3:01 PM
The good news is that, since that is the most incoherent/nonsensical possibel thing from me in the coming decade, he will not be annoyed by it anymore now that it has already under scruinty (and indefintiely sheleved as a result due to roadblock), at least that's what the calculations said
1 hour before AMA
cannot wait

@secret can you help me with a question about group actions?

@Secret The comoving coordinates are the coordinates in which everything in the (isotropic/homogeneous) universe is at rest. So in the usual way the FLRW metric is written everything is at rest forever.

AMA in 1 hour????????????????

@Secret Who is this "we" you're talking about? You don't need to share your inner monologue with us.

@Slereah ??????????????????????????

3:04 PM
While it's true to say observer is basically at rest with the cosmic microwave background this is really the wrong way round.

@ACuriousMind I like it
Makes me seem sane in comparison

@obliv I only know so little about groups, but throw it out (acuriousmind will help spot any mistakes I made if any)

The CMB is at rest wrt the matter that emitted it, and the matter that emitted it was at rest. So you should really say the cosmic microwave background is basically at rest with respect to the comoving observer.

@0celo7 Converting 16:00 UTC to Syd time I got 2:00

I understand the question but I don't understand how to come to the solution. Say you have a group $S_n$ (a group of permutations $\sigma = (1~2~...)$) that act on a subset of $n$. What size does this subset have to be for this action to be faithful (ie injective)? The way I approach it is that there will be multiple $(\sigma,\{...\}) \in S_n \times k$ (k is the subset of n) that map to the same permuted subset if $|k| \ne |n|$ My reasoning is this

3:08 PM
The site says 51 minutes
So that's that

When a symmetry group $S_n$ acts on a subset of $n$, Suppose the subset of $n = \{1,2,3,4,5,6,7\}$ was $k = \{3,4,5\}$ I think that elements $\sigma_1(k) = (1~2) \to \{3,4,5\}$ and $\sigma_2(k) = (6~7) \to \{3,4,5\}$? are not injectively mapped.
the solution says that it's injective for all $k< |n|$

@ john I see

@Obliv I think you are misunderstanding the question. I think you are to consider the action of $S_n$ on the set of all subsets of size $k$ of the set $\{1,\dots,n\}$, where a subset $\{i_1,\dots,i_k\}$ is acted upon by a permutation sigma simply by $\{\sigma(i_1),\dots,\sigma(i_k)\}$.

@Secret this is why you get people saying that distant galaxies aren't really moving away from us. It's because in comoving coordinates they aren't moving. Technically their four velocity is $(c, 0, 0, 0)$.

OH right I made that mistake yesterday too. Okay so $|k|$ is the size of the subsets and $S_n$ acts on a set of these subsets as @acuriousmind mentioned.
I'll see if that changes the outcome

3:15 PM
The claim is of course complete pants as the coordinates we use every day are not comoving and in these coordinates the distance galaxies are moving away from us.

indeed

It is possible to write the FLRW metric in everyday i.e. non-comoving coordinates but you end up with a metric taht has off-diagonal elements. I think Jerry Schirmer wrote the non-comoving form of the metric in one of his answers.

You probably need it for like

The set size is $|n|$ every time then. fuck this makes it so obvious that $k < |n|$. Thanks @acuriousmind saving me everytime

Observations and such

3:17 PM
@Obliv ...what?

For instance I think the CMB is only homogeneous in comoving coordinates

There are not $n$ subsets of size $k$ for every $k$. That's only true for $k=1$ and $k=n-1$.

yeah, wikipedia said in non comoving coord the CMB will redshift or blueshift wrt observer

oh

Don't guess. Think.

3:25 PM
@ACuriousMind
If $S_n$ act on $M$ which is the set of all subsets of size $k$ of the set $\{1,\dots ,n\}$, then how can injectivity be ensured for all elements of $M$ as e.g. for one of the elements of $M$ say e.g. $\{1,2,3,5\}$ and act by an element in $S_n$ that is e.g. $\sigma_8=(4 6)$, then we got the mapping to $\{1,2,3,5\}$ but the identity element in $S_n$ can map $\{1,2,3,5\}$ also to itself?

you're not permuting the subsets in the set. you're permuting the position of the subsets themselves in the set. @secret

@Secret The question is whether the representation of $S_n$ on $M$ is faithful, i.e. whether the map $S_n\to\mathrm{Aut}(M)$ induced by the action is injective.
And that is the case.

@Secret Aha, I found Jerry's answer where he gives the metric in regular coordinates:
1

The standard coordinate system is the mathematically simplest, but I don't think it's actually the most physically intuitive. This is because we live on objects that are gravitationally bound, and admist objects that are electromagnetically defined. This means that our local length scales are n...

Howdy

i'm trying to figure out a formula for the size of these sets of subsets. $\{1,2,3\}$ has 3 2-element subsets. $\{1,2,3,4\}$ has 6, $\{1,2,3,4,5\}$ has 10. What the hell is the pattern, $|2k-\frac{n}{2}|$?

3:29 PM

O wow, pascels triangle..

when is the AMA
@Obliv if you can't vary the Einstein-Hilbert action in your head, just quit physics
this is baby stuff compared to that

15 minutes

> If you see an error message ‘Permission denied’ type chmod 666 * and try again
You literally invoke Satan to get rid of the Linux error

3:45 PM
Probably poor security but when I get such messages I just put maximum access on my files
Because fuck it

who would want to hack you anyway

@0celo7 I can't tell if you're serious. I guess I'll start playing more blindfold chess in case you are lol

@ACuriousMind Ok even that still doesn not add up
I just calculated, We have $S_n$ where $|S_n|=n!$ and $|M|= ^nC_k$ But $^nC_k < n!$ thus the mapping $S_n \rightarrow Aut(M)$ is basically mapping from a set with more elements to a set with less elements, thus how can this be injective?

that's what I thought too @secret since it's not always $|n|$ every time

Well not me
The work computer

3:47 PM
@JohnRennie It's in Weinberg.

I don't use linux at home

@0celo7 loads of people, though they're usually more concerned with hacking Windows installations. They'll try to put bots on your PC that they can then use for their own purposes.

@Secret $\mathrm{Aut}(M)$ does not have less elements than $S_n$.

for subsets of $|n-1|$ the set size is $|n|$ but for $|n-2|$ it's $2|n| - \frac{|n|}{2}$ etc

@JohnRennie Also probably in Stefani

3:48 PM

No
But I didn't plan to be ready

@JohnRennie Yes, and probably in Cosmology as well.

Also wasn't @DanielSank supposed to be here
as the host

@0celo7 Gwen Stefani has written a physics book?

@JohnRennie Yes.
It's very good.

3:49 PM
Ah, yeah, I was going to say, who's going to host the AMA

what is $\mathrm{Aut}(M)$ ?@acuriousmind

@DavidZ I volunteer.

@vzn are you around
It was your idea and all

@0celo7 This book?

> An event is starting in 9 minutes in The h Bar - "Physics chat session"
Oh my god it was on time for once

3:51 PM
@Slereah It appears you are on your own ;P

@JohnRennie No...

Do we need a host?

are you serious?

It's fine
I can ramble on on my own

@JohnRennie Do you not know all the advanced GR books
@Slereah so the AMA is just the usual state of affairs?

3:51 PM
@JohnRennie I guess not really. The plan is that I'll start the chat session as normal, and then after doing introductions (if there are any to do) I'll turn it over to... someone

@Obliv The set of bijections on $M$. $S_{\lvert M \rvert}$, if you will. But shush now, the session is about to start.

Well more rambling I guess

9

This list is extensive, but not exhaustive. I am aware that there are more standard GR books out there such as Hartle and Schutz, but I don’t think these are worth mentioning. Books with stars are, in my opinion, “must have” books. (I) denotes introductory, (IA) denotes advanced introductory, i.e...

@0celo7 All but this one. Which book is it?

@JohnRennie It's in there

3:52 PM
Would have been @vzn if he were here

@0celo7 hi

Missing the first AMA would be pretty unprofessional
Tsk tsk

@0celo7 ah Stephani

@JohnRennie $ph\cong f$

Guys, is there a table of the running of alpha_s (strong)? I need the value of alpha_s at 7 TeV

user116211
3:53 PM
8 min to go

@FrancescoS small :-P

I think it's in Particle Data Group

@DavidZ I need a really precise value

Then particle data group

@FrancescoS 1/137

3:54 PM
@0celo7 you need to review QFT

@FrancescoS then PDG is probably the place. I don't think there is a very precise measured value at such high energy, though.

the host arrives just in time

@FrancescoS Isn't the point of the running coupling that we only need to know it at one scale and then we can compute it at the other scales? oO

@DavidZ and which value of alpha_s do you use when you study partonic processes? for sure, not the value at 7TeV

3:56 PM
It's page 158 in my edition
The 2008 edition of the booklet
Fig. 9.2
It only goes to 100 GeV
189, to be precise

@FrancescoS nah, I use 0.2 usually, or the LO running coupling evaluated at something on the order of 10 GeV

@ACuriousMind Sure, but I want to avoid to spend time everytime to compute the runnings

You might actually have to look up the formula and extrapolate

@DavidZ Is there a rule?

user116211
3 min to go

3:58 PM
I saw vzn, but he is greyed out atm

@FrancescoS uhh... sort of? It's an art :-P

hi all am going to introduce slereah early & let him run the show. thx very much S for agreeing to this new idea. he wrote a great intro here and there are excellent questions for him on meta (thx all for contributing those). S is an expert on CTCs, the meta post has more info & link to his masters thesis. would like to write more, space limited. (thx also DS for efforts wrt the series.)

user116211
Donc, ici présent Slereah

We can discuss this further when there isn't an impending chat session

Gee I dunno about "an expert"
Anyway
I guess I'll start with a little historical overview of the whole thing

4:00 PM
OK, time to start?

@DavidZ Ahahah thank you. Is there an impeding chat session?

Yes, and it starts now ;-)

IS TIME TRAVEL REAL
5

Welcome everyone to our biweekly chat session!

FIND OUT NOW ON
THE $\mathfrak{Biweekly \,\,Chat\,\, Session}$

4:01 PM
Here is our agenda for the day:

1. Intro, welcome newcomers, take policy questions (<5m)
2. AMA event with @Slereah

user116211
people are coming

Oh no

Please keep unrelated discussion to a minimum for the duration of the chat session

Newcomers? Looks like the standard crowd tbh

(note we are scheduled 60m which may go by fast but S is free to hang out later & encourage everyone to hang out further afterwards.)

4:02 PM
So, who's here? Anyone new to the site, new to chat, or new to chat sessions?

I am new to chat sessions. I just clicked on this to see what it is about.

@qmd cool, welcome! Today's is a special event, with the interview style, but we do this every couple weeks to bring the community together.
Usually there is some discussion of physics and/or site policy or whatever needs to be brought up. It's also a good venue for new-user questions

Sounds great. So during the AMA I can ask any physics related question?

I'm actually going to turn it over to @vzn to explain the format and introduce our guest.
Take it away ;-)

@Slereah lol fyi the special spker idea was proposed by DZ originally & am helping carry it out, yes my idea is to start with chat room regulars

4:06 PM
@qmd it's an "ask me anything" after all

@Slereah what is your favorite food

I like me a good steak
With some baked potatoes
Them's good eating

hey all we are still experimenting here, thx for joining, the format is fairly fluid/ still TBD & will let slereah indicate how he would like to handle it :)

Well I guess maybe more questions relating to the topic?

17

Hey I'm the scheduled guest for the june 14th Physics AMA and I was told to maybe make a little presentation and asks for any questions for the AMA people might have, so here it is. About me So I am Samuel Lereah, got a Master degree in particle physics from the university of Nantes. My master ...

4:08 PM
Since questions on physics you can do that all the time here

user116211
yes, go on...

@slereah Do you live in france? (since you got a degree at a french uni)

I do
Around Paris currently

@Slereah Why did you decide to study physics?

@Slereah I just posted a question on the thread

4:09 PM
did your MS thesis have connections to CTCs or did you do a lot of separate research in the area? how did you get into/ specialize in CTCs in particular?

Just an interest of mine for quite a while

@Slereah What advice would you give to an undergrad student or someone who has just started studying physics?

It's a good one that should be answered

user116211
@Slereah: Do you want to go to other country for phd?

@vzn Nothing related to CTCs in my master thesis
It wasn't even relativistic

4:10 PM
@slereah Do you think the job market for physics research in France is more difficult than in the U.S. or other countries?

Even though it was in Riemannian spaces
@qmd I'd say go for something that has some chances of getting you a job :p
@MAFIA36790 It would probably be easier than here certainly

What is the major research topic in riemannian space in your thesis, i.e. what is the problem your thesis is going to address in terms of the GR research field (or differential geometry in general)?

@Obliv Well it's not that great and there is also a big hurdle where legally, you have to be paid to do a PhD, so universities are a lot more picky about who they take
And since they have to spend money on it, theory tends to get pretty ignored
@Secret Wasn't related to GR, as said
It was only that particles evolve on a curved space
Not a curved spacetime
Everything else is non-relativistic

@slereah What other subject of study interests you, outside of physics?

I see, what are the major results you found during your research in the thesis?

4:13 PM
Well I quite like the bronze age period, history-wise
I used to be pretty big into logic and computation theory
@Secret The big result is that only the Stratanovitch integral for the action permits the Lagrangian to remain covariant

@Slereah what is your profile image?

S did you do a masters thesis presentation/ defense with advisors? guess that happens "usually" in US, dont know EU/ france...?

That would be Leonard J. Crabs, Something Awful's fictional lawyer
He used to defend them against legal threats in the early internet days
I did yes
it went alright

@Slereah how long was the session? how many advisors or attendees?

Don't remember too much, I think it was a pretty standard thing
20 minutes presentation, 10 minutes of questions

4:16 PM
@Slereah What will happen if I plot 50 things at once in gnuplot

so, of all types of stochastic integrals, only if the action is formulated with the stratanovitch initegral give a covarient lagrangian?

Depends on your raw computing power, I suppose

@slereah when did you and @0celo7 start a blog together at einsteinandtheevidence.wordpress.com

@Slereah It's a US gov't supercomputer, I think

Yes

4:17 PM
@Obliv long ago

For instance

But its hard to maintain
It's been dormant for a while

Greetings. I see there is a discussion about the system of Master and PhD. May I ask your opinion on how the system works? Would you say you were satisfied in general and maybe if there are certain problems that need to be addressed? Thanks.

If you use the Ito integral
The Lagrangian acquires a term that is

@Slereah Biggie or Tupac

4:19 PM
$$\Delta V = \frac{\hbar^2}{8m} (g^{ab} \Gamma^d_{ac} \Gamma^c_{bd} - R)$$

@Slereah You might want to use the reply feature so it's clearer which responses belong to which questions

Not a very covariant quantity

@0celo7 do you have any plans for blog S? news to me that you (co)contributed. (encourage everyone who chats frequently to experiment with blogs, it doesnt take much more effort & allows math formulas etc) =D

For real, I need Linux help. I need to get all the files that contain "491" and end in ftlgr.gr

@Slereah If you could go back to your first year at university, would you still do physics? why or why not?

4:20 PM
@ConstantineBlack Well, as I could not do a PhD because of the current system, I'd say not overly satisfied
although that is peculiar to France I think
In the US I could have probably done that PhD for free
@qmd Well probably still, yes
I still do physics now
It's just a field that I enjoy

In your opinion, what are the major areas or uunsolved questions that any aspiring CTC researchers need to focus more on?

Well the big one currently is probably the chronology protection conjecture
There's a lot of reasons to think it's true
But no generic theorem regarding it

> Connection to analysis.sns.gov closed by remote host.
Connection to analysis.sns.gov closed.
Crap
What does that mean?

Also it would be nice if there were some demonstrations to prove CTCs impossible or not in the few quantum gravity theories that might allow them

@Slereah "demonstration" = "experiment setup"?

4:23 PM
I don't think there's much in the way of experiments to perform
Just a theoretical proof would be nice
not too much I can say on the topic, though
I'm not that big on string theory
Or Euclidian gravity or Lorentzian gravity or whatnot

@Slereah what would you say about CTC vs string theory researchers? small overlap? compatible somehow?

Currently one of the somewhat active field on the topic seems to be quantum computing with CTCs

5 plots works just fine
let's try 10 now

@vzn Well I can't answer this too confidently

chronology protection conjecture : Do we need a quantum gravity theory to attack this problem and why would you say so? Why GR isn't enough. Thanks.

4:26 PM
@0celo7 no unrelated discussion, remember

I want to say there's no a priori reason that closed strings could not produce closed timelike curve-like configurations

@DavidZ Art is never unrelated :(

If you have a spacetime you could imaging geodesics flowing through it almost like streamlines. A CTC would then look I suppose a bit like a vortex in a flow. Wouldn't there be some very strange behaviour at the stagnation points?
Or is this just a meaningless analogy?

I'm not sure the string theory framework is robust enough yet to confidently talk about CTCs. To my knowledge, the well-understood aspects are string perturbation theory where the backreaction of the "stringy" objects on the ambient metric is largely ignored.

@ConstantineBlack People have tried pretty hard to do chronology protection without quantum theory, with quite a lot of angles of attack, but there are always counterexamples to those
Quantum field theory in curved spacetime offers arguments that are a bit more general against them
Hopefully quantum gravity would give a definitive answer outside of the regime of QFT in curved spacetime
@JohnRennie what is a stagnation point
@ACuriousMind It would be nice, certainly

4:29 PM
@Slereah In fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero. Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object.
(c) Wikipedia

I know there is a vague paper on CTCs with regards to Pauli Fierz quantum gravity
Which is sort of the ancestor of string theory
That's about the extent I can imagine for it
@JohnRennie Well from the raw description I'm not quite sure how it would apply here
Maybe I can look into it later

I haven't really though this through and we should let others ask more relevant questions first.

@ConstantineBlack btw, the three big angle of attacks have historically been
1) The instability of the horizon

@JohnRennie (am also looking into connections/ analogies between fluid dynamics & GR & 0celo7 has mentioned it, & also secret recently posted ideas on it, agree with you it probably connects to timelike curves, but it seems to be "fringe" right now...?)

It was expected that generically, any particles trying to cross a closed timelike curve would disrupt it, making it collapse
Since that is the case for the Kerr metric
So that in effect, closed timelike curves could exist, but no particles could cross them
2) The energy conditions
A big theorem due to Tipler, and a later one due to Hawking, roughly state that any finite sized time machine would have to break some energy conditions
Which were expected to always hold
They do not hold so well in later analysis, though

4:33 PM
@Slereah How can a theorem be found to "not hold so well"

@Slereah whats your take on hawking? modern near-einstein? what about his pop science books/ rep vs hard core science? (0celo7 is lately studying him intensely although will prob not admit it) :P

Well the theorem is valid, but the energy conditions themselves do not hold for all cases
Hawking (and Ellis) did write one of the best book on general relativity of the modern era
And he did a lot of important work on various bits of quantum gravity, quantum field theory in curved spacetime and general relatiuvity
I used to read Hawking's books as a kid
I even had the "video game" version of a brief history of time

@Slereah did you see "theory of everything"? any reaction? have you heard opinions on it from others? etc

I rant a lot about pop science but I think it's important for people to get in science, though

(found a peer reviewed article on CTC on google, figure might just post it here and see what can be added to the discussion iosrjournals.org/iosr-jm/papers/Vol7-issue4/…)

4:35 PM
@vzn I have not
"The elimination of Closed Timelike Curves in Loop Quantum Gravity"
Where spacetime is globally hyperbolic by design

What do causal dynamics triangulation and loop quantum gravity say about CTC geometries? Do they got similar findings as string theory predictions?

See above for LQG
I think CDT also imposes causality in the triangulation, hence the "causal"

@vzn I won't admit it because I'm not studying him intensely

@Slereah there is a "video game" version of "brief history of time"? hadnt heard...?

A lot of quantum gravity theories have the notion of causality baked into them more or less deeply
Because it's pretty hard to do quantum theory without that
@vzn Well a CD ROM version, anyway
Wasn't really a game but it was interactive educative sort of thing

4:40 PM
@Slereah re "not that big on string theory" ... ie somewhat skeptical?

No I think string theory is fine
Don't know if it will be the answer but I just haven't studied it all that much
I know it doesn't have that many testable predictions but to be fair, almost every quantum gravity theory has math so bloody awful that it's hard to make any good one

@Slereah I never read pop science
My first science book was a GR textbook
My first physics book
I guess I had chemistry and biology textbooks before that

@0celo7 (the swaggering physics rapper dude) :P

...what

Is that about all for the questions?

4:43 PM
I broke the server.

Maybe the questions from the thread?

I tried to run a massive program 11 times.

@Slereah lol (nope! still have ~15m!) slereah re your admitted collaboration with 0celo7 on blog, if you could cowrite a book with him, what would it be?

@Slereah did you know much physics in high school?

user116211
4:44 PM
@vzn officially...

@3075 More than the average joe but all that much

(this is more of a general public style question) What is the most important difference between CTCs and the popscience and scifi notion of stable time loops?
http://tvtropes.org/pmwiki/pmwiki.php/Main/StableTimeLoop

@vzn Well the one we usually talk about is just a big book of GR with all the proofs
Because it would sure be nice

I didn't!
I did look into stochastic theory
But for the most part, the exact details of what we are integrating over isn't usually used all that much
There are a few heuristic methods to perform the stochastic integral
Similar to the Riemann sum for normal integrals
Which give the desired results

4:47 PM
At some level then, you learned the mechanics of stochastic integration and use it effectively?
No need to get into the details?

I think the book I'd recommend the most for the topic is probably Demichev and Chaichian "Path integrals in physics"
It does go fairly deep into the whole matter
Starting with Wiener processes and Wiener integrals
But yes

wiener lol

@Slereah re books, youre a big collector, how many do you think you have on physics anyway? do you also use ebooks?

"ofcourse"

4:49 PM
@Slereah you missed my question
http://chat.stackexchange.com/transcript/message/30353912#30353912

@Slereah What level of rigor are we talking here? Would a mathematician agree with this?

And I have about 40-50 science books, all things considered, I think
@ACuriousMind I think a mathematician would run away screaming to be honest
There are sound mathematical reasoning behind the heuristic but you know
Doing physics here

@Slereah lol, hey thats ok, its a physics chat room :P

@Slereah Well, after the mathematician has overcome his usual shock of being confronted with physics at all, of course ;)

@Slereah Would you say this book can be understood by a physicist with typical math background?

4:51 PM
Yes, it is done with physicists in mind

Thank you

Also maybe
Kleinert's book

@Slereah That's the gigantic canonical book on path integrals, right?

Kleinert is probably the most complete book on non-relativistic path integrals
yeah
To be fair, I don't like reading it too much
It's just too big
I broke its spine by accident since it is so big

Is it possible for a waveguide to have impedance higher than the vacuum impedance?

4:53 PM
Also Feynman and Hibbs is probably the best introduction, still

@Slereah pic?

It doesn't get all that much into stochastic theory, though
Well, kinda
Quite informally

Ok, thanks for the recommendations.

Maybe I missed it and you already said something about it, but another question from the meta thread: How could CTCs be observationally confirmed in a universe where they exist?

4:56 PM
I did answer it on Stack Exchange a while back!
There's quite a fair bit of changes in physics once you introduce CTCs
The big one is that you (can) lose determinism

@Slereah lol we already lost that long ago (>~century) with QM though right? :P

Oh I'll get to QM too
It depends on what matter you consider and the exact metric, but overall, it's quite likely that if you have a CTC, you might get particles coming out of nowhere and disappearing into nothing

@Slereah sounds a lot like virtual particles though!

@vzn but since the time evolution of a quantum state is deterministic, then it should not be a big deal?

That's due to the fact that with CTCs, a geodesic might not cross every spacelike hypersurface
Or cross it several time
Although to be fair, this is a feature they also share with naked singularities