694 reputation
622
bio website
location United Kingdom
age
visits member for 3 years, 6 months
seen Dec 7 at 18:05

Manual worker. Live in England. Formal scientific education "peaked" many years ago when I failed maths and chemistry A-level and scraped a bare pass in physics. Still enjoy pottering about in maths and physics.


Apr
3
accepted Very basic question about empty universe
Apr
2
revised Very basic question about empty universe
added 92 characters in body
Apr
1
asked Very basic question about empty universe
Apr
1
comment Why doesn't dark energy vary with time?
thanks for that
Mar
31
accepted Why doesn't dark energy vary with time?
Mar
30
asked Why doesn't dark energy vary with time?
Mar
9
comment Finding the correct units for the energy-momentum tensor?
Thanks, though I find it hard to visualise something (the four components of four-momentum) flowing across a surface of unit time per unit time?
Mar
9
accepted Finding the correct units for the energy-momentum tensor?
Mar
9
comment Finding the correct units for the energy-momentum tensor?
Thanks. Just to be clear. Are you saying the units of area are $m^{2}$ even when the surface is time, ie for all four $T^{\mu0}$ components? Than makes sense if time is measured in $ct\equiv m$ units. If that's right, then that's clearer.
Mar
9
asked Finding the correct units for the energy-momentum tensor?
Mar
6
comment Still trying to understand gravitational potential and Poisson's equation?
self taught I'm afraid, and I know it shows. Last time I studied physics and maths was many years ago at school when I took my A levels (here in the UK). I've been trying to teach myself GTR for the last year or so, starting with the free online Leonard Susskind lectures. It's hard but fun.
Mar
6
comment Still trying to understand gravitational potential and Poisson's equation?
Of course. Thanks.
Mar
6
comment Still trying to understand gravitational potential and Poisson's equation?
Thanks, that makes sense now. I did try to look up IIRC but for some reason I searched for LLRC and ended up with Low Level Radiation Campaign, Lessons Learnt and Reconciliation Committee or Lincolnshire Land Rover Club. D'oh!
Mar
5
comment Still trying to understand gravitational potential and Poisson's equation?
thanks, but this is way over my head. I confess I have no idea what a 3d Dirac delta distribution is.
Mar
5
comment Still trying to understand gravitational potential and Poisson's equation?
Thanks. Bear with me if I do this one step at a time. So$$r^{2}\left(\frac{\partial}{\partial r}\left(\frac{1}{r}\right)\right)=-1$$ $$\frac{\partial}{\partial r}\left(-1\right)=0$$ Therefore $\nabla^{2}\phi=0$ which is what it should be. Finally, did you get rid of the second and third terms of your first equation by assuming $\theta=\phi=0$?
Mar
5
comment Still trying to understand gravitational potential and Poisson's equation?
Thanks. What does IIRC mean? I've just found this in Wikipedia for inside a uniform spherical body$$\phi=\frac{2}{3}\pi G\rho\left(r^{2}-3R^{2}\right)$$which (yippee) I managed to change into your equation. Does all this mean that the equation for the gravitational field $$\mathbf{g}=-\mathbf{\nabla\phi}$$ is only true for points outside of the mass, ie for $r>R$?
Mar
5
accepted Still trying to understand gravitational potential and Poisson's equation?
Mar
5
asked Still trying to understand gravitational potential and Poisson's equation?
Feb
23
comment Why no basis vector in Newtonian gravitational vector field?
@Manishearth - Just showing my colossal ignorance! I'm a self learner and have only recently learned that $y=y\left(x\right)$ means $y$ is a function of $x$ so I was wondering why there's a $\mathbf{g}\left(\mathbf{r}\right)=$ for the first equation but only a $\mathbf{g}=$ for the second.
Feb
23
accepted Why no basis vector in Newtonian gravitational vector field?