The h Bar

@0celo7 Depends. Undergrad students of physics can have that if the do the usual things (career fairs, focusing their studies toward getting an industry job, networking...).

Slereah

4:15 PM

But I'm not from a fancy engineering school

dmckee

For grad students it is also possible, but the game is different. Especially if you are looking for a academic job.

Heck, my department has three students whose internships last summer secured them a job offer for their graduation at the end of this year. All majoring in physics.

Slereah

Lies

Nobody in physics finds a job!

Did I ever tell you the title of my proposed thesis btw

"Application of the Atiyah Singer theorem and supersymmetric quantum mechanics in Kahler manifolds"

dmckee

Forces present when cutting of a robot's arm, a computation study?

Slereah

For some reason it was not funded

dmckee

See, you should have gone with the robots.

ACuriousMind

4:18 PM

Didn't he just give up a job in robotics?

Slereah

Robots are boring

Worked two years with the damn things

Mostly it was "connect remotely to the robot, upload file, reboot robot"

0celo7

robots are incredibly boring

dmckee

@Slereah Every experimental discipline has their version of that, and of course it is tedious. The joy, if any, comes in crunching the number that you collect between reboots.

skull petrol

Come on guys, let's face it: sometimes you have to do "incredibly boring" things in life to pay the bills.

Slereah

Oh boy, an interesting bibliography element

"To be published"

>:|

0celo7

4:26 PM

@Slereah Do you even know the AS theorem?

Slereah

No

I was supposed to read up on it for it

BUT I GUESS WE WILL NEVER KNOW

skull petrol

:D

0celo7

@Slereah An exercise in Wald references an unpublished thesis

and the author is dead, I think

Slereah

shakes fist

skull petrol

fight! fight!

Slereah

4:27 PM

1. Private communication heard in the bathroom at a conference

0celo7

hehe

Danu

Maldacena is coming to give a talk at my university next week.

You jelly, bro's?

0celo7

@dmckee How good are positrons at penetrating things

and messing up humans

@Danu Who?

skull petrol

@Danu you going?

Danu

@skullpetrol No, not much to learn there...

skull petrol

4:31 PM

Not jelly, bro :P

Danu

(I was obviously kidding)

Of course I'm going to Maldacena's lecture.

Slereah

Do all non-time orientable spacetimes have CTCs

I suspect they do but I never see it discussed

0celo7

Woman Dancing To Samba Ey Macalena

►

@Danu This?

Danu

You jelly, methinks

0celo7

@Danu Dude, if I wanted to do String Theory with my life, I would have gone to a different school.

So while I think the talk would be interesting, no, I'm not jealous.

Slereah

4:37 PM

music school

ACuriousMind

@Danu The correct term for someone who wants what you have is envious, not jealous.

Also, what would I be envious if there's "not much to learn"?

0celo7

In any case, shouldn't we all be jealous of @ChrisWhite then? He could feasibly just walk up to the dude and chat with him.

Slereah

@ACuriousMind : He did not say jealous

He said jelly

It is definately what jelly means

obe

no blue jelly?

0celo7

Wonder how many people in academia alive today actually knew Albert.

Slereah

4:39 PM

Well he died in the 50's

So they'd have to be at least 60

Maybe Weinberg?

And more probably 80

Steven is 82.

He could have I suppose

obe

you call them by their first names like you're best friends.

Slereah

4:40 PM

But then Einstein would need to have some interest in some new student :p

FRESH MEAT

0celo7

Albert was a punk in that regard.

obe

imagine if duffield met einstein right now after showing him his chat logs.

lol what would he do

Slereah

He would show him he is wrong with his own book

skull petrol

I asked him that^^

0celo7

Uh, be praised for spreading the word of Albert and the Evidence?

Secret

4:42 PM

need to train more to check my intuition with the maths

But one thing I am kinda curious on is that

Say you have two wormhole mouths that

Slereah

using diagrams is a bad idea

0celo7

@Slereah What are some non-time orientable spacetimes?

Slereah

The method to generate CTCs is complicated

Hm

Not sure

0celo7

@Slereah Look at the non time-orientable spacetime in Wald's 8th chapter. Is there a CTC there?

Slereah

Usually people go through them pretty quickly

Rarely examples

Secret

4:47 PM

differ in time (I forgot the correct term to describe that). If the difference is large enough such that you can plot multiple worldines that can form CTC in between, how many times of that observer's worldine will cross a given region since there's a continuum of many possible ways to form CTC within that region

How do we know the number of time the worldline crossing that event is not going to diverge?

Slereah

Hm

Wald, HE and Visser don't give examples

0celo7

Wald does

Slereah

Well yes

All books have that diagram

But it has no metric given

0celo7

who gives a crap

Slereah

me

0celo7

4:49 PM

is there a CTC

lol

I think so?

What I am trying to said is that, if there are multiple CTC sharing the same endpoints, how do we know the numebr of these will not diverge since it can be any number of CTC that can be formed in that region?

because for each CTC you have a worldline revisiting the same point many times

and so to other observers within the region, it as if seeing many copies of some observer at once

?

dmckee

@0celo7 Positrons have all the interactions electrons do plus a significant cross-section for annihilation, so pretty poor. But the annihilation gammas penetrate reasonably well.

0celo7

4:53 PM

@dmckee Ok. We have a positron source in the lab, but it's behind some lead barriers and a huge wall of cubicles and computers.

Slereah

You can just go forward, take the lightcone heading backward, go back a bit, flip again and go back to the original spot

0celo7

And the people on our side of the cubicles don't need badges, but the people on their side do.

dmckee

Don't take a nap by it. Other than that you should be good. If' you're worried borrow a survey meter...

0celo7

Just wondering if that's legit.

Secret

For the above scenario, it can possibly arise if the mouths of the wormhole are only meters apart but differ in decades

So it is possible for an observer to keep looping back and form a CTC that looks like a spiral in the spacetime diagram

But they are many such spirals that can form for given endpoints , which different turns , so are we expect to see infinite number of observers for that region of spacetime?

Slereah

4:54 PM

I wonder if you can do a simple spacetime like that

Maybe just

dmckee

@0celo7 Probably according to policy, which is always set on the conservative side of "slightly paranoid".

Slereah

Take the flipping in the metric

Like a 2D spacetime like...

0celo7

@dmckee ORNL employees wear three badges inside the reactor building

users only need two

Slereah

$ds^2 = -cos(x) dt^2 + sin(x) dx^2 + f(x) dxdt$

Hm, let's see

0celo7

and apparently they get way more radiation from their yearly chest X-rays

which are legally mandated for "radiation workers"

Slereah

4:56 PM

The signature is $- cos(x)sin(x) - f^2$

skull petrol

Has anybody gotten cancer? @0celo7

Slereah

Or $\frac{1}{2} \sin(2x) - f^2$

0celo7

@skullpetrol It's statistically impossible for someone at ORNL to not get cancer...

Slereah

So if $f = 1$ the signature is guaranteed negative

skull petrol

-_-

Slereah

4:58 PM

Hm, is this spacetime non-time orientable tho

0celo7

@skullpetrol dude go get face surgery or something

Slereah

I think so

But not quite sure

If identified along $x = 0$ and $x = \pi$

0celo7

can you have a non time orientable spacetime where you don't have some kind of identification?

dmckee

@0celo7 Doesn't surprise me. There are several ways for a badge to read wrong and if they need a really reliable record they'll want some redundancy.

Slereah

No

Spacetimes that are simply connected are always time oriented

0celo7

5:00 PM

proof?

Slereah

I forget

skull petrol

Are you trying tell me you wanna rearrange my face pal? @0celo7

Slereah

I think it's in that paper on the line element on Lorentzian manifolds

0celo7

@skullpetrol is Wash playing Oakland this season?

Secret

@Slereah Or is it not very sensible to consider multiple worldlines get cramped between two points hence the problem is not valid?

I am not sure if you get what I am trying to ask, because I am seriosuly lacking terminology

Slereah

5:03 PM

Not quite sure what you are talking about

Hm, let's see

I think a simple curve would be just a half circle in that metric

$U^\mu = (\cos(\lambda), \sin(\lambda))$, I think

Wait or would that be $x$

skull petrol

Nope @0celo7

Secret

@Slereah
Consider two mouths of a wormhole which are 1 lightminutes apart and the difference in time between the mouths are 2 years.

1) Then theoretically an observer can jump into the wormhole to go back to some earlier time coordinate, and then travel towards the entrance again, rinse and repeat until he/she reaches a time coordinate where the wormhole was first converted into a time machine?

Yes.

@skullpetrol then no

:D

5:08 PM

Let's see

0celo7

@Secret what

Danu

@ACuriousMind That was a joke :P

Slereah

$g(U,U) = -\cos(x)^3 + \sin(x)^3 + 2 \sin(x) \cos(x)$

Hm wait

Does that make sense

Danu

@ACuriousMind I'm pretty sure "you jelly, bro" works just fine :)

Secret

@Slereah
2) But there are many ways the observer can travel so that he/she end up going in a CTC

For example, he she can decide to travel back 5 times, then wait 10 years at the entrance to get back to where he/she started

Or he/she can travel back 3 times, then wait 6 years to end up back where he/she started in spacetime

Or he/she can travel back 3 times, then accelerate towards the entrance mouth in a twin paradox fashion to get bakc where he she started

There are literally a continuum many of such trajectories the observer can take, so for another observer who just entered the regio…

Slereah

5:12 PM

There are also many ways to travel without going back in time

skull petrol

Are you going to take a date to the talk? @Danu :P

Slereah

For a halfcircle starting at $t = 0, x = 0$, that would be $x(\lambda) = \frac{\pi}{2} (\cos(\lambda) + 1, \sin(\lambda))$

ACuriousMind

@Danu Oh.

skull petrol

If yes, then I'll change my answer @Danu :D

Slereah

So the velocity will be $\frac{\pi}{2} (-\sin(\lambda), \cos(\lambda))$

And $g(U,U) = -\cos(x) \sin^2(\lambda) + \sin(x) \cos^2(\lambda) - \sin(\lambda) \cos(\lambda)$

Secret

5:17 PM

@Slereah yes but those that does not involve any going back in time will not be an observable consequence because their light signal will never reach us (because it is travelling towards the future direction)

The case is very different for CTC regions, because if one can go back in time, then infromation about them can be transmitted with the velocity bound by the light cone towards the future, where some observers some time coordinate ahead of them can possibly received.

But if there are a continuum number of possible such ways to get back to essentially the same point in spacetime, then…

Slereah

Diagrams won't help you much to tell you what exactly happens to objects along those CTCs

That's when you need to solve the Cauchy problem

ACuriousMind

@Secret Just because there is a possiblity doesn't mean it has to be realized. I have no idea what you are talking about

Slereah

If you want there's some rather low level papers on the topic using billiard balls on CTCs

skull petrol

Your semantic analysis of envy vs jealousy was impressively rigorous @ACuriousMind

Slereah

Yes, it would be impressive

IF

ACuriousMind

5:19 PM

@skullpetrol The Simpsons are educational

Slereah

Hm, let's see

ACuriousMind

@Slereah You mean "if not" ;)

Secret

@Slereah
http://plato.stanford.edu/entries/time-travel/

I have read the stuff here 2 years ago, which involve some of the billard ball example you talked about (which is what inspired me the diagrams)

But as you said, it seems to do this rigorously I need to solve the cauchy problem
I think I will read more about in that direction then (because I am not famialr enough with the term cauchy problem)

Slereah

$g(U,U) = -\cos(x) (1 - \cos^2(\lambda)) + \sin(x) \cos^2(\lambda) - \frac{1}{2} \sin(2\lambda)$

$g(U,U) = -\cos(x) + (\sin(x) + \cos(x)) \cos^2(\lambda) - \frac{1}{2} \sin(2\lambda)$

Fuck how do I show it is negative

Well $x$ is within $0$ and $\pi$ so $\sin(x)$ is positive

And that curve is also in that range

Blargh

Let's just check for a few values

But wait, $x$ depends on $\lambda$

Aaaah

whatever

Secret

Slereah had confirmed my understanding of the following:
1) Then theoretically an observer can jump into the wormhole to go back to some earlier time coordinate, and then travel towards the entrance again, rinse and repeat until he/she reaches a time coordinate where the wormhole was first converted into a time machine

I then ask about the many possible ways an observer can travel via a CTC back to where he/she just started entering the wormhole, such as going back the wormhole 2 times then wait until 4 years has elapsed in his frame

ACuriousMind

5:29 PM

@Secret Of course we expect that the actual world obeys the principle of least action (which may get arbitrarily complicated if gravity is not the only interaction at play).

Slereah

What about

THE PRINCIPLE OF MOST ACTION

Technically we only require that the variation vanishes

It could be a maximum!

"Non-time-orientable lorentzian cobordism allows for pair creation"

Oh boy

Let's see

Maybe a metric!

>sen

ugh

Gonna be a weird function again

ACuriousMind

@Slereah Could even be a saddle point. Technically, it's the principle of stationary action.

Slereah

Oh apparently sen is sin in spanish?

But this isn't a spanish book :O

ACuriousMind

lol

Secret

@Acuriousmind and @Slereah I see (and I hope I have made the question make sense to you)

I should do more reading then

Slereah

5:35 PM

I think it's the metric I wrote?

Not quite sure since he didn't write it

but it seems to be

John Duffield

@Slereah : "Show that electrons are photons on a moebius strip". Wheeler didn't understand the difference between curved spacetime and curved space. Which is why he called them geons: "a geon is an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy. They were first investigated theoretically in 1955 by J. A. Wheeler..." And here we are, fifty years later.

Slereah

Please Duffield

No need to talk to me.

I don't care.

skull petrol

::popcorn::

Want some^

?

Secret

Ok, so my plan:
1. Read about cauchy problem and understand what it is (with all the important math details)
2. solve the cauchy problem in the S^2 # T2 x R spacetime
3. compute the relativistic lagrangian and einstein action
4. Apply variation princple and obtain the Equation of motion
5. check with slereah on results

Sounds like a good plan or am I missing some important steps?

My aim: Trying to understand more on the dynamics of CTCs

Slereah

a bit too ambitious

Perhaps first learn basic GR and math

skull petrol

5:43 PM

^

Secret

I think the word will be revise, as 0celo7 pointed out I am just keep on forgetting stuff I learnt before

but otherwise I should do it

Slereah

if you want to know about the Cauchy problem learn about like

The Picard Lindenhopf theorem first I guess?

Picard–Lindelöf theorem

In mathematics, in the study of differential equations, the Picard–Lindelöf theorem, Picard's existence theorem or Cauchy–Lipschitz theorem is an important theorem on existence and uniqueness of solutions to first-order equations with given initial conditions. The theorem is named after Émile Picard, Ernst Lindelöf, Rudolf Lipschitz and Augustin-Louis Cauchy. Consider the initial value problem Suppose f is uniformly Lipschitz continuous in y (meaning the Lipschitz constant can be taken independent of t) and continuous in t. Then, for some value ε > 0, there exists a unique solution y(t) to the...

The lindy hop

Secret

ok

footnote: The major drive that drives me on researching time travel as my hobby:
I HATE time travel movie PLOTHOLES and I want to get rid of EVERY SINGLE ONE OF THEM ~lol

ACuriousMind

You can do time travel without plotholes without physics.

Secret

but that's not scifi, that's science fantasy

people having been doing that and they still got plotholes everywhere

skull petrol

5:48 PM

Finding the right book in step 1. Is the most important part of your plan imho.

Secret

@skullpatrol any recommendations?

Slereah

amazon.com/dp/1933929936/…

This book

ACuriousMind

@Secret SciFi is really not about the science. You may distinguish between hard science fiction and soft science fantasy depending on how hard they try to stay scientifically plausible, but the presence of plot holes is the fault of the writer either way, and has nothing to do with the science.

Slereah

If you want a somewhat hard treatment of time travel you can try Stephen Baxter's books

ACuriousMind

For instance, I think the time travel arc in Babylon 5 is reasonably well done - no scientific explanation for it is given, really, but a there's stable time loop that doesn't have plot holes in it

Slereah

5:51 PM

Also a lot of times the problem isn't that they are plot holes

They are not plot holes but plot devices

There for the narrative's benefit

skull petrol

@ACuriousMind have you read Edgar Allan Poe's science fiction work?

ACuriousMind

@skullpetrol No

Didn't even know he wrote one till now

Secret

@ACuriousMind I don't say the science causes the plotholes, but it is known that time travel stories tend to have a lot of plotholes, probably because of its complex narrative structure it introduce

and time travel, as my my knowledge goes, is still not very well understood because there are no expt examples to check with

will check babylon 5 out

@Slereah I should also give Stephen Baxter's a read.

@therest Thanks guys

Slereah

A lot of early time travel books were mostly an excuse to have historical settings, really

ACuriousMind

@Secret Yes, many writers are lazy and write bad time travel stories. Still, I'm firmly convinced that the plotholes and inconsistencies have nothing to do with them not knowing physics, and everything with them just not thinking hard enough to make it logically consistent

skull petrol

5:55 PM

Have fun @Secret :-)

Slereah

there was a book where the hero time travels, changes genders, and becomes his own mother and father

It was "All you zombies" I think

ACuriousMind

And then, there's the kind of stories where you're expected to suspend disblief so it's not supposed to make sense of you overanalyze it.

@Slereah Yep, Heinlein,

Slereah

"The man who folded himself" also has a bunch of time travel orgies with yourself

Secret

@ACuriousMind knowing the physics helps on the thinking, IMO, especially the physics help train you the logical thinking needed to join the dots together

time travel is a very complex thing, I myself have spent 3 years on studying the media side of it, and I planned to study the physic side of it the next step

ACuriousMind

@Secret You want physics in your novels - go read Greg Egan. He tries to write really hard science fiction. It's interesting, but his stories tend to be pretty dull if you're not interested in physics.

Slereah

5:59 PM

He wrote a story about spacetime having a different signature

Secret

ok

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