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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Difference between Bogoliubov-Shirkov renormalization group and Wilson's Renormalization group
I just learned the Wilsonian renormalization group from a QFT lecture, I heard that there is another renormalization group called Bogoliubov-Shirkov renormalization group which is a true group instead ...
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Definition of the left-right derivative symbol in the Klein-Gordon scalar product [duplicate]
At the start of QFT, studying the Klein-Gordon scalar field, it is often mentioned that the following is the definition of the scalar product in the space of the solutions:
$$\langle f _{\vec{k}}|f_{\...
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What is momentum exactly? [duplicate]
What is momentum exactly? I am confused that, is momentum a property of particle or something different.
Whenever i look for it's definition it is product of mass and velocity of a particle.
At ...
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Is there a unified conceptual definition of mass?
Last night I posted this question: Since light has inertia and experiences gravity, what does it mean for photons to be massless?, which I now think was overly wordy and didn't properly express what I ...
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On the $\ast$ map in the Osterwalder-Schrader axioms
I'm studying "A Mathematical Introduction to Conformal Field Theory" by Schottenloher and there is one point on the Osterwalder-Schrader axioms that I am a bit confused about. They are ...
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Is there a sharp definition of an unstable nuclide?
This may be a somewhat philosophical question, but here goes.
Wikipedia claims that several nuclides (e.g. hydrogen-5) have half-lives shorter than $10^{-22}$ seconds. This is on the same order of ...
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Why is the angular momentum of photon $\hbar$ if the spin is 1?
I saw in many places that the spin of photon which is a boson is 1. Which we can write as $s=1$.
I also saw that the angular momentum of a particle with spin $s$ is $\sqrt{s(s+1)}\hbar$.
If both is ...
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$SO(3,1)$ is locally $SU(2)\times SU(2)$, what does *locally* mean here?
I am learning Lie group and Lie algebra. I saw in a YouTube video "Supersymmetry lecture 02" from OpenCourseWare (OCW) at University of Cambridge at 11:17 that
$SO(3,1)$ is locally $SU(2) \...
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Formal mathematical definition of radiance and irradiance
Radiance is defined as
$$L = \frac{\partial^2 \Phi}{\cos{\theta} \partial A \partial\Omega}$$
and irradiance as
$$ E = \frac{\partial \Phi}{\partial A},$$
where $\Phi$ is the radiant flux, $\Omega$ is ...
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Mass of an object [duplicate]
The mass is said to be matter content of an object. Is there any detailed definition of mass because the phrase , "matter content of an object" uses the word matter whose definition is ...
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Are all events along a Cauchy surface always space-like separated?
I'm new to relativity. Are all events along a Cauchy surface always space-like separated?
I know that a necessary condition of a Cauchy surface is that every inextendible causal curve intersects the ...
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Why is current defined as $dQ/dt$ even though it is not defined as the rate of 'change' of flow of charges?
I do not understand this definition. $dQ/dt$ represents the rate of CHANGE of charge flow at an instant even though current is defined as only the charge flow per unit time.
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What is the difference between wavefunction renormalization and field strength renormalization?
A while ago I asked a question asking what is field strength renormalization (What exactly is field strength renormalization?). I now have a better way of thinking about this, which is that it relates ...
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Is the vector representation the fundamental representation of the Lorentz Group?
The vector representation of the Lorentz Group has dimension 4, while the group has dimension 6. If it's not the fundamental representation, which one is?
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Are Lissajous figures SHM(s)?
Are Lissajous figures SHM(s)? I have been studying perpendicular superposition of SHM(s). And i understand that 2 shm having same angular frequency and differed by phase pi/2 when superimposed ...
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On the physical meaning of functionals and the interpretation of their output numbers
I am studying about functionals, and while looking for some examples of functionals in physics, I have run into this handout .
Here are two questions of mine.
1- This handout starts as follows (the ...
user386360
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Confusion in definition of Potential Energy
Potential energy is defined like this. $ΔP.E=-W_{AB}$. This means that the potential energy at point A minus potential energy at point B should equal the negative of the work done by a conservative ...
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Understanding Wikipedia's definition of a spinor
I originally asked this question on math SE but I'm asking it again here due to the lack of responses. I should note that I come from a mathematical background and not a physics one so I am not ...
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What is the difference between interaction energy and self-energy?
From my understanding self-energy is the energy required to put charges in a certain charge distribution and interaction energy is the potential energy caused by the interactions between particles, ...
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Physical motivation for the definition of Spin structure
I'm pretty confused about the motivations behind defining a spin structure on a manifold. Let me explain.
In quantum mechanics, particles are represented by irreducible unitary projective ...
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Are electrons and positrons part of a family of 4 (8? 16?) similar particles?
EDIT: Completely rewritten because of the 'needs clarity' tag and some useful related questions appearing in the side-bar. I hope this is clear now
This answer gives a long list of properties of ...
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Relationship between bel and decibel
Bel is a unit of $\log_{10}$ of ratio of two quantities.
$$1\,\text{bel} = \log_{10}\frac{P_1}{P_2}$$
On Wikipedia it says: $1\,\text{decibel} = \frac{1}{10}\,\text{bel}$.
According to this definition ...
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The frequency is off by a factor of $2 \pi$?
I was reading Morin's Introduction to Mechanics, and the following material came up:
At equilibrium point $x_0$ expanding the Taylor series, we see $V(x)=\frac12 V''(x_0)(x-x_0)^2$ so comparing this ...
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What is a hypersurface?
What is the concept of hypersurface in general relativity? I know it could be characterized into three categories but how do we define hypersurface (in general) in physics? I didn't get what thing it ...
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The equipartion theorem and degree of freedom in case of vibration
I have been taught in chemistry that,
the energy of a vibrational freedom is $RT$ (ie, twice that of rotational/translational)
The degree of freedom which I found in chemistry, for the vibrational ...
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Exact Definition of electrostatic field
Some documents define an electrostatic field as the electric field of a stationary charge or steady current.
However, in other documents, they are defined them as electric fields satisfying $\nabla \...
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Can generalised momentum be defined as mass times generalised velocity？ [closed]
The generalised momentum is defined by the partial derivative of Lagrangian. Can we think of this as a mass times a generalised velocity?
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Sign of distance/arm when finding moment/torque
I know that $M=Fd$, but when I have a negative moment, does the sign of the moment matter when I'm calculating for distance? If yes, then what would it mean when distance is negative?
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Is Retardation and Deceleration the same thing? [closed]
Does, Deceleration always have to be Retardation? Or, can there be Deceleration without Retardation.
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What exactly is charge and what does it have to do with anything? [duplicate]
Often charge is defined as a measure of interaction with the electromagnetic field. However, electromagnetic field is often defined as a field produced by moving electrical charges, or relates to ...
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When does a theory decouple?
The question is very broad, but it seems to me that the term 'to decouple' is also used in various contexts. For example, neutrinos decouple from the photons in the early Universe, when the ...
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What is kinetic energy?
I am asking these question on a fundamental level please don't go too deep in explaining.
A very basic definition if energy is that It is the capacity to do work, my question is "on what thing ...
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What was the exact triple point of water in the pre-1954 definition of Celsius and in the post-2019 definition of Celsius?
In 1954 the definition of Celsius was changed in terms of absolute zero and the triple point of water. And the triple point was chosen to be exactly 0.01C. Since there is no physical reason for the ...
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Problem with Center of mass equation definition
I was trying to motivate the definition for the center of mass formula and I am getting a weird result. For simplicity let's assume two-point masses of mass $m$ and $2m$ connected by a rigid massless ...
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What is difference between normal force and reaction force? [duplicate]
according to my book the perpendicular component of reaction force is called normal force when there is contact between two bodies. I can not understand that how for example when we jump we pushes the ...
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What is meant by dimension of the defining representation (and adjoint representation)?
A linear representation of a classical Lie group $G$ is defined by $\rho:g \to GL(V)$ where $g \in G$ is a group element and $V$ is the representation space. The dimension of the representation space(...
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What is normal force and when it acts?
what are contact forces and according to: https://www.physicsclassroom.com/class/newtlaws/Lesson-2/Types-of-Forces
it says there are 6 types of contact forces. I am having doubt with applied force and ...
user379089
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What is a parent Hamiltonian? [closed]
The term is used throughout the literature but I was not able to find a definition or even a paper properly introducing the term. What does a Hamiltonian have to satisfy to be a parent Hamiltonian?
An ...
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What is energy? [duplicate]
What is energy? Some say its just a very useful quantity. But its also a very important concept. Its used everywhete in physics. Did we just some day randomly came up with something like F.S and ...
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Contact terms in Schwinger-Dyson equation and Ward-identity
I am reading Weigand's notes for the derivation of Ward-identity.
The Second last paragraph on page 133, says the following statement
"The Schwinger-Dyson equation and the Ward-identity show ...
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Why does $\omega^2=\frac{k}{m}$?
Why does $\omega= \frac{2 \pi}{T}=\sqrt{\dfrac{k}{m}}$? If we take the differential of $a=-\frac{k}{m}x$ from $F=-kx$ by SHM definition, we get $x=A\cos{(\sqrt{\frac{k}{m}}t+\phi)}$. And we just ...
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Is this motion rotation or curvelinear translation?
Arrow orbit around point C like graph show.
Is motion of arrow, rotation or curvelinear translation according to physics definition?
What is definition and types of translation and rotation?
I find ...
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Is entropy defined up to a constant?
The question was asked long time ago on this site, but not answered properly.
In classical thermodynamics, entropy is defined up to a constant. In statistical thermodynamics, there is no such freedom. ...
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Why is mass nonnegative/is the nonnegativity of mass a convention?
In An Introduction to Tensor Calculus and Relativity, Lawden provides the definition of mass by considering a collision between two massive particles $p$ and $q$ with masses $m_p$ and $m_q$, ...
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Distinction between majorana neutrinos and sterile neutrinos
I am a bit confused about Majorana neutrinos. A generic Dirac spinor can be written in terms of his left and right-chiral components as
$$
\psi = \begin{pmatrix} \psi_L \\ \psi_R \end{pmatrix}
$$
A ...
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How does Newton's First Law show the existence of inertial reference frames? [duplicate]
Newton merely stated the law of inertia as:
An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced ...
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Is Torque always defined about the axis of rotation?
My professor told me torque can be calculated about any point in space, but would such a torque ever make sense even if the particle is rotating about some other axis, on which the point does not lie?
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Newton's second law - local laws and non-local laws
What are local laws? I was reading this line in a book...
Newtons second law is a local law. This means that it applies to a particle at a particular instant without taking into consideration any ...
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What is a symmetry defect?
I found that it is a normal concept appearing in condensed matter physics and especially topological order field. I have been aware of the topological defect. But what is a symmetry defect? Could ...
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Why isn't the charge of an electron taken to be "$-1.6 \times 10^{-19}$ coulombs" [closed]
Even though we refer to electrons being negatively charged, why is it that we don't write the charge as "-1.6 × 10^-19 coulombs"?
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