@StanShunpike most likely because if you post an image then it is not searchable. for example if I need help understanding an equation, i may search it in latex form, and if your answer contains the latex form of the equation, then i will be able to find it.
also images may not display properly for everyone.
@StanShunpike search engines can search images based on some attributes, e.g. the color pattern, so that given one image you can find other URLs where the same image occurs; they can also search by keywords given in the HTML close to where the image is embedded.
But I don't believe search engines do OCR on images. And even if they do, I don't think they can handle math.
Besides searchability, having text as text, not as an image, is important for accessibility measures, like screen readers, and also for the ability to copy and paste.
And this is an SE-wide thing. The whole system is designed with these factors in mind.
BTW @StanShunpike mind if I give you some feedback here on a flag of yours?
@DavidZ no plz do. The flags are meant to be helpful
@DavidZ I eventually typed out my whole question. No image :)
@DavidZ how can I flag better? I'm all ears.
I have been trying to actually look for questions I can answer. And as I read through, I just flag as I see stuff since you said volume wasn't an issue.
whoops sorry, got carried away with some other stuff
@StanShunpike actually this is about the flag you cast for copyright infringement, on that question where someone was asking to get a PDF of an article. We don't/can't handle flags for copyright violations.
Certainly it was fair to close the question, but the reason is that it's off topic to ask for a free source for a paper which the OP already knows how to get at some cost.
It's not off topic because of anything to do with copyright, because (1) that question doesn't actually infringe anyone's copyright, and (2) even if it did, we moderators don't handle that.
I guess the point to take away is that it makes things a lot easier for us if you never mention "copyright" in a flag description.
When operating a vacuum pump, after we have pumped it to its (almost) limit, and want to pump it back down, we normally
Turn off the Turbomolecular Pump first
Turn off the rotary vane pump next
But we do not do the reverse.
So I have a few questions/ideas on the reason why we do this.
I thin...
oh well, I didn't knew that. thought this site should be some archive of answers to certain problems. I just gave an universal answer, since that can be reused
@MarcelKöpke This site is meant to be an archive of answers indeed, but we don't really want to solve problems like "calculate the electric field of this charge configuration".
It's a continual debate on Physics Meta, but the current community stance is that we don't really want such homework-like questions, and we don't want them answered, either.
@MarcelKöpke Conceptual questions like: "What is the reason for X?", "Why do we assume X in this derivation?", "Why is this not a contradiction?" [Then presenting two different, but superfically correct ways of looking at something], "What is X?" [if not answered easily by Wikipedia], etc.
Answers may involve calculation, but the question shouldn't be about how to carry out the steps of the calculation.
@abt I think most calcuations "by hand" use highly symmetric situations, and so use the integral forms to arrive at the result. You mainly use the differential forms when you want to numerically solve for the fields of more ugly charge/current distributions, I think.
@abt: $\varphi \propto \int \frac{\varrho}{|r-r'} d^3r'$ is not the integral form, it's the solution of the poisson-equations, which in turn is the differential form of some maxwell equations
@abt: this are coordinate independent formulations of the maxwell equations, just like using $\nabla$ instead of $(\partial x, \partial y, \partial z)$ in cartesian coordinates. one can proove things without reference to a coordinate system. but usually it's more easy to just stick to one and transfrom to another one afterwards
@abt: this things come in quite handy if you think about it in differential geometry terms, since most math-textbooks stick to the coordinate independent formulation of diff-geometry
@abt: for a lagrangian approch to electrodynamics one usually sticks to the EM-field-tensor instead of E and B fields, since the tensor transforms under Lorentz-transformations "correctly", while E and B cannot be extended to 4-vectors in this sense
@ACuriousMind yeah, because there are simply so many of them they overwhelm everyone else :-P
@MarcelKöpke in no particular order: (1) students who are actually interested in learning concepts, not just passing classes (2) as many researchers as we can convince to contribute
so I believe we have a chat session coming up soon
@DavidZ: that would exclude students with interesst in understanding calculus concepts? e.g. how to solve a specific integral. those tricks applied are concepts too I'd say
let me introduce myself, I'm from Colombia, I'm chemist and in my country I don't have a lot of referenced people to ask for, maybe, silly questions ... and this is why I come to this nice group to ask for help ... The first question I would like to ask is how to ask help from you?
ok and I agree with you that is why I delete, if this is ok, I excuse myself. I get upset because I ask for help to people, no to the ego of the people
@beginner (Assuming you mean on the site, you can ask anything in chat) Read through the homework policy and DanielSank's guide to question titles. After that you should have a good enough idea of what kind of questions we want, I think.
and, in this case, by looking at examples of good questions and bad questions
Another thing to keep in mind is doing prior research. If you ask a question where the answer can be found by a simple web search for the title of the question, for example, people will not take it well.
(this is similar in spirit to what it says in the homework policy)
BTW @ACuriousMind since you brought it up, maybe today we can discuss faq-ifying the guide to question titles
it's just a time when we try to get people into the chat room
because nobody can be here 24/7, but chat sessions are the designated times when we try to make sure there is some activity for people to participate in (or at least follow along with)
Sometimes there is a specific topic to discuss, but usually it's whatever is on people's minds
@TerryBollinger also if you register for the chat session you will get email reminders... though come to think of it they have been less than helpful for me. The email says "chat session in 3 hours" but it's sent 3 and a half hours ahead of time
Big holes, very slow bounces, they look just like what we see in galaxy centers.
Small ones, boom boom, fast fast, I like that, small ones make more sense to me.
... because hair-pin curved space = super hot space = rapid pair production, not just at the surface of the black hole, but also near it (is that heresy? don't mean it to be...)
Here's what I really like though: When I first read Kip Thorne's book, one reaction I had was to wonder whether the true (?) horizon that forms as a point at the center might repel matter (just a postulate). That would result in... well... a boom. So, when I found LMH's paper shortly after that (this was a refind), I was intrigued...
The nicest features of such a boom, which I don't know if LHM addresses or not, is that it would hugely simplify supernova explosions. They are just very short black-hole bounces in that scenario... rebound problem solved.
I'm very cautious about all this semi-classical Hawking radiation business. It seems to me to be comparable to working with the old quantum theory ala Bohr-Sommerfeld and making confident predictions with that.
@TerryBollinger Well, at face value, the paper (journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.101304) is just meaningless unless you're very careful with what you mean by "speed of light" (i.e. what you mean by length and what you mean by time). Which they don't do.
Please, I would like to ask some bibliography to study quantum mechanics approximations (HF, RSPT, MPn, CC) in some easy way, like to beginners, thank you so much in advance for any reference
@EmilioPisanty From that article: "But in some alternative theories of cosmology, the speed of light is not actually constant, but varies throughout time and space." Do you know a (serious) theory where this is actually the case? (The experiment probably still remains a good thing to do even (or especially) if there isn't, I'm just asking)
@EmilioPisanty actually I'm serious, the idea of time having a variable rate is easy pickings for computer types like me, but it requires introducing multiple levels of time. Of course I also like the latter idea -- of multiple time levels -- because I think you can come up with some really nifty resolutions of entanglement under SR. That really is a paper I'm trying to write, need to get around to it.
@ACuriousMind, @EmilioPisanty, it does seem like any theory like that needs to be reduced down to extremely quantitative predictions first.
After that, you might find that variable time is needed... or not. The predictions, even if true experimentally, might have simpler ways to formulate them in singular time.
There is also a synchronization issue with variable time ideas, as with SR when frames collide...
... and I will now, er, shut up and at least look at the paper...
They have a hefty reference section - 10 theory works. And also 7 works on the "controversy" on whether it means anything, which they heartily ignore, "while there is still much controversy about them [4],".
@GlenTheUdderboat, you've got it sir, but with the very strong admonition to all that "I am not a physicist, listen to me at your peril!" But mostly I like to ask oddball and/or annoying questions from the perspective of information structure self-consistency.
@EmilioPisanty, $25 bucks from this locale, not on our internal list of free access. I must alas pass on that one. Couldn't make heads or tails of that abstract.
@GlenTheUdderboat, the paper I mentioned on two-level time for reconciliation of entanglement is serious though. There are some just delightful symmetries in SR that I don't think you can pull out fully until you present them from a computer-ish event modeling perspective.
@EmilioPisanty, thanks, bless be arXiv, they give me hope in a world of insanity!
@TerryBollinger 'Crackpot' theories are almost always serious. (Only pointing out the fact that seriousness is not really a criterion, perhaps unless reversed.)
@GlenTheUdderboat, seriousness is very much akin to certainty, and certainty is, alas, nothing more than an emotion. The emotion of certainty proves only that the person who has it is going to be very difficult to talk to, unless they realize it is an emotion and are willing to discard it in favor of detailed elaboration of their arguments.
well i guess special relativity wasn't quite elaborated when einstein proposed it first. he had to make some serious persuasion on conference to get it started
@EmilioPisanty, interesting. Comments: (1) It's a deep-in astrophysics article, with lingo not known to other classes of mere mortals, and so hard to decipher. (2) It looks very much self-consistent, so I think it's real and specific, just hard to read due to the lingo...
... (3) What I think they are saying seems to be that if you postulate that large regions of space, without being in motion relative to your own frame, can have SR-like time dilations for any activity taking place within them, then you should be able to detect it through some sets of distance-related ratios.
I'm not instantly convinced that "SR-like time dilation in a region of space for which the objects are not on average moving in a different frame" is logically self-consistent, but it does seem like a plausible construct.
It would for light traveling through it be a bit like a region of space with a higher index of refraction... only of time, "time refraction"?
hi all ... like this regular chat session idea ... have a book for you all ... wrt some earlier discussions re bells thm etc ... just bought/ finished reading it.
written by an MIT prof. meticulously researched. cites many top physicists & their connections to the ~1970s "counterculture". found some amazing items in there. eg historically the (QM) no-cloning thm was formulated in response to the "counterculturalists" so to speak. also BB94 crypto was inspired also by their musings. etc
conterpoint for those who continue to dismiss/ devalue focus on QM "interpretations".
there are many blurbs by top physicists & scientists/ writers endorsing it.
a wild, previously hidden story!
user54412
@TerryBollinger Be careful. Supernovae are very complex beasts, and many of them leave behind neutron stars rather than black holes, so any exotica black hole physics can't be necessary for them.
Sorry to change the topic, butwhile people are still here, I wanted to bring up the guide to writing good question titles. I'd like to make it an official FAQ; any thoughts before we do so?
I think I might leave it for a couple weeks and finalize that during the next chat session.
If this question is allowed: Please, I would like to ask some bibliography to study quantum mechanics approximations (HF, RSPT, MPn, CC) in some easy way, like to beginners (I'm chemist), thank you so much in advance for any reference
@ChrisWhite, agree, very much. This is a rare forum where I'll mention chain-of-thought ideas, but for me they are exploration heuristics, nothing more. They led e.g. to my interest and curiosity about LMH's ideas, which otherwise I would have not noticed.
This is so far what I have done with Rayleigh-Schrödinger Perturbation Theory http://physics.stackexchange.com/questions/174614/rayleigh-schr%C3%B6dinger-perturbation-theory-how-to-find-2nd-order-pertubation-to
I haven't done more than skim the work of Laura Mersini-Houghton, but my surface impressions are (1) sure, there are subtleties with defining true horizons and whether or not black holes really "exist" and (2) I can't really believe this has any macroscopic observable effect.
@EmilioPisanty, BAO, wow, it's just the "patterning" of matter in the distant universe, e.g. the sizes and forms of galaxies would give some sale of how far they are if that patterning is consistent.
Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this is probably the place to start.
All the question...
Sorry to change the topic, butwhile people are still here, I wanted to bring up the guide to writing good question titles. I'd like to make it an official FAQ; any thoughts before we do so?
user54412
@TerryBollinger Neither the phys.org review nor the arxiv paper seem to mention it, but this looks like a variation on the Alcock-Paczynski test (a good keyword, though it seems wikipedia has no entry on it)
@DavidZ Good example David Z, I think Feynman thought Wheeler was 99% nuts... but he still took the weirdness and tried to quantify it... and it worked! Also, plate tectonics, wow, is there ever a great quote in the old Earth book by Life-Time.
@JohnRennie Ah, got it, thanks! Good topic. Bad title and all is lost...
Wow, that's not bad at all as an initial FAQ. For whatever it's worth, I'd agree with the idea of FAQifying it and letting it evolve as needed (the Linux Torvalds model).
@GlenTheUdderboat Er... yes? Do I want to? Hmm! Linus Torvalds created Linux. He did it by quickly posting his first hack at a free Linux kernel, which he though would never go anywhere. Instead, it gave a starting point for improvements, and sort of took off.
@DavidZ : As I have also mentioned to @DanielSank before in a comment, I think the meta post should be CWed before it could become a FAQ. Also It would be nice to have a TL;DR on top with the bare essentials and nothing more for the users that couldn't bother to read the whole post.
Gotta go folks, always delightful talking with this group. I'll dig into LMH and maybe have more to ask later. @GlenTheUdderboat, I'll get that long list of random and anecdotal CS refs ready for you and sent it to you (just kidding, just kidding...)
As a casual science reader, I've always found the implications of relativity (inconsistent clocks after near-light-speed travel and various space-time paradoxes) to be confusing and magical-sounding. Yet I know it's accepted as foundational to modern physics.
What are some of the experiments tha...
@Qmechanic The TL;DR I like. What would be the reason to make it CW?
@GlenTheUdderboat no, but it's something to point to when someone does express an interest in improving their question
I think that could be said of the FAQ meta posts in general; they're intended more for us to show people after the fact, not as things someone should read in advance.
Sorry for my absence by the way. Bad internet.
(needless to say the "official" chat session is long closed)
@DavidZ : To e.g. encourage improvements from other editors. Like it is a site-owned post (where all have equal saying) rather than a specific user's post.
@ACuriousMind Can you give a hint for this exercise please? Let $P(M,G)$ be a principal bundle and $\pi$ the projection. Let $X\in V_uP$ be an element of the vertical subspace of $P$. Show that $\pi_*X=0$.
@0celo7 Do you know anybody in the chat room from Harvard University? I just want to know if anybody has some useful information about the Harvard Physics program.
@0celo7 Probably it is weird for me. Can you tell me when you exactly started learning this all and how you handled it? I don't want to troll, just want to know how you managed these stuff, if you are really 17, you have been successful and I want to learn from you!
@0celo7 You know, for instance, knowing GR without knowing Electrodynamics sounds hilarious. Because the main ideas which formed SR & GR have been rooted from the Electrodynamics, specifically, Electrodynamics of moving bodies ( Einstein's Paper )
@FreeMind When I say I don't know electrodynamics, I mean I couldn't calculate the current through a wire or the charge on the surface of the sphere. I know Maxwell's equations...and that's about it.
@ACuriousMind 180 to go. Hopefully, I will get this bounty, but I'm worried it will run out.
@ACuriousMind Ok, some thoughts. Since there is an isomorphism $\sharp:\mathfrak{g}\rightarrow V_uP$, I think that every vertical vector $X$ can be written as a fundamental vector field $Y^\sharp$.
Since $\pi:P\rightarrow M$, I think $\pi_*:T_uP\rightarrow T_pM$ where $\pi(u)=p$. We also have the general rule $(f_*V)(g)=V(g\circ f)$.
@ACuriousMind Let $Y\in\mathfrak{g}$. Then Nakahara defines the fundamental vector field generated by $Y$ as $$Y^\sharp f(u)=\left.\frac{d}{dt}\right|_0 f(u\exp(tY))$$
Then I get $$\pi_* Y^\sharp f(u)=\left.\frac{d}{dt}\right|_0 f\circ \pi(u\exp(tY))=\left.\frac{d}{dt}\right|_0 f(p)=0$$ since $\pi (u)=\pi(u\exp(tY))$ which implies the result.
@0celo7 Well, that's algebra/topology for you. They learned how to distill the essence of most things into definitions in the last century so there's not much room for proofs until you learned the language. The things you could prove are "trivial" because they choose to define stuff the way that most properties are evident.
@Danu If only everyone was so thoughtful ;) I had my landlord once replace the door to the house I live in without contacting me first, so I couldn't get in :D
When operating a vacuum pump, after we have pumped it to its (almost) limit, and want to pump it back down, we normally
Turn off the Turbomolecular Pump first
Turn off the rotary vane pump next
But we do not do the reverse.
So I have a few questions/ideas on the reason why we do this.
I thin...
