Hi, I was hoping someone might be able to help me understand the following statement:
"At a point p of a manifold M, a metric g has a unique signature, sig(g)=(p,n-p). Since, in terms of a chart, the components g_{ab} are continuous and since the matrix of components is invertible at each point, it follows that there is some open set O in M containing p such that sig(g)=(p,n-p) at each point in O."
I understand the first part: that the metric has a unique signature at a point follows from Sylvesters theorem. But I don't understand the logic leading to the second conclusion
@Secret "lack details to allow testing"? lol, what about impracticality or inaccessibility of testing, like, oh, say, string theory? extremely high energy particles? theories of black holes, or the big bang? dark matter/ energy? etc? :P
re kuhnian paradigm shifts also in math, it is not widely accepted/ debated (compared to other fields), but there are many academics who take it seriously and not hard to find a few refs via googling. heres one for this audience-- university senior prj with many refs. (and who else is working on one around here?)
@vzn it is a complicated topic. As you may aware, string theory is also quite shaky nowadays given how superstmmetric particles still failed to show up. But these models at least proposed a way to test them, thus they are testable in principle (even though it is borderline metaphysics and mathematics). Pseudosciences don't even bother doing that, which is why they are often said as not even wrong
I was reading a note of Hojoo Lee on inequality which is written for International Math Olympiad (IMO) participants. Although he writes that “target readers are challenging high schools students and undergraduate students“, it appears to be quite advanced.
It occurred to me to ask, do these IMO ...
Wasn't there someone in here a day or two ago claiming that Motl must be one of the smartest mathematicians on SE because he earned an IMO bronze?
Anonymous
@DawoodibnKareem IMO is a school level examination. Although it requires lots of practice and some bit of ingenuity to get through, it surely isn't related to actual research. The topics are just Euclidean geometry, Combinatorics, Number theory, and some bit of Algebra.
Anyway, my point is, the person who was in here glorifying Professor Motl had it wrong.
It would be interesting to know how many Stack Exchange users hold IMO medals. An absolutely impossible statistic to obtain.
I'm certain Motl is not alone.
Anonymous
Anyhow, my point is, PhD mathematicians not being able to score 42/42 on an IMO paper, is not because they are incompetent, but rather because they haven't been practicing those kind of problems for a long time. Here, students prepare for nearly 4 years for appearing for JEE/Olympiads etc. So they are better equipped for handling those type of problems.
Anonymous
7:10 AM
And almost every year 2-3 people from our institute would go to the IMOs. It wasn't an irregular feat
Anonymous
Out of them, not more than 5% would grow up to be professional mathematicians
Anonymous
One of them even went on to take up a medical degree
@DawoodibnKareem : I like that kind of stuff. I've read a lot of "science history" books like that. Dorigo gave a lot of little anecdotes about the Tevatron, it felt like being there.
But note that I have a special interest, somebody who doesn't might not like the book so much.
I think we have seen enough about Motl. Constructive comments about his physics are still welcome but non-constructive ones about him personally are not.
@DawoodibnKareem : I've done a lot of reading about the history of particle physics in recent months. Enough to say that IMHO there are some issues with the quark model.
Talking of gluons, see Wikipedia: "There are also conjectures about other exotic hadrons in which real gluons (as opposed to virtual ones found in ordinary hadrons)". The gluons in a proton are virtual. As in not real.
if you think about it, particles are kinda weird in that they can interconvert between each other as long they have the same energy and obey $e^2=p^2c^4+m^2c^2$
@DawoodibnKareem In the feymann diagrams, they are internal edges, in reality, they are really a feature of perturbative expansion. It is more likely that in reality, all you have is just some interactions and no notions of particles inside those high energy collisions
So that brings me back to my question. If all particles are mathematical abstractions, and none of them exist, how does one distinguish a virtual gluon from a real gluon?
@DawoodibnKareem : what real gluons? A virtual particle is not a short-live real particle that obligingly pops into existence. It only exists in the mathematics of the model. And the gluons in a proton are virtual. As in not real.
Virtual particles, all virtual particles, are excitations of the quantum field that cannot simply be described as particles. We model them as a sum of different particle states. The states appearing in that sum are the virtual particles.
@DawoodibnKareem when you quantise a quantum field you get field states, just as when you quantise particles you get particle states. In the non-interacting limit i.e. when the particles are too widely separated for interactions to be significant, those states are well defined states called Fock states.
tbh, in QFT, the thing I really interested is the messy interacting regime, but so far our supercomputers are simply not powerful enough to simulate e.g. protoon proton collisions
@DawoodibnKareem the point is that QFT works i.e. the descriptions it provides correspond to experimental results. In fact they ciorrespond exceedingly closely to experimental results.
OK, but humour me for a moment. If you're studying cell biology, you could imagine shrinking yourself down very small, like smaller than a cell, and swimming round inside a cell to see what's there. When you see the pictures of a cell's structure in your biology textbook, you think "those are real things", and you know that if you were small enough, you could run into the occasional mitochondrion, or whatever.
If you're studying chemistry (an anagram of mercy sh** - who knew?) you could imagine shrinking yourself even smaller, say the size of a hydrogen atom, and seeing what the actual molecules are doing.
@DawoodibnKareem Technically, those pictures are just illustrations, they don't 100% reflect what a cell look like. The reason why the above observation works for cell is because they are not small enough for the weirdness of quantum mechanics to play signifcant role that give things counter to our intuition
Then, when you get into "very small physics", you kind of want to be smaller than a proton, so you can actually see what quarks look like. You expect that at some point, you're going to see little red, green and blue balls.
But no, you're not allowed to do that without becoming a figment of a deranged mathematical imagination.
@DawoodibnKareem you can't see molecules. Light doesn't scatter off them. You have to go to some other probe and that immediately ceases to correspond to anything intuitive to humans.
quarks cannot have colors, cause they are smaller than optical wavelength. We call that color charge simply because it has 3 kinds, thus we use the RGB as analogy and the name then sticks
@JohnRennie OK, fair enough, I will grant you that. But it seems that biology and chemistry try to describe actual things, whereas physicists gaze at their navels in the hope of constructing models.
SR is still ok for me because minkowski diagram with a suitable way to interpret angles can pretty much capture most of the maths, but the way it distort spacetime still need some time to get used to
In fact, I kinda thank Johnrennie in one of his PSE describing how the expansion of spacetime is mathematically speaking, the metric itself is changing, thus rather than having more space being produced, the grid itself becomes bigger so tospeak
@DawoodibnKareem you can make a theory better in two ways: 1. predict experiment more accurately 2. keep the current accuracy but make the theory simpler
Both are actively pursued by physicists, and option (2) is a perfectly valid way to spend your research budget. But just saying a theory is bunk because it's too complicated is not physics.
Sometimes it makes me curious, can cantorian infinities/infinite cardinals one day ever spawn a technology that will transform our civillisation like the internet does?
QFT is actually pretty simple in concept, but it turns out we can't compute the field states for an interacting theory. The hideous complexity that characterises modern QFT is all concerned with finding approximate ways to get round this.
Well, let's take an electron as an example. As we learn science at school, we first meet tiny little balls zooming around nuclei. And we think of electrons as particles. Then we learn that they're not particles at all, so we change our mental model of what an electron is like. Then we learn that they don't really have position, or momentum, or any of the things that exist in the picture of little balls zooming around nuclei. But that's OK, because they're still things - just different
types of things. But it's an even bigger leap to say - no these aren't things, they're just a mathematical model that explains how chemistry works, and how electric circuits work, and how all sorts of other stuff works that needs electrons. Nobody wants their electrons taken away from them.
Dawood: Your above discussion reminds of a philosophical discussion between me and my physical professor back in 1st year. We discussed about what exactly is an electron and we then conclude we may never have an answer
@DawoodibnKareem Now imagine turning up the strength of the interaction so the energy is the same as the proton rest mass. Is it best to describe this as two protons plus an interaction, or as an interaction plus two protons?
It is prohibited to kill ants. The Prophet {Peace Be Upon Him} has prohibited the killing of ants and three other creatures:
Narrated Abdullah ibn Abbas :
The Prophet (sallallahu 'alaihi wa sallam) prohibited to kill four
creatures: ants, bees, hoopoes, and sparrow-hawks.
حَدَّثَ...
Speaking as a room owner I think extended discussion of personal theories of philosophy are inappropriate here and I ask that they stop. Unless there is a mass outcry I will start removing future posts of this type.
@Kenshin Chat flags are for seriously offensive posts. They notify every moderator and 10K user on the while SE. Using them to mark posts for deletion will make you very unpopular!
I don't think there's anything per se wrong about talking about philosophy in this room. I just think Kenshin's extended semi-logical babble belongs to a blog of his own, not in this room
@Kenshin There are two types of chat flags: "rude/offensive" flags (using the little flag icon next to a message) and "flag for moderator" (which appears on the drop-down menu for a message). The latter can be used for simply asking that some of your own messages be removed, if it's important. But John Rennie isn't a moderator, so he wouldn't see those flags.
@Kenshin I didn't insinuate anything, I posted a link to a wikipedia page. If you find psychiatric disorder inherently offensive you're part of the crowd that stereotypes it.
Tough choice, given you're the man who is talking about ethics and morality
@JohnRennie why are personal theories of philosophy inapproporiate. They are not actively promoting misinformation as far I knew since philosophy really don't have a notion of accuracy?
@Secret It's not per se inappropriate, it just doesn't belong to a public chat. It's the same argument that's been said against your extended write-ups on algebraic structures and stuff in here.
@Secret it's not that philosophy is banned, but when it appears to be turning into an extended monologue I don't think it serves the main physics chat room well.
But as I said I wouldn't go against the wishes of the chat room regulars.