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location Princeton, NJ
age 25
visits member for 2 years, 3 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).


2d
comment Center of mass frame for massless particles
And this is why various authors say "center-of-momentum frame" instead, where conveniently/confusingly it has the same acronym.
2d
comment Failure of the Steady State Theory
@user36790 Before we settled on the Big Bang, "homogeneous" and "isotropic" would probably be interpreted as you're thinking - homogeneous and isotropic in spacetime. Once we decided the universe was not in steady state, we started implicitly restricting these terms to mean spatially homogeneous and spatially isotropic. People simply got tired of saying "spatially" everywhere, so they omitted it.
Oct
23
comment Direction of pressure in fluids
@jake Yes. My answer to a spinoff of this question touches on this a little.
Oct
22
comment How non-invasive blood glucose concentration measurement work?
This might be more readily addressed on Biology. Also, I think the device clips onto the earlobe, rather than clips the earlobe.
Oct
22
comment Recommended books for undergraduate electrodynamics
I thought the biggest flaw (other than not being Purcell, of course), was the use of that script $r$. Look at all the pain it causes for those of us who typeset undergrad homework.
Oct
22
comment Recommended books for undergraduate electrodynamics
The "level" may feel like it's between Griffiths and Jackson, but after reading Purcell cover to cover, I picked up Jackson and realized there was nothing in the latter I didn't already know (and Jackson is wrong on several points).
Oct
21
comment Does entropy increases due to gravity?
Could you explain why you think being in a stronger gravitational field raises entropy? There may certainly be an effect on energy but in what way are you envisioning the same arrangement of particles being more disordered simply by being in a stronger field?
Oct
19
comment How Earth protected from the gamma rays generated by Sun?
@sugunan Space suits are not capable of stopping gamma rays. Nothing can stop gamma rays except by being very thick.
Oct
18
comment How Earth protected from the gamma rays generated by Sun?
Minor correction to your logic: Many high-energy photons (in the regime where their interaction with matter is dominated by Compton scattering off electrons) basically only care about the column density along their path. And the full atmosphere has the same column density as 1 meter of lead (or 10 meters of water). That is, we only like dense materials because they make shielding more compact.
Oct
18
comment How Earth protected from the gamma rays generated by Sun?
@HDE226868 actually that page has a good image, since the Sun does produce some gamma rays. Its corona is millions of kelvin, so there is a (small) source other than the core.
Oct
18
comment How Earth protected from the gamma rays generated by Sun?
I think what NASA was trying to say (not too clearly) is that charged cosmic rays (not gamma rays of course) hit the upper atmosphere all the time, and this then produces gamma rays, so you want to point your gamma ray telescope away from Earth to avoid contamination.
Oct
18
comment Verifying radiation measurement smart phone applications
"highpass-filters every individual pixel and all three color channel": just to be sure, you're not filtering each image independently, right? When a cosmic ray, for instance, strikes a CCD, the most common outcome is a hot pixel. The only hot pixels you should remove are those that persist from one image to another. Also, I don't know the details of consumer cameras, but I've heard and fully believe they do similar filtering before saving the data (because no one wants grainy images reminiscent of 1990s-era digital cameras).
Oct
17
comment Upside down images on the surface of the water
Are you unclear as to why there is a reflection, or are you unclear as to why the reflection is upside-down?
Oct
16
comment Why do the big nuts always remain at top? The Brazil-nut Effect
Fun fact: this same effect has been proposed to occur in asteroids, many of which are basically rubble piles. Just looking at the surface might mislead you as to the size of the typical rock in an asteroid.
Oct
15
comment What does the “size of the universe” mean if the Universe is infinite?
@Goodies The observable universe can be made arbitrarily small as you go back closer to the big bang, but see the last few sentences I added. If you scale your coordinates with the expansion of space, there is just as much volume then as now. The "point" you envision is the whole observable universe, not a part of it.
Oct
14
comment What experiments compete with BICEP 2, and when are their results expected?
There is also Spider.
Oct
14
comment How did Newton establish his famous third law of motion?
Re "We can prove 2nd law using calculus": see physics.stackexchange.com/questions/2644/…
Oct
11
comment How big can a cube-shaped planet be?
@DavidHammen Interesting too that Lineweaver cites Dave Stevenson's notes, which predict numbers closer to what you are saying.
Oct
10
comment About category theory and physics
possible duplicate of Do Category Theory and/or Quantum Logic add value in physics?
Oct
10
comment Physics First: Where is Science Education Today?
@BenCrowell By "at the calculus level" I think I mean the same as you -- not necessarily knowing calculus, but comfortable enough with solving equations and having a feel for functions' behaviors that one could teach them calculus at that point. In my mind calc isn't a particularly high bar (no reason it needs to come after trig for example).