The h Bar

5:00 PM

In naked singularities, the particle just pop out of the singularity or disappears inside

While with CTCs, the particle just loops back, possibly right back into itself

This also happens when you consider quantum fields

(BTW the hour is up - feel free to keep going but thanks everyone for coming to our chat session, and see you in two weeks!)

Slereah

They will lose unitarity quite generally, both for CTCs and naked singularity

vzn

ok all we hit 1hr, thx much to DZ for this time slot, thx much @Slereah, feel it went great, thx to questioners/ participants, all are free to stick around, & DZ has indicated we will evaluate this session some & decide on the next speaker/ time. :)

Slereah

Due to that exact effect

Secret

(My final question was completely ignored ::: ( )

Slereah

5:02 PM

So that a particle will not have a probability of 1 of existing as time goes by

vzn

@Secret not all good questions have answers, aka physics zen, thx for dropping by... looks like we did have lots of great questions, more questions than answers as can happen in science etc :)

Slereah

If you still have questions I can still answer 'em really

0celo7

I'm quite proud of this

Secret

That's really me attempting to immitate a general public asking that question

David Z

@0celo7 looks nice

Slereah

5:03 PM

Well stable time loops are probably the closest thing to the common opinion on CTCs

You cannot change the past

Secret

i see

0celo7

@DavidZ now if someone can explain the crazy cyan curve to me...

I doubt that curve represents reality

Slereah

One big problem with CTCs by the way, beyond the loss of uniqueness of solutions, is sometimes the loss of existence of solutions

vzn

← also a gnuplot freak, think nothing cant be analyzed with graphs if sliced & diced the right way, even simple ones... & strong tiein with bigdata, 21st century science

David Z

@0celo7 maybe a cat ran through the experiment :-P

Slereah

5:05 PM

There are some matter fields on acausal spacetimes which have no solutions at all

Which is probably not a good sign overall for the existence of CTCs

0celo7

@DavidZ we're looking at a specific exothermic reaction in erbium titanate. I was hoping plotting ALL of the data at once would be illuminating

Besides that one cat plot, it's actually very nice and uniform

No evidence that this reaction has occured

Secret

@Slereah What does it physically mean when there is no solution, does it mean siad configuration of matter fields is not possible near a CTC?

0celo7

The curves are basically in order of increasing temperature

Slereah

Yes

If you allow CTCs, it may mean that some initial configurations of matter are impossible

David Z

@0celo7 Yeah, which I assume is a good sign (and if not, looks good anyway)

vzn

5:07 PM

@DavidZ (since youre around...) any reaction/ feedback re session? what do you think about next speaker in 4wks after std mtg? too soon? @Slereah, whatd you think?

0celo7

@DavidZ I wonder if there are cats in the oak ridge facilities

Slereah

So if they are allowed in our own universe, the initial conditions of the universe would have to be pretty specific

0celo7

@DavidZ No, because you can't tell that anything actually happened :P

Slereah

Well it felt a bit short :p

Could have gone on for quite a bit longer

0celo7

I'm now going to refine the data and see if I can get some interesting structural data from the scattering data

David Z

5:08 PM

@0celo7 If nothing was supposed to happen, that is a good sign

vzn

@Slereah think thats a good sign actually. :) yeah 60m goes by fast on here (thx again)

Slereah

CTCs are quite a rollercoaster, historically

David Z

@vzn actually I wasn't too closely tuned in for most of it. But I did think it seemed quite busy. Which is good, in some sense, but it made it hard to follow.

Slereah

It's a lot of people finding reasons why they can't exist, followed by people finding counterexamples

Secret

@vzn I enjoyed the session, it has the informative property of a conference. I wonder if eventually, we will invite physics professors thus that will be kinda like a TED talk

and that would be cool because not many expect chat sessions can function like an official event

vzn

5:10 PM

@DavidZ yes seemed well attended, think a good sign :) ... alas think this chat room will never be all that coherent no matter what... we could try other venues, had some idea on that... videoconference is obvious possibility...

0celo7

@DavidZ but something did happen, the data is just now showing it!

I mean there's a tiny bump around the 800C mark

vzn

@Secret great, would luv to get very high profile guests in here & am gonna write a meta post on that sooner or later, have been musing a few wks on that, was thinking after 3rd guest or so. ps you are a regular so are also a speaker candidate yaknow! :)

David Z

@vzn There are ways to get around that, e.g. you could ask people to bold their questions, and post answers as replies to the questions

0celo7

But we were hoping for some massive reordering at 800C

David Z

Also, an active moderator could ask people to stop posting new questions when the guest can't keep up

vzn

5:12 PM

@DavidZ answers as replies works well

David Z

These are things the SE team does in their town hall chat sessions and moderator chats (back when we had those)

vzn

@DavidZ ok, think its pretty hard to "moderate" in chat rooms other than kinda awkward approaches with so few visual/ body language/ inflection cues

David Z

We can also brainstorm more ideas

DanielSank

yes

David Z

@vzn p'shaw, you don't need visual cues

It may take some practice

vzn

5:14 PM

@DavidZ challenging, just keep in mind we will have new/ shifting audience all the time. & ppl dropping in middle without any idea re waveguide/ vacuum impedance lol ... anyway thx for being here

David Z

Keep in mind I'm talking about moderating in the sense of guiding the discussion, like a debate moderator, not controlling inappropriate behavior; that latter task still falls to the chat mods and room owners

DanielSank

@vzn Actually that was a very interesting question.

Loong

@DavidZ If necessary, you can temporarily make the room read-only and give the guest explicit write access.

DanielSank

@DavidZ Yes.

vzn

@DanielSank fine but it had nothing to do with CTCs :P

DanielSank

5:15 PM

@vzn No it didn't.

I was surprised how much of the chat session was about CTC's.

vzn

@DanielSank really? youre sure? sorry my mistake thought it was someone random showing up and asking a misc physics question as always happens in chat. anyway, that kind of thing seems unavoidable.

Slereah

Well I tried to pick an interesting topic

If I tried Colombeau algebras in QFT might have been a bit of a snooze

David Z

@Loong yeah, but that takes time, right? Like 20 seconds. In any case it shouldn't be necessary unless people in the room are trying to be disruptive.

Slereah

Nobody even asked me why I became interested in CTCs :p

(I read about Tipler cylinders in a book by Stephen Baxter)

vzn

@Slereah did ask something like that. you dont have to answer questions exactly ofc you can extrapolate/ segue into stuff you want to talk about

Slereah

5:17 PM

I think it was... Ring

ACuriousMind

@Slereah I think everyone assumed "because time travel" ;)

Secret

Well about being a candidate, there are some issues:
1. My physics is kinda undergrad level still, thus there isn't really any good things to talk about
2. My speciality is mostly chemistry thus any thesis level stuff I can talk about is gonna be atmospheric chemsitry and molecular dynamics
3. The most content-ful thing I can talk about that is closest to physics is that scifiphysics mix of my time travel model, which a bottleneck was just identified recently by the h barers thus it is shelved

DanielSank

@vzn Huh? I meant no the question about waveguides didn't have anything to do with CTC's. I was agreeing with you.

Slereah

Well yes but it's a very poorly documented part of GR

Carroll barely mentions it and that's the "standard" GR book for students

vzn

@DanielSank lol ok got mixed up because you usually challenge me so much :P

DanielSank

5:18 PM

@Secret I think AMA questions can go outside of physics per se.

Slereah

Wald mentions it a bit but only in chapter 8

No mention of even any famous CTC metric in it

Secret

@Loong I think that would be a good idea. I have a couple of physics classmates and/or professors that I can invite to join and they can also participate if this is done

hubot

Hello!

Slereah

Hawking Ellis does talk about it a fair bit but nobody has ever read it

Hello

vzn

DZ the thing is, seems AMAs cant really be "moderated" much. like the idea of an interview but think its kinda hard in here... maybe one guest might like to volunteer for an interview style format if they lean toward that? it could certainly be more "coherent"

0celo7

5:20 PM

@ACuriousMind time travel would be cool

I would go back and tell young me to not learn GR

Slereah

Oh btw

One of the question was about applications

0celo7

Oh also not to break up with Nadia

that was dumb

Loong

@Secret Yes, with explicit write access, these users are able to talk even when their reputation is too low.

Slereah

The one usually discussed, outside of the obvious, is quantum computing

Secret

@DanielSank Like a typical TED talk, in todays session, we have gone through important things such as major questions, appraches etc. concerning CTCs, I think that is actually quite informative for a 1st AMA

Slereah

5:22 PM

CTCs would permit to go around a fair amount of no-go theorems in quantum information theory

David Z

@vzn I think they can. Maybe the kind of structure you have in mind is something which encourages people to ask questions freely, and sure, if you want to do it like that, you can't have much active moderation. But it is also possible to have a similar event with more active moderation, where the flow of the conversation is more controlled. Perhaps you wouldn't call it an AMA, but I do think it is possible.

vzn

@Secret understand your objections but personally feel like anyone who likes to hang out in chat room is candidate for speaker as long as they can talk about physics for awhile

DanielSank

@Secret I don't disagree :) I was surprised, not dismayed.

I think the advantage of additional moderation could be a wider variety of topics.

hubot

How to solve trigonometric equation as d=t/2-sin(t)/4 where t is 0 < t < 2pi?

DanielSank

Today we mostly heard technical talk about CTC's. That's great! but it's not the only thing I would find interesting.

vzn

5:23 PM

@DavidZ ok, but sounds like lot of work for moderator, ie collect/ prep/ organize questions. think its doable. more pressure on the moderator though. sort of like megyn kelly lol

Secret

@Slereah One that came to mind is you can use CTCs to bypass no cloning theorem, if I recall correctly

DanielSank

@vzn Dude, that's why we had the meta post with all the questions.

A moderator could simply go through them at a reasonable pace.

hubot

I have recently very hard exercises.

0celo7

No woman who looks like Megyn Kelly is doing physics

vzn

@DanielSank think meta post was good idea & worked out but think its up to slereah to use them at his discretion and he didnt really go that direction.

0celo7

5:24 PM

And certainly not using PSE

Secret

also a CTC computer can be exploited to solve certain NP problems in polynomai time

Slereah

Ah yes

There was an old paper that tried to estimate the maximal computing power of the universe

Allowing for closed timelike curves

It was pretty big, but not as big as you'd think

One of the big problem being inflation

David Z

@vzn Yeah. I'm not sure why you brought up Megyn Kelly in particular, but that's kind of the idea. It's not really that much work for the moderator, anyway, not if people are actually asking questions. Actually in my experience, there are usually too many questions.

Slereah

At some point, there are qubits you cannot reach

ACuriousMind

@DanielSank I think that will vary with the speaker. Slereah was kinda advertised as the CTC guy

Slereah

5:26 PM

Well I could talk about the Bronze Age collapse, if you want

Not a very physics topic tho

Did you know that basically all civilization collapsed at the end of the bronze age

0celo7

When was that

Slereah

At least around the mediterranean

3075

@Slereah the word collapse gives me shivers.

0celo7

implying there was civilization anywhere else

David Z

Anyway I'm off for dinner

Slereah

5:27 PM

Around 1200 BCE

0celo7

@DavidZ toodles

David Z

BTW @Slereah you said you're near Paris? I'll be in your area next week. Small world ;-)

Slereah

Basically Greece went back to farming and popping their small pox boils for nourishment

Egypt stopped trying to expand

vzn

@DavidZ megyn kelly was under a lot of fire for her mod (or "interview") style, and that happens with high profile mods. she was accused of softball questions in her recent interview etc. its a different area, but pt is that sometimes a lot of focus, even pressure can be on whether the mod asked good questions, their style, etc., aka "walking a tightrope" o_O

Slereah

The Hittite coast was in flame

0celo7

5:28 PM

@ACuriousMind I'm meeting with my prof today to discuss Milnor. I wonder if he'll know how to solve these exercises...

Slereah

It was a pretty traumatic period

Not quite sure what other topics I could do if I had to do another AMA

Wormholes, I guess

Maybe physics history

It's pretty interesting, too

David Z

@vzn oh. Well then, not like that.

Slereah

Beyond that I mostly like boring physics stuff

vzn

@Slereah had an idea, re DS point, figured the meta questions might not work out, but think if you feel like answering more questions you could update your meta post with answers to any questions you feel like, loose ends etc

0celo7

@ACuriousMind Facebook wants me to be friends with FLP...Facebook is creepy

Slereah

5:30 PM

Sure

(The fall of Troy happened during the Bronze Age Collapse, btw)

The Iliad gets a bit weirder when you realize that maybe the greeks were the harbringer of doom

Also interesting to note : Writing is basically never mentionned in the Iliad apparently

John Rennie

Wasn't it the Phoenicians? The "Sea People"?

Slereah

(Greece lost the art of writing during the collapse)

3075

@0celo7 because you searched his profile.

vzn

@DavidZ so what do you say about next session scheduling? 4wk from now ok? ask later? we already have a guest lined up, yuggib said hes ready to go, just needs your ok on the timing

Slereah

The Sea People is a generic term for barbarian invaders from the sea

Which included a fair bit of people of various places

^some sea people prisoners

As you can see it's a rainbow of diversity

John Rennie

5:37 PM

The Phoenicians suffered less that the other big civilisations. Their rise was pretty steady from 1500BC onwards. Hasn't that been used as an argument that they, i.e. Phoenician pirates, were the cause?

Slereah

The bronze age collapse's cause is pretty vague

It is probably several causes

Possibly linked

Usually it is assumed to be maybe something like

Ecological collapse -> famines -> invasions

It is known that there was a lot of famine going around during those days

And a lot of invasions

The sea people invaded a bunch of places, Troy is sacked, various places in Greece are invaded

It is also possibly the timeframe for the Exodus, whatever the real historical event was

(Probably pretty far removed from it)

btw, according to greek legends, it is a phonician that did give writing back to Greece!

Cadmus

In Greek mythology, Cadmus /ˈkædməs/; Greek: Κάδμος Kadmos), was the founder and first king of Thebes. Cadmus was the first Greek hero and, alongside Perseus and Bellerophon, the greatest hero and slayer of monsters before the days of Heracles. Initially a Phoenician prince, son of king Agenor and queen Telephassa of Tyre and the brother of Phoenix, Cilix and Europa, he was originally sent by his royal parents to seek out and escort his sister Europa back to Tyre after she was abducted from the shores of Phoenicia by Zeus. Cadmus founded the Greek city of Thebes, the acropolis of which was originally...

That guy

John Rennie

5:56 PM

Incidentally, the stuff about the fluid dynamics analogy - the idea falls down because in FD streamlines can't cross but in GR geodesics can cross. So it was just random babbling and should be ignored :-)

Slereah

Although

If you want a fluid related bit about CTCs

For some acausal spacetimes, the causality violation stems from

THE FOUNTAIN

It's a closed null geodesic from which the Cauchy horizon stems

thusly

Fountains are bad because that is where the divergence of the vacuum energy comes from

John Rennie

hmm, a Google leads me to this paper by Visser that mentions fountains.

One to settle down and read some time I think

Slereah

Basically the problem is that closed geodesics are bad

Because the vacuum will, let's say, "follow" the geodesic

So you get the usual infinite blueshifting of a particle

John Rennie

Why did you say that Hawking's proof that a CTC requires either an infinite structure or exotic matter isn't as solid as it seems?

Slereah

Oh because QFT predicts exotic matter

John Rennie

6:04 PM

The topic was mentioned but I didn't follow the argument.

Slereah

Matter violating energy conditions, anyway

John Rennie

It does?

Slereah

Although probably not enough for it

Yes

There's a few

Casimir effect is probably the most famous

John Rennie

Is that really exotic matter though?

Slereah

Well you don't need exotic matter

ritwik sinha

6:05 PM

Lets see if i can understand anything at all.

Slereah

you just need violation of energy conditions

Of course almost every such violation is extremely small

So unless you want a time machine for a gnat it's probably not relevant

(Or Planck scale, really)

vzn

@JohnRennie dont throw fluid analogies away! :(

@JohnRennie not sure what you mean "geodesics can cross"...

John Rennie

@vzn a spacetime point can lie on more than one geodesic. Actually there are an infinite number of geodesics passing through any point.

Slereah

Welll some geodesics can cross yeah

I think that's why the exponential map can't cover every manifolds

vzn

@JohnRennie (reading wikipedia...) think a geodesic is quite similar to concept of speed of (wave) travel/ propagation through a medium of varying density.

John Rennie

6:11 PM

@Slereah I was thinking that since a geodesic cannot split or join, a CTC would somehow repel other geodesics. But obviously that's not the case.

vzn

also theres a already a known concept of geodesic flow

Slereah

An example of crossing geodesics, btw

Secret

the left diagram basically demonstrate the geometry of a "conjoined CTC" I mentioned a long while ago (which 0celo7 and slereah mistaken it as a butt due to my poor ppt illustrations)

ACuriousMind

@Slereah What exactly is the condition that the Casimir effect violates?

Slereah

Errr... most of them I think?

WEC, NEC, ANEC, quantum inequalities

It's a static negative energy so it violates a lot

It doesn't violate some of the weird energy conditions Visser came up with

Secret

6:27 PM

Is it feasible to simply erect an array of casmir plates in order to concentrate enough negative pressure at the mouth of a wormhole to stablise it?

Or is the negative pressure from the casmir effect not modular thus cannot be amplified simply by adding more modules?

Slereah

No

Casimir effect is very poor for that

It has two giant sources of energy next to it (the plate themselves)

So anything sustained by it would be microscopic

ACuriousMind

@Slereah Well, most of these conditions seem hung up on the sign of the energy - but the QFT in which one calculates this "negative" energy has this negative sign relative to a vacuum energy renormalized to zero, which is something one cannot really do in theories with gravity, I think.

Slereah

Well yeah that's one of the difficulty

For the most part it is assumed that the vacuum energy is either 0 or, following the cosmological constant, very close to it

ACuriousMind

So I'm wondering whether the apparent violation here comes from using a non-gravity result unchanged in a theory with gravity.

Slereah

But it's hard to check experimentally

Although!

I will say one thing, at least

The expansion of the universe depends on an energy condition violation

So there's that

Secret

6:32 PM

Slereah

Accelerated expansion, at least

Secret

so you mean because of the two large sources of positive energy, having a configuration like that is not going to supply enough negative energy in the region to stablise the wormhole?

Slereah

Well Visser did the calculations for that

But he ignored the energy of the plates

It might work, but your wormhole would be extremely tiny

For a start because the distance at which casimir plates have a negative energy density is tiny already

Also that energy itself is tiny, as well

So probably something like Planck scale or not far

Secret

hmm... make sense

ACuriousMind

Do you know what kind of contribution the gravitons give to the Casimir effect?

I'd expect attractive, but...one never knows.

Slereah

6:36 PM

Dunno

Not sure it's been calculated

Also I don't think a metal plate is a good conductor of gravity

ACuriousMind

Oh, right

Slereah

The Point of Casimir is that we can assume that the fields are 0 at the plates

Which is also false btw, since there's no perfect conductor

ACuriousMind

@Slereah I think there are calculations with "realistic" metal plates

Slereah

there are, yeah

Much more interesting, though

There's calculations with domain walls

That way it's purely QFT

No approximation

ACuriousMind

I have no clue what a domain wall is, actually

Secret

6:39 PM

@ACuriousMind is the notion of resistivity for gravity defined, as gravitation is determined by the geometry of spacetime, thus I cannot imagine any scenario that e.g. gravitational waves can be slowed down?

Slereah

Also there's a little AQFT theorem that states that the expectation value of any operator is always bounded for a "reasonable" quantum state

So there should be a lower bound for the energy of any quantum state, at least

ACuriousMind

@Secret There's a superconductor effect, called the Heisenberg-Coulomb effect, that predicts certain arrangements of superconductors are able to expel gravitational fields just like they do with magnetic fields.

Slereah

A domain wall is the interface between two different vacua

Domain walls are not concerned by that theorem btw because I think that they are those unreasonable quantum states

0celo7

@ACuriousMind WHAT

Link, proof???

Slereah

arxiv.org/abs/0812.4760

I think that's the one

ACuriousMind

6:47 PM

@0celo7 arxiv.org/abs/0903.0661

0celo7

damn physics paper

where are the theorems

Slereah

^the reasonableness condition

That sounds reasonable, right?

Trying to remember what it means

0celo7

A net?

A topological net?

Slereah

Net of algebra

0celo7

good lord

Slereah

6:49 PM

I... I think it's a functor?

The functor mapping open sets of manifolds to the C* algebras

0celo7

@ACuriousMind What's something that's not a functor

Slereah

love

0celo7

it's a functor of deez nuts

Slereah

From what I remember it's basically that the quantum state is like

Of finite extent, in some capacity?

I forget

Or rapidly decreasing or something

ACuriousMind

Sigh...why do all these people asking bad questions think that I am critical of specifically their questions...

Slereah

6:53 PM

because their questions are terrible

0celo7

I wonder what the prerequistes for Bott and Tu are

I need to stop buying books

That third name is made up

ACuriousMind

All names are made up.

0celo7

Wrong.

Slereah

What is that SR book with a made up name

0celo7

functors on page 19

great

Slereah

7:00 PM

Grogmo the destroyer or something

0celo7

@Slereah Gorgoulhon

Slereah

Gorillaman

Yeah, that guy

0celo7

@Slereah did you say your HE says $5 on the back or something

Slereah

Lemme see

£3.95

$10.95

0celo7

Pic

Mine says nothing

Slereah

7:07 PM

0celo7

Slereah

>website

Pretty recent copy

0celo7

Yeah it's a year old

I am 18 you know

Slereah

I think mine is the original paperback

0celo7

I think yours has better print quality

The library has an original hardback

It looks gorgeous

Slereah

7:10 PM

yeah but it's even more expensive

0celo7

Springer >:( you owe me a book today

Slereah

what didja get

0celo7

Lee's topological manifolds book

So I can have a reference for topological shit

That's not Bredon

Bredon is too functorial

But Bredon has good stuff too

Slereah

Isn't there a Spivak topology book

There's a Spivak book for all math

0celo7

Dunno

0celo7

7:27 PM

@ACuriousMind What is $\pi_p(S^2)$?

ACuriousMind

@0celo7 Damn hard to compute

0celo7

@ACuriousMind Apparently, yeah

Why?

ACuriousMind

Because higher homotopy groups are not particularly well-behaved (e.g. they're not a nice functor in any way, and there's no variant of e.g. Seifert-van Kampen for them)

0celo7

God I need to learn algebraic topology

ACuriousMind

Well...there is actually some way to compute these things, but it's so horrible it's infeasible in practice

0celo7

7:32 PM