725 reputation
616
bio website
location United Kingdom
age
visits member for 3 years, 3 months
seen Sep 19 at 18:31

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
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?
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?