# Tag Info

18

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

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 ...

16

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 ...

14

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 ...

13

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 ...

12

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 ...

12

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 ...

11

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 ...

11

There are 5 points relative to an orbiting body in a mostly circular orbit which are gravitationally stable, meaning that a small body placed in such a location would remain there. These are called Lagrangian points. There are 3 such points along the axis between the planet and star called L1 (between), L2 (behind the smaller body), and L3 (opposite the ...

10

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.

10

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. ...

10

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 ...

10

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 ...

9

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 ...

8

I found a general, qualitative answer in David Blackstock's book Physical Acoustics, on page 46: Impedance is often described as the ratio of a "push" variable $q_p$ (such as voltage or pressure) to a corresponding "flow" variable $q_f$ (such as current or particle velocity). I also received a nice answer to this question on another Q&A site ...

8

The term "God Particle" is used only by journalists. It's a wholly inappropriate term and I'd be very surprised if any physicist used it (outside of the lower end popular science TV programmes). General Relativity tells us that inertial and gravitation mass is the same thing. The Standard Model isn't going to say anything directly about gravitational mass ...

7

This is a very good question and we have very good discussions. I feel that a meaningful scientific question should satisfies the following condition: The different answers to the question should have different measurable consequences. Also a meaningful scientific statement should satisfies the following condition: The statement being true or false should ...

7

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, ...

7

Proabably not, at least not that I've ever heard of. Since no one has ever lived there :), there has never been any sort of calendaring system needed. Even the longest Apollo missions were only there a few days. I'm sure if there was ever a permanent base (or bases) there, some sort of time keeping system would be devised but it would also make sense to ...

7

You are correct that the standard for naming exoplanets is normally the lower-case letter after the star name in the order of discovery. So in our system, Earth would be Sol b. If there are multiple stars in the system, like 16 Cyg (which has 16 Cyg A and B), then the planet's lower-case letter would be appended to the star's, such as 16 Cyg Bb. So the ...

7

"Host star", or "host" for short, seems to fit the bill.

7

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 ...

6

Any discussion of entanglement implies you are thinking of (at least) two separate systems. Superpositions on the other hand, can also apply to situations where you are only looking at a single system. Keeping this in mind: "Can a state be entangled without also being a superposition? (Please give an example)" No. An entangled state is by ...

6

I would generally say that most physicists mean "speed of light in a vacuum" when they say "speed of light," and therefore would say that the "speed of light is constant." If it is in a field that often deals with light propagation in materials (optics, condensed matter), people are usually pretty careful to say "speed of light in a vacuum" when they mean ...

6

A nonperturbative theory means a theory where all the results can be calculated in principle to arbitrary accuracy on a computer. This is really just the same as a well-defined theory, a theory which is mathematically ok, and which makes sense, which isn't incomplete in some way at some high energy or when you do measurements at some high accuracy. An ...

6

If I create an electron on earth and someone else creates an electron on Andromeda, they're identical particles. They have the same quantum numbers, they're both excitations of the electron field. However they're distinguishable by means of their spatial separation. Their wavefunctions don't overlap. Edit: perhaps I should add that not everyone uses the ...

6

INCLUDING AN EXTENSION $\psi_o$ is, as mentioned previously, the normalisation constant which is calculated by doing the integral $\int_V|\psi|^2dV$ and setting its value equal to 1 (hence normalization). This will give you the equation for $\psi_o$. If your interest is to find the probablity amplitude for a particle in a volume V, for example, then you ...

6

Great question! I don't think there is anything obvious at play here. In quantum mechanics, we assume that that state of any system is a normalized element of a Hilbert space $\mathcal H$. I'm going to limit the discussion to systems characterized by finite-dimensional Hilbert spaces for conceptual and mathematical simplicity. Each observable quantity ...

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