Questions tagged [definition]

The definition tag is used in situations where the question is either about how some term or concept is defined or where the validity of an answer depends on a subtle definition of some term or concept used in the question.

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Fermat's Principle: no first-order change in time?

I was reading the chapter on Fermat's principle in the Feynman lecture series. The principle is stated along these lines: "The correct statement is the following: a ray going in a certain ...
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0answers
18 views

What is recessional nonlocality?

In laymen’s terms, what is recessional nonlocality? I understand recessional means to recede or retract away, and (correct me if I’m wrong) I understand locality has something to do with quantum ...
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1answer
34 views

How does flavor change in weak interaction works with $Z$ and $W^{\pm,0}$?

How does flavor change in weak interaction works with $Z$ and $W^{\pm,0}$? I'm completely confused of how could weak interaction just "approximately" conserve the flavor, but $u,d,s,c$ could be just ...
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3answers
1k views

When exactly is a dimension spatial?

I every so often hear claims like: M-Theory predicts that there are 10 spatial dimensions! Now I'm not really sure what these claims mean. There are three spatial dimensions that I normally ...
-1
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0answers
17 views

Is the word 'acceleration' used for both increasing and decreasing velocity? [duplicate]

the question was: "if a particle is accelerating, it is either speeding up or speeding down. do you agree with the statement ?" and this is what our teacher told us: "This statement cannot be ...
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2answers
122 views

How could 'rest mass' and 'invariant mass' be the same?

The terms rest mass and invariant mass are often interchanged, however i cannot reconcile these concepts: Consider a photon ...
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1answer
37 views

What does the notation $\mathcal{O}\left(\frac{1}{r^2}\right)$ mean? [duplicate]

I was reading a text about quantum scattering, and I faced a notation I don't understand. The equation is the following: $$ \nabla \psi_{\text{scattered}} = \frac{i k f(\theta) e^{ikr}}{r} \mathbf{\...
2
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1answer
55 views

Conceptual definition of the $H$-field

I am looking for a conceptual definition or statement for H to help ground my understanding as I work through the math. I often see it explained that D and H are just the components of the fields ...
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2answers
66 views

Definition of fluid

What's fluid? The definition, I read isn't clear to me,it says, "A fluid is a substance which will yield to any continued shearing stress however small." But, what's shearing stress? What does ...
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1answer
41 views

What is definition of motion? [closed]

I know this question is too basic and silly. But if you really contemplate about the definition of motion which say that it is "change" in position over time then you will find a loop. "Change" is ...
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4answers
573 views

What is the difference between a translation and a Galilean transformation?

What is the difference between a translation and a Galilean transformation?
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0answers
87 views

Is Fermi energy the same as chemical potential?

In all of the textbooks , the Fermi Energy was interpreted as the energy level below which all possible energy states are filled. However with a Fermi-Dirac type of distribution, which has asymptotes ...
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0answers
37 views

Holonomic basis

Is the following definition correct? Given a differentiable manifold $M$ and an ordered basis $\{e_j^m\}$ of the tangent space $T_m M$ with $m\in M$ (they are vectors and not vector fields). An ...
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0answers
21 views

Difference between resonance and the synchronization of coupled oscillator

I was reading about phase synchronization of coupled oscillator where the oscillators are synchronized by an applied field. Now the coupled oscillators are synchronized. So my question is that what is ...
2
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2answers
83 views

Difference between left- and right-handed, helicity and chirality

What is the difference? I know there is the (almost) same question What's the difference between helicity and chirality? but when a particle is given as left-handed. Is it helicity or chirality?
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0answers
24 views

What is Wedge Constraint? (Definition)

What is Wedge Constraint? How can we apply it in the following problem? The block of mass $m$ slides on a wedge of mass $m$ which is free to move on the horizontal ground. Find the accelerations ...
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1answer
115 views

The invariance for time-reversal: some clarifications

On the Physics.SE I haven't found a simple definition of the invariance for time-reversal. Is it possible to have a simple explanation (also using differential equations) of the invariance for time-...
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1answer
70 views

What's the difference between canonical quantization and second quantization?

I am wondering the difference between the canonical quantization and the second quantization in quantum field theory. For example, a harmonic chain, one can write down its lagrangian density $\...
-1
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2answers
67 views

What does the term $\mathcal O(\epsilon^2)$ mean?

In the highest upvoted answer to Where does the $i$ come from in the Schrödinger equation? the author writes the following equation: $$ U^\dagger U=(\mathbb I+\epsilon^* A^\dagger)(\mathbb I+\...
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3answers
55 views

Is a rotating object moving or stationary?

Because it rotates in situ, its center of mass does not move, so it is static, but it is rotating, so it is not static, then is it static or moving?
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0answers
36 views

Rigorously define degrees of freedom

I want to understand if there is truly a rigorous definition for the degrees of freedom in a system. Say all of a system's physical states are contained in some set $S$. A seemingly acceptable (and I ...
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0answers
49 views

Mathematical formulation of the concept of temperature

We were taught the following. Consider three systems $A$, $B$ and $C$, which have exactly two independent variables each, $(X,Y)$, $(X’,Y’)$ and $(X’’,Y’’)$. Thermal equilibrium for $A$ and $B$ is ...
2
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1answer
26 views

Sensitivity to initial conditions and predictability

Are there examples of chaotic systems that are predictable and at the same time sensible to initial conditions? or would that violate the notion of sensibility to initial conditions? Lets imaginge A ...
3
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0answers
67 views

General definition of symmetry in physics?

I've looked at a number of questions on what symmetries are in physics, such as this one, this one and this one. However, I found the questions and answers to be not completely satisfying because they ...
2
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1answer
70 views

What is the most general conception/formalization of “energy”? [duplicate]

(disclaimer: I am not a physicist). I have been taught about the concept of energy in a fairly ad-hoc way. i.e. I was simply "told" that kinetic energy equals $\frac 1 2 m v^2$, and similarly I was ...
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1answer
38 views

A doubt regarding Modelling physical phenomena and position uncertainty

For example, in velocity, when we say $v=\frac{dx}{dt}$, there is no proof for it. Its almost like an axiom. Something taken to be true, without a proof. How do I know that for every $x=f(t)$, $v=f'(t)...
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0answers
48 views

What is Rectangular Simple Harmonic Motion (SHM)?

While studying Simple Harmonic Motion and Lissajous figure, I found a term called Rectangular SHM. But what is it actually? Is it something like square wave? What is the difference between Linear and ...
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2answers
53 views

Soft X-rays and Hard X-rays

How to know how much of kilovolts are in soft X-rays, and in hard X-rays? (Ex: 80kV is in soft X-rays or hard X-rays?
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1answer
37 views

Specific total enthalpy VS Specific enthalpy

What is the difference between specific enthalpy and specific total enthalpy in the context of fluid flow?
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2answers
53 views

What is a mass moment?

I am currently reading through a document Finding Moments of Inertia from MIT, page 4, and I am a little confused as to one of the concepts that they use. In this document, there is mention of a mass ...
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0answers
95 views

A concise definition of a frame of reference in Newtonian mechanics?

I've read Wikipedia's entry on frame of reference and also followed all of the references cited in the text (Salençon, Brillouin, Norton, etc) but I'm struggling to find any concise definition in all ...
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1answer
37 views

Definition of non-conservative force [duplicate]

In defining conservative force, we say that "The potential energy difference is path independent." However, as far as I understand, potential energy only exists when there is a force field. ...
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3answers
278 views

Equivalent definitions of total angular momentum

Consider the equality \begin{equation}\exp\left(-\frac{i}{\hbar}\boldsymbol{\phi J}\right)\left|x\right>=\left|R(\phi)x\right>,\end{equation} where $\left|x\right>$ denotes a position ...
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2answers
78 views

Definition of “specific gravity”

I've learnt that a specific quantity is an extensive quantity divided by the mass. How does the definition of specific gravity fit into this scheme?
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3answers
89 views

WHY did physicist defined velocity as displacement divided by time, why not displacement * time? [closed]

V=S/T. As per my knowledge i think ratio as division and it don't give any meaning like this much displacement in this much time. So i think physicists only used division as notion for velocity. But ...
5
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0answers
65 views

Physicist path integral and cylinder set measures

Path integral via discretization So let me start with what seems to be the point of view of physicists (corrections are highly appreciated since this is what I understood!). Let a quantum system with ...
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0answers
44 views

Off-shell vs half off-shell vs fully off-shell $T$-matrix

I know what are on-shell particles, but I want to know what are off-shell, and half off-shell, and fully off-shell states? and how we decide to consider one of these states in evaluating $T$-Matrix?
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0answers
20 views

Physics Equivalent of IUPAC Gold Book

I wanted to look up a few definitions and found them to vary from source to source so I wondered if there was a book such as IUPAC Gold Book in Chemistry which formally lists and defines almost all ...
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0answers
46 views

Why definition of potential energy and law of conservation of mechanical energy is misleading several times?

I regularly see 1 or 2 questions on this website about the definition or application of potential energy.The users fundamentally ask the same thing in every question. What I have learned till now is:-...
3
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1answer
93 views

Is four velocity always given by $U^{\mu} = d x^{\mu}/d\tau$?

I was taught that four-velocity is defined as $${\bf U} = \frac{d \bf x}{d\tau}$$ and that it has the components $$U^{\mu} = \frac{d x^{\mu}}{d\tau}$$ where $d\bf x$ is the four displacement and $\...
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3answers
105 views

Is there a better definition of magnetic field than this?

It may seem a trivial question but the definition of the magnetic field in everyday books is misleading. "It is the region or area around a magnetic material in which its magnetic force can be felt." ...
3
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2answers
62 views

What is irrotational flow? How to judge?

For example, when the wing moves horizontally, the direction of fluid flow changes first to upward at the leading edge of the wing and then to downward at the trailing edge. Does it rotate? If the ...
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1answer
36 views

What is a pseudopure state?

In the paper titled "Experimental Implementation of the Quantum Baker’s Map" by Weinstein et al. (Phys. Rev. Let. 89 (2002)), the author says something like [...] the pseudopure state corresponding ...
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3answers
186 views

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

I want to understand what 1 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 $0$K) emits, if it's ...
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2answers
96 views

What is a Hamiltonian of a System?

What is a Hamiltonian of a System? When learning about Hamiltonian for first time it is an object introduced as Legendre Dual Transform of Lagrangian of the same system. And we learn further that it ...
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3answers
77 views

How do I understand Kinetic energy formula? [duplicate]

$$\frac{mv^2}{2}= Kinetic Energy$$ Can you explain me? What is purpose of $v^2$, $mv^2$, I am trying to understand the formula.
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1answer
36 views

Klein-Gordon equation propagators: intersection with the support of the source

Let $(M,g)$ be a globally hyperbolic. Let $P = \Box - m^2$ be the Klein-Gordon differential operator. Following Fewster's notes, we may define the retarded/advanced propagators $$E^\pm : C^\infty_0(M)\...
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2answers
119 views

How does one obtain $\hbar$ as $\frac{h}{2\pi}$?

I'm reading Dirac's Principles of Quantum Mechanics. He defines $\hbar$ to be the real number satisfying the following relation $$ uv - vu = i\hbar[u,v]$$ where $u$ and $v$ are dynamical variables, ...
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1answer
70 views

What do you mean by Newtonian space? [closed]

What do you mean by Newtonian space? When you see this question, most of you might be thinking that I am trying to crack a joke or something..but no. This was a genuine doubt which one of my friends ...
2
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2answers
77 views

In which sense equations of motion are covariant?

I read lots questions about what covariance is and I found out that, according to this topic Lorentz invariance of the Minkowski metric, we say an object is covariant if it doesn't take the same value ...