Hertz showed... the... rays were not appreciably deflected by electrified metal plates. This seemed to rule out... electrically charged particles... … - Steven Weinberg

A superconductor of any kind is nothing more or less than a material in which a particular symmetry of the laws of nature, electromagnetic gauge invariance, is spontaneously broken. ... These rotations act on a two-dimensional vector, whose two components are the real and imaginary parts of the electron field, the quantum mechanical operator that in quantum field theories of matter destroys electrons. The rotation angle of the broken symmetry group can vary with location in the superconductor, and then the symmetry transformations also affect the electromagnetic potentials ... The symmetry breaking in a superconductor leaves unbroken a rotation by 180°, which simply changes the sign of the electron field. In consequence of this spontaneous symmetry breaking, products of any even number of electron fields have non-vanishing expectation values in a superconductor, though a single electron field does not. All of the dramatic exact properties of superconductors – zero electrical resistance, the expelling of magnetic fields from superconductors known as the Meissner effect, the quantization of magnetic flux through a thick superconducting ring, and the Josephson formula for the frequency of the AC current at a junction between two superconductors with different voltages – follow from the assumption that electromagnetic gauge invariance is broken in this way, with no need to inquire into the mechanism by which the symmetry is broken.

Consider... [the formula given by special relativity for the magnitude of the ]<math>P \equiv m_0 \sqrt{g_{ij}\frac{dx^i}{d\tau}\frac{dx^j}{d\tau}}</math>...where <math>d\tau^2 = dt^2 - g_{ij} dx^i dx^j</math>. [This holds because in] a locally inertial Cartesian coordinate system, for which <math>g_{ij} = \delta_{ij}</math>, we have <math>d\tau = dt\sqrt{1 - \mathbf {v}^2}</math> where <math>v^i = \frac{dx^i}{dt}</math>... [The <math>P</math>] is evidently invariant under arbitrary changes in the spatial coordinates, so we can evaluate it... in Robertson-Walker coordinates. ...[T]o save work ...adopt a spatial coordinate system in which the particle position is near the origin <math>x^i = 0</math>, where <math>\tilde{g}_{ij} = \delta_{ij} + \mathit0(\mathbf{x})</math>, and we can therefore ignore the purely spatial components of <math>\Gamma_{jk}^i</math> of the . General relativity gives [the momentum]... with a metric <math>g_{ij} = a^2(t)\delta_{ij}</math>...<math>P(t) \propto 1/a(t)</math>... for any non-zero mass, however small... Hence, although for photons both <math>m_0</math> and <math>d\tau</math> vanish... [the momentum relation] is still valid.

I'm offended by the kind of smarmy religiosity that's all around us, perhaps more in America than in Europe, and not really that harmful because it's not really that intense or even that serious, but just... you know after a while you get tired of hearing clergymen giving the invocation at various public celebrations and you feel, haven't we outgrown all this? Do we have to listen to this?