-1 completely misleading answer. Metric spaces don't have anything to do with it. In fact, turning a manifold with metric tensor into a genuine metric space involves calculating the length of continuously differentiable curves, but calculating lengths only requires one to consider the norms of the tangent vectors, i.e. only inner products of the form $g(u,u)$. Thus, $d(A,B) = d(B,A)$ would be correct regardless of whether there's asymmetry in the metric. — Stan Liou3 mins ago
@Sofia - I would have stuck through NASA, even with all their garbage (better: BS), had their goals with regard to humans in space been realistic. I was so excited ~five years ago when NASA finally wanted to return to the Moon. That was deemed a dumb idea for some reason. I was still excited when the goal changed to mining asteroids, with human help. Sending humans to Mars? That's such a dumb idea.
For one thing, it's going to take a long time. All it takes is one anti-science Congress or President, and poof, the project is canceled. (There are a lot of people, left and right, in the US that are anti-science.) For another, what if there is life on Mars? Humans have done a lot to hurt the Earth, but we've never committed planetocide before. There are a number of people high-up in NASA who say humans will never go to Mars if Mars still support life.
@Danu When you impose the physical requirement that the distance between points should be symmetric, that distance is defined as the infimum of the length functional. The length functional only involves $g(u,u)$, and no metric product between two different vectors, so the path distance metric on M is symmetric even if the metric on the tangent spaces is not.
@Danu I think ACuriousMind said it better than I would. I might also add that my intuition for the spacetime metric is that it is more concerned with angles than lengths. The metric means something even in just $T_p$, before considering the structure of a whole manifold.
So my question is I have this Hamiltonian:
$$ H = \sum_i \epsilon_i \sigma^+_i\sigma^-_i + \sum_{i\neq j} V_{ij} \sigma^+_i \sigma^-_j, $$
and I want to write it out for two site.
Is this correct?:
$$\sigma^+_1 \sigma^-_1 + \sigma^+_2 \sigma^+_2 + V_{12} \sigma^+_1 \sigma^+_2 +V_{21} \sigma^+_2 \...
A class mate of mine gave a presentation on Thursday to the entire department about his research on quantum gravity
He showed some of his mathematical work in deriving his results
First slide contained 4 sheets of paper with about 10-20 lines per page
Then a big "WHAT!?"
(indicating it was wrong)
He then showed another set of 5 pages with an "EH?" at the end
Then he showed a series of 12 pages with a correct result
I imagine that he neglected a few more mistake sets of papers in that presentation as well
You seem to be trying once, maybe failing, and then asking for more help. My suggestion is to be more like my class mate and try it a few times before asking for help.
@DavidHammen Best of luck in your new endeavors. Every job comes with its share of BS, but every NASA employee I've talked with reports even more than goes on in the national labs I've worked at, and they're no slouches.
@Danu Yeah, but once you pass your courses nobody in grad school cares what grades you got (at least, not in my experience).
Four votes to close? Really guys? I see questions that literally ask why such-and-such group has so-and-so property and y'all don't VTC (I know because I do VTC and then nobody else does). How can you honestly think a circuit topology question is off-topic but pure group theory is not?
@DanielSank in fact, this question should be closed as an exact duplicate. I know I've seen it several times here. I'll find a duplicate and vote to close.
In my previous Phys.SE question, situated here, I asked about finding the equivalent resistance of the following circuit :
I got some very good answers and some tips.
Now what if the same circuit is modified by adding two more resistors on the free connecting wires like this :
How should I fi...
So I am trying to write a matrix for a Hamiltonian and am having problems figuring it out. So the Hamiltonian is
$$\epsilon_1\sigma^+_1 \sigma^-_1 + \epsilon_2\sigma^+_2 \sigma^-_2 + V_{12} \sigma^+_1 \sigma^-_2 +V_{21} \sigma^+_2 \sigma^-_1$$
I heard from my friend that the matrix would be so...
@TAbraham Surely, you will not like anyone poking you again and again asking you to talk about something when you're busy with something else. Think about it, you'll understand. People will not take you seriously if you continue to do so. Please don't take it negatively.
So maybe it's just me, but is it really coincidence that somebody who has gained just enough rep to complain on Meta about our policies in the past 23 hours of activity also has PO as the website in their profile?
The project is still (officially) alive, though. I found the PO idea interesting, but only in a sociological sense. They broke away from an established community to found their own community with different rules, different power hierarchies.
Yeah. I'm super tired of every post becoming the exact same thing
Doesn't matter what it starts out as
@innisfree And ironically, fell to problems more significant than the community they left. They left what they considered to be heavy-handed moderation (which, in many of the discussions now, they admit at least was mostly open) to go to a place that was more heavy-handed and much more secretive about it
@dmckee should this post be closed as a duplicate of this other question? Or more generally, are these kind of things (i.e. about matters clearly spelled out by network-wide policies) debatable?
What is the community consensus over the current "thank you" policy of www.physics.stackexchange.com and meta.physics.stackexchange.com. According to the current policy a user is not allowed to say "Thank You" in their question. Should it be changed? I've provided an answer whose net score will d...
@ACuriousMind a question: does flagging as duplicate a question which has clearly already been flagged (and/or voted for closure) by others has some value? Or is it just redundant?
@ACuriousMind yea well, one could ask if that is useful in some ways. But I wonder how does that mechanism work on your (i.e. >10k rep users) side. Where does the "flag" go? Is it an annoying notice or does it just put the question on the close queue, eventually specifying who flagged it?
@ACuriousMind hahahaha I guess this answers you question: "@ACuriousMind This question is not duplicate, because it asks for re-establishing a policy, whereas the one you linked asks for the current policy."
@ManishEarth I don't understand the distinction made between homework questions which "concern a particular physics concept" and those "which ask of methods to get to the answer" as mentioned in the homework policy. My reasoning is that the latter is no less in educational value than the former since it does demand an understanding or involvement of a particular physics concept. Can you please clarify this to me?
@Gaurav "Methods to get to the answer" is primarily intended to mean mathematical manipulations of formulae. There are quite a few homework questions that have the correct equations to start from and just ask how to get to the correct answer - which involves no physics at all.
My latest question Re-establishing the “thank-you-policy-in-only-questions” of www.physics.stackexchange.com and meta.physics.stackexchange.com has been marked as the duplicate of my previous question. The two questions are different. So my question to user ACuriousMind, user Alfred Centauri, use...
@EmilioPisanty One of the things I do when I teach our "Physics for Poets" type class is give the class a number of "case studies": stories about episodes in the history of the physical sciences where the science process went off the rails for a while (and one where the laymen might have concluded that , but they really hadn't).
This might be a candidate in a year or two, when the fall out starts to become apparent.
Proposed Q&A site for people interesting in scientific questions on a non-professional level, non-fictional literature, and sociological phenomenona around science.
Currently in definition.
I can't say I see where an adequate base of experts would come from for that.
@DavidHammen : it's very worrying what you say that Nasa's research program is not realistic. You say ". . . had their goals with regard to humans in space been realistic. I was so excited ~five years ago when NASA finally wanted to return to the Moon. . . . Sending humans to Mars? That's such a dumb idea." But, we were on the Moon. We can't do "terra-forming" there. No air, gravitation by far insufficient. But there may be chance of terra-forming on Mars.
Neither object has an atmosphere, so you'd have to build a dome of some sort
We were lucky to land Curiosity on Mars (I recall that England had a failed attempt several years ago), landing a shuttle full of people is going to be pretty hard, if not entirely impossible
Actually that should say "useful atmosphere." Mars' atmosphere is almost entirely CO2 and is about 2 orders of magnitude less dense than Earth's atmosphere
I suspect that David's comments were not all about the manned mission (or lack of one), but about the overall situation. NASA has not been able to maintain the congressional support to stay at the front of the pack for decades.
They recently retired the only program that made them at all unique and the proposed replacement is some year away even if all goes well.
There has been a long-standing lack of vision in the agency. Or at least a lack of vision brought to fruition; I suspect that lots of people there have had clear visions of what could be done it...
I I'm new here and hoping I'm doing this right? I'm asking to get help with building a proper pinewood derby car for my son as we have a year to build and looking and checking out all our options. The reason is simple the answers may not be? My son just started cub scouts a tiger cub at 5yrs old....
@KyleKanos And typically the resignation would be in the form of someone senior accepting a early-retirement package that they'd been considering any way. A head to display, but not much actual carnage. (To use a metaphor that strikes too close to home these days. Must find a better one.)
There is a saying "heads will roll" (i.e. be cut off) when something goes really bad. It used to mean that people would get fired in a way that really hurts their career.
Proposal: Popular Science
From the Example Questions, it appears that most of these questions would actually be on-topic at Physics.SE. In some cases, they actually have been asked and answered:
http://physics.stackexchange.com/questions/40985/what-does-it-mean-how-is-it-visualized-for-a-parti...
@Danu You need a little cynism to get through the day if you have a clear view the brutality that goes on. And some folks need to have that clear view. So it's mostly a good thing.
I've been looking through the new Physics Overflow beta¹ and looking back at some of the discussion about the closure of the Theoretical Physics SE. From a purely personal perspective I would prefer research level questions to be asked here because though it's very unlikely I could answer any it'...
@KyleKanos I used to hang out on alt.peeves on the USENET. They used exclusively the British spelling of whine and I picked it up from there. Sticking "fest" on the end was a flourish born of frustration.
AFAIK, the claim of axion DM in the sun wasn't widely accepted (and was derided in some quarters as an elementary blunder). i don't think it ought to be in an encyclopedia entry about DM.
@innisfree I think you are right - it shouldn't be there
Yet, as written, it is not wrong to speak of "possible detection", and it is sourced, so I don't think deleting it would achieve anything - it would probably be restored.
Really? i don't have much experience in editing wikipedia. my impression is that it's working incredibly well, but it's still somewhat suboptimal.
i mean, there are millions of articles, but many of them could be improved, or have a few strange edits. there isn't the same level of interest in eg maintaining the DM wiki page as there is in answering questions on DM here.
I think I've edited WP articles exactly twice, so I'm a long way from an expert. But I think that NPOV rules would allow you to hang a "but the result has been challenged as flawed [new citation(s)]" off of that even if you can't get rid of it.
@innisfree Research strategy: most journals and preprint sites make it possible to identify what papers cite a known paper. Scanning the titles of citing articles would let you find any that are obviously critical and review articles. If none are obviously critical then scan the review articles ...
Unfortunately, we have identified three distinct flaws in the analysis by Fraser et al. which ultimately make it totally irrelevant both for axions and for cold dark matter.
The section should definitely go, thanks for the paper
@kyle i don't think every small claim about DM should be addressed, especially not on an equal footing as established, undisputed experimental evidence. something should be widely accepted by the community before it appears in an encyclopedia.
Maybe it could be added again and put in the direct detection section, along with eg DAMA.
I definitely think the article is better without it.
but you're free to start a revert war with me on wiki ;)
@TAbraham: Note that "check my work problems" are considered off-topic by the community; this is how you were asking your questions. The community is supportive of you learning something about QM & such (hence all the help you've received thus far!), but you can't ask your questions in that particular manner
@innisfree I have no interest in doing that! But leaving the comment you did in the edit history certainly addresses your reasons for deletion!
I just am hesitant to approve of rash decisions! (though content can always be reverted on Wiki!)
@dmckee - You hit the nail. NASA has had plenty of ideas. Congress and multiple administrations have nixed them. Congress thinks they are better rocket scientists than the very few real rocket scientists at NASA, so NASA has to build this atrocity called the Senate Launch System (the official name is the Space Launch System; nobody calls it that). NASA has very few rocket scientists. What NASA does have is a lot of (mis)managers.
@Sofia The most optimistic plans for sending humans to Mars call for a trip in 2035 or so. That's three or four administrations and ten congresses in the future. Twenty years is too long a span of time to be realistic. All it takes is one money-tightening, anti-science administration or congress and poof! it's canceled. A realistic plan is something that can be done in a decade or less. NASA sent people to the Moon for a two day visit. Nobody has sent people to the Moon to live there.
@DavidHammen A reasonable assumption would be that if something happened to a Moon-based habitat, we'd be able to send help in a more reasonable timeframe than if something happened to a Mars-based. Am I wrong in that assumption?
DH interesting stuff re NASA but keep in mind with congress its nothing personal, look what happened to the supercollider. US not so interested in "big science" anymore after kennedy. & the politics makes it difficult. US focused on just trying to have a halfway decent economy since 2008. lots of internal problems (eg inequality) make any big ambitions difficult. US (govt) spends most of its $$$ on so-called "defense"....
there is some significant innovation going on in private industry re spaceflight...
in recent times NASA has done quite well with all the robotics probes to mars, thats not so gigantic but defn something to be proud of. (some run Java, yay!) :)
afaik US hasnt put all that much $$$ into the european LHC...
US congress can barely agree on the validity of global warming! its a sort of major breakthru the senate resolved recently its real... but that small gesture maybe all that can be achieved politically on the issue in the near future!
@DavidHammen : I see. But, I apologize for naivety, what's our business to visit the moon more? It is useless. I would understand if the gravitation on it were better, but, what can one do there? (Military use? No chance.)
@KyleKanos Exactly. Suppose someone on the Moon or Mars breaks their back and needs to come home. (I broke my back 25 years ago. I feel their pain.) A return to Earth from the Moon means launching NOW and then drifting for three to five days. A return to Earth from Mars means waiting a long, long time and then drifting for six to nine months.
@vzn Don't overlook that the supercollider had deep management problems aside from the funding issues.
They "designed" the fool thing simply assuming that they would get a break-though in superconducting magnet technology: the design called for magnets stronger than those on the LHC.
They didn't have in place management structures adequate to manage a billion dollar project--something that we do have now, but it has taken twenty years to seep into the culture and added a whole extra layer of complexity to experiments.
Money was mis-allocated all over the place, which was one of the causes of the overruns.
@Sofia We went to the moon as part of a national greatness stunt. To show up the soviets, mostly. To be first at something in a really big, visible way.
There never was a long term plan, and the science goals were tacked on.
@dmckee - Controlled fusion is another one of those pipe dreams. (Aside: I mistyped "pipe dreams" and my autocorrect wanted me to write pipe dramas.) When I was a kid, fusion supposedly was but a decade in the future. A decade passed, and that decade had become two decades. Now its five.
@Sofia To what purpose? Who are you going to threaten with what? Or what are you protecting from whom?
I suppose I should note that over the long run the lunar mission science outcomes have been really, really good. People keep finding new things to look for in those rocks. BUt that was beyond anticipation in the 60s.
agreed moon has little military value. and @#$& lets not militarize space more than its already been (eg @#%& chinese satellite destruction experiment few yrs ago, lobbing space junk all over the place, no intl warning/ agreement whatsoever)
re controlled fusion, was just going to mention that, (aka "feasible in roughly 20years" ad infiinitum) there was a really neat article on that recently ... re european challenge ... gotta dig that up ... yeah feel that billions are being blown on too-big experiments, think smaller scale fusion ideas should be pursued...! current large funding balance is near madness
@dmckee Yup. One of my many jobs involved things that went way high up, came back down, and ended with the grandest possible finale. That was thirty+ years ago; I had to cleanse my soul after that. Anything more, deponent say nought.
Getting back to the mundane, yesterday I looked at the map I hung over my bed decades ago, and I told my wife "We need to go back there!" She is my financial manager, and she said yes! Where? The Boundary Waters Canoe Area (US) / Quetico Provincial Park (Canada). Permits are extremely limited, and this is the time to reserve a permit. I'll check with my new employer whether its okay to completely escape the modern world for two weeks in July or August.