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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Differentiating Propagator, Green's function, Correlation function, etc

For the following quantities respectively, could someone write down the common definitions, their meaning, the field of study in which one would typically find these under their actual name, and most ...
Nikolaj-K's user avatar
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115 votes
11 answers
49k views

What Is Energy? Where did it come from?

The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does ...
Anna's user avatar
  • 1,697
110 votes
6 answers
13k views

Are Newton's "laws" of motion laws or definitions of force and mass?

If you consider them as laws, then there must be independent definitions of force and mass but I don't think there's such definitions. If you consider them as definitions, then why are they still ...
user5402's user avatar
  • 3,023
95 votes
3 answers
33k views

Why was carbon-12 chosen for the atomic mass unit?

The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit? Why not ...
Dieblitzen's user avatar
  • 1,637
92 votes
9 answers
50k views

Are matrices and second rank tensors the same thing?

Tensors are mathematical objects that are needed in physics to define certain quantities. I have a couple of questions regarding them that need to be clarified: Are matrices and second rank tensors ...
Revo's user avatar
  • 17k
73 votes
12 answers
24k views

What is a tensor?

I have a pretty good knowledge of physics, but couldn't deeply understand what a tensor is and why it is so fundamental.
0x90's user avatar
  • 3,316
65 votes
5 answers
23k views

What does it mean for a Hamiltonian or system to be gapped or gapless?

I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean? Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
Jordan's user avatar
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60 votes
3 answers
27k views

What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?

What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$? I know one is a partial derivative and the other is a ...
CuriousAutomotiveEngineer's user avatar
60 votes
10 answers
9k 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 ...
Jasmeru's user avatar
  • 1,138
52 votes
4 answers
12k views

What's the real fundamental definition of energy?

Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our ...
Gold's user avatar
  • 35.9k
52 votes
4 answers
36k views

What's the difference between the five masses: inertial mass, gravitational mass, rest mass, invariant mass and relativistic mass?

I have learned in my physics classes about five different types of masses and I am confused about the differences between them. What's the difference between the five masses: inertial mass, ...
QEntanglement's user avatar
48 votes
14 answers
27k views

What is entropy really?

On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as ...
user avatar
45 votes
2 answers
2k views

Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and ...
John Rennie's user avatar
44 votes
12 answers
87k views

What do people actually mean by "rolling without slipping"?

I have never understood what's the meaning of the sentence "rolling without slipping". Let me explain. I'll give an example. Yesterday my mechanics professor introduced some concepts of rotational ...
pppqqq's user avatar
  • 4,594
44 votes
3 answers
8k views

History of interpretation of Newton's first law

Nowadays it seems to be popular among physics educators to present Newton's first law as a definition of inertial frames and/or a statement that such frames exist. This is clearly a modern overlay. ...
user avatar
43 votes
3 answers
23k views

What is a mode?

The word mode pops up in many fields of physics, yet I can't remember ever encountering a simple but precise definition. After having searched fruitlessly on this site as well, an easy to find place ...
Janosh's user avatar
  • 1,254
41 votes
5 answers
8k views

Hilbert space vs. Projective Hilbert space

Hilbert space and rays: In a very general sense, we say that quantum states of a quantum mechanical system correspond to rays in the Hilbert space $\mathcal{H}$, such that for any $c∈ℂ$ the state $\...
user929304's user avatar
  • 4,645
40 votes
2 answers
4k views

Is our definition of entropy unique?

Suppose we don't know anything about statistical mechanics, not even the existence of atoms. Why is entropy defined as $$\delta S=\frac{\delta Q}{T}$$ instead of, say, $$\delta S=\frac{\delta Q}{T^...
oamer's user avatar
  • 565
37 votes
8 answers
8k views

Is the Big Bang defined as before or after Inflation?

Is the Big Bang defined as before or after Inflation? Seems like a simple enough question to answer right? And if just yesterday I were to encounter this, I'd have given a definite answer. But I've ...
Jim's user avatar
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36 votes
5 answers
82k views

What's the difference between Fermi Energy and Fermi Level?

I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the ...
Eddy's user avatar
  • 517
35 votes
10 answers
7k views

What is wrong with the high-school definition of a vector?

Why is the high-school definition of a vector as "a quantity with a magnitude and a direction" incomplete? For example, Griffiths Introduction to Electrodynamics book says: The definition of ...
Solidification's user avatar
35 votes
7 answers
253k views

What is the difference between diffraction and interference of light?

I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?
Abdul Wajid Lakhani's user avatar
35 votes
4 answers
4k views

The physical definition of work seems paradoxical [duplicate]

So this is possibly a misunderstanding of the meaning of work, but all the Physics texts, sites, and wiki that I've read don't clear this up for me: In the simplest case with the simplest statement, ...
Addem's user avatar
  • 1,229
35 votes
2 answers
9k views

What is the definition of a timelike and spacelike singularity?

What is the definition of a timelike and spacelike singularity? Trying to find, but haven't yet, what the definitions are.
user23071's user avatar
  • 353
34 votes
4 answers
139k views

What is the difference between center of mass and center of gravity?

What is the difference between center of mass and center of gravity? These terms seem to be used interchangeably. Is there a difference between them for non-moving object on Earth, or moving objects ...
DarkLightA's user avatar
  • 1,422
34 votes
4 answers
21k views

What exactly is a virtual displacement in classical mechanics?

I'm reading Goldstein's Classical Mechanics and he says the following: A virtual (infinitesimal) displacement of a system refers to a change in the configuration of the system as the result of any ...
Gold's user avatar
  • 35.9k
33 votes
11 answers
9k views

Why does work depend on distance?

So the formula for work is$$ \left[\text{work}\right] ~=~ \left[\text{force}\right] \, \times \, \left[\text{distance}\right] \,. $$ I'm trying to get an understanding of how this represents energy. ...
Dominic Roy-Stang's user avatar
33 votes
5 answers
6k views

Is this statement of conservation of charge circular?

According to Wikipedia: A closed system is a physical system that does not allow certain types of transfers (such as transfer of mass and energy transfer) in or out of the system. According to my ...
Max von Hippel's user avatar
33 votes
8 answers
148k views

What is the difference between stress and pressure?

What is the difference between stress and pressure? Are there any intuitive examples that explain the difference between the two? How about an example of when pressure and stress are not equal?
Armadillo's user avatar
  • 1,395
33 votes
5 answers
6k views

What is the definition of temperature, once and for all? [duplicate]

Can someone please explain to me what the formal definition of temperature is? Neither my textbook, nor my professor, nor any of the online sources I've checked are able to give me a proper ...
math_lover's user avatar
  • 4,536
33 votes
11 answers
35k views

What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage and electromotive force (EMF)?

This is a confused part ever since I started learning electricity. What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage and electromotive ...
new her's user avatar
  • 439
32 votes
15 answers
5k views

What is the fundamental definition of force?

As I pick up more physics I see that the definitions of force commonly provided in books and classrooms are misleading. "A force is a push or pull." This seems to be a "correct" ...
Ethan Dandelion's user avatar
31 votes
5 answers
11k views

What is a state in physics?

What is a state in physics? While reading physics, I have heard many a times a "___" system is in "____" state but the definition of a state was never provided (and googling brings me totally ...
Manish Kumar Singh's user avatar
31 votes
3 answers
2k views

Is there any truth to interpreting definition of a second as corresponding to oscillations?

As far as I understand the definition of a second, the Cs-133 atom has two hyperfine ground states (which I don't really understand what they are but it's not really important), with a specific energy ...
Meni Rosenfeld's user avatar
31 votes
2 answers
6k views

What is the Wilsonian definition of renormalizability?

In chapter 23.6, Schwartz's quantum field theory book defines renormalizability as follows, paraphrasing a bit for brevity: Consider a given subset $S$ of the operators and its complement $\bar{S}$....
knzhou's user avatar
  • 102k
31 votes
5 answers
64k views

Why is the potential energy equal to the negative integral of a force?

Why is the potential energy equals to the negative integral of a force? I am really confused with this negative sign. For example, why there is a negative sign in the gravitational potential energy ...
Omar Ali's user avatar
  • 706
30 votes
10 answers
6k views

What is a joule? I find the definition confusing

This is the definition on Wikipedia: It is equal to the amount of work done when a force of 1 newton displaces a body through a distance of 1 metre in the direction of the force applied. I take that ...
Guye Incognito's user avatar
30 votes
3 answers
7k views

How is temperature defined, and measured?

In questions like this one, temperatures of millions of degrees (Celsius, Kelvin, it doesn't really matter at that point) are mentioned. But, what does it mean exactly? What is measured, and how? As I ...
Martin Argerami's user avatar
30 votes
9 answers
98k views

Inertia Vs Momentum

At my recent lesson on kinematics, my teacher taught about inertia and momentum. This is what she said. Inertia: a characteristic of an object that resists changes to its state of motion. Momentum: ...
Simon-Nail-It's user avatar
29 votes
8 answers
7k views

Why is Microgravity called "Microgravity"?

I find the term "microgravity" to be misleading, how was it coined? NASA provide this definition: Microgravity is the condition in which people or objects appear to be weightless. The ...
dangerousdave's user avatar
29 votes
5 answers
5k views

What does the concept of an "infinite universe" actually mean?

When physicists talk about the universe being infinite, or wondering whether it is or not, what do these two options actually mean? I am not interested whether the universe is infinite or not, I am ...
Nohus's user avatar
  • 406
29 votes
1 answer
5k views

Operator Ordering Ambiguities

I have been told that $$[\hat x^2,\hat p^2]=2i\hbar (\hat x\hat p+\hat p\hat x)$$ illustrates operator ordering ambiguity. What does that mean? I tried googling but to no avail.
Greta's user avatar
  • 471
29 votes
3 answers
13k views

Why are they called "cyclic" coordinates?

In Lagrangian formalism, when $\frac{\partial L}{\partial q} = 0$, the coordinate $q$ is called cyclic and a corresponding conserved quantity exists. But why is it called cyclic?
iblue's user avatar
  • 632
25 votes
11 answers
541k views

What is the difference between weight and mass?

What is the difference between the weight of an object and the mass of an object?
Hobbs's user avatar
  • 675
25 votes
5 answers
10k views

In what sense can a complex number be a scalar?

A definition of a scalar like A scalar is a one-component quantity that is invariant under rotations of the coordinate system (see http://mathworld.wolfram.com/Scalar.html) seems to exclude ...
asmaier's user avatar
  • 9,804
25 votes
6 answers
5k views

What is the connection between a mathematician and physicist's definition of a tensor?

I study mathematics but I have a deep interest in physics as well. I have taken a course in smooth manifolds where a tensor is defined as an alternating multilinear function. Recently I have learned ...
CBBAM's user avatar
  • 3,258
25 votes
5 answers
13k views

What is the difference between a functional and an operator?

What is the difference between a functional and an operator? When we define an operator in physics, e.g. the momentum operator as $\hat{p} = i \frac{d}{dx}$, it is said this operator acts on the wave ...
asmaier's user avatar
  • 9,804
24 votes
6 answers
73k views

Why is the speed of light defined as 299792458 m/s?

Why is the speed of light defined as $299792458$ $m/s$? Why did they choose that number and no other number? Or phrased differently: Why is a metre $1/299792458$ of the distance light travels in a ...
Xplane's user avatar
  • 421
24 votes
7 answers
3k views

How many types of inertia are there?

I was looking for types of inertia, but I am confused. My book says there are three types of inertia, namely inertia of rest, inertia of motion, and inertia of direction. But when I searched for these ...
Shinnaaan's user avatar
  • 1,239
24 votes
4 answers
2k views

Definition of mass

In high school physics, I was taught that mass was just how much "stuff" or matter there is in an object. However, now that I am learning physics again in college, I am taught that mass of an object (...
baker's user avatar
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