# 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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### If I were to change the definition of refractive index [closed]

We know that the Refractive Index is just a number that gives an idea of how much velocity of light decreases in comparison to vacuum. So, it makes sense to define a variable, n as: n=c/v where v ...
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### Why is work equal to force times displacement?

This is how I think of what work is.I am sure I am wrong somewhere because I shouldn't be coming to the conclusion that I am coming to.It would be helpful if you would point out where this conceptual ...
101 views

### Diffeomorphic but physically inequivalent spacetimes

In the last few years there has been a considerable endeavor in understanding the asymptotic symmetries of quantum gravity on Minkowski Spacetime. This has been tied to a study of the BMS group that ...
107 views

### Why the need for defining the velocity as a derivative? [closed]

Something intuitive and fundamental as the concept of velocity (of a particle for example) in classical physics is defined as a derivative, a concept to me quite vague and strange, although i know its ...
31 views

### What is meant by homogeneous in $x$ in $n$'th degree?

I'm reading about classical mechanics by Goldstein, and in the section about Hamiltonian mechanics it is stated that in the expression: $$H(q,p,t)=\dot{q}_ip_i-L(q,\dot{q}, t)$$ the Lagrangian ...
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### What is a quantum system?

I heard that a wavefunction applies to a quantum system. But what is a quantum system? I am new to quantum mechanics, sorry for asking a basic question.
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### Standard Definition of speed of light and metre

The speed of light is the speed at which lightwaves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per ...
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### The difference between exact solution and analytic solution

My mother tongue is not English so I am confused with the difference between exact solution and analytic solution. Are these the same?
33 views

### What is a free parameter?

Soft question here, but I was wondering just what exactly free parameters are? I have a murky understanding on the concept but I would much appreciate someone shedding some light on the matter. Is ...
75 views

### Transformation of metric by diffeomorphism: pushforward or pullback?

Let $(M,g)$ be a smooth manifold with a metric tensor of signature $(p,q)$. The signature isn't really important for this question so we leave it general. If $\Phi : M\to M$ is a diffeomorphism we ...
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### What is the true quantitative definition of “force”? [duplicate]

Newton's second Law of Motion states that for a point mass, $\vec F = m \vec a$. This is a law and not a definition. So, this law only makes sense if all the physical quantities appearing in this law ...
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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 ...
20 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 ...
41 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 ...
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 ...
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### 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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### 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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### 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 ...
50 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 ...
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### 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 ...
71 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 ...
71 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 ...