Recent content by Roberto Pavani

Just keep in mind that charge conservation doesn't hold in such a universe, and since it follows from Maxwell's equations, you'd have to specify the new law that replaces it. Any choice you make there can give any outcome you like.

In empty space, from Maxwell: light speed =c, Respect to what? -> Einstein said respect to nothing -> SR

Undergraduate answer: There are two different "c"s. One is just a unit conversion between space and time, its numerical value depends on your choice of units, so "changing it" can be nothing more than a unit redefinition. The other is the physical principle that there's a single invariant...

For myself, I find Newtonian dynamics interesting even though it's 300+ years old and "superseded" by GR. And the same for Maxwell's electrodynamics.

It is said that two particles (e.g. two photons) with the same quantum numbers are indistinguishable, particles carry no label. So take two runs in Victor's Φ⁺ group with identical recorded outcomes: (A1, V1, B1) and (A33, V33, B33). Since the photons carry no label, A1 and A33 are...

Agreed, I was imprecise, I left the boundary conditions implicit. With a single mass and asymptotic flatness (metric → Minkowski at infinity), the solution is unique: Schwarzschild if static, Kerr if rotating. On your second point: as a novice I was staying within standard GR as we all share it.

I'd put it this way: the question "where was Feynman wrong?" isn't well-defined, because there's no single "the" explanation of the twin paradox to be right or wrong about. Feynman gives the qualitative marker (acceleration breaks the symmetry); Maudlin insists on the quantity (proper time along...

I think it's simple: do we want to keep GR, or not? If we keep GR, the answer is unique, change the mass distribution, solve Einstein's equations, get the geometry. If we don't keep GR, then we first have to define the new physical laws, and then derive the outcomes from them.

@A.T. I didn't mean removing them now; I meant a universe that never had any other matter, a single object from the beginning of time. So the imprinting story doesn't apply: there's no past epoch when those stars were nearby and dense, because they never existed at all. And GR still answers...

A universe with a single mass admits either the Schwarzschild or the Kerr metric, depending on whether it is static or rotating, and the two metrics are different.

@timmdeeg @Dale, Let me put it more carefully by superposition. To linear order the local inertial frame gets two contributions: the bucket's own stress-energy, plus the integrated effect of all distant matter (Lense–Thirring summed over the cosmos): ##\Omega_{\text{local inertial}} =...

The Riemann tensor of the spinning bucket is a coordinate-invariant fact, sourced by the bucket+water's own ##T_{\mu\nu}## (the angular momentum shows up as gravitomagnetic terms). The distant stars contribute nothing to it, so removing them changes nothing local: the water surface stays curved...

Still, Maxwell's equations do not require an arrow of time

I know this is the Science Fiction section, so we're allowed to bend the laws of physics a little. But simply changing an electron into a positron (and nothing else) would be taking it a bit too far: you'd be violating electric charge conservation. If it's only a relabeling, assign the electron...

Agree