# Tag Info

## Hot answers tagged terminology

55

Velocity is a vector. Speed is its magnitude. Position is a vector. Length (or distance) is its magnitude. A vector points in a direction in space. A negative vector (or more precisely "the negative of a vector") simply points the opposite way. If I drive from my home to my workplace (and then defining my positive direction in that way), then my velocity ...

30

Which year? The sidereal year? The tropical year? The anomalistic year? The calendar year (and whose calendar)? The sidereal year is the average amount of time it takes the Earth to make one complete orbit about the Sun with respect to the fixed stars. The tropical year is the amount of average amount of time between successive spring equinoxes. The ...

26

A second-order tensor can be represented by a matrix, just as a first-order tensor can be represented by an array. But there is more to the tensor than just its arrangement of components; we also need to include how the array transforms upon a change of basis. So tensor is an n-dimensional array satisfying a particular transformation law. So, yes, a ...

25

The main distinction you want to make is between the Green function and the kernel. (I prefer the terminology "Green function" without the 's. Imagine a different name, say, Feynman. People would definitely say the Feynman function, not the Feynman's function. But I digress...) Start with a differential operator, call it $L$. E.g., in the case of ...

20

From here: Higgs is an atheist, and is displeased that the Higgs particle is nicknamed the "God particle", because the term "might offend people who are religious".Usually this inappropriate nickname for the Higgs boson is attributed to Leon Lederman, the author of the book The God Particle: If the Universe Is the Answer, What Is the Question?, but the ...

20

Those code names all come from certain catalog. For example, NGC means 'New General Catalogue'. There are various catalogs aiming at different objects, like stars, nebulae, galaxies, etc, but not for the Earth, at least not yet. You can find almost all known astronomical catalogs and tables at CDS

20

Theoretical physics is the field that develops theories about how nature operates. It is fundamentally physics, in that the ultimate goal is to describe reality. It is informed by experiment, and at the same time it extends the results of experiments, making predictions about what has not been physically tested. This is accomplished using the language of ...

19

To pick up on twistor59's point, time is not a vector but a time interval is. The confusion arises because you have to define carefully what you mean by the word time. In special relativity we label spacetime points by their co-ordinates $(t, x, y, z)$, where $t$ is the time co-ordinate. The numbers $t$, $x$, etc are not themselves vectors because they just ...

19

In classical mechanics "kinematics" generally refers to the study of properties of motion-- position, velocity, acceleration, etc.-- without any consideration of why those quantities have the values they do. "Dynamics" means a study of the rules governing the interactions of these particles, which allow you to determine why the quantities have the values ...

18

The reason people say this is because all particles you see are absorbed after a finite time, and the notion of on-shell is asymptotic. The finite time means that they are really internal lines in a diagram, and so ever-so-slightly off shell. The exactly on-shell S-matrix is an asymptotic quantity, relevant only in the holographic limit.

17

This principle is called "positivism". But I prefer the term "logical positivism". Positivism is a basic principle of thought--- it distinguishes questions which are meaningful and meaningless. It is not meaningful to ask "How does Argentinian property law taste?", it is not meaningful to ask "How many angels can dance on the head of a pin?", and it is not ...

17

A "Trojan" object is any smaller object that shares the same orbit as a larger body but leads or trails it by about 60 degrees in the orbit. These positions are the L4 and L5 Lagrange points (respectively) in the larger body's orbit about its parent object. The L4 and L5 Lagrange points are locations of stable gravitational equallibrium between the larger ...

17

In physics101, scalar quantities are defined to be ones which have magnitude only, and no direction, where "direction" in this context means a direction in three dimensional space. Time clearly has no such direction. However, in slightly more advanced physics, where special relativity is applied "scalar" is used as a shorthand for "Lorentz scalar" - a ...

17

Actually there are terminological subtleties when you are talking about that. Particles in the accelerator's tube are gathered in a sequence of little "bunches". For the proton-proton mode there was roughly ~3000 bunches per beam. And each bunch contained roughly $\simeq 10^{11}$ protons. So, at the largest level what you actually have during the ...

17

Matrices are often first introduced to students to represent linear transformations taking vectors from $\mathbb{R}^n$ and mapping them to vectors in $\mathbb{R}^m$. A given linear transformation may be represented by infinitely many different matrices depending on the basis vectors chosen for $\mathbb{R}^n$ and $\mathbb{R}^m$, and a well-defined ...

17

The definition of a wave is not that it is the oscillation of a medium. Waves are called waves because they are solutions to a wave equation, which is, for a generic "excitation" $A(t,x)$ depending on the time $t$ and some spatial coordinate $x\in\mathbb{R}^n$, of the general form $$\frac{\partial^2 A}{\partial t^2} = c^2\Delta A$$ where $\Delta$ is the ...

16

The Wilsonian effective action is an action with a given scale, where all short wavelength fluctuations (up to the scale) are integrated out. Thus the theory describes the effective dynamics of the long wavelength physics, but it is still a quantum theory and you still have an path integral to perform. So separating the fields into long and short wavelength ...

15

It's not really a single principle - it's a philosophy and in the context of philosophical discussions about science, it is usually known as positivism. http://en.wikipedia.org/wiki/Positivism As any philosophy, it cripples the penetrating power of science if it is extended too far - and every philosophy ultimately fails. The thought experiment about ...

15

The dual of a tensor you refer to is the Hodge dual, and has nothing to do with the dual of a vector. The word "dual" is used in too many different contexts, and in this case it is even used the same $*$ symbol. One usually specifies "Hodge dual", or "Hodge star operator", to avoid confusion. Both these "duals" are isomorphisms between vector spaces endowed ...

15

A fermion is any particle, elementary or composite, that obeys Fermi-Dirac (as opposed to Bose-Einstein) statistics relating to how identical particles behave when you swap two of them. Due to an important but complicated result, this is taken to amount to having half-integer spin. A lepton is one type of elementary particle with spin 1/2. The only leptons ...

14

Statics: Study of forces in equilibrium without consideration of changes over time. Kinematics: Study of motions (position, velocity, acceleration) Kineto-statics: Study of forces in equilibrium, with the addition of motion related forces (like inertia forces via D'Alembert's principe) one instant at the time. Results from one time frame do not affect the ...

13

An experimental take Exclusive implies that you have measured the energy and momenta of all the products (well, with an exception I'll discuss below). Inclusive means that you may have left some of the products unmeasured. This applies to scattering processes as well as decays. Some things to note: Exclusive measurements allow you to nail down one, ...

13

What is “special” and what is “general” in Relativity? The "special" in special relativity refers to the fact that it is not a universal theory. Predictions made by special relativity only apply under certain special circumstances. Those special circumstances are where gravitation is not present or can essentially be ignored. Initially I thought in ...

13

If I saw the word "amp" written as such in a paper in my field (astrophysics) it would strike me as a bit informal. I would expect to see the full "ampere" written. That said, it is rare to actually write out the full name of a unit; usually it follows a number and is given its standard abbreviation. When abbreviated to e.g. "$5\ \mathrm{A}$", I would ...

12

A particle is said to be on-shell if it satisfies the relativistic dispersion relation, $$E^2 = p^2 +m^2$$ in units wherein $c=\hbar=1$. If you graph it, you obtain a parabolic surface for massive particles, and a cone for massless particles, like a photon. This is known as the mass shell, it is quite literally a shell or surface. The momentum of a real ...

12

OP wrote (v1): What does "the ${\bf N}$ of a group" mean? 1) Physicists are referring to an irreducible representation (irrep) for whatever group $G$ we are talking about. The number ${\bf N}$ refers to the dimension of the irrep. The point is that irreps are so rare that irreps are often uniquely specified by their dimension (modulo isomorphisms). ...

12

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which: Is in orbit around the Sun, Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and Has "cleared the neighbourhood" around its orbit. A non-satellite body ...

12

To pretty much everything you stated in your question, "no". That convection requires a medium is not the main difference, it is simply the most obvious aspect of what is a fundamentally different mechanism for transfering energy. Convection is the transfer of energy by movement of a medium, whereas radiation is the transfer of energy by, well, thermal ...

12

Within power systems such as regional or national electricity grids, $\frac{\mathrm{d}^2E}{\mathrm{d}t^2}$ is called the slew rate: it's used to denote the rate of change of power demanded from, or supplied to, electricity grids. It's typically either expressed as MW/s or GW/h, being two time periods of interest in balancing electricity grids. ...

Only top voted, non community-wiki answers of a minimum length are eligible