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seen Aug 26 at 8:22

Dec
2
answered How to show that every Killing vector field is a matter collineation?
Dec
1
comment What are the borders of determinism?
@jinawee: I went ahead and 'fixed' it; as far as stackexchange goes, I'd say it's better to ask forgiveness than wait for permission - after all, it's easy to roll back my changes if anna disagrees...
Dec
1
revised What are the borders of determinism?
added latex and fixed some minor stuff
Dec
1
comment Why doesn't Length of cycloid depend on speed of rolling?
Thus, the arc lengths don't have to be the same - shouldn't that read Thus, the arc lengths don't have to be different?
Dec
1
comment Examples of singularities in classical physics
obligatory link to Wikipedia: en.wikipedia.org/wiki/Rational_function#Applications and de.wikipedia.org/wiki/Rationale_Funktion#Anwendungen ; the German one lists some examples, but their singularities aren't particularly interesting as far as I can see
Dec
1
comment Are all elementary particles of the same type exactly the same?
@akhmeteli: in a way, yes: in the Dirac basis, the spinor components are related to the particle/anti-particle and spin up/spin down distinction; the gamma matrices are responsible for mixing them and can be decomposed as $\gamma^0=\sigma^0\otimes\begin{pmatrix}1&0\\0&-1\end{pmatrix}$, $\gamma^k=\sigma^k\otimes\begin{pmatrix}0&1\\-1&0\end{pmatrix}$; note that the second factor of $\gamma^0$ is diagonal, consistent with the idea that the particle/anti-particle distinction is related to projection onto the time axis
Nov
30
comment Are all elementary particles of the same type exactly the same?
@akhmeteli: well, we do describe both particles and anti-particles with a single Dirac field; the idea of a single worline for all electrons is not really workable (despite conjectured proton decay involving positrons), but I like it nevertheless ;)
Nov
30
comment Are all elementary particles of the same type exactly the same?
see also en.wikipedia.org/wiki/One-electron_universe
Nov
30
comment Continous spectrum of black body radiation
things like doppler shift and compton scattering can broaden the already present natural line width of spectral lines, which get supplemented by things like dipole radiation and Bremsstrahlung
Nov
29
revised How does the Einstein Equivalence Principle imply a spacetime with a metric (and a connection)?
restructured anwer
Nov
29
comment How does the Einstein Equivalence Principle imply a spacetime with a metric (and a connection)?
@AstoundingJB: see edit
Nov
29
revised How does the Einstein Equivalence Principle imply a spacetime with a metric (and a connection)?
answer actual question
Nov
29
answered How does the Einstein Equivalence Principle imply a spacetime with a metric (and a connection)?
Nov
29
comment Proof that QCD is the theory describing strong interactions?
on an unrelated note, where does the abbreviation TOF (instead of TOE) come from? I'm guessing it's not 'Totally Obvious Fraud' ;)
Nov
29
comment Is the standard model so full of holes it is time to start again?
@MattLuckham: we haven't exhausted the possible dark matter canidates yet; take the Higgs boson: we excluded large ranges of possible masses before we finally found it; keep in mind that the universe doesn't have to play nice with the scientists: let's assume that the matter/antimatter imbalance is due to the fact that there's an antimatter bubble that inflation pushed out of the cosmic horizon; if we cannot come up with some indirect way to prove or exclude that possibility (I don't know what the current status on that it), we might never be able to conclusively decide one way or the other
Nov
28
awarded  Yearling
Nov
27
revised How to prove the raising/lowering indices operation?
added 23 characters in body
Nov
27
answered How to prove the raising/lowering indices operation?
Nov
27
comment How to prove the raising/lowering indices operation?
nothing to show here, it holds by definition: tangent and cotangent space are not canonically isomorphic, so we need to manually specify an isomorphism - lowering and raising indices via metric tensor and its inverse
Nov
27
awarded  Necromancer