# Tag Info

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 ...

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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 ...

1

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 ...

4

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 ...

1

Newton had much precedent. He didn't devise the 1st and 2nd Postulates in a vacuum. Regarding the 1st Postulate: John Philoponus (ca. 490-570) first devised the notion of inertia. …rest is found in all things. For the perpetually moving heavens partake in rest, because the very persistence of perpetual motion is rest.[In De anima, 75, 11]. …the ...

3

Going by a magic 8-ball a brief web search, the most important steps towards the geometrization of electromagnetism (ie its formulation as a classical Yang-Mills theory in terms of principal connections) should be: Maxwell's equations: James Clerk Maxwell, A dynamical theory of the electromagnetic field (1865) differential forms: Élie Cartan, Sur certaines ...

5

It's my understanding that the invention of the metric system during the turbulence following the French Revolution also included a switch to decimal time, with ten hours per day, etc., but that it didn't take. There's a certain amount of cultural inertia that has to be overcome; as you're probably aware, those of us in the United States still have many ...

2

The French Revolutionary Gov't did try to move towards a decimalized system of time measurement, with a second defined as one-one hundred thousandths of a day (along with decimal hours, minutes and a new calander), around the same time as it introduced the proto-metric system. But unlike the rest of the metric system, the new time keeping system and ...

0

Dirac already considered it in a 1925 paper(Open access). (Thanks to Alex Nelson for the comment to the OP's question. I found it at the Wikipedia's article on Groenewold. =]) Operator ordering issue also exists in path-integral formalism, and of course Feynman was aware of it. See for example "Techniques and applications of path integration" by Schulman. ...

1

I believe that in Physics, when introduced to a law, we must first ask what observation does it predict. In the case of the gravitational two body problem, we have a Hamiltonian $H = \frac{p_r^2}{2\mu} + \frac{L^2}{2\mu r^2} - \frac{G \mu M}{r}$, where $\mu=m_1 m_2/(m_1+m_2), M=m_1+m_2$, $L$ is the total angular momentum, and we have done a transformation ...

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 ...

3

It is by no means necessary to introduce the Dirac delta. For instance, $G(x,x')$, singular for $x=x'$ is a Green's function of $\Delta$ if, defining, $$\phi(x):= \int_A G(x,x')f(x') dx'$$ we have $$\Delta \phi(x) = f(x)$$ where further details on the behaviour of $\phi$ and the regularity of $f$ and the nature of the integration domain $A$ are assumed and I ...

0

Special Relativity, and a proper understanding of time, can be confusing if you just look at the effect rather than both the cause and the effect. In other words, to start with, you have an absolute foundation or cause, that due to its structure, produces a relativistic outcome. But today's school system in general only teaches you of the relativistic ...

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