The h Bar

You might like it, @heather. It's essentially a enormous labyrinth, but set up so that you can only go certain places after finding the right equipment.

heather

Super Metroid?

DanielSank

Yeah.

heather

oh, that does sound cool.

DanielSank

1:46 AM

Ninja Gaiden 2 is from the original Nintendo, so you can play that on emulator. Not sure you'd like that though.

You can probably play Smash Brothers Melee on an emulator too, but to be honest that game is really only fun in multiplayer.

It's my favorite multiplayer game ever... like... including board games and sports etc.

Smash Brothers Melee is simply the best one-brain-versus-the-other game I've ever played.

heather

didn't John Rennie say awhile back that you could play it on his server?

DanielSank

@heather o_O

h- how?

heather

@BernardoMeurer sorry I didn't respond right away...section 1.4?

@DanielSank oh, maybe not, I don't know.

i must be misremembering.

heather

2:08 AM

hmm, I really need to clean something up somewhere

i'm having storage space troubles and i don't even know where almost all of it is going.

it lists 372 MB in use, but apparently I overall have 9.4 out of 10.2 GB in use (!) which doesn't make sense.

i guess it's all the stuff on the linux side of my computer?

DanielSank

yikes

A lot of it could be stuff like the arduino IDE.

Look up how to ask your computer how much storage is used by various parts of the fiel system and sniff around a bit.

heather

I deleted the arduino IDE because I started having problems after that was installed, but I'm still having problems.

i'll google that.

DanielSank

Yeah just look around.

I'm having a hard time imagining how you've used that much space. Ubuntu is probably a couple G though.

I have no idea what the Chromebook stuff uses up.

You could try to acquire a regular laptop with, you know, 250G hard drive space...

heather

that would be nice =)

DanielSank

2:24 AM

Yah.

heather

hmm...there's a couple weird lines - it says 885M ./Downloads/.Trash-1000/files/arduino-1.8.0

DanielSank

kill it

heather

but that's the thing

i've already emptied trash

i looked in downloads

DanielSank

what's your working directory?

heather

there's nothing there but crouton and I sure don't want to delete that.

DanielSank

2:25 AM

./Downloads is only meaningful if we know where you are.

heather

/home/heather

is where I'm at.

DanielSank

I don't know why there's a .Trash directory under Downloads, but I'm pretty sure you can nuke it.

heather

but there's no trash directory under downloads - i just cd'd into downloads, typed ls, nothing came up except crouton.

DanielSank

It might be where your browser puts stuff you've downloaded if you "delete" it from within the browser. That's just a guess.

Do ls -la

-l means "list" and -a means "all, including hidden stuff".

In Linux, a file or directory starting with a dot is "hidden", which literally just means "doesn't show up in ls unless you add -a.

heather

drwx------ 4 heather heather 4096 Dec 24 19:54 .Trash-1000

is one of the lines

so there's the mysterious file.

DanielSank

2:29 AM

Well there you go.

heather

how do I delete it?

DanielSank

rm .Trash-1000

rm means "remove"

If it's a directory containing other stuff it will complain.

heather

rm: cannot remove '.Trash-1000': Is a directory

yep

DanielSank

yah

ok, so to remove a directory, it's rmdir, but...

that only works if the directory is empty.

heather

curses

DanielSank

2:30 AM

To remove a directory and all of its contents, you do this:

rm -rf <name of directory>

BE CAREFUL WITH THAT

heather

not typing it in yet

DanielSank

-r is for "recursive", meaning "this and all subfiles/subdirectories.

heather

why's it so dangerous?

oh gosh

recursive deleting sounds...scary.

0_0

maybe I'll cd into the directory and see what all's in there.

DanielSank

Yeah, if you rm -rf the root directory of your system... you'll be really, really sad.

heather

i'm sure.

yep, it's all arduino stuff

::takes deep breath::

well, if I'm out of this chat for a wee bit, you'll know why

DanielSank

2:34 AM

lol

heather

i'll run that command ::winces::

DanielSank

there are ways to be safe about it.

heather

oh, there are?

like what?

@Kaumudi.H, hallo

DanielSank

For example, let TAB completion fill in the file name so you know you're deleting something in the working directory.

heather

tab completion? what is this magic?

DanielSank

2:35 AM

Also, don't ever try to rm -rf an absolute path, i.e. don't do this:

rm -rf /some/directory/somewhere/with/a/file

^ DO NOT EVER DO THAT

3

heather

^WON'T EVER DO THAT

3

DanielSank

Instead, cd to where the file is, and then do

heather

why am I not doing that?

user228700

Oops. Hey, you guys :-)

DanielSank

rm -rf file

heather

2:36 AM

okay

DanielSank

Anyway, tab completion just means that if you type

rm -rf fi

and then hit TAB, it should finish fi into file, assuming you have a file named file, lol.

This is nice because it guarantees you're deleting something in your local directory.

heather

hmm, it isn't doing that.

DanielSank

Oh weird, that might be that TAB completion doesn't work with hidden files.

heather

wait, can i just delete each individual file, then each directory, and so on until I'm done?

this command is scaring me.

DanielSank

I gotta go for a bit. Best of luck.

heather

2:37 AM

okay

thanks for your help

DanielSank

Oh, yeah you could do that, but yuck, that would take forever.

Two more things:

heather

true

DanielSank

1) You can use the graphical file browser if you have one. That way, you know exactly what you're deleting.

heather

i dunno what that is, unfortunately. hmm...

DanielSank

2) Don't be too scared of rm -rf. I'm just telling you all the things that can go wrong.

heather

2:38 AM

"file manager"

DanielSank

Yeah that.

heather

okay, how do you see hidden files in that?

DanielSank

Probably some option somewhere.

I forget exactly where, but click around the options a bit.

See you a bit later.

heather

yup, found it

under view

okay, thanks!

"unable to trash file - invalid argument"

user116211

3:05 AM

@DanielSank, hats off for asking this; we never got the chance to get the swag.

user116211

I think SE people do make few if you demand for them; maybe some are left too. Recently there was sort of some contest to get one left over T-shirt in Movies&TV too.

user116211

33

Q: 5 Years of Movies & TV - Free T-shirts!

Movies & TV Stack Exchange celebrates its 5th anniversary! We'd like to take this opportunity to thank the community of this wonderful site for the years of dedication and hard work they put into it. And luckily we can even thank you materially, since as it turns out, we still have a T-shirt lef...

David Z

@heather are you running KDE?

(for your graphical desktop environment)

user116211

@DavidZ, you should wear some hat!

user116211

Before they are gone :(

David Z

3:10 AM

I have a hat :-P

user116211

uff ;/

user116211

You people celebrate Christmas in China, right @DavidZ?

David Z

Not really

user116211

:(

David Z

It's sort of a shopping holiday but not an official holiday. E.g. schools and businesses don't close or have special hours.

user116211

3:12 AM

oooh :|

David Z

There are decorations though.

user116211

:)

user116211

@DavidZ Merry Christmas!

David Z

@MAFIA36790 you too

user116211

0

Q: How to prove that the average of kinetic operator is positive for bound state?

How to prove that the average of kinetic operator T is positive for bound state?

quantum-mechanics

user116211

3:14 AM

Homework?

user116211

No efforts too.

David Z

I'm actually back in the US so it's not quite Christmas yet, not that it matters

DanielSank

@DavidZ Lol

David Z

@MAFIA36790 seems like it

DanielSank

@heather weird. Use the command line I guess.

user116211

3:15 AM

Could you please attach what you have tried? — Unnikrishnan 45 secs ago

user116211

Glad that new people are coming forward and understand how the site works :)

David Z

Indeed :-)

DanielSank

@MAFIA36790 And saying it nicely.

o/

user116211

@DanielSank yes.

user116211

@DanielSank :P

user116211

3:17 AM

\o

user116211

7

user116211

Two important things happened today:

1. Sir Newton was born.

2. The Soviet Hammer and Sickle flag was lowered for the last time leading to the collapse of USSR.

DanielSank

Interesting.

user116211

indeed

DanielSank

3:34 AM

I need to go wrap the rest of my gifts.

user116211

4:07 AM

0

Q: Are these correct definitions for a quantity called 'center of shape'?

Today, I thought of a new quantity. I don't have a proof but I think it should give the geometrical center of any body. The position vector of the center of shape of any body is given by the sum of the position vectors of all the particles constituting the body divided by the number of particles ...

mass

user116211

Don't know what to say about this.

Swapnil Das

Merry Christmas all!

user116211

Merry Christmas.

Swapnil Das

@MAFIA36790 What plans for today?

user116211

4:23 AM

My exam is just concluded; no plans for now; have to submit some sheets and return books to the library; might have a feast in the evening.

Swapnil Das

Cool, enjoy!

Oh, Newton's B'day today.

Physics Meta

5:12 AM

0

Q: may i ask a question about philosophy of physics?

is it permitted or not or frowned upon? i couldn't find a tag for it. but i hope you'ld rather see discussions about "what is physics?" and "falsifiability" (and some of the stuff that Smolin complains about) here among physikers, than in a purely philosophy-bent forum. can we ask in the physi...

discussion

Bernardo Meurer

Merry Christmas folks

@heather We can work on it tomorrow :)

user116211

@BernardoMeurer Hey, bernardo; merry Christmas and a happy Hanukkah.

Bernardo Meurer

@MAFIA36790 You too man!

user116211

@PhysicsMeta The tag has been burned down.

user228700

6:14 AM

@JohnRennie@Kenshin: Merry Christmas :-)

Physics Meta

6:37 AM

0

Q: How to prove that the average of kinetic operator T is positive for bound state?

How to prove that the average of kinetic operator is positive for bound state?

support homework

Swapnil Das

Hi.

Secret

google.com.au/…

Something that has no future

Swapnil Das

Philosophy of physics, indeed a cool topic.

Secret

7:05 AM

Physics Guy

Merry Christmas

Secret

7:25 AM

user228700

7:41 AM

@SirC: Hey, Merry Christmas! :-)

Sir Cumference

@Kaumudi.H Thanks, Happy Chanukah ;)

user228700

Oops, um, I'm not very familiar with these holidays. I'm sorry for supposing that you celebrate Christmas :-/

Sir Cumference

@Kaumudi.H Nah, I'm just kidding around

But I am actually Jewish :3

user228700

Oh, okay. So you don't celebrate Christmas?

Sir Cumference

Nope. Christian holiday.

user228700

7:45 AM

Ohh, so what dyou guys celebrate?

Sir Cumference

Well on Chanukah, I think it celebrates a war the Jews won when they were severely outnumbered and outmatched. Then they lit some candles with olive oil and the candles miraculously stayed lit for 8 days

So now we light one candle for each of the eight days of Chanukah. And we give a present on each day.

user228700

Oh, that's cool, thanks for telling me. I've always wondered but never bothered to actually look it up :-P

Sir Cumference

Say, you have any holidays you celebrate where you give presents?

user228700

For the majority of Indians, that would be Diwali, I guess. "We" burst firecrackers, wear new clothes, eat many many sweets and give gifts, I suppose.

Sir Cumference

That sounds nice. What about for you?

user228700

7:49 AM

But notice that I said "for the majority". My family doesn't...

Sir Cumference

:/

user228700

We only celebrate two extremely boring festivals and I don't live in my hometown so I've never actually celebrated either of them the traditional way.

user228700

And no, no gifts :-( Just birthdays gifts, once a year.

user228700

Dyou guys celebrate any other festival?

Sir Cumference

@Kaumudi.H Well Jews celebrate Passover, where we have to go a week without eating anything with flour

So no bread :(

We also celebrate Yom Kippur, where we have to go 25 hours without food, water or electricity

user228700

7:51 AM

@SirCumference Oh, I see. We have that for one day every month, I think.

user228700

@SirCumference That's absolutely crazy. What's the celebration even about?

Sir Cumference

@Kaumudi.H I think it's about God

I'm not too sure on the details XD

user228700

:-P Okay. Sheesh, not even water?!

Sir Cumference

Nope

user228700

What's with 25?

Sir Cumference

7:53 AM

@Kaumudi.H Day and an hour? Not sure lol

Guess that shows how knowledgeable I am regarding my own religion

user228700

Okay :-P I was wondering what that extra hour is about.

user228700

I'm not too knowledgeable either, so you're in good (hmm, bad? :-P) company. On second thoughts tho, I'm not sure if I even identify with my religion so that's that, I guess...the only thing I've learned from my religion are the values of being a vegetarian, I guess.

user228700

So when's Chanukah? The same day as Christmas..?

Sir Cumference

@Kaumudi.H Well the Jewish holidays use the lunar calendar, not the solar calendar people use nowadays. So because the lunar calendar doesn't exactly match up with the solar calendar, the dates for the holidays shift every year.

This year it started yesterday (the 24th) :)

user228700

Ah, I see...

Sir Cumference

7:58 AM

It usually takes place around Christmas time

@Kaumudi.H Yep, it confuses everyone

Especially me, when I'm never sure when the holiday will start until the last minute

user228700

Cool. Have u gotten any nice presents so far?

Sir Cumference

I got a watch that doubles as a star chart

Looks like this one

redditgifts.com/gallery/gift/amazing-star-chart-watch

user228700

Whoa my Gawd, that looks very very cool.

Sir Cumference

Yup :D

Gonna wear it every day

user228700

:-D Nice.

Sir Cumference

8:01 AM

Gotta head to sleep now, it's 3:00 am

'Night

user228700

G'night, have a great holiday season :-)

Secret

[Zero term algebra] Whatever these semigroups equipped with a distributive law things are, it seems the closest structure they resemble are near-semirings.
In fact, they are kinda "inverted" versions of near-semirings in that instead of having the + structure a monoid and * structure a semigroup, we have + structure a semigroup (which is almost like a "one sided version" of group since all elements have one sided inverses and a opposite sided identity) and the * structure a monoid instead (which commutative examples exists) while the usual distributive law holds.

Secret

8:15 AM

For all your timeline editing needs

newgrounds.com/portal/view/685942

koolman

8:27 AM

Merry christmas to everyone !!!!!

2

Swapnil Das

Merry Christmas.

2

Physics Guy

8:57 AM

Merry Christmas

2

Secret

CTCs!

If CPT symmetry holds, a particle travelling back in time is indistinguishable to an antiparticle moving forward in time

GR does not allow this: You cannot travel back to a time before the wormhole time machine is made. By symmetry, you cannot ever dreamt of finding a wormhole mouth in the present that sent you the future where the wormhole is first formed.

Pissed off layman

9:20 AM

Do you @Secret know how to post a YouTube video in here to start at particular time?

For example @t=5:30

Secret

no idea, I don't think the chat room has a plugin that work similarly to timesteps in youtube

Pissed off layman

Thanks for replying :-)

Secret

Ok, this one is so strange, might be Slereah worthy material to analyse:

If the arrow is pointing this way, then it means in 3 seconds, the astronaut will enter the wormhole and appeared 3 second in the past, which is right now. Therefore, another astronaut appeared, which is strictly speaking, his future self at the present, now heading towards the exit pad

It is also interesting that this might be able to be handled by GR because literally all changes occurs in the present time. Experimentally, one can replicate this scenario as follows:

1. Let the starting point be $t=0$

2. At t=4.5 the pad was flipped, directing a ball bearing towards the location where a wormhole time machine is going to be formed

3. At t=5, the wormhole time machine forms

Now, depending on which time travel model, many things can potentially happen:

1. If this is type 1 (i.e. GR CTCs), it means all of thise is predestinated and the trajectory of the ball bearing will be part of a CTC

2. If this is type 4 (branching timelines), then there will be no duplicates of the ball bearing appearing instantly at the other end of the wormhole no matter what circumstances

3. If this is type 3 (Ripple dynamics or back to the future malleable timelines), then there will be a nonzero delay before the ball bearing duplicate appearing at the other end of the wormhole

4. If this is type 3 (instataneous overwritting malleable history), then the outcome will be indistinguishable from 1.. However, should one decided to alter the trajectory of the ball before it reached the wormhole, then there's a nonzero probability the duplicate will instantly disappears

Secret

9:38 AM

conclusion: Time travel might be experimentally testable, provided one can figure out how to produce the wormhole time machine in the first place

Secret

Proposed experimental setup for the above arrangement:
1. Create/Find a wormhole
2. Send a ball bearing, particle whatever towards a spot where the past mouth of the wormhole is going to be positioned.
3. (If found instead of create), move the wormhole so that one mouth is somewhere in deep space, and the other mouth is somewhere in a massive body such as the earth's surface
4. If our spacetime allows time travel of type 1 or type 3 (instantaneous), then there should be a duplicate appearing at the other mouth of the wormhole before the original entered it

*6. If chronology protection conjecture holds, then it should be impossible to
A) create a wormhole time machine
B) move an existing wormhole such that the aformentioned "overwritting the future" scheme can be carried out
C) There are no duplicates because regardless of number of trials, the original can never enter safely into the wormhole

typo: future mouth, not past mouth

Secret

9:55 AM

More simply, anything that is sent towards the future mouth of a wormhole before the wormhole forms (or move into position) later, the past mouth of the wormhole should have the duplicate appearing very near the present time as seen in the frame of the object being first sent along a trajectory towards the future mouth.

Thus to the object, the wormhole time machine arranged in this manner will act like a duplicating machine

Scenario: There's a wormhole CTC already in position in the area, represent by those 'teleporter' looking pads

user116211

-3

Q: Hardest 3 multicorrect questions

Strong reflection by plate. initially compressed then released switch closed at t=0

homework-and-exercises visible-light

user116211

This poster is blatantly posting homework questions.

user116211

I tried to make him aware and so one other user; but he doesn't seem to understand.

user116211

Some non-constructive comments are also there.

user116211

This is a blunt homework query; Physics.SE is not a homework-solving site; show some effort to work through the problem. And also be nice in your comments, users here use part of their valuable time from their precious schedule to help the others; moreover using @user wouldn't work unless he/she commented here. Be constructive. — MAFIA36790 9 mins ago

user116211

10:02 AM

$\uparrow$ left this.

user116211

but of no avail ;(

Secret

The astronaut (alternately can be replaced by a beam of photons) triggers a switch 3 seconds later, which not only lowers the door (which for our simplified experimental setup can be removed), but also flips the arrow pad (which in our experimental setup will be some kind of optical element that splits or direct the laser towards the future mouth

user116211

What are you doing @Secret?

Secret

As soon this happens, the path of the laser to the future mouth is set, thus laser will start to sprew from the past mouth (which is positioned right at the present in our frame due to how the experiment is set up (meaning that the wormhole is moved into position not long before it first formed)

There's a rewind unity game with interesting time travel mechanic. On this particular level, everything seemed to be able to reduce to GR stuff and one can easily trigger a "duplicate making scenario" by flipping a switch.

This suggest should wormholes were found, and they are not formed not very long, one can set something towards it and depending on the nature of time travel in our universe, a duplicate may or may not be observed at the other end (where the past mouth is roughly at the same time as our present)

Or in short, this game might provide an experimental setup to test the nature of time travel in our universe since all the changes took place in the present or future lightcone of our present

user116211

ooh. Sounds interesting; enjoy.

Secret

10:11 AM

When slereah is on, I am going to ask him about the plausibility. Since this one does not involve any notion of "overwritting the past", it might be actually possible and well within the domains of GR. The onyl issue, of course is to make or find a wormhole in the first place, moments after it formed

Experimental scheme to be posted shortly

Secret

10:30 AM

Experimental procedures:
1. Find or create a wormhole. This wormhole has both mouths "comoving" as they are both in the present.
2. Set up the experimental setup as shown in the diagram.
3. Subject anything except photodiode 2 and the mouth labelled past mouth under a sufficiently strong gravitational potential (This can be done relatively easily by having these components located on the surface of the earth, and the mouth labelled future mouth placed in near earth orbit. A time dilation in the order of seconds is more than enough for the experiment)

Pissed off layman

10:43 AM

@Secret "time steps" don't work in YouTube anymore :-/

Secret

Better diagram

The blocker is used to mimic the grandfather paradox (Duplicate at the past mouth, which should be the future version of the original laser pulse, should vanishes when the blocker is raised)

Swapnil Das

10:58 AM

hi

Secret

11:11 AM

t2=Time in theory to travel from the future to the past wormhole from photodiode 1
All taus measured wrt t0. Tau is the proper time of the experimental setup other than the future mouth.

$\tau_1$ = Time taken for the pulse to reach photodiode 1 (Obtained by setting x close to 0)

John Rennie

@Kaumudi.H Merry, erm, December 25th :-)

BTW your cable should be delivered today.

Swapnil Das

@JohnRennie Is it 26th there?

John Rennie

Here in the UK it's 11:20 on Dec 25th. About 17:00 in India?

user228700

@JohnRennie Huh? I thought u sort of do celebrate x'mas..?

John Rennie

@Kaumudi.H you have mail

user228700

11:21 AM

@JohnRennie Oh, cool! (Just saw the mail)

Swapnil Das

@JohnRennie Yeah. x mas time.

Secret

John Rennie

@Kaumudi.H Yes, we do. But I can't really say Merry Christmas to a non-Christian

user228700

@JohnRennie Ohh, um, why not..?

Secret

John Rennie

11:22 AM

@Kaumudi.H because you don't celebrate Christmas ... ?

user228700

The logic checks out but I guess I'm just used to wishing people a merry Christmas and well, they wish right back, for some reason.

Swapnil Das

True. And who doesn't want a merry Christmas?

user228700

@JohnRennie: Anyhoo, how's ur Sunday and Christmas going? :-)

John Rennie

@Kaumudi.H I've only just got to my mother's house (three hour drive!) and I've been busy installing her new PC. So I haven't had a chance to relax and enjoy Christmas yet.

user228700

Ah, I see. Installing the P.C sounds like fun :-) Wokay, u have fun, Sir!

Ramanujan

11:32 AM

Who got what from santa claus?

Just asking

Secret

The following might be nonsense because without QG we cannot predict anything about it:

Presumably, this may be calculated by assigning a wavefunction to each history that depends on the mirror angle and then perform a feymann integral for each $\theta$

But since I have virtually no rigorous background in QFT, what I said might be nonsense

user228700

12:30 PM

@DanielSank: Merry Christmas :-) Remember how I went on and on about Kerala, the other day? Well, this what I was talking about:

user228700

holidify.com/places/thenmala/photos.html and thenmalaecotourism.com

heather

Merry Christmas everyone!

user228700

12:46 PM

:-) @heather: Merry X'mas!

user228700

@JohnRennie:Ooh, cool hat! :-D

heather

@Kaumudi.H, thanks! I know you don't celebrate Christmas, but, Happy Holidays, then how about? =)

user228700

Thanks :-) (Although I'm not on holiday)

heather

oh, I'm sorry, how about Happy Not-On-Holiday?

=P

I'm pretty sure that's just have a good day.

=)

@DanielSank, deleted that file and all went well, I now have 1.2 GB of space left. I still don't know where all of that's going, though, I'll keep looking.

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