The h Bar

user218912

4:00 AM

yes so it's not the same as a spacetime.

user218912

because spacetime can be minkowski/flat or curved.

0celo7

Gourgoulhon is going for axiomatic SR

that's different than GR

user218912

okay w/e :)

0celo7

@IceLord do you even know what book?

user218912

@0celo7 yes.

0celo7

4:01 AM

which one?

user218912

the one that you said cost $500

user218912

white cover.

0celo7

not 500

it was $200, and was a Christmas gift

@JohnRennie I had something to talk to you about

oh, Gauss's law

user218912

Do you mean the one by Beem?

0celo7

Yes

John Rennie

4:03 AM

@0celo7 Morning

0celo7

@JohnRennie Never mind.

It's dumb physics stuff I think

user218912

@0celo7 just read tong's notes.

0celo7

on?

user218912

em

0celo7

no those are too advanced

user218912

4:05 AM

lolwut

user218912

says the guy who's like advanced af.

0celo7

I do not yet have the bullshit skills necessary for advanced physics

user218912

bullshit skills?

user218912

care to describe them?

0celo7

yes

@IceLord $e^x=1+x$ type stuff

user218912

4:06 AM

lel

user218912

I love tong's notes.

0celo7

@IceLord what are you doing in analysis?

user218912

I'm going to read all of them.

GPhys

@ACuriousMind I'm going to double down and say it's not clear to me that $p^2$ is only going to change by a total time derivative at most

user218912

@0celo7 same stuff as before.

0celo7

4:08 AM

what does that mean

user218912

we're following spivak's calculus but in a more rigorous way than in the book.

user218912

we did the last chapter and currently on ch 3 or 4 i think.

user218912

I don't attend the lectures.

0celo7

I don't have Spivak

@IceLord is the homework that easy?

what are some problems from it?

user218912

@0celo7 let me find it

user218912

4:10 AM

and no the problems are not obvious.

GPhys

Does anybody here know a counterexample to this?: Suppose $V=V(q_1,\dotsc ,q_n)$ is a potential that depends only on the generalized coordinates. Then, suppose $\zeta[\epsilon]$ is a coordinate transformation parametrized by $\epsilon$ that does not include a time transformation. If $\zeta[\epsilon]$ transforms the potential like $V\mapsto V$, then it corresponds to a symmetry.

user218912

it takes a while to do.

user218912

that's why I'm considering dropping it for regular calc so I don't have to worry about the homework. since idgaf about real math.

user218912

because my work load is too much

0celo7

I would slap you if I could

Consider yourself slapped

user218912

4:12 AM

why? :O

0celo7

"idgaf about real math"

user218912

well I'm an aspiring physicist.

user218912

it's the right mindset for me.

0celo7

Trump would say you're "sad"

I agree

user218912

lol

0celo7

4:19 AM

@IceLord I will give you a list of books to read if you want to stay in my good graces

it is the bare essentials

user218912

okay tell me and I'll read them in 2017.

0celo7

Jost - Postmodern Analysis. Lee - Topological Manifolds. Spivak - Intro to Diff Geo 1. HE.

user218912

in that order?

0celo7

Yes. It might help to know single variable analysis before you start Jost.

user218912

so like the class I'm in right now?

0celo7

4:23 AM

Yes.

user218912

dude this book is too advanced.

user218912

the other books are okay because I've seen them before.

0celo7

Which book?

user218912

jost.

0celo7

Jost is quite reasonable.

What is advanced?

user218912

4:25 AM

it begins with differentiation in banach spaces?

user218912

I much prefer folland's modern analysis book.

0celo7

No, it begins with a review of sequences on $\Bbb R$.

user218912

oh

user218912

wrong book lol

0celo7

Folland is much more advanced.

@IceLord What book were you looking at?

user218912

4:26 AM

it was the right book but the site didn't show the other chapters.

user218912

okay this is actually quite okay.

user218912

I'll read it if I ever want to learn analysis.

user218912

which is probably in an arbitrarily long time from now.

GPhys

4:39 AM

uhhh

is $\dot y^2$ a total time derivative of something?

:o I should go to bed

0celo7

@GPhys Don't think so!

unless it's $(y^2)^\dot$ ofc :)

GPhys

nope

0celo7

then in general not

else the kinetic term of a Lagrangian would not contribute to the action

GPhys

indeed

DanielSank

Hi, everybody

0celo7

4:42 AM

hi

DanielSank

@0celo7 Did you figure out travelling waves?

0celo7

nope

DanielSank

I realized later that I could probably convince you by pointing out that $\hat{p}$ on the functions we were talking about returns either $-p$ or $+p$

0celo7

currently working on transformation groups

GPhys

then, I imagine there are transformations such that $V\mapsto V$ that still change the system's motion

0celo7

4:43 AM

@DanielSank No, I've thought of that.

I don't know how to connect $p$ to motion.

DanielSank

@0celo7 And you're still not convinced?

@0celo7 :|

Dude, $\hat{p}$ generates translations.

GPhys

even if $V$ depends only on the generalized coordinates (not their time derivatives) and the transformation only changes those coordinates (so not time)

0celo7

In QM, it seems $p$ has nothing to do with motion. Ehrenfest's theorem aside.

DanielSank

@0celo7 :| :| :|

0celo7

@DanielSank But there's no time involved there.

DanielSank

4:44 AM

@0celo7 k

Let me know when you figure out a way of thinking that makes sense to you. I'm curious.

0celo7

I had this same problem when reading Shankar

I guess this is obvious to physicists but I don't see it

GPhys

:o I have a copy of Shankar next to me

0celo7

I do too

GPhys

because sakurai makes you want to commit sudoku

sometimes

0celo7

part of my desk right now:

Note Arnold too

GPhys

4:47 AM

I was about to say I recognized arnold as well

Does your copy of sakurai have incredibly poor binding?

0celo7

define "incredibly poor"

GPhys

I have pages that literally come out of the book with regular page turning pulling

0celo7

o.o

GPhys

in two copies

0celo7

nope mine is perfectly good

smells a little funny

@GPhys see

5:14 AM

5:27 AM

@GPhys Sakurai's angular momentum chapter is good.

@GPhys My Shankar does that.

user228700

5:46 AM

Hello everyone :-) Still learning circular motion and I came across the term "reference line". It was my understanding that the reference line is attached to whichever particle in question and that this line is used to measure the angular position of the particle w.r.t a fixed "origin" line. Is this correct?

John Rennie

I don't think reference line has a universally agreed meaning. It will depend on the context.

user228700

@JohnRennie Oh. Then how is the term "angular position" defined..? Every definition of this that I have come across has to do with a reference line...

John Rennie

If you're going to measure an angle then obviously you need to define where the zero is. It would certainly make sense to choose a line then measure angles relative to this line. It's just that I don't think the term reference line is universal.

user228700

@JohnRennie Hm, OK. I'm learning about relative angular velocity and there is some ambiguity about this term reference line. That's why I asked.

John Rennie

If you wanted me to look at a specific problem I'd be happy to do that, but I'm not sure I can usefully make any general comments.

user228700

5:57 AM

@JohnRennie It's not a problem, exactly. I'm trying understand the concept of relative angular velocity. I will look into it some more first...

NeuroFuzzy

@JohnRennie clicked on the U2 video, ten or twenty minutes later I'm on Macklemore thrift shop

John Rennie

I do like Macklemore. In a genre where performers take themselves very seriously Macklemore is a breath of fresh air :-)

And I think he's a great lyricist.

NeuroFuzzy

Haha all I know is "thrift shop"!

Sanya

6:14 AM

I need all of your help

0

Q: Is this a way to go to the past?

I know that one can time travel to the past through wormholes. But what if I use special relativity for that. I mean, If one has to go to the future, he has to increse his velocity, but what about going to the past. I propose that if we increase the velocity of the surroundings of a system while ...

user116211

@Sanya Duplicate... wait...

user116211

@JohnRennie answered this thing earlier also.... lemme dig up old posts...

Sanya

what could one flag this for? I am pretty positive from my physics understanding that this question is not a sensible question because it simply arises from a misunderstanding of the fundamentals of SR - but I do not know which flag to rise

@MAFIA36790 ah, cool

user116211

19

Q: Is time travel possible? Is it possible to go back in time?

I read somewhere that according to relativity, black holes and other space related stuff it is possible to jump into past. Is it possible for anything to go back in time either continuously or by jumping? I doesn't fit my mind and seems totally illogical, unreasonable and even stupid thought bec...

time relativity time-travel faq

user116211

11

Q: Is time travel possible?

Time travel -- often featured in movies, books, or facetiously in conversation. There are also theories treating time as simply another dimension, which to the layperson might imply forward and backward movement is possible at will. But what do we know scientifically with respect to the possibil...

relativity time-travel

user116211

6:18 AM

@sanya ^^^

Sanya

7:22 AM

@MAFIA36790 thanks

user116211

@Sanya sure.

g g

7:45 AM

re;
re: ok now lets say that a photosensor just 2 feet away from this spot / point of interaction is installed so will it detect the red and the green light (it cant see the yellow colour because colour mixing is human vision, humans can see colour) just by the scattering of the light in the air, scattering being caused by dust ??

John Rennie

@gg OK ...

user228700

@JohnRennie: You are indeed "un-pingable" at the CSE chat. Are u familiar with the concepts of Lewis and Brönsted acids and bases?

John Rennie

@KaumudiHarikumar in principle yes, though I might need a quick memory refresh. The Brönsted approach treats them as proton donors and acceptors doesn't it?

user228700

@JohnRennie Yep!

John Rennie

Ha, I'm not as senile as I thought :-)

@gg is that a question? If so I don't understand what you are asking. The sensor will detect light scattered by dust if that's what you're asking.

@KaumudiHarikumar do you want me to join the Chem chat room so we can discuss it there?

user228700

7:55 AM

:32592873 Why would u think that?! :O

user228700

Can you help me to understand this:

user228700

John Rennie

I would guess the OP means that all Lewis bases can accept a proton so all Lewis bases are also Brönsted bases. The point being that a Lewis base $B$ can always donate electrons to a proton to form $BH^+$.

lanzariel

have often read of famous people (and also some physicists) explaining how relativity can be counterintuitive to the layman since its effects are not part of common everyday experience. Now that I'm looking for some quotes I can not find any. Do you know any quote on the topic from any phisicist?

John Rennie

Hi @lanzariel. There's hardly anyone around at the moment. The Physics chat tends to start getting busy around 12:00 UTC i.e. in about four hours.

user228700

8:00 AM

@JohnRennie OK...what about acids? I'm not able to do understand how the two concepts are the same with regard to acids...

John Rennie

@lanzariel I wonder if it's worth asking in the History of Science Stack Exchange. They'd probably know more about quotes from historical figures.

To be honest most working physicists tend to be impatient with quotes from people who haven't made the effort to learn the (basically simple) maths required.

@KaumudiHarikumar well a Lewis acid is something that can accept a pair of electrons from a Lewis base. Anything that can act as a proton donor obviously fits this definition because the donated proton will always accept the electrons froma Lewis base.

user228700

@JohnRennie Sure, but we could make a similar argument for bases and say that the two concepts are the same for bases too, right?

user228700

We're only using two different words-"proton acceptor" and "electron donor"...

John Rennie

The distinction, if any, between the Lewis and Bronstead approaches is pretty small. I suspect most chemists would regard them as basically the same thing. I have a vague recolletion that there are Lewis acids that don't donate protons, though I'd have to do some Googling to check this.

The reason we have the two theories is mostly historical. Both Lewis and Bronstead were attempting to formulate a nice clear theory of acid and base behaviour. They just had slightly different priorities.

user228700

@JohnRennie Hm, I see. Maybe reading this would help to see where I'm coming from:

user228700

8:09 AM

quora.com/Are-all-Brønsted-acids-Lewis-acids

Sanya

@JohnRennie sorry John, but that's a huge difference

John Rennie

Daniel James Berger's answer looks pretty good. What in those answers is giving you a problem?

@Sanya I take it my memory is correct then, and there are Lewis acids that don't donate protons?

Sanya

the Bronsted approach only allows to talk about reactions involving proton exchange, the Lewis approach allows you to talk about reactions with electron pair transfer - that's a much wider class of reactions

@JohnRennie yes

take AlCl3 for example

user228700

@JohnRennie It's Max Tube's ans. that I'm more concerned with...

John Rennie

@Sanya At the end of the day it comes down to pecisely what you mean by the terms acid and base, and it's far from clear to me that this matters in practice.

Sanya

8:13 AM

desperately trying to recall my 1st year of chemistry before I changed to physics :D

@JohnRennie if you can categorise more reactions according to one scheme, you can understand more reactions with one principle/model/method

so in the end, it is an advantage to have broader concepts

John Rennie

@KaumudiHarikumar I think the point is that all Bronstead acids are Lewis acids, but not all Lewis acids are Bronstead acids because not all Lewis acids donate protons.

user228700

@Sanya OK, that makes sense. But my question is, is it fair to say that the two concepts are the same for acids? That is what's given in that quora link that I referenced...

John Rennie

@KaumudiHarikumar No Max Tube is wrong because Lewis and Bronstead acids aren't the same.

But Bronstead acids are a subset of Lewis acids.

user228700

Have a look at this:

user228700

Sanya

8:17 AM

@JohnRennie that's actually not true; so disregard what I said before

user228700

@Sanya OK, great.

Sanya

but HCl is not a Lewis acid - no chance to put an electron pair there

whereas it is clearly a bronsted acid

because it can easily give away a proton

user228700

@Sanya So it's not a subset, but the two sets do intersect(obviously) and that's it..?

John Rennie

@KaumudiHarikumar the answer says Lewis is a generalisation of Bronstead, which is what I've just said.

Does it really matter? Sanya says HCL isn't a Lewis acid, but I'd argue that it is because it can bond to a Lewis base. e.g. if you do a gas phase reaction between $NH_3$ and $HCl$ you get $NH_4Cl$.

There may be fringe cases that make strict categorisation impossible, but should we really be lying awake at night worrying about this?

user228700

@JohnRennie No, I agree that it may seem like we're all overanalyzing this but I'll ask just one more question regarding this and then I think it'll become clear for me.

John Rennie

8:24 AM

OK

Secret

I am the one who compare the dependence of statistics with time, however it is not included here

user228700

When we say "electron acceptor", we don't mean "proton donor" in that it loses a proton and hence, now has an extra electron, right? We mean that the molecule actually acquires an extra electron from outside, right?

John Rennie

@KaumudiHarikumar Yes

An internal electron transfer e.g. $HCl \rightarrow H^+ + Cl^-$ doesn't count.

user116211

Let me try if \mhchem works here.

user228700

Then it's obvious that the Lewis concept is not a generalization of the Brönsted concept, right?

user228700

8:30 AM

At the CSE chat, this is what Orthocresol had to say: chat.stackexchange.com/transcript/message/32593330#32593330

John Rennie

@MAFIA36790: that doesn't render for me.

user116211

@JohnRennie wait.... I'm giving you something...

John Rennie

@KaumudiHarikumar I don't see why not ...

Ah OK, I've just read the thread in the CSE. If you want to claim HCl isn't a Lewis acid then well I'm not going to scream and shout. But it seems a fine, possibly petty, distinction.

user116211

Name:

javascript:(function(){if(window.MathJax===undefined){var%20script%20=%20docume‌nt.createElement(%22script%22);script.type%20=%20%22text/javascript%22;script.src‌%20=%20%22http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML%22‌;var%20config%

John Rennie

@MAFIA36790 Nope

user228700

8:35 AM

@JohnRennie Hm, alright. But OK, I think I've got it now. Thank you! :-)

John Rennie

In any case we've just moved to the CSE :-)

user116211

@JohnRennie Why?

John Rennie

user116211

@JohnRennie I was typing wait... ;P

user116211

Address: `javascript:(function(){if(window.MathJax===undefined){var script = document.createElement("script");script.type = "text/javascript";script.src = "http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML";var config = 'MathJax.Hub.Config({' + 'extensions: ["tex2jax.js"],' + 'tex2jax: { inlineMath: [["$","$"],["\\\\\\\\\\$","\\\\\\\\\\$"]],

user116211

8:38 AM

displayMath: [["$$","$$"],["\\\[","\\\]"]], processEscapes: true },' + 'TeX: { extensions: ["mhchem.js","cancel.js"] },' + 'jax: ["input/TeX","output/HTML-CSS"]' + '});' + 'MathJax.Hub.Startup.onload();';if (window.opera) {script.innerHTML = config} else {script.text = config} document.getElementsByTagName("head")[0].appendChild(script);(doChatJax=function‌()

user116211

{window.setTimeout(doChatJax,1000);MathJax.Hub.Queue(["Typeset",MathJax.Hub]);})‌();}else{MathJax.Hub.Queue(["Typeset",MathJax.Hub]);}})();`

user116211

And now, @JohnRennie, use the name and the address in your book mark bar ;)

John Rennie

user116211

@JohnRennie ;/

user116211

It should be like this:

user116211

8:44 AM

user116211

Wait, Manish indeed wrote a post on it; let me dig it up for @JohnRennie....

John Rennie

@MAFIA36790 I wouldn't go to the trouble. It's not something I'm terribly concerned about.

user116211

Aha!!

user116211

13

Q: MathJax in chat (ChatJax offshoot)

This is an offshoot of ChatJax, which enables MathJax along with mhchem on chat. Copy the text below: javascript:(function(){if(window.MathJax===undefined){var%20script=document.createElement("script");script.type="text/javascript";script.src="https://d3eoax9i5htok0.cloudfront.net/mathjax/lat...

discussion mathjax chat

user116211

Now, here it as @john ;))

user116211

8:50 AM

@JohnRennie :(

John Rennie

$\ce{HCl => H^+ + Cl^-}$

user116211

@JohnRennie use ->

John Rennie

$\ce{HCl -> H^+ + Cl^-}$

user116211

Did it render for you?

John Rennie

It's not rendering ...

user116211

8:54 AM

WTH ;/

John Rennie

I think we should move on at this point :-)

user116211

@JohnRennie Did you bookmark it in the bookmark bar as Manish wrote above in the post?

user116211

hmm.

John Rennie

Yes. Well I put the bookmark in other bookmarks not on the bar, but it should work just the same.

user116211

8:57 AM

@JohnRennie Did you press it?

John Rennie

Yes. Of course I did. But please, I am now losing the will to live.

user116211

@JohnRennie sure; sorry for the inconvenience ;/

user116211

But still there must be some reason why it is not working for you....

John Rennie

:: John slumps forward onto his desk, a broken man ::

user116211

hmm. Anyways, I need to upgrade my Opera...

user228700

9:07 AM

@JohnRennie Noooooo (:P)

Philippe Gaucher

9:35 AM

(I am mathematician, not physicists) I read in standard newspapers, so not in scientific journals, that physicists in elementary particle physics were very worried about the future of this field of research because only machines having the size of our galaxy could help to choose among all possible extensions of the Standard Model. Is it serious ? More seriously are there survey papers giving a general picture about the theories beyond the Standard Model ?

John Rennie

Hi Philippe.

The accelerator the size of the galaxy idea is that to directly probe theories like string theory experimentally we would need colliders with an energy of around the Planck energy.

Philippe Gaucher

Hi John, my question is I guess naive, I would like just to know if the journalists who wrote that really understood their subject. In the same vein, I read that some free parameters in SM (hopefully I am using the right words) could never be explained by a theory.

John Rennie

As a general rule you can assume that science journalists don't really understand the subject. That's inevitable because quantum field theory (and string theory) are incomprehensible to anyone not prepared to take the years required to learn them.

Note also that there are no BSM theories that have any experimental support.

All the BSM ideas like supersymmetry, grand unification and string theory are currently just theories.

They are fun to talk about, which is why science journalists do it, but there is no hard evidence for any of them.

As for:
> In the same vein, I read that some free parameters in SM (hopefully
>I am using the right words) could never be explained by a theory
the truth is that no-one knows.

Philippe Gaucher

Yes it is what I guessed (about science journalists). That all these theories (BSM, Supersymmetry, etc...) are only mathematical theories ; is there any survey paper somewhere that could give a general picture ? For example, for a given theory, what experience could be done ? what does it solve ? etc...

John Rennie

You could try looking at these articles, but in general there are only articles written by physicists for physicists and articles written for laymen by science journalists, and very little in between.

Philippe Gaucher

9:48 AM

thanks that gives me something to read :)

hys.0515

I believe there're something for undergraduates? Those gave some general ideas and won't be so hard to read.

Sorry for my intrusion, BTW.

Philippe Gaucher

10:06 AM

no you're welcome. A few years ago, I had tried to read a survey paper (I don't remember the author) giving a survey about quantum loop gravity and string theory, comparing the two theories, the advantages and drawbacks, what was solved and not solved by these theories, but that was before the recent discovery (actually the non-discovery) in particle theories.

Danu

10:34 AM

Hi @PhilippeGaucher

@PhilippeGaucher There are many, many, theories that theoreticians have written down---the problem is that, on Earth, it will be hard to have particle experiments that go to energies more than, say, hundredfold the current LHC energies. This will make it hard to distinguish extensions of the standard model, especially when many of them will have noticeable effects only in ranges outside that...

Finding a general review of beyond-the-standard-model ideas might be hard, because so many things have been tried.

Basically, the standard model was finished somewhere in the 70s, maybe 80s. A lot of work since then has been purely BSM "speculation"

There are perhaps two main branches that one can and should distinguish: Extensions of the SM that try to basically take the SM and build something extra on top, and extensions that say---screw this, we're going to start from a "better" framework, and try to recover the SM in an appropriate limit.

The former would be things like this---basically using a bigger gauge group and seeing if that could work.

The latter would be things like string theory.

The first branch has major problems because it turns out to be very hard to extend the SM without breaking some part of the particle physics that we have already established to be true experimentally, while the latter approaches are just very far off from connecting to the real world (i.e. obtaining realistic theories from it has proven to be very hard; especially in string theory a LOT LOT LOT of effort has gone into this)

I hope that this is at all helpful @PhilippeGaucher

Also, I'd love to hear a little bit about what homotopy theory is about ;D

Oh, by the way, about loop quantum gravity---it's a theory purely of gravity, so it doesn't (aim to) include the quantum field theory stuff that we use in the LHC or anything like that. In that sense, it's not a BSM theory.

Fermi paradox

11:03 AM

I want to buy some polarizing sheets to give to my students as a gift. What exactly should I be looking for in amazon/ebay etc? (I don't want anything that has a circular polarizer along with the linear.) Is there any cheap option?

John Rennie

Something like this?

Fermi paradox

@JohnRennie do those for smartphones have only a linear polarizer?

John Rennie

@Fermiparadox I think polarisers for smartphones tend to be circular, but I wouldn't swear to this. Unless the ad specifically says its a linear polariser you'd be well advised to steer clear.

user507974

hey guys, grandpa here is forgetting something kinda easily

how do you take the gradient of something like $f(x+y-z)$

Secret

11:20 AM

Well in cartesian coord. $grad(f)= \begin{pmatrix} \partial_x f\\ \partial_y f\\\partial_x f\end{pmatrix}$

John Rennie

@user507974 Do you mean $\nabla f$? If so it's a vector function $(df/dx, df/dy, df/dz)$.

user507974

@JohnRennie yea

I feel like its just some chain rule where you do something like $w=x+y-z$ then can group together all the $\frac{\partial f}{\partial w}$

user228700

@JohnRennie: I uploaded a picture on the CSE chat, which differentiates b/w $H_2O$ (l) and $H_2O$ (aq). Exactly what is the difference b/w these two when talking about equilibrium constants..?

Secret

@JohnRennie I am trying to drag a friend of mine full of questions into this chat. What is the quickest way to get 20 reputation for him/her?

user228700

@JohnRennie is a poor man. He is being bombarded by questions. I hope he makes it :P

user116211

11:33 AM

@Secret Answer questions?

Secret

Ok

Philippe Gaucher

I found the survey paper I mentioned above: its this one arxiv.org/abs/hep-th/0303185, "How far are we from the quantum theory of gravity?" by Lee Smolin (which is already difficult for me to read), with tables like the one of page 60.

Secret

12:09 PM

[Topology musings]

in Mathematics, 1 hour ago, by Secret

Danu

12:32 PM

@PhilippeGaucher Note that Lee Smolin is heavily invested in Loop Quantum Gravity---he'll most definitely paint a bad picture of strings when possible, and is likely to push the LQG program

0celo7

Intellectual garbage according to our favorite string theorist

Danu

There are many people with strong opinions ^^

One thing that is for certain is that the LQG community is a lot smaller than the string community.

And that both approaches are quite stuck when it comes to explaining experimentally verified physics.

@PhilippeGaucher A small excerpt from [a little piece](http://math.ucr.edu/home/baez/week195.html) John Baez wrote about Smolin's article: "Smolin's paper also gives a critical summary of various standard conjectures in string theory, along with the evidence for these. This makes good reading for anyone wondering how much of what one hears about string theory is hype and how much is solid. To make this clear, Smolin states an amusing "minimal string theory conjecture" describing the worst possible scenario consistent with everything that's actually been shown so far! The gap between this an…

In any case, don't believe anyone when it comes to theories of quantum gravity :)

0celo7

Don't believe anyone when it comes to physics, I say.

Danu

@PhilippeGaucher Baez recommends this review as more "fair", though he also says it doesn't do string theory justice.

ACuriousMind

@Danu That review is 15 years old. Regardless of how fair it is, it's pretty outdated by now

Then again, so is the Smolin paper

Danu

12:47 PM

@ACuriousMind Yeah, of course.

Secret

MisanthropiK - Timeless & Colorless (dark organ ambient)

►

This music sounded ... empty...

Philippe Gaucher

1:19 PM

Well I just wanted to illustrate what I meant by a paper giving a general picture, not to start a polemical discussion about quantum gravity :-).

ACuriousMind

There are no non-polemical discussions about quantum gravity ;)

Danu

@PhilippeGaucher Also, don't worry, we're all converted string-believers ;)

(just kidding; I think the younger generation is generally unconvinced about any approach being "the right one")

John Duffield

@PhilippeGaucher : they're worried about the future of their field, but not because they need bigger and bigger colliders. Instead it's because they've painted themselves into a corner with the Standard Model, there's been no significant progress for forty years, they know that the public and politicians have lost faith in stamp collecting. That's the "discovery" of a succession of particles, each of which has a lifetime so short that it can hardly be said to exist.

@PhilippeGaucher : there is no quantum theory of gravity, and never will be. When two charged particles like a proton and an electron attract one another they "exchange field", such that the hydrogen atom has very little in the way of an electromagnetic field. This underlies the QED exchange-particle idea. But gravity doesn't work like that. Two gravitating bodies don't exchange field, their combined field is the sum of their individual fields.

Danu

As you see, John has his own brand of strong opinion on these matters...

Secret

1:34 PM

For me, I used to be a string believer, but now I am more agnostic

Danu

How can you be a string believer without having studied string theory, though?

Similarly, I think it's ridiculous to strongly criticize theories of quantum gravity without having any idea about how they work.

John Duffield

Only it isn't my own brand of strong opinion. You can find plenty of references to stamp collecting elsewhere: "Experimental Particle Physics has been derided as stamp collecting by some, and I would like to illustrate here why this comparison, although understandable, is wrong. The unstable and massive particles that have been discovered at various particle accelerators since the 1960s can decay to..."

Secret

@Danu This is why pop sci are unreliable and I always want to learn all gravity candidates. currently I am on my way to this by first trying to get my head around topology, and then bundles, that will allow me to understand GR. On the other end of the spectrum, I am beefing up my QM to prepare for QFT

Danu

@Secret You don't need either topology or bundles to understand basic GR, by the way.

John Duffield

@Secret : GR is easy. You just have to know how to approach it. Read the Einstein digital papers for that.

Secret

1:38 PM

but you need these kinds of concepts to go more advanced GR, and also QFT and gauge theory

It is the advanced GR that really interested me because of the scifi connections

John Duffield

@Secret : you don't. They get in the way to be honest.

ACuriousMind

@JohnDuffield It's rather straightforward to show that, in quantum field theory, forces mediated by fields of even spin will always be attractive. The metric perturbation that plays the role of the quantum "field of gravity" is spin-2, so quantum field theory already correctly predicts that gravity is universally attractive. So gravity works very much like a quantum force. The difficulties of quantum gravity are different and more subtle than that.

I know you it's futile to tell you this, but I won't let you mislead people who come here asking questions and have faith they will be competently and truthfully answered.

4

Secret

Only QM is the subject that I dared to say I learn not because it can help me to realise scifi

Danu

@PhilippeGaucher It's always good to listen to @ACuriousMind :)

user116211

And Danu always speaks the right thing.

John Duffield

1:41 PM

@ACuriousMind : I'm not misleading people, I've read the Einstein digital papers, I know how gravity works, and there's not a single mention of a spin-2 field. You're misleading people with a postdiction.

Danu

Einstein + modern QFT = non-intersecting timeframes

John Duffield

The predictions concerning gravity came from GR, not QFT.

Danu

Riiight

John Duffield

See The Confrontation between General Relativity and Experiment by Clifford M Will.

Would you like to provide some QFT predictions for gravity?

ACuriousMind

@JohnDuffield True enough. That doesn't mean there's no quantum description of gravity. We can do gravity as an effective QFT well enough, the trouble is that most of the "testable" predictions involve absurdly strong fields like black holes, to which we have no experimental access.

John Duffield

1:44 PM

Note that "QFT predicts that gravity is always attractive" doesn't quite cut it.

@ACuriousMind : the trouble with that is that Einstein's GR predicts that the strength of the gravitational field at the black hole event horizon is zero.

2

Danu

Riiiiight...

John Duffield

Yes, "the strong curvature regime" is misguided.

I will write about this.

Meanwhile, for now, I have to go.

ACuriousMind

@JohnDuffield A simple look at the Schwarzschild metric proves you wrong. What are you talking about?

John Rennie

1:57 PM

@ACuriousMind I suspect it's not widely realised that a QFT can be only an effective theory and still be an excellent description of the physics in the range where it's applicable.

Danu

@JohnRennie It's widely realized in the research community, of course...

I mean, nobody really views the standard model as anything more than an effective theory.

ACuriousMind

@JohnRennie Yes, that's a general problem with the perception of physical theories. Many expect them to explain everything, when that's neither what they can do nor what they were designed to do

John Rennie

@Danu ^^^ Too many people think they know how QFT works but don't.

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