# Tag Info

15

Surely there is a story behind the print on Feynman's tee, these are the CM-1/CM-2 T-shirts. Quoting Tamiko Thiel: The geometric boxes and their 'hard' connections represent the 12-dimensional 'cube of cubes' that forms the internal hardware network connecting all processor chips with each other in a maximum of 12 steps. Feynman is the one who suggested ...

9

Most physics publications nowadays (at least in my experience) include a link to the paper's DOI, which is the easiest way to get to the reference's abstract page. If I am reading a printed-out paper and care about the references, I'll usually have the references section in a browser window and use that to go to each reference. Absent that, the best bet is ...

5

The "topological" in "topological order" and the "topological" in "topological insulator" have different meanings. The 'topological' in topological order means 'robust against ANY local perturbations'. The "topological" in "topological insulator" means 'robust against some local perturbations that respect certain symmetry'. In fact the properties of ...

5

Those Greek letters are indices indexing the components of $g$. Generally if one expresses a rank-2 tensor like $g$ as a matrix, the first index indexes the rows, the second the columns. In your example, we have $g_{rr} \equiv g_{11} = 1$, $g_{\theta\theta} \equiv g_{22} = r^2$, $g_{r\theta} \equiv g_{12} = 0$, etc. As you can see, we sometimes use numbers ...

4

Quantum electrodynamics is needed to describe Nature instead of classical electrodynamics because quantum phenomena are observed – and have been observed at least since 1900 – which prove that classical physics in general and classical electrodynamics in particular is incorrect as a description of Nature and a better theory is needed. The quantum phenomena ...

2

Math is a tool for describing stuff in e.g. physics. I could say "Energy conservation means that any change in internal energy must be exactly the same as all heat gained but withdrawn the work done". Or I could say: $$\Delta U=Q-W$$ It is even hard to express in words without using words like "equal to", "added", "plus" and "minus". Math should be ...

2

The line J. Stat. Phys. 148 513-547 contains all the information needed to locate the article. It appeared in the Journal for Statistical Physics, issue 148 on pages 513-517. In fact, if you just type the stuff in my quote into Google, the third hit (at least on my search) is a .pdf version of the paper.

1

This sounds like a fun game and seems to me to be a highly evocative analogy for microscopic domain behaviour. Ideally, there should be a north pole face, a south pole face and the other four faces are all magnetically equivalent to one another. That is, the cube's magnetic behavior is invariant with respect to 90, 180, 270, ... degree rotations about an ...

1

There are such things are large gauge transformations, which I think are related to your question. For example, consider general relativity where the gauge invariance is diffeomorphism invariance. Typically gauge transformations are considered that leave the boundary invariant, but there are also large gauge transformations that for example rescale the ...

1

In YM theory, you generally don't take the constant transformations to be gauge transformations, since the constant transformations are generated by the charge operator. If the charge operator generated gauge transformations, it would act trivially on all physical states, which would mean that you couldn't have charges.

1

Higher voltages allow conduction paths that can do less than kill. You can get serious electrical burns and even arcs that pass through the body and leave burn channels, as in lightning strikes, or sitting on a charged laser capacitor (fatal for a Stanford grad student many years ago). Nerve damage and permanent heart arrhythmia can result. Might be good for ...

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