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5h
comment How is it that distortions in space can be measured as distances?
@brightmagus: No. In the rest frame of the interferometer the changes are purely spatial, just as in our rest frame the change in the geometry of the universe is purely spatial.
5h
comment LIGO discovery: if the space “time” metric is changed, how is it measured?
The gravitational wave changes the length of the arms. This is a real change that you could measure with a ruler if you could do it in 4 ms. The gravitational wave does not change the wavelength of the light. The end result is a change in the number of wavelengths along the arm, which is why there is a phase change.
5h
comment What is time, does it flow, and if so what defines its direction?
@CuriousOne: I profoundly disagree since I think time doesn't flow and the arrow of time is an artefact of the (wrinkly grey) measuring device humans use. Why not post your view as an answer? The point of a canonical question is to gather all the arguments into one place.
8h
comment What is time, does it flow, and if so what defines its direction?
@CuriousOne: I don't understand what you mean by what time is. Time, or rather $ct$, is the same as space - just a coordinate. What more are you asking for?
8h
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?
12h
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.
13h
comment Light from the big bang
And the universe doesn't have a centre
13h
comment Light from the big bang
The Big Bang didn't happen at a point
18h
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?
18h
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.
19h
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?
19h
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?
1d
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?
1d
comment Producing negative energy/exotic matter with particle accelerator for warp drive
Related: Exotic Matter — What is it?
1d
comment What is the isochronic law of pendulums?
I'm voting to close this question as off-topic because it shows insufficient prior research
1d
comment Can speed of light be $c$ in air or other medium?
@igael: In practice you would only work with polaritons when the coupling is very strong e.g. in a BEC. Nevertheless the principle applies. Once the light starts interacting with a dielectric is is no longer just an EM wave and therefore doesn't travel at $c$.
1d
comment induction of voltage from power lines to telephone lines
Hi and welcome to the Physics SE! Please note that this is not a homework help site. Please see this Meta post on asking homework questions and this Meta post for "check my work" problems.
1d
comment Producing negative energy/exotic matter with particle accelerator for warp drive
No. Next question?
1d
comment Does the speed of light change?
@DerbR. this is a site for, and I quote from the tour, a question and answer site for active researchers, academics and students of physics and astronomy. Everyone is welcome, but we expect you to be willing to put some effort into your questions and a basic search of this site is part of the required effort. In relativity the speed of light is constant because it's a geometrical property. Have a browse through the search I suggested in my first comment to learn more.
1d
comment Does the speed of light change?
I'm voting to close this question as off-topic because it shows insufficient prior research.