980 reputation
216
bio website github.com/spraff
location London, United Kingdom
age 95
visits member for 2 years, 11 months
seen yesterday


1d
asked How does $p=fv$ hold when $v=0$ and an engine is working?
Mar
4
comment Why does fusion stop at iron when nickel is most tightly bound?
Any idea why Fe is so often quoted as being the stop point?
Mar
3
asked Why does fusion stop at iron when nickel is most tightly bound?
Jan
29
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
I mean $\Phi(a,b)+\Phi(p,q)\neq\Phi(a+p,b+q)$, right? And because of this fact I can't simply transform $\nabla\Phi(r,z)$ into Euclidian coordinates to get the Euclidian vector which corresponds to $\nabla\Phi(x,y,z)$ can I?
Jan
28
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
I'm having trouble with the concept of a non-Cartesian vector. Vectors are defined by linearity, aren't they?
Jan
26
awarded  Nice Question
Jan
26
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
I can transform into cylindrical coordinates no problem, but when I apply $\nabla\Psi$ how do I interpret this equation? What do those hats mean? Is the result in cartesian coordinates? If not how do I interpret addition of terms?
Jan
26
accepted What is a reasonably accurate but simple model of the Milky Way's gravitational field?
Jan
26
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
Problem is the kinematics are calculated in rectangular coordinates. How do I translate this? Also, G has those units normally but these equations are in kpc and solar masses so won't the constant change?
Jan
26
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
I've just read this in more detail. Can you please confirm I've got this right? The potential will always be directed in the north-south axis so I should take $\frac{\partial}{\partial z}$ to get the acceleration, yes? Also, what are the units of G? Thanks a lot.
Jan
26
comment What does velocity relative to CMB mean?
Well, that is just about the most mind-blowing thing I've read in a while.
Jan
26
comment What is a reasonably accurate but simple model of the Milky Way's gravitational field?
Excellent resources, thank you. Quick sanity check -- since the positions of stars are usually quoted in heliocentric coordinates, will I have to rotate them all such that the sun is lifted up from the central plane in order to be in the same coordinate system as is used in your second resource?
Jan
26
asked What does velocity relative to CMB mean?
Jan
26
asked What is a reasonably accurate but simple model of the Milky Way's gravitational field?
Jan
25
comment Does the payload of an Alcubierre drive have to be on board during the preparation phase?
Thanks. Gotta love the no-big-deal tone of all this :-)
Jan
25
accepted Does the payload of an Alcubierre drive have to be on board during the preparation phase?
Jan
25
comment What does it look when we travel at the speed of light?
IIRC Feynman constructed a result about collisions between quarks and photons by modelling the quarks as flat disks, justifying this as, from the photon's point of view, everything becomes perfectly thin and flat in the forward direction.
Jan
25
comment Does the payload of an Alcubierre drive have to be on board during the preparation phase?
So, if point A sets up two drives to go to B many light years away, and meanwhile B also sets up one drive to go to A, there could be an exchange of messages like this which happened in a matter of seconds from A's point of view? A: "Do you want X or Y?" B: "X please." A: "Okay, here's X."
Jan
24
asked Does the payload of an Alcubierre drive have to be on board during the preparation phase?
Jan
17
comment Why do crystals grow in preferred directions?
I get that selection is happening, but brief calculations make me skeptical. Suppose we divide each of the main arms on my sketch into 10 segments, all points within each section being "equivalent" for the purposes of symmetry. The secondary arms have 10 candidate attachment points, and positing one secondary arm on each side of each main arm, gives $10^{12}$ permutations of even this basic shape. Did someone really search through that many to find the one in the photo?