The h Bar

Secret

12:01

http://physics.stackexchange.com/questions/391/is-anti-matter-matter-going-backwards-in-time

QUOTE
> If you apply the time reversal operator (which is a purely mathematical concept, not something that actually reverses time)

mind=blown

yuggib

12:15

that is what it would happen with a time reversal concrete machine

'All time machine systems nominal ... T-minus ten ... eleven ...'

not very useful

Secret

well, from the comic stripe, it seems time reversal symmetry is obeyed, and the user ends up doing nothing

yuggib

how would you have a different outcome?

Secret

I am however, tempted to see what would happen if the user flip the switch when the time machine is in relative motion...

yuggib

of course a machine like that reverses the proper time :-P

Secret

I am speculating, but if
1. (Lack good terms to describe it), if we can somehow invoke a twin paradox like effect so that the time elapsed as the user move forward in time was slower relatively to the time machine moving back time, then something interesting will happen
2. Place the whole thing under a magnetic field, since magnetic fields break time reversal symmetry (Ok I don't really understand the specifics of the time reversal operator on magnetic fields enough to comment further on proposal 2...)

Slereah

12:31

Magnetic fields don't break time reversal symmetry

Secret

then I must be confused with something else, but I remember somethign that ferromagentism break time reversal

Slereah

EM in general preserves time symmetry

Not that it means anything for time travel

yuggib

I still think that time travel by time-reversal is not viable (problem above ^). Of course time travel by other means (like the CTC that @Slereah like so much) is not a priori discarded

Secret

Then proposal 2 is nonsense then
*Back to non time travel stuff*
Well... I guess I am not even familiar with time reversal operator enough to understand time reversal symmetry in its full glory

David Z's answer that the operator actually has nothing to do with reversing time just makes it more mysterious to me without some in detail investigation

yuggib

and to avoid causality problems, you could think of CTC that leads you in a past space-time point only if it is causally disconnected from your existing worldline...

you could see the past, but only of very distant worlds

(more and more distant the more and more back in time you get)

Slereah

12:39

Time reversal symmetry is more about processes

Time reversal symmetry tells you that the process $e^+ + e^- \rightarrow \mu^+ + \mu^-$ is the same process as $\mu^+ + \mu^- \rightarrow e^+ + e^-$

But reversed in time

Secret

Is all process must have time as a parameter?
What does a process without time look like?
(I am not sure if I am making sense...)

Slereah

not too much

Secret

I am not sure if you can understand what I am trying to ask
For example electric potential V from a static point charge has dependence on position x, but we won't call something like
$$V(x_1)\rightarrow V(x_2)$$
as a process since it does not depend on time

Slereah

Well it is static

All of classical physics is invariant under time reversal, basically

It is one of the big thing about time

Secret

(In naive laymen thinking) I think this might be possible if the time elapsed in moving forward in time does not get offset completely by moving backward in time

For example, if we have A activating the time reversal machine in a moving train, and meanwhile B is nearby. Then B should theoretically 'see' A switching off the machine again in slow motion. What would happen if B then decided to hop into the train and push A away before the switching off thing can be completed

(This scenario has a loophole because I am not actually sure what B saw if A reverses time (effectively travelling bac…

Secret

12:50

I learnt form the answer that ferromagnetism is a quantum thing and it breaks time reversal symmetry

Condesned matter physicists also frequently work with materials that breaks this symmetry
It's a bit confusing to me at the moment because time is not physically being reversed in these systems, but rather the time reversal operator acting on the (forgot) and give a different answer

(separating these posts because they are not related)

more reading is needed

Slereah

QED is also time symmetric

Secret

then why people say ferromagnets break time reversal?

Slereah

Do they

Secret

http://physics.stackexchange.com/questions/176141/meaning-of-time-reversal-symmetry/176166#176166

For example

and there are many others

Secret

13:10

RpfnoRTTV 26

The Lagrangian should have the final verdict

RpfnoRTTV 26b

RpfnoRTTV 26c
It seems naively, causality is preserved (red arrow)

Balarka Sen

13:41

out of curiosity : isn't relativity a good reason one should not view time as an independent dimension? I mean, instead of viewing time as a 4th dimension, and spacetime being foliated by codimension 1 submanifolds parameterized by the time dimension, one should view the spacetime as some sort of line bundle on the space, right?

coming from a person who knows 0 physics.

Slereah

Not all spacetimes can be foliated though

Although there is a description of GR that is just the evolution of a 3-manifold, yes

It's called the ADM formalism

It's the GR version of the Hamiltonian formalism

Balarka Sen

ok, I see.

Slereah

As long as your spacetime is reasonable you can use it

0celo7

14:05

ACM's point of "how do you know if spacetime is reasonable" has not been answered yet

Secret

RpfnoRTTV 26d

Worldlines and light cones made for each light second passed that theoretically will be seen by O when it was 8 light seconds ago to 3 lightseconds ago
The result is weird and require computing an EOM to make sense of
Causality seemed to naively make sense

0celo7

@ACuriousMind how do I get over an epistemological crisis

I'm not even sure what it means

ACuriousMind

14:22

Not sure you "get over" the realization that knowledge and theory isn't all you thought it should be...more like: You get used to it.

skill patrol

perhaps a paradigm shift is in order

ACuriousMind

@yuggib Oh god...how did they get that crap published

0celo7

@ACuriousMind It's probably correct.

ACuriousMind

@0celo7 Look at it. They use some formula involving gamma functions, use that they already know $\Gamma(0.5) = \sqrt{\pi}$, and are then surprised that one can solve the resulting formula for $\pi$?!

0celo7

@ACuriousMind You reading the paper?

I'm reading.

lol

ACuriousMind

14:30

"Journal of Mathematical Physics, since 1960, publishes some of the best papers from outstanding mathematicians and physicists. It was the first journal in the field of mathematical physics and publishes research that connects the application of mathematics to problems in physics, as well as illustrates the development of mathematical methods suitable for such applications and for the formulation of physical theories."

My respect for them just dropped a few notches.

0celo7

J. Math. Phys. is forever ruined

Slereah

Hm

Can a stationary cylindrical spacetime have CTCs not everywhere

0celo7

-2

Q: What is the physical significance of the area $e>mc^2$ in the graph $e=mc^2$?

What is the PHYSICAL significance of the area above the straight line graph $e=mc^2$ ( because $c$ is constant ) , of the inequality $e > mc^2$ , where $e,m,c$ have the same meaning of Einstein's equation $e=mc^2$. (This area $e>mc^2$ is considerable because $tan^{-1} c^2$ is not infinity and...

lol what

Slereah

The condition is $g_{tt}g_{\phi\phi} > g_{t\phi}$

For CTCs to develop

0celo7

@Slereah where are you getting this stuff?

Slereah

14:32

That's from Tipler's paper

0celo7

I still haven't opened that huge file you sent me

Slereah

CTCs developing is $g_{\phi\phi} < 0$

0celo7

can't unpack it

you have some crazy compression on it

Slereah

But $g_{tt} g_{\phi\phi}$ has to be always negative

I think the CTCs have to be everywhere

That is, there's no distance for which you won't get them as long as you turn around the axis

The problem really is that nice time machines are basically never very symmetric

I don't think any of them are spherically symmetric and stationary (and topologically trivial)

0celo7

what is Tipler's paper

@ChrisWhite What are the standard references for 3+1/numerical GR?

Slereah

14:37

F. Tipler - Rotating cylinders and the possibility of global causality violation

0celo7

I procured a legal copy of Weinberg

Now it is gone

Freaking NSA

@Slereah linku?

yuggib

@ACuriousMind Shit...I published a couple of paper in that journal

Slereah

use the google you numbskull

yuggib

once (in the 60s-70s) was leader in the field

0celo7

@yuggib link?

@Slereah how

yuggib

14:39

and nowadays is still reputed to be a rather good journal in math.phys.

Slereah

open the google machine

0celo7

@Slereah D:

yuggib

not the best, but a reasonably good one

Slereah

Input the string "Rotating cylinders and the possibility of global causality violation" into the search bar

0celo7

where

Slereah

14:39

Press the button to google

If you do not know where google is, perhaps you should google it

yuggib

@0celo7 do you want to read my papers??

they're not interesting...you know, maths :P

Slereah

M.A.T.H.S. you say

Mathematical Anti-Telharsic Harfatum Septomin

0celo7

@yuggib interesting?

maybe, maybe not

I don't know anything beyond some algebra

so we'll see how much I get

Slereah

If you know that you know nothing

Then you know everything

yuggib

I'll post the link to the "lighter" of the two

0celo7

14:41

wtf weinberg is on my computer but it refuses to tell me the file path

there we go!

Weinberg

you scoundrel

ACuriousMind

@yuggib Well, what is a bit strange is that that pi paper is freely accessible while most of the others are behind a paywall...perhaps they published it as some sort of joke or "recreational" paper? :P

yuggib

scitation.aip.org/content/aip/journal/jmp/55/10/10.1063/…

@ACuriousMind Maybe it is an open access publication

0celo7

Marco

yuggib

i.e. they paid

0celo7

Sounds Yugoslavian to me

yuggib

14:43

with a "k" maybe

marko

with a c, not so much

however the referral process should be more or less the same

ACuriousMind

hmmm "Received 13 August 2015 Accepted 14 August 2015" for the pi thingy

That does not look like a review process to me :P

0celo7

@ACuriousMind hehe

yuggib

o.O ok, then indeed it is because of some open access option

or something strange

ACuriousMind

I can't find any indication that it is different from the "proper" papers.

0celo7

@yuggib why do you say this

this kinda PDE stuff is what I'm interested in

yuggib

14:46

with my papers there was an "usual" refereeing

ACuriousMind

Yeah, all the ones behind a paywall took their sweet time

But there's no disclaimer there that it's not peer reviewed or something

0celo7

why is your EM paper classified as "algebras"

yuggib

where??

0celo7

yuggib

no idea

0celo7

14:48

> Your access is provided by:
Univ of Tennessee

Well that's nice

yuggib

@ACuriousMind I think that in J.Math.Phys. all the articles are available for free in the first month however

is this very new?

ACuriousMind

Yes: Published online 10 November 2015

yuggib

ok

so maybe it is not a problem of open access

0celo7

why can't you peer review in one day

this is a pretty simple paper

tho revolutionary

yuggib

these are the things that could ruin a journal's rep in my opinion

0celo7

14:50

is this a proof of the quantum theory?

ACuriousMind

@yuggib Yeah, it's completely unclear to me what happened here

yuggib

@0celo7 actually it is a mixed ODE/PDE system...and for that motivation it has not been studied so much in the literature

as a friend of mine said "I would have expected your result to have already been done in the 70s"

Slereah

I like how in Fallout 4, when you shoot people in the face, they do a little "ow my face" kind of animation

Even when it's giant crabs

It is quite expressive for a giant crab

It is like "Jeez dude why did you do that"

ACuriousMind

@Slereah Well, why did you do it!

yuggib

I read the $\pi$ paper

and now I am sad

like the crab that was shot in the face

Secret

15:00

RpfnoRTTV 26e

yuggib

@ACuriousMind a quick $C^*$-algebraic question for you:
let $\omega$ be a state for the Weyl algebra; and $(H_\omega,\pi_\omega,\Omega_\omega)$ the associated GNS representation. Suppose now that $\omega$ is normal with respect to the Fock representation

i.e. there is a $\rho$ trace class in the Fock repr such that $\omega(W(f))=\mathrm{tr}[\rho \pi_{Fock}(W(f))]$ for any $f$

this does not yield that $(H_\omega,\pi_\omega,\Omega_\omega)$ and $(Fock, \pi_{Fock},\Omega_{Fock})$ are unitarily equivalent, am I right?

0celo7

@yuggib Why?

@yuggib why wouldn't you \mathrm words

@ACuriousMind mirelurks man

yuggib

@0celo7 too long to write

0celo7

they're just annoying

yuggib

they're just slanted

Slereah

15:09

Especially baby mirelurks

0celo7

@yuggib more like evil

Slereah

they are hard to hit the buggers

0celo7

@Slereah I use the punching thing from Swan

I wish it had better upgrades

Slereah

I usually use the knife if I can be bothered

0celo7

like a bundle of razors

a knife?

Slereah

15:09

Tho really the worst are the super mutant suicide bombers

0celo7

like a switchblade?

Slereah

They're babies, who cares

I could stamp them with my boot

0celo7

sadly that's not in the game

Slereah

Suicide bombers are basically like you have to deal with them immediatly

Or you'll have to hire a cab to find your ass

Secret

ACuriousMind

15:11

@yuggib Indeed, I don't see what the isomorphism would be.

Secret

RpfnoRTTV 26f
A lamp looks like it is outputting a continuous stream of photons despite all photons move along null curves is because... (see above) strictly speaking (if we can label them), they are not the same photon

ACuriousMind

@yuggib I would intuitively say that that could mean that $H_\mathrm{Fock}$ is a traced-out/reduced version of $H_\omega$.

I.e. $H_\omega$ is the space in which $\rho$ is purified.

Slereah

I still don't know how to do Klein Gordon in AQFT :(

yuggib

@ACuriousMind I see your point...

however it would be very disappointing if that thing would be true

but I really don't think so

ACuriousMind

@yuggib What would be disappointing? That two spaces in which a state that is pure in the one and (probably) mixed in the other are not unitarily equivalent?

It's definitely false in the finite-dimensional case, but it may be that the infinte dimensions and/or some special property of the Weyl algebra might make it true here

yuggib

15:17

no no, that they would be unitarily equivalent

ACuriousMind

@yuggib Ah, yes, that would be boring.

Secret

ACuriousMind

1

Q: Why does the vacuum permeability have the value of pi in it?

The vacuum permeability, or the capability of the vacuum to permit magnetic field lines, contains the value of $\pi$. Why? What does this have to do with the ratio of a circle's circumference to its diameter?

electromagnetism

Can we stop with the $\pi$ already?

yuggib

luckily for the moment we don't have the tag $\pi$

Secret

RpfnoRTTV 26g
Even if the previous ones does not make sense unless it can be proved, at least I can now see how length contraction and time dilation arises graphically

yuggib

15:18

I could always create one :-P

0celo7

@yuggib how does one make a new tag?

ACuriousMind

@yuggib That's evil.

0celo7

@ACuriousMind why

@Slereah I still can't solve quadratic equations

get in line

I'm the worst here

ACuriousMind

@0celo7 To me, it would mean that the mixed states in the Fock space aren't actually mixed, meaning there's no real difference between statistical and quantum effects for it

@0celo7 You just type it into the tag box

0celo7

@ACuriousMind i have no clue what any of that means

@ACuriousMind making a pi tag

ACuriousMind

15:22

@0celo7 No, don't do that

It's annoying to get rid of tags

Qmechanic will be angry with you

0celo7

@ACuriousMind Well, there are a lot of pi questions.

@ACuriousMind Robots don't get angry.

How does one get rid of a tag

ACuriousMind

You untag everything tagged with it and hope no one uses it for the next 24 hours, then it's automatically deleted.

0celo7

@ACuriousMind well

yuggib

well, today you could not have get rid of the pi tag :-D

0celo7

either people use it

ACuriousMind

15:25

In cases where it's necessary to get rid of it but untagging doesn't work because people always create it again you need to call The Burninator(TM)

0celo7

then it's useful

or people don't and it goes away

now how do I put this

I don't see the issue.

Ich sehe kein Problem.

^that's how you know I'm serious from now on

ACuriousMind

@0celo7 Look at it like this: It's not a useful tag for physics. There's no discipline of physics that is especially concerned with "pi", it would be exclusively used by people who are for some reason unrelated to physics obsessed with it.

0celo7

@ACuriousMind the who?

ACuriousMind

@0celo7 Yeeeeeah, right

Secret

http://physics.stackexchange.com/questions/176141/meaning-of-time-reversal-symmetry/176166#176166

@ACuriousMind What are the physical consequence of broken time reversal symmetry besides ferromagnetism?

David Z said in another answer that time reversal is a mathematical thing and is not really about reversing time

Slereah mentioned how time reversal concern about processes in that $A\rightarrow B$ is the same as $B\rightarrow A$

But while mathematically one can easily do the algebra to see how some physical quantities differ when t is replaced by -t, what are the examples of the effects…

0celo7

15:27

@ACuriousMind Well, this paper might change that...

ACuriousMind

53

Q: What does it mean to "burninate" a tag?

There are requests to "burninate" tags all over the place! What does this really mean; what happens to a burninated tag? How is it different than removing a tag or black-listing a tag? Would it be useful to include this definition in the Stack Overflow Glossary?

support meta

Secret

@ACuriousMind Basically, my question is e.g. if we could reverse time in a naive sense (i.e. just considering physical processes running backwards), how will ferromagnetism behave such that it indicate time reversal symmetry is broken?
(PS Just beofre someone decided to tie me, up, the motive of this question has nothing to do with time travel)

Slereah

What's the current memory of quantum computers

On how many qubits can we operate

0celo7

Better than mine.

It's so boring in here without JD

Slereah

"DD-Wave Systems Inc., the world's first quantum computing company, today announced that it has broken the 1000 qubit barrier"

Oh man

Almost a kb

Or... 125 qubyte

ACuriousMind

15:32

@Secret I don't know of any interesting effects except ferromagnetism. Why ferromagnetism breaks time reversal is easily seen: It means all the spins are aligned. But spins switch their direction under time reversal, so a time-reversal symmetric object cannot have aligned spins.

Slereah

Almost enough memory to run a quantum pong

IIRC there's a 126 byte version of Pong

yuggib

@0celo7 Let's do a paper to prove how by a conformal transformation you can link every number to the same physics of $\pi$'s paper...a project like the polymath of T.Tao but to obtain awful physical "results"

Slereah

pouet.net/prod.php?which=24841

Nope

256 bytes

We're not quite at Pong levels

Secret

@Acuriousmind I see
...
Hmm, I should continue on doing that exercise on Dirac equation, to get some idea on how relativistic quantum mechanics will derive spin

ACuriousMind

I can't play Quantum Pong? :(

yuggib

15:34

and where would you operate the OS?

Slereah

Not yet!

I don't think there's any game for 126 bytes

ACuriousMind

Will the pong ball tunnel through the player occasionally? ;)

yuggib

yes

Slereah

pouet.net/prodlist.php?type[]=128b&page=1

yuggib

but in real life

ACuriousMind

15:35

Nice.

Slereah

Here's what you can do with that much memory

yuggib

you need also the memory for the operative system

I don't think these are binary code games

Slereah

They are

All in assembly

yuggib

ah ok

Slereah

Although, to be fair

That's x86 assembly

I don't think quantum computers have that complex instructions

yuggib

15:37

I have no clue

0celo7

@yuggib what

pls derive $\mathrm{e}$ from quantum theory

yuggib

@0celo7 it's up to the sillyphys group to do that

0celo7

What

yuggib

the group formed to solve silly physical problems and publish papers accepted in one day

0celo7

Like what

yuggib

15:39

the aforementioned one

on the quantum mechanical derivation of $\pi$

0celo7

Oh that's really a troll paper?

yuggib

I may semiclassical derivation of the product formula for $\pi$

ACuriousMind

@yuggib That should be Silph Co. :)

yuggib

@ACuriousMind Nice :)

@0celo7 Well, let's say that it makes me very sad to see that it was accepted

0celo7

@ACuriousMind uh you never played those games

yuggib

15:41

in one day

ACuriousMind

@0celo7 lolwat

Of course I did.

0celo7

You never played them. We've been over this.

ACuriousMind

Just not the newer ones you kiddies are so hyped about

0celo7

You never played any Nintendo

Literally not a kiddie

ACuriousMind

@0celo7 I am completely certain I spent ridiculous amounts of time playing Pokemon Blue and Yellow.

And slighly less ridiculous amounts playing Gold, Silver, Crystal, then I stopped.

0celo7

15:43

Bah.

You've repeatedly said you never played Nintendo.

ACuriousMind

@0celo7 Find one instance of that

0celo7

[private correspondence]

it appears you've deleted all evidence

you're a clever one...

Slereah

15:56

Wait

No

You can totally have integral curves changing from CTCs to not CTCs

0celo7

What

Slereah

Since the $M^2$ bit can compensate it

0celo7

What does that even mean

Slereah

The thing about the Tipler metric

0celo7

A CTC is always a CTC

Slereah

15:58

Integral curves of $\varphi$ going from closed timelike curves to closed spacelike curves

Tho that means that $g_{\phi\phi}$ has to go through 0

Not quite sure what that means

Other than the curve becoming a CCC

0celo7

That doesn't make any sense

Slereah

Why

Although

Hm

Wait

If L goes from negative to 0 to positive

Then F has to as well, otherwise the determinant becomes positive

Still fine, though, I suppose

It's not a horizon or anything

0celo7

Are you saying there's a vector field which sometimes has CTCs and sometimes not

Depending on the initial data?

Slereah

Well yeah

It's not too uncommon?

That's how you build compact time machines

And the boundary between the two has closed null curves

0celo7

I thought you meant there was a curve that was a CTC at some points and not a CTC at others

ACuriousMind

16:29

2

Q: Are some gravitational wavelengths forbidden by causality?

Consider a gravitational wave in linearized gravity $d_{\mu \nu}(X_{\eta}) = D_{\mu \nu} e^{i X_{\eta} K^{\eta}}$ with $K^{\eta} = (-\omega t, \textbf{k})$. Let $d=| \textbf{D}|$ the scalar maximum amplitude of the wave, measured in distance units. A gravitational wave makes in a single oscillat...

general-relativity causality gravitational-waves

Someone understands what this question is about?

Three upvotes, but I don't get the argument at all. What has the amplitude of a gravitational wave to do with a velocity?

Just because you can measure it in distance units, that doesn't mean it has something to do with actual distance!

0celo7

16:41

@ACuriousMind FWIW I have no clue either.

But then again I don't know anything about GR

ACuriousMind

Don't be so dramatic :P

0celo7

@ACuriousMind Why not?

I don't understand the very basic structure of the theory

Also using η as an index is a Schande.

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