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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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Meaning of the word 'canonical' in physics

I often encounter the term canonical in my study of physics. What does it mean? There is canonical momentum, canonical transformations and I have even heard the phrase 'proving something more ...
0
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1answer
26 views

What is the definition of the moment of inertia tensor?

I can find volume integrals for the moment of inertia in 2D and 3D, but is there a definition that works in an arbitrary number of (spatial!) dimensions?
0
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0answers
53 views

Phase space meaning [duplicate]

In the field of medical physics, specifically in monte carlo simulation of radiation beams produced by electron accelerators, people call ‘phase space’ to a file that contains the data of a large ...
2
votes
2answers
73 views

Minkowski spacetime conformal infinity: why not allow the full range of $R$?

Let $(\mathbb{R}^4,\eta)$ be Minkowski spacetime. We want to describe infinity as a place in some bigger manifold containing Minkowski spacetime. The idea is to work with null incoming/outgoing ...
51
votes
10answers
4k views

Quantum made easy: so what *is* quantum mechanics all about? [closed]

Being a physics grad student, I got used to the weird concepts behind quantum mechanics (used to doesn't mean I fully understand it though). What I mean is that I'm not surprised anymore by the fact ...
1
vote
1answer
53 views

Tracing over a Fock space?

Suppose you have a bosonic Fock space with a vacuum $|0\rangle$. A particular state is labeled by the parameter $N \in \mathbb{Z}$. You can construct states like $$ | n_{N} \rangle = \frac{ \left( \...
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1answer
23 views

Are angles for vectors always measured from the horizontal?

I have a vector which has been stated to have a force of: 96.0 N at $51.3^\circ$. I had a different answer because I was measuring my angles from the north. By default if an angle is given by itself ...
6
votes
3answers
1k views

What does the 'displacement' refer to in the definition of work?

The definition of work given in books is The work is said to be done by a force on a body, when the body is moved by the force through some 'displacement'. Now let a body of mass $m$ at rest. When a ...
0
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2answers
122 views

Distinction between spacetime and vacuum

In GR, is spacetime just a mathematical abstraction, and in reality, it's the vacuum - whatever that is!!- is what curves, bends, and warps? In other words, is the distinction between spacetime and ...
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0answers
24 views

Definition of Galilean structure in Arnold's book?

I am reading Arnold's Mathematical Methods of Classical Mechanics. He quickly introduces the notion of Galilean structure. The universe is defined as the affine space $A^4$ and time is defined as a ...
3
votes
0answers
41 views

Is Force a fundamental concept just like position and time? [duplicate]

The doubt arises from my understanding of Newton's second law. First I though it was just the definition of force but it actually states the relation between force, acceleration and mass (which can be ...
1
vote
2answers
225 views

What does “degrees of freedom ” mean in classical mechanics?

The definition I come up with is 3M - N ...where N is the number of constraints. I assume M is the number of distinct points. In what context is it used ? According to Wiki it says "an ...
12
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3answers
1k views

Definition of an operator in quantum mechanics

In J.J. Sakurai's Modern Quantum Mechanics, the same operator $X$ acts on both, elements of the ket space and the bra space to produce elements of the ket and bra space, respectively. Mathematically, ...
2
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3answers
134 views

In general, how are representations used in physics?

I want is a basic overview, if there is one, of the meaning (and purpose) of the word representation in general terms. I have looked up sources such as Particle Physics and Representation Theory, but ...
2
votes
2answers
27 views

Distinction between “assumption”, and “build in definitions” - What is considered an assumption in a physical model?

This question I want to deal with the basics of modelling a physical theory: Let's say we start with observing in the world (be it little bubbles in the water, a particle moving, a pattern in the ...
2
votes
1answer
49 views

Inertial and gravitational Mass

Why definition of mass is not stated as " the property of object to change radius of curvature of space time fabric is called mass"
0
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2answers
47 views

Work Energy misunderstanding

Suppose I have to displace a body to a height $h$ and I applied force more than its weight. So, there will be an acceleration on the body. Then the body will reach the height $h$. Then I let the ...
2
votes
5answers
96 views

Why is voltage described as potential energy per charge?

Voltage is often called an electromotive force since it causes a flow of charge. However, it is described in terms of Joules per Coulomb or Potential Energy per Charge. Question: How does the ...
1
vote
2answers
60 views

How to interpret resistivity and its unit?

I was wondering which interpretation could we find for the resistivity, what image correspond to the concept. Furthermore, how to interpret its unit $\Omega \:\rm m$ . Why is it more logical than a $ ...
2
votes
3answers
103 views

Linear and non-linear systems

When I read about the superposition principle, it says that it works only on linear systems, my problem is that I cannot really understand the difference between a linear and a non-linear system. I ...
1
vote
1answer
42 views

Is a displacement a vector, a line segment, or something else?

It probably seems ridiculously naive of me to ask such a basic question, but I have a need to use accurate language. Typically, I think of a displacement as a directed line segment whose end-points ...
-1
votes
1answer
53 views

What is difference between rolling, kinetic and static friction? [duplicate]

If there is a toy train on the floor moving linearly then would the friction acting be rolling or static?
0
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1answer
56 views

Stellar Isochrones, what are they?

So I have been reading and I am trying to understand what stellar isochrones are and what relationship they have to the Hertzsprung-Russell diagram. My understanding at the moment is that, the ...
-2
votes
1answer
34 views

Is mass a property or a quantity?

From definition mass is the amount of matter that an object has. So why we call it a property? What is the difference between a property and a quantity in the dictionary of physics? Wiki says physical ...
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0answers
26 views

What is the difference between a defining equation and a word definition? [closed]

For example, can we define kinetic energy as the product of 1/2mu^2 and also as the energy that an object has due to motion ? are these definitions equivalent ?
1
vote
2answers
71 views

What is the definition of force? [closed]

Wikipedia says a force is any interaction that, when unopposed, will change the motion of an object. But in a site i find this , force is a "quantitative" description of an interaction that causes a ...
0
votes
1answer
29 views

What is the definition (not on the basis of energy level) of scattering state?

A question is given in Griffiths. Consider the "step" potential V(x) which rises to Vo (at x=0) which is 0 for x<0. For the scattering states and E less than Vo, the reflection coefficient comes ...
2
votes
1answer
41 views

What is the difference between dual CT and spectral CT?

I'm reading about different types of CT scans and I'm frequency seeing the words "Dual x-ray CT" and "Spectral x-ray CT" being used to describe different things but I can't find a single explanation ...
3
votes
2answers
589 views

Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
3
votes
1answer
114 views

What is the difference between thermodynamic free energies and the Landau free energy?

How and why is the Landau free energy any different from thermodynamic free energies? It is written on page 140 of Nigel Goldenfeld's book Lectures on Phase Transitions and The Renormalization Group ...
4
votes
2answers
159 views

Difference between physicist's vector and mathematician's vector

Mathematically a vector is defined as an element of vector space which obeys certain properties. While reading about the special theory of relativity, I came to know about another definition of ...
0
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2answers
99 views

What is the precise relationship between enthalpy and heat?

I am struggling to resolve the definitions of heat and enthalpy of reaction. This is based on research I have been doing around the production of course materials. I have found the following ...
2
votes
1answer
74 views

Cross product of vectors

I am unable to comprehend the following lines given in page 657 of Shankar's Principles of Quantum mechanics: One tricky point: The cross product is defined to be orthogonal to the vectors in the ...
1
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0answers
58 views

Adjoint of Weyl Spinor

Given a (Dirac), spinor in the Weyl basis, $\psi = \begin{pmatrix} \psi_{L}\\ \psi_{R} \end{pmatrix} $ , where $\psi_{L}$ and $\psi_{R}$ are Weyl spinors we define the adjoint of the Dirac spinor as; ...
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0answers
22 views

Clarifying the physical significance of the attenuation coefficient

I have seen several sources define the attenuation coefficient as the fraction of a beam's intensity which is attenuated per unit distance e.g. "the fraction of attenuated incident photons in a ...
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2answers
91 views

What is the “displacement” of the object in the definition of work?

Work in physics is mathematically defined as force $F$ applied on an object multiplied by the displacement $d$ it covers in the direction of the force. In a system where, a restrictive force exists ...
3
votes
3answers
96 views

Why are electric and magnetic susceptibilities defined in such an unintuitive way?

When I studied electromagnetism the $\mathbf{B}$ and $\mathbf{E}$ fields were introduced as fundamental quantities to me, and the $\mathbf{H}$ and $\mathbf{D}$ fields were introduced as something of ...
1
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2answers
51 views

Confused with heat as a form of energy [duplicate]

I have quite a simple question. Energy can be defined as capacity to do work. But I have read When energy is exchanged between thermodynamic systems by thermal interaction, the transfer of energy ...
0
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2answers
24 views

Definition of a system in flow processes

In classical thermodynamics the state of a system (pressure, temperature etc.) can only be specified if it is in equilibrium. My understanding of equilibrium leads me to believe that the state is ...
4
votes
4answers
155 views

Difference between pure quantum states and coherent quantum states

In the post What is coherence in quantum mechanics? and the answer by udrv in this post it seems to imply that a pure quantum state and coherent quantum state are the same thing since any pure state ...
2
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2answers
67 views

Definition of physical quantities

Physical quantities are often defined in textbooks as measurable quantities. I find this definition confusing. For example, if you think about it, the number of clothes in a cupboard is also a ...
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0answers
40 views

Local and Non-Local Potentials

Can anybody explain the difference and concepts between local and non-local potentials in light of quantum mechanics?
0
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2answers
16 views

Can speed vary within a given interval of uniform motion in a straight line?

The definition of 'Uniform motion' in a straight line says that the body covering equal distances in equal intervals of time is to be understood to be in uniform motion. However, can the speed vary ...
2
votes
1answer
42 views

Does vacuum has another meaning from quantum vacuum?

According to QFT. The quantum vacuum is a particular state of quantum fields; it's not a "place" where quantum fields "exist in. No problem with that. But does the word "vacuum" without the quantum ...
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1answer
39 views

How are these multipole moments related to the ones from electrodynamics?

Let $f : \mathbb{R}^3\to \mathbb{R}$ be a continuous function of compact support. Its Fourier transform is $$\mathfrak{F}[f](k)=\int f(x)e^{ikx}dx=\int f(x)\sum_{n=0}^\infty \dfrac{i^n}{n!}k_{a_1}\...
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1answer
38 views

Acoustic, optical, ferromagnetic and antiferromagnetic spin-waves?

In the context of spin-waves I have seen the following words as descriptors*: Acoustic Optical Ferromagnetic Antiferromagnetic which I have seen used together e.g. "acoustic ferromagnetic spin ...
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votes
1answer
57 views

Concept of infinity in limits [closed]

Can somebody explain as to what is difference between an expression being undefined ( such as 0/0) and one being an an infinity?
0
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1answer
64 views

Definition of temperature ambiguity

Temperature is defined as \begin{equation} \frac{1}{T} \equiv \left(\frac{\partial S}{\partial U}\right)_{N,V}. \end{equation} However, a system with a fixed temperature does not have a fixed amount ...
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votes
1answer
52 views

How is $\gamma^{\mu}$ defined in the anti commutation relation $\{{\gamma_{5},\gamma^{\mu}}\}$?

how is $\gamma^{\mu}$ defined in the anti commutation relation $\{\gamma_{5},\gamma^{\mu}\}$? does it make a difference if you write the index ${^\mu}$ lower? what does usually change if the index is ...
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3answers
76 views

Why does the Hamiltonian define symmetry/invariance?

In Sakurai's Modern Quantum Mechanics, in Chapter 4, he effectively states that the operation of rotation or translation, represented by a unitary operator $U$, is customarily called a symmetry ...