10,779 reputation
3078
bio website
location
age
visits member for 3 years, 3 months
seen May 26 at 11:12

I like physics.


May
26
comment How many observations are needed to determine a comet's orbit?
AcidJazz, I would note that while CuriousOne's answer is nicely succinct and accurate in the sense that no finite number of predictions can predict infinitely into the future, I would note that I think you might want to reconsider your Answer vote given that @WetSavannaAnimal has given you a very specific answer in terms of how you do it in practice. But also: If you were thinking e.g. "How many photographic snapshots of a comet or asteroid or dwarf planet are needed to give you a fighting chance of knowing where to look for it next?", then you might want to look again at Martin HM's answer.
May
26
comment How many observations are needed to determine a comet's orbit?
I retract my down vote and bow to the far greater understanding of this area of @WetSavannaAnimal! Alas, the stupid system won't let me, something about some kindo of one-hour limit expiring (I'm expecting flying monkeys at any moment...) My apologies, Martin, I'll reverse it when I can.
May
26
revised If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
Addendum to clarify the peculiar fact that silver atoms are bosons, not fermions.
May
26
comment Can something contain itself?
This is not really a physics question. However, it can be interpreted as a topology question, which is a discipline of mathematics. Take a look at both the Möbius strip and the Klein bottle for some ideas on how to reframe your question. I'm voting to close this one since I can't figure out how to make it into a physics question, but please don't be discouraged by that. You are poking hard questions at what is and is not possible, and that's always a good thing. You just need to find the right forum.
May
17
comment If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
[3 of 3] The SciAm article asserts that experiments have shown that quarks only contribute "30 percent" of the total $p$ spin of $\frac{1}{2}$. That seems odd, since its sounds as though the experiments were measuring the spins of individual quarks. Those would not necessarily add up to $\frac{1}{2}$, since by measuring quark spins separately the coercive effects of baryon-level spin measurement must disappear. If you separately measured proton and electron spins in para hydrogen using probes smaller than atoms, I doubt they would add up to spin $0$. Has that experiment ever been done?
May
17
comment If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
[2 of 3] For baryons, if the positive spin vector is made coincident with a coordinate axis such as $z$, conservation of angular momentum combined with quantization of spin permits only two possible spin outcomes, e.g. $uud\rightarrow\{+\frac{1}{2}(p),+\frac{3}{2}(\Delta^+)\}$. To model the effect of this baryon-level spin quantization you would need to trace its coercive effect all the way down to the quarks, where it would necessarily cause the three quark spins to "line up" precisely (and only) at the exact moment when the whole-proton spin measurement is made. That's not QCD, that's QM.
May
17
comment If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
[1 of 3] Actually, even in silver atoms the strong force impact on spin cannot be totally ignored, since strong-force-bound nuclear protons and neutrons add more $\frac{1}{2}$ spins to the total than do the electrons. But your excellent point about the easier mutual isolation of QED-controlled electron spins is well taken.
May
15
comment If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
@ACuriousMind, the online teaser preview for the article is here: scientificamerican.com/article/… My copy of the article came as a strange slurry of metallic and organic selective light-absorbing compounds smeared on to finely flattened fibers from I think large cellulose-rich plants, which together work amazingly well to emulate web pages. It must be some new technology, kind of cool, but alas hard to share over the Internet... :)
May
15
comment If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
Thanks John, that also helps me realize why the problem bugs me: Aren't all the virtual particles in that seething mass also by definition paired in ways that ultimately exactly cancel all of each other's properties, including spin? If so... well, where's the problem exactly? Only the unbalanced real-particle spins will be left. I must be missing something subtle?...
May
15
asked If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?
May
11
accepted Are electrons just incompletely evaporated black holes?
May
11
comment Are electrons just incompletely evaporated black holes?
Ben Crowell, I am truly sorry to do this, but in the pursuit of overall accuracy for other readers reading these answers I feel I need to change the accepted answer to the new one by @LubošMotl, which is both fascinating and deeply relevant to the question. Yours is still very good, and would have remained "the" answer without the new one.
May
11
comment Quantum Entanglement - What's the big deal?
Dear @LubošMotl, nice to hear from you! Your reply made my day, I laughed out loud reading it! Your two main points appear to be: (1) non-local correlations of course exist, and (2) folks who find such correlations interesting are hairy-knuckled orangutans barely capable of sentient thought. I love it! Just curious: Have you noticed that the encryption-breaking power of quantum computing is closely tied to non-local correlation? From that alone I would say John Bell had a far more real and profound impact on physics from "worrying" about such issues than has the entire corpus of string theory.
Apr
6
awarded  Popular Question
Mar
30
awarded  Nice Answer
Mar
23
awarded  Popular Question
Mar
10
comment Gravity and the Speed of Light
On the other hand, the questioner is God, so...
Feb
24
awarded  Nice Answer
Feb
16
comment Is quantum uncertainty a function of how matter is distributed in the universe?
I hope others will look at and weigh in on Jan Lalinsky's answer! It is a interesting one, and it provides a route for resolving my question that is, at least for me, nicely unexpected.
Feb
16
comment Is quantum uncertainty a function of how matter is distributed in the universe?
Jan Lalinsky, that is fascinating and consistent with the odd transition Feynman made when he went from his PhD work to QED. Perhaps his efforts to quantize his PhD work led him simply to abandon the oddly complicated half-retarded, half-advanced framework and move to the conceptually far simpler electron photon interactions of QED? I'm pretty sure I've seen comments by Feynman where he was assuming a photon-based interpretation of the half-and-half model, but that's a lot different from producing the actual model.