# Tag Info

1

The common definition of "time" is a type of measurement, like size. No. The common definition of "time", certainly in the context of physics, is as one indication of one participant, or also as the ordered set of all indications of one participant. As Einstein put it: "[... that instead] of "time" we substitute "the position of the little hand of my ...

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In terms of physics time is a coordinate and defines a coordinate system . In the newtonian world we live our lives in, time is fixed by clocks and space by rulers. Slowing or acceleration of time is a personal perception, old people feel time passes very fast, in crisis situations it passes very slowly in the observer's perceptions. If one goes into ...

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A frame is one image (produced by some imaging device such as a computer monitor). Frames per second is therefore the measure of how many unique images are produced in one second (i.e. the frequency of the frames). Hertz is the SI unit of frequency, typically used as a measure of cycles per second. When you're referring to cycles or frames, you're not ...

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"Frames" is not a unit of anything. A frame is a thing. FPS in Hertz measures frames in one second. More generally Hertz can be used as the unit of any "thing" per second. In the case of an oscillating wave we measure cycles per second in Hertz. When I was young, there was no "Hertz", and the units were "cps" and "fps". Those old designations were ...

2

As user ACuriousMind correctly writes: What Goldstein calls the principle of least action $\int p~\mathrm{d}q$ is usually called Maupertuis' principle or the principle of abbreviated action. What Goldstein calls the Hamilton's variational principle is often also called the the principle of least/extremal/stationary action $\int L~\mathrm{d}t$. This is ...

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The more common names for what you are talking about are the abbreviated action $$S_0[q] := \int p \mathrm{d}q$$ versus the action $$S[q] := \int_{t_1}^{t_2}L(q,\dot q,t)\mathrm{d}t$$ Both are used in different formulations of classical mechanics, and deliver a different "flavor" of solutions. On both one can do variations calculus and obtains the ...

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I suppose it is related to hyperbolic partial differential equations.

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Steve and Emilio have provided nice technical descriptions of how a quantum error correcting code is defined. I thought it worth adding where the terminology comes from and how quantum codes and Hamiltonians are connected. The toric code is called a code because it is a quantum error correcting code. Quantum error correcting codes are quantum analogues of ...

1

I'm not familiar with ISO 5725 (a 1994 revision of a 1986 document, apparently "reviewed and confirmed" in 2012), and it seems that I have to buy it to read it. A 2008 vocabulary of metrology put out by the BIPM and also cited by Wikipedia has definitions much closer to my intuition, and to common usage among folks I know who specialize in precision ...

7

The "shift in the meaning" refers to some attempts to reinterpret the terminology that were made by a metrological document, ISO 5725, in 2008. That may be described as a bureaucratic effort by a few officials – really bureaucrats of a sort – and as far as I know, the "shift in the meaning" hasn't penetrated to the community of professionals. The people ...

2

Work. Potential energy exists because of some force that exists, and moving an object relative to that force causes work to be done. And by the work-energy theorem, the work done on an object is equal to the change in kinetic energy of that object.

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This is not specific to converting potential energy to kinetic, but the term you're looking for might be "transduction" or "to transduce"- the process of converting energy from one form to another.

3

I like alemi's suggestion in a comment of crash density, by analogy with the linear "mass density" for a rod. Among other advantages, this frees up "crash rate" to mean the number of crashes per million hours driven. Alternatively you could invert the crash density to talk about the "mean distance between crashes."

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You could use the generic/boring term ratio, and precede it with CMK for crashers per million kilometers. The entire thing then becomes the CMK ratio. Though CMK rate can also work as others have said so long as your definition is clear. Then there's CMK factor if you want to use a fancier but more ambiguous term.

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Collider physicists actually use a quantity called "luminosity" which has inverse areal units, and they just quote as such (note that a "barn" is a (very small) measure of area), so when they say "inverse-femtobarns" they mean the inverse of an exceedingly small area which equates to a very high luminosity. There is nothing fundamentally wrong with "inverse ...

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The term "Crash Recurrence" comes to mind. http://dictionary.reference.com/browse/recurrence an act or instance of recurring. return to a previous condition, habit, subject, etc. It captures the concept of an interval without implying that it is regular or predictable, which seems to be what you're looking for.

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For a fission chain reaction to spontaneously happen in uranium there are some requirements to be fulfilled. A) there should be at least 4% U-235 in the mix of uranium isotopes. B) there should be a moderator to decrease the energy of the neutrons which result from the fission of a uranium core. 2 billion years ago the percentage of U-235 was large ...

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There isn't really a difference between "natural" or "artificial" reactions. All reactions are just "things that can happen". Some things only happen in certain circumstances, and those circumstances may be very unlikely to occur without being specifically engineered, but there is no reason in principle why they could not happen naturally. There is evidence ...

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$r^2 \dot{\theta}$ is known as the specific angular momentum. Also, the correct formula for the 2 body Lagrangian is actually: $$\mathcal{L} = \frac{\mu}{2} (\dot{r}^2+r^2 \dot{\theta}^2) + \frac{GMm}{r} = \frac{Mm}{M+m} \frac{\dot{r}^2+r^2 \dot{\theta}^2}{2} + \frac{GMm}{r}$$ where $\mu = \frac{Mm}{M+m}$ is the reduced mass.

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Spontaneous parametric down-conversion converts a single incoming photon to two outgoing photons. I think the article is saying that that if you measure one photon coming out there must be a second photon as well. The author is referring to the second as a heralded photon in the sense that measurement of the first photon is a sign that the second (heralded) ...

1

From a little bit of reading, I believe that "Supersymmetry" refers to the relativistic formulation of the extra symmetries between bosons and fermions, and "supersymmetric quantum mechanics" refers to the non-relativistic formulation of the same symmetry. In other words, one is compatible with special relativity and deal with fields (SUSY) and one ...

1

Here's a parallel answer to Luboš's but purely classical. Start by noting that the momentum vector of a plane wave with wavelength $\lambda$ is: $$\vec{p} = \frac{2\pi}{\lambda}$$ In some elastic scattering experiment, e.g. X-ray or some other diffraction measurement, we have something like: where $\vec{p}_{in}$ is the momentum of the incoming wave ...

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Microscopically, i.e. in the quantum theory the scattering with radiation is a collision of particles with photons such as $$e^- + \gamma \to e^- + \gamma$$ The momentum vectors of the particles above are $$\vec p_1+\vec p_2= \vec p_3 + \vec p_4$$ where the identity holds due to momentum conservation. But in general $\vec p_1\neq \vec p_3$ and $\vec ... 4 As described in the link you provided, Raman scattering is any scattering that changes the frequency/wavlength/energy of the light by transfer of energy to or from the matter that scatters it. If the matter absorbs energy it is called Stokes Raman scattering (sometimes shortened to just Stokes scattering). If the matter loses energy it is called anti-Stokes ... 1 To my knowledge, there isn't a specific term for these types of gasses. In your question you name "substance" while you list elements. Many different molecules are gaseous at room temperature; however, only a few of the elements are. I'll look at both. They come from different parts of the periodic table but do have a couple of features in common: ... 1 If you only have two particles, they only have a mutual angular momentum — there's only one$L$. If you have many particles orbiting a central potential, like electrons in an atom, or if you can get away with pretending like you do, as in the nuclear shell model, then the eigenvalue of the system under parity inversion is the product of the eigenvalues of ... 3 The terms "Landau gauge" and "Feynman gauge" (among others) were introduced by Bruno Zumino. I accidentally learned about it an hour ago from David Derbes http://motls.blogspot.com/2014/06/bruno-zumino-1923-2014.html?m=1 in this blog post about a sad event, Bruno Zumino's death a week ago. David Derbes wrote: I met Bruno Zumino at the Scottish ... 1 It's the total product. The famous example is the spin of the deuteron. We have evidence that the two-nucleon isospin triplet with$I=1$is unbound because we do not observe diprotons or dineutrons in nature, so we expect the deuteron to have isospin$I=0$. We know that the deuteron has positive parity, so we require$L$even; by antisymmetry the deuteron ... 1 A inertia tensor$I$. $$I \equiv \begin{bmatrix} I_{1,1} & I_{1,2} & I_{1,3} \\ I_{2,1} & I_{2,2} & I_{2,3} \\ I_{3,1} & I_{3,2} & I_{3,3} \\\end{bmatrix}$$ A product of inertia is an off-diagonal entry in the tensor:$I_{1,2} = I_{2,1}$,$I_{1,3} = I_{3,1}$, or$I_{2,3} = I_{3,2}$. True. A principal moment of inertia is ... 0 They are just the equation above rewritten to a shorter format using the definition of the moment of inertia about an axis: $$I = \sum_{i=1}^N{m_ir_i^2}.$$ The equation above uses$r_{j,2}$and$r_{j,3}$, which correspond to$I_2$and$I_3$. 1 There is a mapping from dimer (and loop) coverings of planar lattices to the configurations of$2-D\$ elastic membranes; the "height" is the displacement of the membrane. I'm aware of the work that Chen Zeng and Jane Kondev did exploring this relationship in the late 1990's, e.g.: Chen Zeng et al. Statistical Topography of Glassy Interfaces Phys. Rev. Lett. ...

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