The h Bar

Abcd

11:00

@Kaumudi.H "magnitude is the same as the maximum limit of static friction" :( . It's less according to 2 books

user228700

Oh, crap. Hang on.

user228700

Oh, right, I'm so sorry, it is a little less.

Abcd

@Kaumudi.H Why does it come into play?

user228700

Oh, are you asking about why friction exists?

Abcd

@Kaumudi.H No, why does kinetic friction exist?

user228700

11:05

@Abcd Hmm, OK, do you know why static friction exists?

Abcd

also why is coefficient of static friction = $f_{max}/N$

Oh sorry for the above question because @JohnRennie has already proved that in one of his answers.

user228700

@BalarkaSen Which chapters?

Abcd

@Kaumudi.H No

user228700

Do you want to?

Abcd

@Kaumudi.H Yes, briefly.

Balarka Sen

11:09

@Kaumudi.H Electrostatics, currents, electromagnetism and a small bit of optics (reflection/refraction).

I know the first two reasonably well, and almost comfortable with the third.

I haven't read optics.

John Rennie

@BalarkaSen given your mathematical sophistication I wonder if you would be best advised to jump straight into Maxwell's equations rather than the usual route of studying basic EM.

Balarka Sen

No less than U(1) gauge theory, eh?

I actually learnt some SU(2) gauge theory in the workshop :p

John Rennie

Yes, it's a classical gauge theory.

Maybe some of the more accomplished gauge theorists would like to comment ( @ACuriousMind maybe?) but I suspect that you'd master Maxwell's equations very easily and the simpler EM problems beloved by JEE question setters would then be child's play.

Balarka Sen

Really? Huh

user228700

@Abcd I'm sure you'll be able to find an "explanation" if you Google this. Let me know if you're having any trouble with it.

user228700

11:13

@BalarkaSen Oh, wow, they've started optics already?!

user228700

@BalarkaSen Cool :-) Are you reading the textbook?

Balarka Sen

@Kaumudi They haven't started anything; it's just that they're throwing whatever at us just because they can :P

Abcd

Thanks @Kaumudi.H and @JohnRennie for your help!!

Balarka Sen

Yeah, going through it.

I don't really know if I should focus on the math or the theory

user228700

@Abcd No problemo :-) Good luck.

user228700

11:16

@BalarkaSen Oh, wtf, that sucks, man.

user228700

@BalarkaSen I can only speak from my own experiences so bear that in mind when I tell you that I spent way too much time trying to understand the Math (I'm sure it wouldn't take you that long but still) and it wasn't super helpful.

user228700

But again, I dunno though, it was helpful in that I felt incredibly confident writing my final exam.

Balarka Sen

Mhm.

@JohnRennie Actually it's a good suggestion. I am a little skeptic about the fact that learning gauge theory in full abstraction would help in my high school studies, but I might actually do that because (1) that'd keep to motivated to look at electromagnetism - which constitutes a big part of my physics syllabus - now and then, (2) I want to learn some gauge theory because hell yeah, (3) I'll take math + physics next year if I get in the uni I want to get in.

user228700

I know nothing about gauge theory but I gotta say that that does sound like a great plan!

Balarka Sen

The more I think about it, the more good it sounds. Downright evil good suggestion.

John Rennie

11:23

@BalarkaSen bear in mind that when physicists talk about a gauge theory they mean a quantised local gauge symmetry. Classical EM is a unquantised global gauge symmetry.

But it's still an excellent starting point.

Balarka Sen

Hm, I am unfamiliar with those words. What does quantized mean in this context?

@Kaumudi.H I can tell you why topologists care about gauge theory in a few sentences.

user228700

Sure.

user228700

Hey @JohnR:

user228700

user228700

What happened?

John Rennie

11:28

@BalarkaSen if I attempt to answer that I will mislead. Try asking ACM.

ACuriousMind

@JohnRennie I like gauge theories but I find electromagnetism terminally boring :P

Balarka Sen

@JohnRennie Hmm, fair enough.

Ah, speak of the devil.

John Rennie

@Kaumudi.H it looks fine here. Try refreshing the page.

user228700

I did. Multiple times too. Not working :-/

Shing

anyone would recommend a good place to start learning gauge theories?

ACuriousMind

11:30

@BalarkaSen "quantized" as in "quantum physics not classical physics". I think if you only know what a mathematician calls "gauge theory" it's a bit difficult to explain why physicists care about it

John Rennie

@Kaumudi.H I've just tried from work and the web site is still fine. It looks like a problem at your end ...

user228700

@JohnRennie Hmm, I clicked on the link once more and now it seems to be working. I wonder what happened there...

user228700

Ah. Episode 6 isn't there anymore :-)

John Rennie

@Kaumudi.H shrug - as long as it's working

@Kaumudi.H ah, let me check ...

Fawad

@BalarkaSen you didnt said

user228700

11:32

^ :-P

John Rennie

@Kaumudi.H it might have been a problem with the file name. I've renamed the file so check again.

user228700

Yep, yep, that was it! :-) THANKS.

Balarka Sen

@ACuriousMind Yeah I don't know what a physicists' gauge theory is.

John Rennie

@BalarkaSen neither do they :-)

Balarka Sen

lol

John Rennie

11:33

No, I'm serious!

ACuriousMind

@Shing What sort of gauge theory and at what level?

Shing

@ACuriousMind I would like introductory gauge theory.

ACuriousMind

@Shing Do you know QFT?

Shing

@ACuriousMind nope

Balarka Sen

@Fawad @Kaumudi.H Say $X, Y \subset \Bbb R^n$ are two subsets of the Euclidean $n$-dimensional space. That means just geometric objects; for $n = 3$, $X$ can be the sphere and $Y$ the surface of the cube. If $f : X \to Y$ is a continuous map between them, it's called a "homeomorphism" if there is a continuous map $g : Y \to X$ such that $f \circ g : Y \to Y$ and $g \circ f : X \to X$ are the identity maps.

What this means, in English, is simply that $X$ can be deformed to $Y$ by stretching, shirnking, crumpling, distorting but not cutting or gluing.

ACuriousMind

11:36

@Shing Then learn QFT. The physical significance of gauge theories lies mainly in the quantum realm.

Balarka Sen

So for example, the surface of the sphere (subset of R^3) and surface of the cube (surface of R^3) are homeomorphic: just inflate the cube by pumping air inside it so it becomes the surface of the sphere.

Does that make sense?

ACuriousMind

And any of the standard texts on QFT includes a chapter on gauge theories, since QED and QCD are gauge theories

Shing

thanks man

user228700

@BalarkaSen Sort of.

ACuriousMind

@BalarkaSen Maybe start with this question, then

Fawad

11:38

@BalarkaSen everything sounds simple. Whats challenging?

Balarka Sen

@Kaumudi.H Here's two examples of things which are not homeomorphic. Let $X$ be the disk of radius 2 in $\Bbb R^2$: $X = \{(x, y) : x^2 + y^2 \leq 2\}$ and $Y$ the annulus of thickness 1 in $\Bbb R^2$: $Y = \{(x, y) : 1 \leq x^2 + y^2 \leq 2\}$. $X$ and $Y$ are not homeomorphic, precisely because $Y$ is $X$ with a puncture introduced in it.

I can't deform one to the other without cutting a hole out of the disk (or sewing the hole off in the annulus)

user228700

Oh, right, I see.

Balarka Sen

A classic problem in topology is to classify all closed (closed means no boundary: disk is not closed, surface of sphere is closed) surfaces upto homeomorphism: constructing a list of surfaces where no two surfaces in the list are homeomorphic, but all surfaces are homeomorphic to some surface in the list. Here is a part of the list (the list is infinitely long but not too complicated).

And yeah, the reason people want to construct a list like that is precisely because every scientist wants to classify their objects; chemists construct periodic tables classifying elements, biologists construct classification charts of species, etc etc

user228700

Right, right...

Balarka Sen

Anyway, that's a list in 2 dimensions. What about higher dimensions? You need to know what an analogue of "surface" in higher dimensions is, of course. But, given a reasonable definition, turns out classification is very icky business, for the following reason:

user228700

11:51

Uhh, I didn't understand that completely but OK, what reason?

Balarka Sen

Given $X$ and $Y$ smooth surfaces in $\Bbb R^3$ (roughly saying that the surface have well-defined tangent planes at each point; surface of the cube does not count, it has bad kink points at the corners), if $X$ and $Y$ are homeomorphic they are also "diffeomorphic". That means, there is a homeomorphism $f : X \to Y$ such that both $f$ and it's inverse $g$ are differentiable.

Geometrically this means you can deform $X$ to $Y$ very smoothly, by not introducing bad kink points

I know ACM is cringing from behind the computer, but well

user228700

Ohh, right, I got that a little bit, OK...

Balarka Sen

In that case, punchline: there exists subsets $U \subset \Bbb R^4$ such that $U$ is homeomorphic to $B = \{(x, y, z, t) : x^2 + y^2 + z^2 + t^2 < 1\}$ (that's a ball in the 4-space; in 3-space that's like interior of the sphere), but not diffeomorphic to $B$.

So even if you can deform $U$ to the 4-ball continuously, you can't do it smoothly.

Geometrically $U$ is a very fractal-like subset of $\Bbb R^4$.

These are called "exotic 4-balls", and only exist in dimension 4.

user228700

Yep, you've lost me now.

Balarka Sen

$B$ is a reasonable object, though, isn't it?

Like, what is $\{(x, y, z) \in \Bbb R^3 : x^2 + y^2 + z^2 < 1\}$?

the inside of the unit sphere $x^2 + y^2 + z^2 = 1$, right?

user228700

12:00

The inside, hmm, ohh, right, yep.

Balarka Sen

Yeah, points in $\Bbb R^3$ with distance from $(0, 0, 0)$ smaller than $1$.

user228700

Yep, yep.

Balarka Sen

Similarly $B$ is points in $\Bbb R^4$ with distance from origin smaller than $1$. Of course you can't visualize it, but it's a 4-dimensional analogue of the inside of the sphere.

user228700

...OK.

Balarka Sen

All that's saying is there exists subsets $U$ of $\Bbb R^4$ which can be continuously deformed to $B$, the 4-ball, but not smoothly. Which is surprising, because you'd think it's true in 3 dimensions (indeed, it is).

existence of such $U$ are given by gauge theory (Taubes, not Donaldson, sorry)

Now you know everything I know about exotic 4-balls lol

user228700

12:03

@BalarkaSen Oh, what?

user228700

If I've understood it, my reaction is "Wtf, why?" but hey, I only understood it in a vague sense so.

Balarka Sen

yeah it's kind of weird

user228700

@BalarkaSen Certainly not even remotely close to it but I do know something now :-P Thanks!

The Raiders of Las Vegas

Was the focus of your workshop Topology?

Balarka Sen

It means $U$ is a super fractal like object in $\Bbb R^4$. Think of it like the interior of the Osgood curve: upload.wikimedia.org/wikipedia/commons/thumb/7/74/…

Well, that is only the upper half of the whole curve. Reflect the Osgood curve along the x-axis to obtain a full closed curve, and it's interior.

(Yes, that's a curve. It's a curve which does not intersect itself ever in fact. These are examples of space-filling curves, but that's a story for another day)

user228700

12:07

@BalarkaSen I...what? Wtf, wow.

Balarka Sen

Crazy, I know, right?

user228700

Yeah... O_O

Balarka Sen

Now... the interior of the Osgood curve is diffeomorphic (can be smoothly deformed) to the unit disk $\{(x, y) \in \Bbb R^2 : x^2 + y^2 < 1\}$ :3 Not as obvious anymore, is it?

user228700

To the unit disk?!

Balarka Sen

Yes. Well, at least, after you think of the Osgood curve as what's in that image plus it's mirror image (after reflecting along the x-axis), so it becomes a closed loop (shit, it doesn't look like a loop but...)

user228700

12:17

Sorry about that; my laptop finally ran out of juice and my house doesn't have power atm.

user228700

@BalarkaSen Uh, OK...

Balarka Sen

it's ok, i was done talking :p

I am angry that internet does not have a picture of the Osgood loop

I am in the process of making one

0celoñe7

The osgood curve is a nasty one

It doesn't have measure zero

Balarka Sen

user228700

@BalarkaSen And I am on the process of wondering whether or not I should jump:

Balarka Sen

12:23

@Kaumudi.H That is the Osgood loop. The interior of it is diffeomorphic to the unit disk.

@0celoñe7 I still don't know why it's a curve

but so it is

user228700

djsmiley2k

how far's that drop?? 4feet? 6?

user228700

No idea.

0celoñe7

@BalarkaSen What are you trying to explain to smoothie above?

user228700

10/11, perhaps.

user228700

12:28

My adrenaline says "Please do it" but my parents scream.

user228700

@BalarkaSen I...OK.

Balarka Sen

you're trying to commit suicide after knowing that fact?

djsmiley2k

10ft is a pretty big drop dpeending on how well your versed in landing safely

Balarka Sen

@0celoñe7 exotic 4-manifolds :3

Slereah

Exotic 4-manifolds are kind of awful

They don't even have an explicit construction

user228700

12:30

@BalarkaSen x'D No, man, I happen to be sitting on this sunroof and now feel like jumping off.

Slereah

IIRC the imbedding of exotic $\Bbb R^4$ in $\Bbb R^8$ is supposed to be a fractal or something

user228700

Especially since someone said "You could jump off, couldn't you?"

0celoñe7

that seems pretty explicit to me

although I guess it depends if you consider limits constructive

Balarka Sen

@Slereah I am pretty sure they do

some of them, at least

Slereah

@BalarkaSen Well you have a method to construct them

But you won't get their atlases

0celoñe7

12:32

i.e. is the usual proof of Banach's contraction theorem constructive?

Balarka Sen

oh. well you're F-ed if you want to construct a manifold atlas for them :p

0celoñe7

@BalarkaSen Do you want a hard RG exercise?

Slereah

F-ed in the A

Balarka Sen

@0celoñe7 I can answer that only after seeing it :)

Mithrandir24601

@Kaumudi.H I jumped off a 20-foot drop just last week. I was perfectly fine. I enjoyed it so much that I went back about an hour later and made the same jump again. It wasn't quite as thrilling the second time, but I was still perfectly fine. Jumping is fun, but I wouldn't want to land on that surface :P

ACuriousMind

12:35

What are 20 feet in the units of the rest of the world? :P

0celoñe7

Let $(M,g)$ be a Riemannian manifold with nonempty boundary. Let $r:M\to[0,\infty)$ by defined by $r(p)=d(p,\partial M)$, where $d$ is the Riemannian distance function. Then $r$ is $C^\infty$ in a neigborhood of $\partial M$, and for $p\in \partial M$ we have
$$\mathrm{grad}\, r(p)=-n(p),$$
where $n$ is the outward unit normal field to $\partial M$.

Balarka Sen

latex error: missing \begin

user228700

@Mithrandir24601 Wow, I see! Yeah, no, I climbed off.

Mithrandir24601

@ACuriousMind about 6 metres :P (I've also just realised that it was 2 weeks ago, not last week)

Balarka Sen

@0celoñe7 sounds like a fun question

0celoñe7

12:41

@BalarkaSen what?

Balarka Sen

Is that true for any codimension 1 submanifold of a closed manifold and distance function from it?

Mithrandir24601

The bit I didn't mention: there was another about 1.5m of an unbelievably soft landing :P

0celoñe7

@BalarkaSen I needed to prove this in $\Bbb R^n$, and when I had that proof I realized the Riemannian one wasn't much harder.

ACuriousMind

@Mithrandir24601 I was about to ask what was at the end of those 6 m ;P

0celoñe7

@BalarkaSen Yeah.

If it's oriented I think.

Balarka Sen

12:42

Right. Hm

0celoñe7

If it's oriented you can just cut off one side of the manifold and call the submanifold the boundary of the result.

Balarka Sen

I was thinking about isometry type of collar neighborhood of boundaries a few days ago actually

@0celoñe7 Uh, well, the submanifold need not disconnect the manifold.

But I guess you can still cut it off and get two boundaries, each that submanifold.

0celoñe7

@BalarkaSen Well, no, but you can just cut out some neighborhood that it will disconnect, probably.

Balarka Sen

No.

Think meridian in torus.

0celoñe7

It will disconnect a tubular neighborhood...

Balarka Sen

12:46

Oh, I'm confused now. N does disconnect tubular nbhd of N in M; that's what you wanted to say?

(Tubular nbhd is just N x I, for the oriented case)

0celoñe7

@BalarkaSen In the codim 1 case, exactly.

Balarka Sen

Ok, I agree.

0celoñe7

@BalarkaSen Yeah, take a tubular nbhd, then pick one half of it. The submanifold is then the boundary of that half.

Balarka Sen

Right, right, fair enough.

0celoñe7

@BalarkaSen Now if you want complete smoothness for submanifolds you need to use a signed distance function.

For instance $d(x,\{0\})=|x|$ for the standard distance function in $\Bbb R$

But if you adjust signs then it can be $\pm x$

Balarka Sen

12:49

Right, so locally on either half it's smooth

0celoñe7

For the boundary you only approach from one side so it's fine

And you need a suitable notion of what smooth up to the boundary means

Emilio Pisanty

13:14

@Qmechanic this is the kind of question we wanted to have reviewed before it got the tags it did, right?

-1

Q: Why and Who wrote rules of universe.(I mean physics)

I am always wondered that why the universe and it's rules were created.what is the goal of this universe. From newtonean mechanics to theory of relativity to electromagnetism to gravitational waves to atom to supernoval to living things to human being to Robots, all of the things around us worki...

0celoñe7

13:24

@EmilioPisanty Opinion based? Really?

Shing

I think "Why are there some rules that the universe follows?" (I interpret it as why the universe is not the other way around) is a good physics question. "What is the goal of this universe? " is opinion based.

assuming the universe has a will is quite a big assumption.

ACuriousMind

@Shing No, "Why are there laws of physics?" is the realm of meta-physics, not physics.

djsmiley2k

if there was no rules, there'd be no universe

pretty simple really.

ACuriousMind

You can ask why a particular law of physics holds. The traditional physics answer is to come up with a more "fundamental" law from which that law is derived. This is perfectly fine physics. But why there are any laws at all is not something that can be answered in this vein.

@djsmiley2k An extreme skeptic who rejects inference could posit that there are no laws, just a giant load of coincidences and that at any time now, the laws of physics could stop describing the universe and you could do nothing to disprove that.

djsmiley2k

a pewrson like that isn't worth talking to

after al, we may all cease to exist momentarily.

ACuriousMind

13:34

I.e. the "existence" of the laws themselves is not an objective fact even if they were absolutely accurate. But it gets even murkier once you realize that physical laws are generally just approximations: Newtonian mechanics does not really describe how objects behave, it's just a pretty good approximation in the non-relativistic regime. So does the universe "follow Newton's laws" or does it not?

Qmechanic

@EmilioPisanty : Yes, that is an example of a misuse of the math phys tag.

heather

hello

Qmechanic

@heather : Hello.

Shing

so after all physics is really all about modeling?

but I found it quite unsatisfying.

ACuriousMind

@djsmiley2k Yes, precisely. Which is why "if there was no rules, there'd be no universe" is not true - the viewpoint that there are no "rules" at all even in our current universe is a possible, if admittedly very rare, viewpoint. Additionally, it is unclear what "no rules" even means - how do you distinguish a system that has "no rule" from a system whose rule you simply haven't understood yet.

Mithrandir24601

13:39

@heather Rytsas!

Secret

We cannot even determine if there can exists systems where the rules are simply not understandable even in principle to us (of course this assumes that us, being humans, are fundamentally limited in the way by our brain hardwiring in comprehending something)

Mithrandir24601

@Shing you might be interested in studying philosophy of physics

heather

@Mithrandir24601 rytsas! =)

Secret

So, yeah, rules, no rules, incomprehensible rules, set of coincidences, I don't think there is any way they can be distinguished

> ()djsmiley2k An extreme skeptic who rejects inference could posit that there are no laws, just a giant load of coincidences and that at any time now, the laws of physics could stop describing the universe and you could do nothing to disprove that.

My favourite way to think of this is that perhaps there really is no maths, but a bunch of classical correlations such that everytime A happens, B then happens (and vise versa)

But that is a very unlikely possibility

Mithrandir24601

@Secret I'm afraid I don't follow this sentence :/

Secret

13:43

Well, let's take newton laws as an example to illustrate

Newton's 2nd law said that F=ma

ACuriousMind

@Secret You are aware that abstract "math" as such is divorced from reality, and that "correlation" is a mathematical concept, yes?

Mithrandir24601

ah - do you mean 'maybe there is no physics'?

Balarka Sen

what are we talking about

ACuriousMind

@BalarkaSen I'm not sure.

Balarka Sen

Can I throw in an unrelated quote then

""Nothing is true, everything is permitted" - Hassan I. Sabbah" - William S. Burroughs

user685252

13:47

As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality.

ACuriousMind

@BalarkaSen Funnily enough that's often misattributed to Dostoevsky.

Balarka Sen

huh really

It's from the novel "Alamut"

Secret

Acuriousmind: Yes, here I sort of build my argument by saying the inference skeptics scenario could well be pairs of classical events in spacetime just happened to be correlated in such a way to reproduce the outcome of the laws we observed without them, which should be similar to what you said as a bunch of concidences, I think

Mithrandir24: So if you push an object, it accelerates depends on how hard you push it and thus you exert a stronger force, right? But it is equally valid to say that it just happens that the object accelerates more because of how hard you push it, not the other wa…

Emilio Pisanty

@0celoñe7 yes.

Not a question that can lead to constructive discussion within the site's topics.

heather

@BalarkaSen my personal favority unrelated quote is "Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read." - Groucho Marx

Emilio Pisanty

13:51

@Qmechanic point is, this is the kind of question that we would've wanted to end up as please-remove-this-tag, but for which the new system failed

user685252

lol

Balarka Sen

my favorite unrelated quote from Marx is "Workers of the world, unite!"

Mithrandir24601

@Secret Generally, physical laws are about correlations. That's the point of a lot of them. Especially the likes of Bell's inequality

Secret

Ok, here's a better example: You have some putty which when you squeeze it, it deforms. In physics, we described as how the net forces acting on small regions of the putty and thus push the molecules in a way to deform the putty.

However, an inference skeptic that is mentioned by Acuriousmind, would have said it just happens that everytime you squeeze the putty, it deforms, and it cannot be reduced to any more fundamental mechanism on how it deforms (it could be that the putty spontaneously deforms when something squeezing it, instead of the usual way of thinking about where its the squeez…

Mithrandir24601

As in "a bunch of classical correlations such that everytime A happens, B then happens (and vise versa)" is a physical law

ACuriousMind

13:54

@EmilioPisanty (cc @Qmechanic) Since there are more questions tagged with the + version than with the ordinary version, it appears first (leftmost) in the tag suggestions. I'm not sure why we would expect people to use the other version instead.

Emilio Pisanty

@ACuriousMind yeah, that was my worry from the start

ACuriousMind

@Secret No, that's not at all what I mean by an "inference skeptic".

0celoñe7

@Slereah is "Hawking's 92 paper" an acceptable citation?

ACuriousMind

I mean someone who refuses the idea that we can conclude the existence of universal laws from empirical evidence, i.e. they reject the inference step that allows us to go from "The last 10000 times I dropped this rock it fell down" to "Everytime I drop this rock it falls down".

Slereah

It is great and so am I

0celoñe7

13:57

@ACuriousMind I reject that.

Secret

Ah ok I see, in that case it is the same idea along the lines of the famous philosophical question of "if I saw the sunrise today, must it sunrise everyday"

0celoñe7

After all quantum mechanics tells us there is a possibility the stone goes up.

Mithrandir24601

@0celoñe7 Erm... There seem to be 3 papers by Hawking in 92...

Qmechanic

@ACuriousMind @Emilio Pisanty @David Z : Yeah, it appears to be a failure. Let me remove the plus again and end the experiment....Done.

ACuriousMind

@0celoñe7 Proof?

0celoñe7

13:57

@Mithrandir24601 Exactly.

heather

@0celoñe7 and Boltzmann's law tells us there's a possibility entropy could increase.

0celoñe7

@ACuriousMind That path has nonzero action so contributes to the path integral.

Balarka Sen

@ACuriousMind Good imitation there.

ACuriousMind

@0celoñe7 So? Superluminal paths also contribute to the path integral.

Secret

@Mithrandir24601 Would that be still a physical law if it is uncomputable (i.e. the law exists, but you can never write it down using constituent of the universe)?

0celoñe7

13:59

@ACuriousMind Ok, and superluminal paths are possible on small scales

ACuriousMind

@0celoñe7 Proof?

0celoñe7

it is well known that particles can leak out of their light cones

ACuriousMind

@Secret What about "everytime A happens, B happens" is supposed to be (un)computable, exactly?

Mithrandir24601

@ACuriousMind Arguably tunnelling. I'm still not sure where I stand on that one

Emilio Pisanty

@Qmechanic fair enough

Let's give this a try, though

chat.stackexchange.com/rooms/63070/tag-review-for-misused-tags

ACuriousMind

14:00

@Mithrandir24601 The answer is that there are no paths in the quantum world. Just because a path "contributes to the path integral" does not mean it "happens".

0celoñe7

I think ACM is fundamentally misunderstanding quantum physics

3

Secret

@ACuriousMind I am not sure how will a physicists wrote down the law "everytime A happens, B happens" can be wrote in the form of an equation if no deeper mechanism is known: Something like Pr(A|B)=Pr(B|A)=1?

Balarka Sen

classic

ACuriousMind

@0celoñe7 I should've known you weren't being serious when you started to discuss physics, right? :P

user685252

> ::shots fired::

ACuriousMind

14:02

@Secret Why do I have to write down laws in the forms of equations? Who said anything about that being a requirement for a law?

0celoñe7

@ACuriousMind I am being serious

Mithrandir24601

@ACuriousMind It's not really a 'path' in any sense to begin with... Just that time taken to tunnel is independent of barrier width, so under certain situations a particle appears to tunnel faster than light would take to travel the same distance. Then it gets messy :P

0celoñe7

Until there is a full quantum theory of gravity I maintain that one cannot know that stones always fall down

5

That's a pretty damn bold claim

Mithrandir24601

(i.e. That's pretty much all I know/remember)

ACuriousMind

@Mithrandir24601 What does "time taken to tunnel" mean, exactly?

Mithrandir24601

14:04

As I said, it gets messy :P

Balarka Sen

@ACuriousMind Oh, just to acknowledge that I didn't miss the link you messaged me; I'll look at it later today. Perhaps I'll read some high school physics before that :P

ACuriousMind

@0celoñe7 That's different from the claim that "quantum physics tells us there's a probability the stone falls up"

Secret

Well, it may be no surprise I am not aware of that, caused almost all laws we encountered in physics are often expressed in some sort of formulae, inequalities, relations etc., from the simplest F=ma, to very complicated path integrals, and then the einstein field equation in GR

This is why when "everytime A happens, B happens" is actually considered as a law, it seems weird to me

Mithrandir24601

There are issues with that - the particle wavefunction bunches up (according to simulations) against the barrier, then suddenly, it's more likely to appear at the other side of the barrier than the incident side

(by 'appear', I mean a measurement gives it to be at one side or the other with >50% probability)

0celoñe7

@ACuriousMind I am currently moving, this is maybe not the best time to think about quantum physics

ACuriousMind

14:07

@Secret Aren't you a chemist? Isn't chemistry full of laws of the form "If you put X and Y together, Z happens."? And although one might describe that with stochiometric equations nowadays, the content of such laws is clearly not diminished by not expressing them in equations

0celoñe7

but I resent your physics comment

Balarka Sen

#rekt

user685252

23 mins ago, by Balarka Sen

what are we talking about

Mithrandir24601

As far as the stone thing goes, I'd just give the usual argument that a stone is a classical object, so while it does have a chance of 'falling' upwards, the probability is so small that you'd have to wait a time greater than the age of the universe...

0celoñe7

@ACuriousMind can't wait to see the cat!!

ACuriousMind

14:09

@Mithrandir24601 Yeah - that's not superluminal motion. That's just quantum objects not conforming to our classical expectation ;)

@0celoñe7 I'm sure he misses your GR books to lie on, too ;)

Are you bringing him some?

Secret

Well, we do tend to express them in stochimetric equations and the word description is usually treated as a statement that describes the law itself, or noting of an experimental observation

Sure, there is always a way to wrote all of these in words (which is often done in the literature), but ultimately, my point is I don't recall seeing any law that is not expressible as some form of equation or more generally a relation

(Wait a sec.., maybe there are exceptions after all, in organic chemistry, the heuristic of "electrons tend to move from electron rich groups to electron poor groups" is…

0celoñe7

@ACuriousMind of course!

Mithrandir24601

@ACuriousMind Exactly. There have been experiments looking at this with weak measurements, but again, I don't see how that really means anything in terms of 'this has/has not travelled faster than light'

ACuriousMind

@Mithrandir24601 See, I don't agree it has a "chance of falling upwards", why would it have? Consider a free particle wavepacket travelling in some direction - it doesn't have a "chance of moving backwards", does it?

Secret

(and yes, there are many more... too focus on the physics and forgot that there are other examples in other disciplines)

user685252

14:12

This is a bold statement

Emilio Pisanty

Hmmmmm, that tag-review room got a heck of a lot of incoming feeds

not all of them from after its creation

ACuriousMind

@EmilioPisanty I think it will settle down after the bot does whatever it needs to do to arrive in the present

Mithrandir24601

@ACuriousMind Thinking of it in thermo terms, why not?

ACuriousMind

@Mithrandir24601 What do you mean by "thermo terms"?

Secret

he probably means second law of thermodynamics

Emilio Pisanty

14:16

@ACuriousMind hope so, yes

ACuriousMind

I'm just thinking of a wavepacket travelling in a certain direction - it doesn't have a definite location, but I don't see how its direction of motion could be construed as ambiguous in any way.

0celoñe7

@ACuriousMind I've gotta get the cat, a GR book and the PC in one picture

Maybe the dog too but he's less cool

Mithrandir24601

As in, using a simplistic model, the probability of having something of mass $m$ in the atmosphere decreases exponentially with height

ACuriousMind

@EmilioPisanty I do wonder why it decided to post some questions from the end of last year, though

Mithrandir24601

only on top of that, the mass of a stone is orders of orders of magnitude more than hydrogen and helium atoms, so this probability becomes absurdly tiny, but not exactly $0$, only negligible

ACuriousMind

14:21

@Mithrandir24601 But...that's not a quantum probability

Mithrandir24601

@ACuriousMind Well... You were talking about stones (i.e. not-quantum things)...

ACuriousMind

I was. And then the ocelot claimed QM said there was a probability of the stone falling upwards. I thought that was what we were talking about

But yes, I guess there is a small classical probability that all the air molecules conspire to bump the stone upward... :P

0celoñe7

@ACuriousMind Please spell my name correctly...

@ACuriousMind Aha!

So you admit defeat

Mithrandir24601

@ACuriousMind Ah, OK - I must have missed the bit were you said you were talking about quantum stuff

skullpatrol

Defeat?!?

Shing

14:24

maybe after all, there is no quantum theory for gravity. Nature just doesn't have it

ACuriousMind

@0celoñe7 I can't, don't know how to put the tilde over the n

skullpatrol

ñ

Secret

So today, because of building some really gigantic structure in maths chat to sketch a proof of the mathematical existence of solutions to some equations, I end up revisiting some of the reddit that talks about Tegmak MUH

Emilio Pisanty

@ACuriousMind no idea

Secret

(Paragraph deleted as the intensity of metaphysics will skyrocket so much that h bar will implode into a black hole)

and then shortly after, I saw you guys discussing about the laws of physics

Secret

14:37

Chat died again, bleh >D(^)< ...

Secret

If there is no quanutm gravity, black holes will forever be a mystery...

Emilio Pisanty

@0celoñe7 shouldn't your name have an accent on the o anyways?

i.e. 0celóñe7

0celoñe7

@EmilioPisanty idk, you are the mexican in the room

should it?

Emilio Pisanty

@0celoñe7 yes

cf

Ordóñez

Ordóñez or Ordoñez is a Spanish family name that may refer to: Ordóñez (bullfighter family) Antonio Ordóñez (1932–1998) Carmen Ordóñez (1955–2004) Cayetano Ordóñez Francisco Rivera Ordóñez (born 1974) Anderson Ordóñez, Ecuadorian footballer Angel Gil-Ordoñez, Spanish conductor Bartolomé Ordóñez (1480-1520), Spanish Renaissance sculptor Diana Ordóñez, also known as LeDania, Cololmbian street artist Diego Ordóñez (1903-1990), Spanish olympic sprinter Francisco Fernández Ordóñez (1930–1992), Spanish politician who became Minister for Foreign Affairs in the PSOE García Ordóñez, Spanish medieval nobleman...

0celoñe7

@EmilioPisanty Updated.

Emilio Pisanty

14:42

@0celoñe7 'ppreciated

0celoñe7

@ACuriousMind Why does Windows ask me if I want to open certain applications even though I've said yes a thousand times

is there a way to disable that for specific ones?

ACuriousMind

@0celoñe7 Supposedly yes, but I've not tried any of these (google "turn off UAC for specific programs" or something similar to see them)

Emilio Pisanty

14:57

brilliant

John Rennie

15:29

Wow, get a look at SciHub!

5

Mithrandir24601

"For some major publishers, such as Elsevier, more than 97% of their catalog of journal articles is being stored on Sci-Hub’s servers" :D

John Rennie

There won't be many of us shedding tears about Elsevier :-)

Mithrandir24601

"each legal challenge resulted in a spike in Google searches [for the site], which suggests the challenges are basically generating free advertising for Sci-Hub"

:D

This is Glorious

0celóñe7

@JohnRennie I shed tears on principle.

Mithrandir24601

(Of sheer joy and happiness, in this case)

Emilio Pisanty

15:41

I do hope they made the full list of 81.6 million articles available to Elbakyan

it's a good resource to know what else needs liberating

make that read

> more than 97% of their catalog of journal articles is being stored on Sci-Hub’s servers so far

Secret

If there's something about the world that is very similar to my personality, is the increasing decentralisation in many sectors

John Rennie

On an unrelated issue, I found these in my garden today:

0celóñe7

@Mithrandir24601 no

John Rennie

I'm trying to work out if they are psilocybe i.e. magic mushrooms. Though I suspect not - the caps are a little too rounded and there are a lot of small brown mushrooms.

0celóñe7

@JohnRennie eat a handful and report back

(or set a dead man's switch for if you, well, die)

John Rennie

15:46

I shall have to continue getting my hallucinogenic experiences from studying quantum mechanics.

0celóñe7

@JohnRennie if you want to see real shit then you've gotta go for category theory

John Rennie

Even if they were the psilocybe mushrooms there aren't enough of them for a trip. As I recall you need about 20-30 of them.

Secret

definitely not magic mushrooms, though you need a mycologist to be absolutely certain

Meanwhile, I already have a lot of thoughts that are considered trippy to most people and the only drug I really took is my kidney medication

in Mathematics, May 4 at 15:52, by Eric Stucky

Secret, I'm about 98% sure you're rediscovering concrete categories.

in Mathematics, Jul 23 at 10:02, by Secret

pretty sure this is not category theory

ACuriousMind

@JohnRennie Eating random mushrooms - what could possibly go wrong?

John Rennie

@ACuriousMind :-) though I think only small percentage of mushrooms are really toxic so in practice you'd be unlucky to get into too much trouble. But I won't be eating these - there aren't nough of them and I quite like my mind the shape it is.

They grow around my plum tree so I suspect they might be from a symbiotic fungus living around the tree roots.

ACuriousMind

15:55

@JohnRennie I think the problem is less how common the toxic ones are, but that some of them look really similar to the delicious ones

Or, in this case, the mind-altering ones

Secret

Eat this = die

(Ps google image)

John Rennie

Maybe I should ask on the biology SE, though I suspect they'll tell me I should have photographed the gills and also taken a spore pattern so perhaps I won't ...

Mithrandir24601

reverse image search?

Emilio Pisanty

@JohnRennie well, it's in your garden, right?

if they do ask for more pictures, just go out and shoot the mushroom again

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