John Rennie
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 Feb 13 answered Is the cosmological time grosso modo isochrone? Feb 13 comment If magnetic force (hence torque) does no work, how does a dipole placed in a magnetic field have a potential energy associated with it? Magnetic dipole or electric dipole? Feb 13 revised LIGO discovery: if the space “time” metric is changed, how is it measured? Oops Feb 13 revised Is there any issue/concern for a slight wavelength shift of the laser light in the LIGO experiments? Oops Feb 13 revised How is it that distortions in space can be measured as distances? Oops Feb 13 answered LIGO discovery: if the space “time” metric is changed, how is it measured? Feb 13 comment How is it that distortions in space can be measured as distances? @JoeBlow: I've extended my answer to address your and related comments. Light is stretched by changes in the metric, but in the LIGO experiment the timescale for this is much slower than new unstretched light is shone into the arms. Feb 13 revised How is it that distortions in space can be measured as distances? Extend answer Feb 13 answered Is there any issue/concern for a slight wavelength shift of the laser light in the LIGO experiments? Feb 13 comment Light from the big bang Feb 13 comment Light from the big bang The Big Bang didn't happen at a point Feb 13 answered Schwarzschild manifold Feb 13 comment Is the cosmological time grosso modo isochrone? igael, are you asking whether any regions of the universe differ significantly from the comoving frame? That is, are there regions of the universe with significant peculiar velocities? Feb 13 comment Exercise 18b in Schutz's First course in GR @MathematicalPhysicist: Oh I see, you're arguing that any null vector normal to $A$ must be zero. I would say instead that to be normal to $A$ any (non-zero) vector $B$ must have the form $(0, b_1, b_2, b_3)$ and therefore must be spacelike not null. But either approach is fine. Feb 13 comment Exercise 18b in Schutz's First course in GR @MathematicalPhysicist: typo? Did you mean $\vec{B}^2 \gt 0$ i.e. $\vec{B}$ is spacelike not null? Feb 13 comment Is the cosmological time grosso modo isochrone? I don't understand what you mean by grosso modo isochrone and googling doesn't help. Can you clarify what you are asking? Feb 13 answered Exercise 18b in Schutz's First course in GR Feb 13 awarded Guru Feb 12 comment Spring constant and a spring's decompressed length Related and possibly a duplicate: Does the stretching of a spring depend on its rest length? Feb 12 answered General Relativity, Curvature, and extra dimensions