?

This question is about terminology for physical quantities. When we talk about magnitude (while talking about scalars and vectors) do we refer to just number or Number along with units? example: If ...
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1answer
123 views

A terminological question about work and energy

Work is force applied over distance. Is it also reasonable to say that work is (the same thing as) the transfer of energy? When work is done, the equivalent energy is transferred. But if energy is ...
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1answer
112 views

What does $L^2(S^1,\mu_H)$ mean?

It's a Hilbert space, $\mu_H$ stands for the Haar measure on $U(1)$, but what does $S^1$ mean? I found it in one of my quantum mechanics books which approaches from a very 'mathematical' way.
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4answers
35 views

What is the distinction between a “ray” and a “wave” in optics?

What is the distinction between a ray and a wave in optics? From what I can gather, the only discernible difference is in nomenclature, where a ray simply refers to an EM wave with short wavelengths. ...
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1answer
46 views

Difference between escape velocity and speed [duplicate]

What is the difference between escape velocity and escape speed , is their any error if we use them interchangeably?
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1answer
58 views

What is difference between polarization and polarizability and how do we define it? [closed]

The book of physics that I have, uses the word "polarization" sometimes and sometimes uses the term "polarizability" and I am getting confused. And I even checked the dictionary for the term ...
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4answers
119 views

What does “the fabric of space and time” actually mean? [closed]

I've heard the term "the fabric of space and time" in both physics and science fiction, and although I know it has something to do with general relativity, I don't understand what, specifically, ...
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1answer
59 views

Is harmonic oscillator continuous variable system?

In the literature I have seen that the notions "our system is continuous variable system", "Hilbert space of our system is infinite" were used as if they were equivalent. For example for harmonic ...
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151 views

Gravitational acceleration

'What is difference between free fall acceleration g and gravitational acceleration a?***a is with subscript g.In my textbook it is written that "free fall acceleration = gravitational acceleration - ...
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1answer
76 views

What is meant by “the superpotential is not renormalized”?

Reading about supersymmetry I often read the phrase because of the non-renormalization theorems the superpotential is not renormalized. I would like someone to be more explicit on what is ...
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1answer
462 views

Holonomic constraints and degrees of freedom?

Can we see that a constraint can decrease the degrees of freedom of a system if and only if it is holonomic. Either way please can you explain why?
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2answers
174 views

What does the “T” stand for in T-duality?

First of all, I am not a physicist. I'm a graduate math student and recently I came across the concept of T-duality. Actually I'm studying generalized complex geometry, which according to this paper ...
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66 views

Is a constant transformation still considered a gauge transformation?

I've never even considered the possibility that a constant transformation would not qualify as a gauge transformation. But I'm reading a paper that seems to make exactly this distinction. In ...
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1answer
90 views

What does it mean the term “probe brane”?

What does it exactly mean the term "probe brane"? People say for example: We put a stack of N branes at some point and then a probe brane ..." How do they appear in AdS/CFT? Can you give me an ...
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121 views

Statistical mechanics vs. many-body theory

Where is the basic difference of statistical mechanics with many-body physics? What are the systems which cannot be studied in statistical mechanics but in many body theory? After all we know ...
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2answers
4k views

What is the difference between electromagnet and solenoid?

What is the difference between electromagnet and solenoid? Both these terms seem as the same thing to me. The only difference that I can find seems to be that an electromagnet contains a soft iron ...
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149 views

What really is “inertial force”?

In Fluid Mechanics we often see the term inertial force when discussing Reynolds number. The problem is, I didn't really get what's this inertial force. Basically, the notion of inertia I have is that ...