I was pondering throughout the whole night thinking why don't we feel the push of the Sun when the Earth comes too close to it.
The Newtons Law says that when we are moving with the uniform velocity, we seldom experience its motion, however when the velocity changes, we accelerate hence we feel...
Earth's orbit is a slight ellipse, so to conserve momentum its speed increases when it is closest to the Sun. If the speed changes there is an acceleration. If there is an acceleration there is a force. Even if the change is small and gradual, wouldn't we experience a force because the Earth is s...
Modified Newtonian? No it's approximate solutions to the field equations of GR. first order approximation can look after most of the relativistic correction needs of the geodesy communities and higher order approximations can be used to study coalescing black holes. @Secret
@MathematicsAminPhysics It's a good idea for this room to be the first place to look for help with physics. If people want to take a prolonged discussion off to separate rooms then we usually create a room specifically for that purpose. This already happens all the time.
@ACuriousMind it is interesting that interest in the subject is low. I feel it is currently the only way researchers can truly model astrophysical situations in a relativistic framework that are not necessarily heavily endowed with symmetry properties BUT it seems to be laden with a clique of experts with little room to manoeuvre for petty commoners like myself.
@MathematicsAminPhysics I have already said that I do not understand what such a "workshop", lacking the real-world component of being close to the other participants all day, would do differently than what just talking about physics in here already does. Until you clarify that I refuse to even make the judgement of it being a good or bad idea and am not interested in discussing this further.
@Rumplestillskin To my eyes, pretty much every research field in physics is filled with experts with little room for commoners! That's how specialisation works.
@MathematicsAminPhysics if a group of students want to have a workshop with one of the more experienced members of the site they just have to ask here. But it's up to those students to get together and ask.
@EmilioPisanty: I have to say I find this sort of thing more dispiriting. It's the complete unwillingness to listen to anything that doesn't already fit their own views.
@Secret for that to be a well-defined question, you need to say what you mean by "playing well", given that the problem is renormalizability, and not some sort of obvious inconsistency
@ACuriousMind Yeah, if I very vaguely remember, the issue of naively trying to combine GR with QFT is that something mass term something blow up to infinity in the perturbative expansion something, which is why gravity cannot be renormalised under this naive combination of the two. The number of "somethings" in the sentence is why I cannot comment further
As others have already told you, a majority of real-world workshop attendees is likely to tell you that the answer to that last question is "cripplingly few"
but it may be better if you see that for yourself.
@JohnRennie I rather suspect MathematicsAminPhysics is operating under two concurrent but divergent definitions of the term. One which other people say is really good, and the other which they want to implement.
user84215
9:19 AM
@EmilioPisanty Thanks for your suggestion. Yes, I also think our definitions are not the same.
Without sophisticated VR stuff, I doubt one can organise a workshop online
user84215
According to Google, workshop is a a meeting at which a group of people engage in intensive discussion and activity on a particular subject or project.
@MathematicsAminPhysics If that's what you want it to mean, go ahead. However, if you want to draw support from anybody that says that IRL workshops are really good, then you can't use the term to mean online workshops.
@EmilioPisanty: this paper says: Thus, complete analytical solutions of the eigenstates of H2+, in areas of molecular interest, such as e.g. the region near the equilibrium internuclear distance (bond length) of the ground state remain elusive.
> If the answer to this is negative, then that's probably a very tall order to prove, since statements of the kind "there is no result of that type in the literature" are inherently hard to tie down. In that case, though, I will settle for a thorough exploration of the literature pointed at by the Wikipedia claim, and an explanation of what it does and does not provide.
@ACuriousMind @JohnRennie If I want to learn gauge theory in physics, what's the best way to go about that? I suspect I should learn Maxwell's theory of electromagnetism. I appreciate gauge theory (at least what I know about it, which is embarrassingly little) from a mathematical point of view, so I'd like a mathematically motivated excursion if possible
@BalarkaSen I suspect that when most physicists say gauge theory they mean a local gauge theory, which is a very different beast to classical electrodynamics.
@BalarkaSen I think Jose Figueroa-O'Farrill (hope I spelt that right) has some excellent papers on that at different levels. Maybe look at his gauge theory notes and/or electromagnetic duality for children
@JohnRennie Hm? EM is the archetypal example of a free gauge theory of Yang-Mills type, albeit one that lacks the features of the non-American ones, obviously. Adding "local" to gauge is superfluous, there's no such thing as a purely global gauge theory.
@ACuriousMind Really? Wow, I wish we had that - the only actual physics we were taught (except, I suppose, classical gravity and that electromagnetism is a thing) was taught outside the curriculum :/ Although we did have good teachers :)
I enjoyed school once I got into the later (A level) years, but before that, it mostly felt like a bit of a waste of time :/
@skullpatrol Here (in the UK) a 'public school' is a (usually extremely expensive) fee-paying school, so I'm not sure if that's what you mean or not - I went to what's called a grammar school
I suppose part of the problem was that one of my friends in primary school was absurdly bright for his age, so there was always that "look at [friend] - he's doing so much better - could you do that?" (it wasn't competitive, just something to aim towards and try for) happening there, which suddenly went away at secondary school, so I had no real goals until A-level (though I didn't realise this was happening at the time)
@Mithrandir24601 I was always top of the class with little effort, but I don't enjoy competitiveness or the idea that I should worry about wasting my time - I liked school for being a place where one could socialise and occasionally learn stuff ;)
@EmilioPisanty An elusive type of theory that autocorrects all constants to be exactly right, thus solving the fine tuning problem :P
@skullpatrol Haha! Nope (although, something I didn't mention for my AMA is that I've met Hawking several times, have a photo with him and Wolpertinger and have had tea in his College office more times than I could count. Another one of my friends is now doing a PhD in his group). Said friend is now studying for a Masters (in law, I think)
@BalarkaSen @ACuriousMind I have a finite-dimensional $k$-algebra $A$, where $k$ is a closed field. Now $S$ is a simple $A$-module. Is $S$ a finite-dimensional vec sp.?
@skullpatrol The automatic "many comments" flag is raised if there are more than 20 comments posted in three days. However if you're involved in a comment discussion that's going to a place you don't like, feel free to raise a custom flag to bring it to our attention.
@0ßelö7 If the algebra is unital, then every ideal is a vector space over $k$. Since $S$ is simple and therefore cyclic with generator $s$, it is $A/N$ for $N$ the annihilator of $s$ in $A$, and since $N$ is an ideal, it is a vector space. The quotient of two vector spaces is a vector space, therefore $S= A/N$ is a finite-dimensional vector space.
@0ßelö7 By definition of simplicity, every non-zero submodule of $S$ is all of $S$, therefore pick any non-zero element $s$ - the module it generates must be $S$
@0ßelö7 The set of all $a\in A$ such that $as = 0$.
@0ßelö7 In the claim that $N$ is a vector space. If $A$ were not unital, $N$ being an ideal would not imply it is a vector space
Currently, the system suggests a chatroom after 20 comments over 3 days.
If this was lowered to 10 comments over 2 days it would make better overall use of the chatrooms, and raise awareness that this option exists.
What's the difference between say $T_{ij}$ and $T^{ij}$? I know that for a vector you have $$v^i = \begin{bmatrix}v_1\\v_2\\v_3 \end{bmatrix}$$ and that $$v_i = \begin{bmatrix}v_1 & v_2 & v_3\end{bmatrix}$$ but what happens with more than one index?
In mathematics and mathematical physics, raising and lowering indices are operations on tensors which change their type. Raising and lowering indices are a form of index manipulation in tensor expressions.
== Tensor type ==
Given a tensor field on a manifold M, in the presence of a nonsingular form on M (such as a Riemannian metric or Minkowski metric), one can raise or lower indices to change a type (a, b) tensor to a (a + 1, b − 1) tensor (raise index) or to a (a − 1, b + 1) tensor (lower index), where the notation (a, b) has been used to denote the tensor order a + b with a upper indices and...
@JohnRennie You said that flaming glass made it lower energy. I came across flamed glass in an article that used corona treated glass (which, they say, is a very high energy surface) for their experiments, and then said it could also be performed on (among other surfaces) flamed glass. How does using flamed glass help in this case ? (the end goal was to have a surface with a spreading parameter greater than 0 for water)
Corona treated glass won't be annealed so it will have lots of dangling Si-O- bonds on its surface. In air these will immediately adsorb water and you get a surface covered in Si-OH groups. These hydrogen bond with water so water will spread on this surface i.e. have a low contact angle.
If you heat the surface the silicon atoms will rearrange to eliminate those dangling bonds and form mainly (strained) Si-O-Si bonds. The only polarity comes from the polarity of the Si-O-Si bond, and that isn't very polar.
So an annealed surface won't interact strongly with water and water won't spread on it i.e. the water will have a high contact angle.
Flame polishing is a method of polishing a material, usually thermoplastics or glass, by exposing it to a flame or heat. By melting the surface of the material, surface tension smooths the surface out. Operator skill is critical with this method. When done properly, flame plastic polishing produces the clearest finish, especially when polishing acrylic. This method is most applicable to flat external surfaces. Flame polishing is frequently used in acrylic plastic fabrication because of its high speed when compared to abrasive methods. In this application, a torch burning hydrogen and oxygen is...
@JohnRennie I'm not sure of technical terms, the paper just says "flaming glass", and the videos (like this one near 0:25) just say to heat glass over a flame. But that description does seem to match the process.
@Hippalectryon Hmm, that's a very light heat treatment. I was imagining something more like flame polishing which actually melts the surface of the glass. I'd have to say I'm not sure exactly what's going on there.
@JohnRennie Oh, I've already dome some experiments some days ago, but I didn't heat the glass a lot (it wasn't microscope slides and I feared it might break under intense heat), thus the water didn't really spread and the droplets barely moved
I just noticed that at the really end of the video I linked above (near 4:33), the speaker says that heating the glass creates a high energy surface... not sure why though
@Hippalectryon that's pretty much what I'd expect i.e. it doesn't make the water spread
@Hippalectryon I've just listened to that bit of the video and the presenter says "the flame is breaking open bonds on the surface of the slide". If that's true you'd get a surface rich in Si-OH groups and water would spread well on it. I'm just not convinced it's true.
@JohnRennie Do you know by any chance a relatively simple experiment that could determine that ? For instance, a chemical that I could put on glass to reveal Si-OH bonds ? (doesn't need to be really precise, I'm just interested in knowing roughly what's going on)
Measuring the contact angle is the usual way of studying the surface. The only trouble is that glass surfaces usually have a layer of organics adsorbed from the environment and you need to clean it very carefully to remove this layer.
I don't know of any simple method to measure the concentration of Si-OH groups.
To be fair the daily max is much easier to hit on Stack Overflow because the audience is so much larger. On the PSE you are only likely to hit the limit if a question you've answered gets on the HNQ list.
and frankly, there's a steep price to rep-cap days in the sheer number of people up for giving you grief over minor details on answers that ended up as HNQs for whatever reason
Under the holographic principle, information is encoded on the event horizon, but is it completely scrambled such that even if we somehow scan the structure of the event horizon, we cannot use that to recover exactly the properties of the original matter that fell into the black hole?
@Secret If you're looking at the arrows pointing to the bottom (classical) register, those numbers are just the number of the classical bit of the information being stored - so it's storing the result in the 3rd classical bit (it starts at 0)
@ACuriousMind It's not the measurement that's destroying the information, but point taken
@rob Then (check (myphysicslab.com/index-en.html) how do I do serious simulations (like in the link) ? VPython, for some reasons, is way toooo slow in my pc