A lot of times after asking a question I can think of some people on StackExchange who may be interested in my question, or even able to answer my question. It would be very convenient if on the some page of my question, there is a button to message those people about my question.
@JohnRennie suppose you use SI units ($m$,$kg$,$s$) and don't know anything about natural units. And I use the natural units ($\hbar=c=1$)
can you find the relation between my unit for mass and yours(which is $kg$)?
@JohnRennie taking this into account: "It is interesting to note that with the help of the [above constants] it is possible to introduce units [...] which [...] remain meaningful for all times and also for extraterrestrial and non-human cultures, and therefore can be understood as 'natural units'." by planck
Anyhow, the simple answer is that I don't know. I would have to get out a pen and paper and work out what the mass unit is in natural units. To be honest I'm struggling to raise the enthusiasm for that.
you know, I'm following a video course and I don't understand it that when we use natural units how can we define the unit for mass, or why we use eV and some questions like this
Using eV is just for convenience. It's a convenient unit for particle physics - using kg would mean lots of multiplying by ten to the minus large number.
@yuggib I'm not talking about calculating numbers, I'm talking about finding units. I mean for example c=1[Lp]/[Tp] = a(whatever we measure in SI units) m/sec and $\hbar$=1[Mp][Lp]^2 /[Tp] = b(whatever we measure in SI units for $\hbar$) kg m^2/sec and then we cant find any relation between masses units of these two systems using these two eqs
but if we also take the gravitational constant $G=1$ into account, then we have three eqs and three unknowns and we can find the mass unit also. but in particle physics we just consider $\hbar=c=1$ and I don't understand how this conversion can be made between two systems
The massive dinosaurs existed because surface gravity around the globe at that time was not uniform. Surface gravity on Pangea was lower than it is today and higher antipodal to Pangea. The reason this happened is that the Earth's core elements moved off-center and away from Pangea. The Gravity T...
they probably changed something at gravatar, everybody got spikier
anyways I am reading nonstandard analysis
it is rather interesting what you can do with it; however it seems more a pedagogical/curiosity tool than something radically "new" (apart from the fact it was done in the sixties)
@ACuriousMind not exactly, it was meant to give a richer environment on which you can do more things
however, it proves theorems well within ZFC so in that sense it can't provide anything new
and the environment is at times too rich: given any unbounded hypernatural $N$ (i.e. a "natural" that is bigger then all standard naturals), there are uncountably many unbounded hypernaturals both bigger and smaller than $N$
@ChrisWhite if you're out there and don't mind random pings, I'd love to get your input on how to pragmatically couple point particles to an electromagnetic field in (for example) a simulation! physics.stackexchange.com/questions/278328/…
@yuggib I think nonstandard analysis suffers when it comes to reproducing some theorems in integration, however I am not sure how reliable my memory is
There is a traveling Matisse exhibit in OKC for another couple of weeks. Worth it if you can get there easily. But you have to buy ticket for a fixed time slot, so do some reaserch before jumping in the car.
Well, my only data is from the chemistry sector. Various academics have been talking about the pros and cons of open access, but they also ramble about how some editors went through a pay to open access style which is not very healthy
There are some peer reviewed journals which allow the authors to make their paper open access, but it requires paying a certain sum of money, around 50 AUD if I recall
They also talked about the issues of using researchgate (most don't favor or found no need to use it), and also how high impacts journals tend to get more cites, thus becoming higher in impact and so on in a positive feedback loop
and also how funding tend to bias on any journal article that made its way to Nature
At least for chemsitry, they wonder whether the impact factor is a good measure of the quality of the review process
@DanielSank Yeah, but publication isn't free either.
APS has a very nice website which does help get research done faster, and it takes money and other resources to maintain a stable server.
I'm not saying those are necessarily services that we as a community should be buying (like e.g. paying illustrators at Nature to re-do scientists' hand-drawn horribilia into prettier stuff), but there's definitely nonzero value being provided by the publisher.
As to whether the price is right, then that's a separate argument. Probably on the side of "they're charging too much", if you ask me, but it's not an easy area to go into.
@JohnRennie I thought he requires a little help. And his question is interesting/useful on the elementary level. I absolutely didn't have any intent to annoy you with it. If I had known it, I hadn't done it. Btw, I voted your posts more times up, as many times more reputation you have.
@peterh "he requires help" is not a reason to upvote an answer. "The answer is good" is a reason to upvote an answer. Please stop breaking windows on the site: if you upvote bad content, more bad content will follow.
@EmilioPisanty What about compensation votes? For example: if your opinion is mainly neutral about the usability of a question, althoug is highly up- or downvoted. You think it doesn't deserve it. What do you think, is it okay to vote into a direction with the intent to make the voting count of the question more near to the value you estimate?
@DanielSank Also I don't have any possibility to know, who votes me up/down, or who gives me stars on the chat. The stars don't have any significant effect to my behaviour, although I am happy if I get one.
user54412
@NeuroFuzzy That's an interesting question, but alas I don't have any good answer. I don't do particles myself, and the only particle codes I know about assume no self force. Moreover, they generally smooth out particles for numerical purposes, and this turns out to be a good thing because the "particles" in the code are actually meta-particles representing many point charges.
@peterh Thanks for allowing me. So some months back in chemistry class they told me that Rutherford's atomic model is broken because, Maxwell's laws says that moving electron will lose energy and thus will spiral and smash into the nucleus. So to overcame this problem we have energy levels in quantum mechanical model. So my doubt is where does this energy come from ? how does shells have their energy ?
I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.
In particular,
I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the electron actually orbits the nucleus...
There are a lot of questions posted which ask about what mainstream physics says about something, but imply that the OP has a completely different opinion. For example:
Let's get back to the Ether. What is it?. The OP has some theory about the ether, as shown in their answers on other questions...
@A---B Are you literally asking "how come the Bohr orbitals have a fixed energy level?". That's one of the axioms of Bohr, so it doesn't make much to ask to prove an axiom. I think the answer in the greatest level of generality is that the energy levels are quantized because they correspond to eigenvalues of the Schroedinger equation of the system (I think this only works for one-atomic systems). Take this as a grain of salt, I am not a physicist.
The axiom does contradict much of our knowledge of classical physics, agreed. But it explains phenomenons which classical physics cannot (spectral lines, e.g.).
@BalarkaSen apart from the fact that Schrödinger QM is of course also an approximation, the answer that the energy levels appear as eigenvalues of the schrödinger hamiltonian is completely correct
My personal intuition of quantization is that "eigenvalues come in integers; 2.41-many or 3/2-many eigenvalues is nonsense". Good to know that's not a bad intuition :P
@A---B each shell that is occupied has an electron having that energy - each unoccupied shell does not have any energy as there is nothing "supporting" that energy
@A---B Take a hydrogen atom. The one and only electron is spinning the one and only orbital and thus have a specific kinetic energy in the Bohr model. But since the hydrogen atom is stable, the orbital which contains the electron must have energy = negative the energy of the electron which is spinning: this is conversation of energy.
If the orbital didn't have an energy, the total amount of energy couldn't be 0.
"The orbital" is a synonym for "solution to the Schrödinger equation", i.e. it's a possible state for an electron. It has no existence apart from being such a state.
2
It's not that the electron and the orbital balance each other
@BalarkaSen well, in the Bohr modell there are orbits, i.e. circular paths around the atom core, that are - for some yet unknown reason - only stable in certain radii around the core and to each radius there corresponds an energy IF an electron is rotating the atom with that radius - can we put it like that if we do not want QM?
@BalarkaSen I think the moment "we" (physicists) stop talking to "other people" about what "we" are doing, we would be doing a big mistake - so I quite disagree with you :>
@ACuriousMind Oh, ok, so you're defining an orbital to essentially be a state for the electron which is in it, and defining the energy of the orbital to be energy of the electron which is in it.
@BalarkaSen This isn't so even in a classical model: the energy of the system is $T+U$ where $T$ is the kinetic energy of the system and $U$ is the (negative in the usual gauge) potential of the system.
Somewhere I've heard a rumor that a post gets out of the review queue if a 3k+ user edits it as part of the review. It may be an unintended side-effect, or a non-trivial interpretation of the spirit of the review votes from the software development side of the SE engine.
What happens if a 3k+ us...
In a non-relativistic quantum context you can identify $-\frac{\hbar^2}{2m}\nabla^2$ as the kinetic energy opperator, but the orbital is not an eigenfunction of that opperator (though in the hydrogen atom the separated angular parts of the wavefunction are, that's the spherical harmonic part of the solution).
@Sanya Heh, there's always the fear of not understanding what the person next to me says. I know the "ideas" behind "classical" quantum mechanics, but never really read it thoroughly from a book or something (although I definitely want to at some point), partially because most QM books I have seen (not a lot) is essentially a lot of mathematics, giving less physical motivation.
Also I have to admit that the mathematics in those books are "physics people mathematics", hence most of the time not rigorous. That sort of repels me, since I am a mathematical person.
@BalarkaSen you might want to try Ballentine for the mathematical part, I think that is not thaaaat awful in terms of mathematical correctness (it is still a physics book); as for the physics part ... well, I have no clue ... Maybe even the Feynman lectures? And well, about that part of not understanding ... that's basically the other persons job to make sure that you can understand - that's my take on it
@BalarkaSen One of the professors at our university does mathematical physics-style quantum optics and he recommended it in his lecture ... But it might not be enough for a mathematician, dunno. If you want a real math take on classical QM, I have heard the original von Neumann book is supposedly quite decent, but I've never read it
@0celo7 I will be indebted to you if at some point later when you're done with the project, you'll find the time to hint me at somewhere I can look those weird things up; until then I'll also give it a more thorough read then I've done just now
Ah, I see now that @DavidZ deleted it. For the record, I would rather it stayed up. It provides a very valuable record of how that theory actually looks when challenged. If it just provokes too much animosity then I feel a historical lock would be better.
@SpaceOtter Your question was: "Why is at a flat universe will slow it's expansion,". It is not true. Here is a flat universe, accelerating its expansion.
@SpaceOtter Maybe you could ask this also on the Astronomy SE, there is would have a better chance for survival, but lower chance to get useful answer.
@EmilioPisanty I suspect first you read somehow only the first some words of this, then you read but you have somehow forgotten this, finally ignored this.
@SpaceOtter I'm not sure I understand your question. It's not the shape of the universe that controls expansion, but the ratios of dark energy to ordinary matter and energy desnity.
@ACuriousMind I was watching this lecture and he says "a flat universe will slow it's expansion, an open one will expand forever, and a closed one will slow and crunch" So I wondered why he said that when I knew that the accelerated expansion was due to Lambda
@SpaceOtter I wouldn't overexamine a statement made in a public lecture. Unless you can find another reference that back up this claim, I'd suspect he misspoke.
@0celo7 If you're not talking about a formula, yes.
So it's perfectly scientifically ok for a flat universe to have accelerated expansion? I doubt he would get that mixed up he said it's the reason he got into cosmology
@0celo7 Of course you can say "Lambda" if you're talking about a Lambda in a formula. Just don't substitute "Lambda" for cosmological constant in a normal sentence.
@SpaceOtter Sure, but I would urge you to find a better reference than a YouTube video. The link can rot and make your question non-sensical. That's the second reason I advised you to find another reference to back that statement up
@0celo7 Mods can see even the deleted comments, but even they can't easily search in them. If a cage happens, the CMs automatically check this in the background. This is why the mod could show from your comments and earlier messages what you have long forgot.
No, I answered the question a while back, and now I'm writing a piece where I talk about the Poynting vector. I've got a fair few references, and was hoping this picture was from some authoritative/trusted website or textbook.