690 reputation
616
bio website
location United Kingdom
age
visits member for 3 years, 1 month
seen Jul 26 at 16:16

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.


May
5
asked Zero divergence of energy-momentum tensor and gravitational energy
Apr
26
accepted Confusion regarding geodesic of thrown ball - curved or Cartesian coordinates?
Apr
26
comment Confusion regarding geodesic of thrown ball - curved or Cartesian coordinates?
Thank you, though I find your first sentence a little ambiguous. Are you saying it's a valid approximation to plot the ball's path using Cartesian coordinates because (1) it's moving relatively slowly and/or (2) is in a weak gravitational field and/or (3) is not being thrown very far? So if I was throwing the ball at a velocity near the speed of light on the surface of a neutron star, for example, I couldn't plot its path using Cartesian coordinates but would need to take into account spacetime curvature and use curved coordinates - whatever they are?
Apr
25
asked Confusion regarding geodesic of thrown ball - curved or Cartesian coordinates?
Apr
25
comment Why do objects follow geodesics in spacetime?
Thanks for the replies, though I'm afraid they are way over my head. I still don't get it.
Apr
24
comment Why do objects follow geodesics in spacetime?
@David Zaslavsky Both, but of course I can't promise I'll understand the mathematical one.
Apr
24
asked Why do objects follow geodesics in spacetime?
Apr
8
accepted Hubble time, the age of the Universe and expansion rate
Apr
8
comment Hubble time, the age of the Universe and expansion rate
thanks very much.
Apr
7
asked Hubble time, the age of the Universe and expansion rate
Apr
7
comment How does the critical density decide the fate of the Universe?
of course! Funny how some things are obvious when they're pointed out. I was getting the strangest results trying to feed my equation into the WolframAlpha differential equation calculator. Thanks.
Apr
7
accepted How does the critical density decide the fate of the Universe?
Apr
6
comment How does the critical density decide the fate of the Universe?
thanks. Ryden uses parametric equations to get her big bang to big crunch graph of a matter only universe on p87. Just out of interest, do you know why I can't set the first term on the rhs of the Friedmann equation to a constant $C$, $k=+1$, $c=1$ to obtain $$\left[\frac{1}{R}\frac{dR}{dt}\right]^{2}=C-\frac{1}{R^{2}}$$ $$\frac{dR}{dt}=\left(R^{2}-1\right)^{1/2}$$ but when I try to solve this, I don't get the nice Big Bang to Big Crunch graph that Ryden does?
Apr
5
asked How does the critical density decide the fate of the Universe?
Apr
3
comment Very basic question about empty universe
Thanks for that. I looked at the paper - superluminal galaxies are not travelling faster than light in our or any other observer's inertial frame so don't contradict STR. That is amazing.
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?