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location Princeton, NJ
age 25
visits member for 2 years, 9 months
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I am a graduate student studying astrophysics at Princeton. I received my bachelor's in physics and mathematics from Caltech (2011).

My primary interest is in general relativistic magnetohydrodynamics simulations of black hole accretion.


Jan
20
comment What is the mathematical nature of the stress-momentum-energy tensor?
Regarding the usefulness of perfect fluids: many (most?) fluids in astrophysics have negligible viscosity (Reynolds numbers of millions). And for cosmological purposes all nonrelativistic matter -- all hydrogen, stars, planets, galaxies, and dark matter -- is dust to an excellent approximation.
Jan
19
comment Are we seeing the past when we look at the stars?
I'm not sure where the 200 million figure comes from -- some telescope no doubt. For naked-eye visible stars, there are only a few thousand from even the darkest places on Earth (and fewer than 10 for those who live in cities).
Jan
19
comment Open source computer algebra systems for general relativity
"Open source" is becoming synonymous with "free." The primary reason to prefer such is because most researchers and universities don't have enough money to give everyone all the proprietary software they might ask for. Note that all science is done in collaboration, and there's no better way to annoy collaborators than to tell them they have to spend their own money to use the code you've written.
Jan
17
comment Are there any theoretical limits on the energy of a photon?
@Ruslan It's not like there is a continuum between GR and QM -- their domains of applicability are rather orthogonal.
Jan
16
comment How would the explosion from a Pure Fusion Bomb differ from the explosion from a Fission Nuclear Bomb?
@Jimmery What's confusing is your wording makes fusion sound hypothetical. In fact most nuclear weapons today are fusion based.
Jan
16
comment How would the explosion from a Pure Fusion Bomb differ from the explosion from a Fission Nuclear Bomb?
Just to be clear, the comparison is between pure fusion and fusion with a fission detonator? You're not trying to compare against pure fission?
Jan
16
comment Group notation $\otimes$ and $\oplus$ used for representations of quarks and mesons
Something useful to keep in mind: when mathematicians say "introductory group theory" they are thinking of stuff like the Sylow theorems. When physicists say "group theory" they really mean representation theory or module theory or linear algebra -- basically most of introductory abstract algebra except group theory proper.
Jan
15
comment The mas-energy equivalence for rest mass
Word of warning: never, never, never use the "relativistic mass" as a single symbol. Rest mass is $m$, "relativistic mass" is $\gamma m$. Hiding the $\gamma$ factor in with the $m$ is something physicists did about a century ago, before they knew any better, and it only leads to confusion.
Jan
13
comment How to demonstrate frame dragging through the Kerr metric?
What form of the metric do you have then? And what have you tried? For instance, what do you take frame dragging to be mathematically? You should show what you've done so we know where to start with an answer.
Jan
12
comment How do we stabilise satellites so precisely?
If you're impressed with Hubble's pointing, you may be interested in Gaia.
Jan
11
comment Curvature of spacetime as a real thing?
@barongbaron While it is true (but highly nontrivial to prove!) that one can always embed spacetime in a higher-dimensional flat manifold, this is emphatically not how anyone thinks about or does general relativity. Curvature can be measured without venturing into other dimensions, just as you can empirically prove the Earth is round without ever leaving the surface.
Jan
6
comment Sign of the totally anti-symmetric Levi-Civita tensor $\varepsilon^{\mu_1 \ldots}$ when raising indices
But your first formula and my sign change from ${}^{**}H$ to $H$ depend on the dimension of the space, the number of indices on $H$, and the number of negative signs in the metric signature, so I don't guarantee this works if those things change.
Jan
6
comment Sign of the totally anti-symmetric Levi-Civita tensor $\varepsilon^{\mu_1 \ldots}$ when raising indices
@Marion That's very confusing notation without Hodge stars, since $H^{\mu\nu}$ should by all rights be $g^{\mu\alpha} g^{\nu\beta} H_{\alpha\beta}$. In any case, we have $({}^*H)^{\mu\nu} = -(1/2)\epsilon^{\mu\nu\rho\sigma}H_{\rho\sigma}$. Then $(1/2)\epsilon_{\mu\nu\rho\sigma}({}^*H)^{\rho\sigma} = (1/2)g_{\mu\alpha}g_{\nu\beta}\epsilon^{\alpha\beta\rho\sigma}({}^*H)_{\rho\sigm‌​a} = -g_{\mu\alpha}g_{\nu\beta}({}^{**}H)^{\alpha\beta} = g_{\mu\alpha}g_{\nu\beta}H^{\alpha\beta} = H_{\mu\nu}$.
Jan
6
comment Measurement of blueshift from Andromeda galaxy
Slipher's paper says only that he used "the region of spectrum from F to H." I wonder if those are references to Fraunhofer lines, which Wikipedia tells me would be 486-397 nm and would include Hβ-Hδ and some iron and calcium. This makes some amount of sense given he used a spectrum of (sunlight reflected off) Saturn for reference.
Jan
6
comment Could we make things out of newly discovered particles?
I'm curious about your footnote. The neutron was discovered in 1932, the proton c. 1920, and the electron in 1897. 83 years old isn't that much younger than 113.
Jan
5
comment What does 1714 mean in hydraulics?
The same thing occurs in any unit system. You could measure lengths in meters on one side of the equation and in light years on the other side, and then the dimensionless (but not equal to 1) quantity $(1\ \mathrm{ly})/(1\ \mathrm{m})\approx9.5\times10^{15}$ will probably appear somewhere in the equation.
Jan
5
comment What does 1714 mean in hydraulics?
@Dennis The problem is that $1\ \mathrm{HP}$ is not the same magnitude as $(1\ \mathrm{psi})(1\ \mathrm{gpm})$, even though they both have dimensions of $\mathrm{(mass)(length)^2/(time)^3}$. You could use Imperial units such that $k = 1$; one such way is to measure pressure in psi, flow rate in cubic inches per second, and power in inch-pounds per second.
Jan
5
comment When did people start to regard “time” as a physical quantity?
It's not clear to me the whole "5 senses" thing is accurate either cognitively or historically. Certainly there is a sense of time, and philosophers hundreds of years ago (and maybe thousands of years ago; I'm less familiar with the literature) had no problem saying it was natural to perceive the passage of time and to order things in time just as they are located in space.
Jan
5
comment Does the stationary object deform more than the moving one after a collision?
@Nagora Has it right (and this was known to be true many centuries before Einstein). By the way, AlJo, the way comments ping people is complicated, but note I only saw your message above coincidentally.
Jan
5
comment Walter Lewin's physics lecture 8.03 Waves and Oscillation
@alarge I'm aware of the backstory. As Kyle Kanos said, it's an issue of whether we become a "please give me this file" site, copyright and lecture quality aside. This can always be brought up on Meta for further discussion.